Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device. The secondary battery comprises a positive pole piece and electrolyte, the positive pole piece comprises a positive current collector, a positive material layer and a protective layer, the positive material layer and the protective layer are both located on at least one surface of the positive current collector, the positive current collector comprises a main body area and a tab area in the width direction of the positive current collector, the positive material layer is arranged in the main body area, and the tab area is arranged in the main body area. The protective layer is arranged in the tab area; the area proportion of the protective layer covering the tab region is S, 0.6 < = S < = 1, and the contact angle theta between the protective layer and the electrolyte is greater than 90 degrees and less than or equal to 165 degrees. The secondary battery provided by the invention has good safety performance.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a secondary battery and electronic device. Background Technology

[0002] Secondary batteries, such as lithium-ion batteries, are widely used in smartphones, wearable devices, consumer drones, and electric vehicles due to their high energy density, long cycle life, and lack of memory effect. Among these, compounds containing fluorosulfonyl groups (e.g., lithium bisfluorosulfonylimide (LiFSI) and lithium bistrifluoromethanesulfonylimide (LiTFSI)) exhibit superior performance compared to traditional lithium salts like LiPF6 in terms of thermal stability, electrochemical stability, conductivity, solid electrolyte interphase (SEI) film formation, and reduced gas generation when used in lithium-ion battery electrolytes. Therefore, they hold great promise for high-performance lithium-ion batteries. However, the use of fluorosulfonyl groups in electrolytes can corrode the positive electrode tabs and current collectors, leading to deterioration of the electrochemical performance of lithium-ion batteries and increasing the risk of safety issues, thus affecting the overall safety performance of lithium-ion batteries. Summary of the Invention

[0003] The purpose of this application is to provide a secondary battery and an electronic device to improve the safety performance of the secondary battery. The specific technical solution is as follows:

[0004] The first aspect of this application provides a secondary battery, including a positive electrode and an electrolyte. The positive electrode includes a positive current collector, a positive electrode material layer, and a protective layer. Both the positive electrode material layer and the protective layer are located on at least one surface of the positive current collector. Along the width direction of the positive current collector, the positive current collector includes a main body region and a tab region. The positive electrode material layer is disposed in the main body region, and the protective layer is disposed in the tab region. The area ratio of the protective layer covering the tab region is S, where 0.6 ≤ S ≤ 1. The contact angle between the protective layer and the electrolyte is θ, where 90° < θ ≤ 165°. By designing the structure of the positive electrode and controlling the values ​​of S and θ within the above ranges, this application can reduce the contact between the positive electrode tab, the positive current collector, and the electrolyte, mitigate the corrosion of the positive electrode tab and the positive current collector by the electrolyte, extend the service life of the secondary battery, and thus improve the safety performance of the secondary battery.

[0005] In one or more embodiments of this application, the secondary battery satisfies at least one of the following characteristics: (1) 0.8 ≤ S ≤ 1; (2) 120° ≤ θ ≤ 165°. By adjusting the values ​​of S and θ within the above ranges, it is beneficial to further improve the safety performance of the secondary battery.

[0006] In one or more embodiments of this application, the electrolyte includes a compound containing a fluorosulfonyl group, which includes at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, or lithium bis(pentafluoroethanesulfonyl)imide. The concentration of the fluorosulfonyl compound in the electrolyte is c mol / L, where 0.2 ≤ c ≤ 2. When the electrolyte includes the aforementioned fluorosulfonyl compound and its concentration in the electrolyte is within the above range, its use in conjunction with the positive electrode sheet provided with a protective layer in this application can reduce the corrosion of the positive electrode tab and the positive electrode current collector by the electrolyte, thereby giving the secondary battery good safety performance.

[0007] In one or more embodiments of this application, the protective layer includes an oleophobic material, which includes at least one of polytetrafluoroethylene, perfluoropolyether, siloxane, silane coupling agent, nano-silica, or fluorinated silane; based on the mass of the protective layer, the mass percentage of the oleophobic material is Ws%, 80≤Ws≤99%. Using the above-mentioned oleophobic material in the protective layer and controlling the value of Ws within the above range is beneficial to improving the safety performance of the secondary battery.

[0008] In one or more embodiments of this application, the secondary battery satisfies at least one of the following characteristics: (1) the weight-average molecular weight of polytetrafluoroethylene is 10. 6 Up to 10 7 (2) The weight-average molecular weight of perfluoropolyether is 5×10⁻⁶. 5 Up to 5×10 6 (3) The weight-average molecular weight of siloxane is 2 × 10⁻⁶. 3 Up to 5×10 4 (4) The weight-average molecular weight of the silane coupling agent is 150 to 300; (5) The average particle size of the nano-silica is 5 nm to 100 nm; (6) The weight-average molecular weight of the fluorinated silane is 300 to 600; (7) 90 ≤ Ws ≤ 95. The secondary battery meets the above characteristics, which is conducive to further improving the safety performance of the secondary battery.

[0009] In one or more embodiments of this application, the protective layer further includes a dispersant and a binder. The dispersant includes at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium dodecyl sulfate, or polyethylene glycol. The binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber, polyvinyl alcohol, polytetrafluoroethylene, epoxy resin, or polyurethane. The polytetrafluoroethylene has a weight-average molecular weight of 10. 5 Up to 5×10 5 Based on the quality of the protective layer, the mass percentage of the dispersant is Wf%, 0.5 ≤ Wf ≤ 10%, and the mass percentage of the binder is Wn%, 0.5 ≤ Wn ≤ 10%. Using the above-mentioned dispersant and binder in the protective layer, and controlling the values ​​of Wn and Wf within the above ranges, is beneficial to improving the safety performance of the secondary battery.

[0010] In one or more embodiments of this application, the thickness of the protective layer is H1 μm, the thickness of the positive electrode material layer is H2 μm, and the thickness of the positive electrode current collector is H3 μm, where 10 ≤ H2 ≤ 3000, 0.1 ≤ H1 / H2 ≤ 1, and 0.2 ≤ H1 / H3 ≤ 15. By adjusting the values ​​of H2, H1 / H2, and H1 / H3 within the above ranges, it is beneficial to improve the safety performance of the secondary battery while maintaining high energy density and good encapsulation performance.

[0011] In one or more embodiments of this application, 50 ≤ H2 ≤ 500, and / or 0.2 ≤ H1 / H2 ≤ 0.5, and / or 1 ≤ H1 / H3 ≤ 10. By adjusting the values ​​of H2, H1 / H2, and H1 / H3 within the above ranges, it is beneficial to further improve the safety performance of the secondary battery while maintaining its high energy density and good packaging performance.

[0012] In one or more embodiments of this application, the width of the protective layer along the width direction of the positive electrode sheet is L1 mm, where 3 ≤ L1 ≤ 60 mm. By adjusting the value of L1 within the above range, it is beneficial to improve the safety performance of the secondary battery, while also achieving a higher energy density.

[0013] In one or more embodiments of this application, 3 ≤ L1 ≤ 20. By adjusting the value of L1 within the above range, it is beneficial to further improve the safety performance of the secondary battery, while also achieving a higher energy density.

[0014] In one or more embodiments of this application, the porosity of the protective layer is f%, where 0 < f ≤ 20. Adjusting the porosity f% of the protective layer within this range is beneficial for improving the safety performance of the secondary battery.

[0015] In one or more embodiments of this application, a ceramic layer is disposed between the protective layer and the positive electrode material layer along the width direction of the positive electrode sheet, and the ceramic layer is disposed in the main body region. Disposing of a ceramic layer between the protective layer and the positive electrode material layer helps reduce the probability of side reactions occurring when the oleophobic material in the protective layer comes into contact with the positive electrode active material. It also helps reduce the possibility of the metal burrs of the negative electrode current collector piercing the separator and coming into contact with the positive electrode material layer or the positive electrode current collector, thus reducing the risk of short circuits between the positive and negative electrodes, thereby improving the safety performance of the secondary battery.

[0016] The second aspect of this application provides an electronic device that includes the secondary battery provided in the first aspect of this application. The electronic device of this application has a long service life and good performance.

[0017] The beneficial effects of this application are:

[0018] This application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode and an electrolyte. The positive electrode includes a positive current collector, a positive electrode material layer, and a protective layer. Both the positive electrode material layer and the protective layer are located on at least one surface of the positive current collector. Along the width direction of the positive current collector, the positive current collector includes a main body region and a tab region. The positive electrode material layer is disposed in the main body region, and the protective layer is disposed in the tab region. The area ratio of the protective layer covering the tab region is S, where 0.6 ≤ S ≤ 1. The contact angle between the protective layer and the electrolyte is θ, where 90° < θ ≤ 165°. By designing the structure of the positive electrode and controlling the values ​​of S and θ within the above ranges, this application can reduce the contact between the positive electrode tab, the positive current collector, and the electrolyte, mitigate the corrosion of the positive electrode tab and the positive current collector by the electrolyte, extend the service life of the secondary battery, and thus improve the safety performance of the secondary battery.

[0019] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0021] Figure 1 This is a schematic diagram of the positive electrode sheet along its length and width directions according to one embodiment of this application;

[0022] Figure 2 This is a schematic diagram of the positive electrode sheet along its width and thickness directions according to one embodiment of this application;

[0023] Figure 3 This is a schematic diagram of the positive electrode sheet along its length and width directions, representing another embodiment of this application.

[0024] Figure 4 This is a schematic diagram of the positive electrode sheet along its width and thickness directions, representing another embodiment of this application.

[0025] Figure 5 This is a schematic diagram of the positive electrode sheet along its length and width directions, representing another embodiment of this application.

[0026] Figure 6 This is a schematic diagram of the positive electrode sheet along its width and thickness directions, representing another embodiment of this application.

[0027] Figure 7 This is a schematic diagram of the positive electrode sheet along its length and width in another embodiment of this application;

[0028] Figure 8 This is a schematic diagram of the positive electrode sheet along its width and thickness directions, representing another embodiment of this application.

[0029] Reference numerals: positive electrode 100, positive current collector 110, positive electrode material layer 120, protective layer 130, ceramic layer 140, main body region 111, tab region 112. Detailed Implementation

[0030] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0031] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0032] The first aspect of this application provides a secondary battery, including a positive electrode and an electrolyte. The positive electrode includes a positive current collector, a positive electrode material layer, and a protective layer. The positive electrode material layer and the protective layer are both located on at least one surface of the positive current collector. Along the width direction of the positive current collector, the positive current collector includes a main body region and a tab region. The positive electrode material layer is disposed in the main body region, and the protective layer is disposed in the tab region. The area ratio of the protective layer covering the tab region is S, where 0.6 ≤ S ≤ 1, and optionally, 0.8 ≤ S ≤ 1. For example, the value of S can be 0.6, 0.65, 0.7, 0.75, 0.8, 0.82, 0.85, 0.88, 0.9, 0.92, 0.95, 0.98, 1, or a range consisting of any two of these values. The value range of S can be 0.6 to 1, 0.7 to 1, 0.8 to 1, 0.9 to 1, and all such ranges, as well as sub-ranges. The contact angle between the protective layer and the electrolyte is θ, where 90° < θ ≤ 165°, and optionally, 120° ≤ θ ≤ 165°. For example, θ can be 91°, 100°, 105°, 110°, 115°, 120°, 125°, 130°, 135°, 140°, 145°, 150°, 155°, 160°, 165°, or any two of these values. The range of θ can be 91° to 165°, 100° to 165°, 110° to 165°, 120° to 165°, 130° to 165°, 140° to 165°, 150° to 165°, and all of these ranges, as well as sub-ranges.

[0033] The inventors discovered that the contact angle θ between the positive electrode tab and the uncoated area of ​​the positive electrode current collector in a secondary battery and the electrolyte is usually less than or equal to 90°. This makes it easy for the electrolyte to fully contact the surfaces of the positive electrode current collector and the positive electrode tab. When the electrolyte contains corrosive components such as compounds containing fluorosulfonyl groups, it will cause corrosion of the positive electrode current collector and the positive electrode tab, which will deteriorate the safety performance of the secondary battery and shorten its cycle life.

[0034] This application designs the structure of the positive electrode sheet by setting a protective layer in the tab region along the width direction of the positive current collector, and controls the values ​​of S and θ within the aforementioned range. This protective layer has a special oleophobic structure, its surface has a strong oleophobic effect, and its contact angle θ with the electrolyte is large. It can repel or not absorb the electrolyte, reducing the contact between the positive electrode tab, positive current collector, and electrolyte in the secondary battery. This reduces the corrosion of the positive electrode tab and positive current collector by corrosive components in the electrolyte (such as compounds containing fluorosulfonyl groups), thereby helping to reduce corrosion byproducts. The deposition of substances on the negative electrode can induce lithium dendrite growth, which can penetrate the separator and cause a short circuit between the positive and negative electrodes. It can also help reduce electrolyte consumption, reduce the probability of the positive electrode tab breaking or falling off due to corrosion, and reduce the risk of thermal runaway and local overheating of the secondary battery under abuse conditions (such as impact, overcharge, and high temperature). In addition, it can reduce the risk of metal elements (such as aluminum) in the positive electrode current collector and positive electrode tab dissolving and causing side reactions or deteriorating the negative electrode interface inside the secondary battery, thus extending the service life of the secondary battery and improving its safety performance.

[0035] When the value of S is too small, for example, less than 0.6, the area of ​​the protective layer covering the tab region is too small, resulting in excessive exposure of the positive electrode tab. This leads to severe corrosion of the positive electrode tab by the electrolyte, thus affecting the safety performance of the secondary battery. When θ is too small, for example, less than 90°, the electrolyte has too good wettability to the protective layer. Even with a protective layer, the electrolyte may still penetrate the protective layer and contact the positive electrode tab, corroding it and affecting the safety performance of the secondary battery. When θ is too large, for example, greater than 165°, the manufacturing process becomes very difficult.

[0036] Therefore, by designing the structure of the positive electrode and controlling the values ​​of S and θ within the above range, this application can reduce the contact between the positive electrode tab, the positive electrode current collector and the electrolyte, reduce the corrosion of the positive electrode tab and the positive electrode current collector by the electrolyte, extend the service life of the secondary battery, and thus improve the safety performance of the secondary battery.

[0037] In this application, the tab region refers to the area in the positive electrode sheet where the positive electrode tab is disposed, and the main body region refers to the area excluding the tab region. The area S of the protective layer covering the tab region is the ratio of the sum of the areas of the protective layers to the sum of the areas of the positive electrode tabs. In one or more embodiments, the main body region is located in the middle region along the width direction of the positive electrode current collector, and the tab regions are located on both sides of the main body region along the width direction of the positive electrode current collector. The positive electrode material layer is disposed in the main body region, and the protective layer is disposed in the tab region. In another one or more embodiments, the tab region is located on one side of the main body region along the width direction of the positive electrode current collector, the positive electrode material layer is disposed in the main body region, and the protective layer is disposed in the tab region.

[0038] For example, in this application, the length direction of the positive electrode sheet is defined as X, the width direction as Y, and the thickness direction as Z. The unfolded positive electrode sheet has a long side and a short side, and the aforementioned length direction refers to the extension direction of the long side of the positive electrode sheet. It should be understood that the above definitions of directions are for the convenience of describing this application; the length direction, width direction, and thickness direction of the positive current collector, the positive electrode material layer, the protective layer, and the ceramic layer are the same as those of the positive electrode sheet.

[0039] Figure 1 This is a schematic diagram of the structure of the positive electrode 100 along its length X and width Y directions according to one embodiment of this application. Figure 2 This is a schematic diagram of the positive electrode 100 of one embodiment of this application along its width Y and thickness Z directions. Figure 1 and Figure 2 As shown, the positive electrode 100 includes a positive current collector 110, a positive electrode material layer 120, and a protective layer 130. The positive electrode material layer 120 and the protective layer 130 are both located on the two surfaces of the positive current collector 110. Along the width Y direction of the positive current collector, the positive current collector 110 includes a main body region 111 and a tab region 112. The main body region 111 is located in the middle region along the width Y direction of the positive current collector, and the tab region 112 is located on both sides of the main body region 111 along the width Y direction of the positive current collector. The positive electrode material layer 120 is disposed in the main body region 111, and the protective layer 130 is disposed in the tab region 112.

[0040] Figure 3 This is a schematic diagram of the positive electrode 100 along its length X and width Y directions, according to another embodiment of this application. Figure 4 This is a schematic diagram of the positive electrode 100 along its width Y and thickness Z directions, representing another embodiment of this application. Figure 3 and Figure 4As shown, the positive electrode 100 includes a positive current collector 110, a positive electrode material layer 120, and a protective layer 130. The positive electrode material layer 120 and the protective layer 130 are both located on the two surfaces of the positive current collector 110. Along the width Y direction of the positive current collector, the positive current collector 110 includes a main body region 111 and a tab region 112. The tab region 112 is located on one side of the main body region 111 along the width Y direction of the positive current collector. The positive electrode material layer 120 is disposed in the main body region 111, and the protective layer 130 is disposed in the tab region 112.

[0041] In one or more embodiments of this application, the electrolyte comprises a compound containing a fluorosulfonyl group, which includes at least one of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTf), or lithium bis(pentafluoroethanesulfonyl)imide (LiBETI). The concentration of the fluorosulfonyl compound in the electrolyte is c mol / L, where 0.2 ≤ c ≤ 2. For example, the value of c can be 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, or a range of any two values ​​therein. The range of c can be 0.2 to 2, 0.4 to 1.8, 0.6 to 1.6, 0.8 to 1.4, 1.0 to 1.2, and all such ranges and sub-ranges. When the aforementioned compounds containing fluorosulfonyl groups (e.g., lithium bisfluorosulfonylimide LiFSI, lithium bistrifluoromethanesulfonylimide LiTFSI, etc.) are used in secondary battery electrolytes, they outperform traditional lithium salts such as LiPF6 in terms of thermal stability, electrochemical stability, conductivity, solid electrolyte interphase (SEI) film formation, and reduction of gas generation. However, these compounds containing fluorosulfonyl groups are corrosive. When their concentration in the electrolyte is within the aforementioned range, the likelihood of corrosion of the positive electrode tab and positive current collector is high, increasing the risk of safety problems. In this case, using them in conjunction with the positive electrode sheet with a protective layer provided in this application can reduce the corrosion of the positive electrode tab and positive current collector by the electrolyte, reduce the probability of the positive electrode tab breaking or falling off due to corrosion, and reduce the risk of thermal runaway and local overheating of the secondary battery under abuse conditions (such as impact, overcharge, high temperature). In addition, it can also reduce the risk of metal elements (such as aluminum) in the positive current collector and positive electrode tab dissolving and causing side reactions or deteriorating the negative electrode interface inside the secondary battery, thus giving the secondary battery good safety performance.

[0042] In one or more embodiments, the electrolyte comprises a lithium salt, which includes compounds containing fluorosulfonyl groups and other lithium salts, wherein the fluorosulfonyl group compounds constitute 20% to 100% of all lithium salts in molar proportion. This application does not impose any particular limitation on other lithium salts, as long as they achieve the purpose of this application. For example, other lithium salts may include, but are not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate.

[0043] In this application, the electrolyte also includes a non-aqueous solvent. This application does not impose any particular limitation on the non-aqueous solvent, as long as it achieves the purpose of this application. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents.

[0044] The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonate compounds may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). The aforementioned fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0045] In one or more embodiments of this application, the protective layer includes an oleophobic material, which includes at least one of polytetrafluoroethylene, perfluoropolyether, siloxane, silane coupling agent, nano-silica, or fluorinated silane. Based on the mass of the protective layer, the mass percentage of the oleophobic material is Ws%, 80≤Ws≤99, and optionally, 90≤Ws≤95. For example, the value of Ws can be 80, 82, 84, 86, 88, 90, 91, 92, 93, 94, 95, 96, 98, 99, or a range of any two of these values. The range of Ws can be 80 to 99, 82 to 98, 84 to 97, 86 to 96, 88 to 95, 90 to 95, and all such ranges and sub-ranges. The aforementioned perfluoropolyethers may include, but are not limited to, at least one of perfluoropolyether oils, perfluoropolyether alcohols, or perfluoropolyether siloxanes. The siloxanes may include, but are not limited to, at least one of polydimethylsiloxane, polymethylphenylsiloxane, polymethyltrifluoropropylsiloxane, polymethylvinylsiloxane, or polymethylhydrosiloxane. The silane coupling agents may include, but are not limited to, at least one of perfluoroalkylsilane coupling agents, methylsilane coupling agents, vinylsilane coupling agents, aminosilane coupling agents, or epoxysilane coupling agents. The fluorinated silanes may include, but are not limited to, at least one of perfluorooctyltrimethoxysilane, perfluorodecyltrimethoxysilane, perfluorohexyltrimethoxysilane, or perfluorobutyltrimethoxysilane. The aforementioned oleophobic materials lack affinity for electrolytes and tend to repel or not absorb electrolytes. By selecting the aforementioned oleophobic material for the protective layer and controlling the value of Ws within the aforementioned range, the protective layer possesses a special oleophobic structure and a strong oleophobic effect on its surface. This facilitates maintaining the contact angle θ between the protective layer and the electrolyte within the range specified in this application. The protective layer repels or does not absorb the electrolyte, reducing contact between the positive electrode tab, positive electrode current collector, and electrolyte in the secondary battery. This mitigates the corrosion of the positive electrode tab and current collector by the electrolyte, lowering the probability of the positive electrode tab breaking or detaching due to corrosion. It also reduces the risk of thermal runaway and localized overheating of the secondary battery under abuse conditions (such as impact, overcharge, and high temperature). Furthermore, it helps reduce the risk of metal elements (e.g., aluminum) dissolving from the positive electrode current collector and positive electrode tab, leading to side reactions within the secondary battery or deterioration of the negative electrode interface. Simultaneously, the special oleophobic structure of the protective layer also exhibits good stability, resulting in a low probability of side reactions in the secondary battery. This improves the electrochemical performance of the secondary battery, thereby enhancing its safety performance.

[0046] In one or more embodiments of this application, the weight-average molecular weight of polytetrafluoroethylene is 10. 6 Up to 10 7 For example, the weight-average molecular weight of polytetrafluoroethylene can be 10. 6 2×10 6 3×10 6 4×106 5×10 6 6×10 6 7×10 6 8×10 6 9×10 6 10 7 The weight-average molecular weight of polytetrafluoroethylene can range from 10 to a range of any two of these values. 6 Up to 10 7 2×10 6 Up to 9×10 6 3×10 6 Up to 8×10 6 4×10 6 Up to 7×10 6 5×10 6 Up to 6×10 6 And all of these ranges, as well as sub-ranges. Selecting polytetrafluoroethylene with a weight-average molecular weight within the above range is beneficial for further improving the safety performance of secondary batteries.

[0047] In one or more embodiments of this application, the weight-average molecular weight of the perfluoropolyether is 5 × 10⁻⁶. 5 Up to 5×10 6 For example, the weight-average molecular weight of perfluoropolyether can be 5 × 10⁻⁶. 5 1×10 6 1.5×10 6 2×10 6 2.5×10 6 3×10 6 3.5×10 6 4×10 6 4.5×10 6 5×10 6 The weight-average molecular weight of perfluoropolyether can be a range of any two of these values, and can be 5 × 10⁻⁶. 5 Up to 5×10 6 1×10 6 Up to 4.5×10 6 1.5×10 6 Up to 4×10 6 2×10 6 Up to 3.5×10 6 2.5×10 6 Up to 3×10 6 And all of these ranges, as well as sub-ranges. Selecting perfluoropolyethers with a weight-average molecular weight within the above range is beneficial for further improving the safety performance of secondary batteries.

[0048] In one or more embodiments of this application, the weight-average molecular weight of the siloxane is 2 × 10⁻⁶.3 Up to 5×10 4 For example, the weight-average molecular weight of siloxanes can be 2 × 10⁻⁶. 3 1×10 4 1.5×10 4 2.0×10 4 2.5×10 4 3.0×10 4 3.5×10 4 4.0×10 4 4.5×10 4 5×10 4 The weight-average molecular weight of siloxanes can range from 2 × 10⁻⁶ to a range of any two of these values. 3 Up to 5×10 4 1×10 4 Up to 4.5×10 4 1.5×10 4 Up to 4×10 4 2×10 4 Up to 3.5×10 4 And all of these ranges, as well as sub-ranges. Selecting siloxanes with a weight-average molecular weight within the above range is beneficial for further improving the safety performance of secondary batteries.

[0049] In one or more embodiments of this application, the weight-average molecular weight of the silane coupling agent is between 150 and 300. For example, the weight-average molecular weight of the silane coupling agent can be 150, 180, 200, 220, 240, 250, 260, 280, 300, or a range consisting of any two of these values. The range of the weight-average molecular weight of the silane coupling agent can be 150 to 300, 180 to 280, 200 to 260, 220 to 240, and all such ranges, as well as sub-ranges. Selecting a silane coupling agent with a weight-average molecular weight within the above range is beneficial for further improving the safety performance of secondary batteries.

[0050] In one or more embodiments of this application, the average particle size of the nano-silica is from 5 nm to 100 nm. For example, the average particle size of the nano-silica can be 5 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any two of these values. The range of the average particle size of the nano-silica can be 5 nm to 100 nm, 10 nm to 90 nm, 20 nm to 80 nm, 30 nm to 70 nm, 40 nm to 60 nm, and all of these ranges, as well as sub-ranges. Selecting nano-silica with an average particle size within the above range is beneficial for further improving the safety performance of secondary batteries. In this application, the average particle size refers to the average diameter of the circumscribed circle of the particle.

[0051] In one or more embodiments of this application, the weight-average molecular weight of the fluorinated silane is 300 to 600. For example, the weight-average molecular weight of the fluorinated silane can be 300, 350, 400, 450, 500, 550, 600, or a range consisting of any two of these values. The range of the weight-average molecular weight of the fluorinated silane can be 300 to 600, 350 to 550, 400 to 500, 420 to 480, and all such ranges and sub-ranges. Selecting a fluorinated silane with a weight-average molecular weight within the above range is beneficial for further improving the safety performance of the secondary battery.

[0052] In one or more embodiments of this application, the protective layer further includes a dispersant and a binder. The dispersant includes at least one of sodium carboxymethyl cellulose (CMC-Na), lithium carboxymethyl cellulose (CMC-Li), sodium dodecyl sulfate, or polyethylene glycol. The binder includes at least one of polyvinylidene fluoride (PVDF), styrene-butadiene rubber, polyvinyl alcohol, polytetrafluoroethylene, epoxy resin, or polyurethane. The weight-average molecular weight of the polytetrafluoroethylene is 10. 5 Up to 5×10 5 Based on the quality of the protective layer, the mass percentage of the dispersant is Wf%, 0.5 ≤ Wf ≤ 10, and the mass percentage of the binder is Wn%, 0.5 ≤ Wn ≤ 10. For example, the value of Wf can be 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or a range of any two values ​​therein. The value range of Wf can be 0.5 to 10, 1 to 9, 2 to 8, 3 to 7, 4 to 6, and all of these ranges, as well as sub-ranges. Similarly, the value of Wn can be 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or a range of any two values ​​therein. The value range of Wn can be 0.5 to 10, 1 to 9, 2 to 8, 3 to 7, 4 to 6, and all of these ranges, as well as sub-ranges. By selecting the above-mentioned dispersants and binders for the protective layer and controlling the values ​​of Wn and Wf within the aforementioned ranges, it is beneficial to form a protective layer with a special oleophobic structure and a strong oleophobic effect on its surface. This ensures that the contact angle θ between the protective layer and the electrolyte is within the range of this application. The protective layer repels or does not absorb the electrolyte, reducing the contact between the positive electrode tab, positive electrode current collector, and electrolyte in the secondary battery, thus mitigating the corrosion of the positive electrode tab and positive electrode current collector by the electrolyte. Furthermore, it facilitates the uniform dispersion of the oleophobic material during the preparation of the protective layer slurry, improving the coating uniformity of the protective layer slurry, resulting in a protective layer with uniformly distributed components. Additionally, the adhesion between the protective layer and the positive electrode tab is high, reducing the risk of detachment during cycling, thereby improving the safety performance of the secondary battery.

[0053] In one or more embodiments of this application, the thickness of the protective layer is H1 μm, the thickness of the positive electrode material layer is H2 μm, the thickness of the positive electrode current collector is H3 μm, 10≤H2≤3000, 0.1≤H1 / H2≤1, 0.2≤H1 / H3≤15, optionally, 50≤H2≤500, and / or, 0.2≤H1 / H2≤0.5, and / or, 1≤H1 / H3≤10. For example, the value of H2 can be 10, 30, 50, 80, 100, 150, 200, 300, 400, 500, 800, 1000, 1200, 1500, 1800, 2000, 2200, 2500, 2800, 3000, or a range of any two of these values. The range of H2 can be 10 to 3000, 50 to 2500, 50 to 2000, 50 to 1500, 50 to 1000, 50 to 800, 50 to 500, 50 to 400, 50 to 300, 50 to 200, 50 to 100, and all of these ranges, as well as subranges. For example, the value of H1 / H2 can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or any range of two values ​​from these values. The range of H1 / H2 can be 0.1 to 1, 0.2 to 0.9, 0.2 to 0.8, 0.2 to 0.7, 0.2 to 0.6, 0.2 to 0.5, 0.2 to 0.4, and all of these ranges, as well as sub-ranges. Similarly, the value of H1 / H3 can be 0.2, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or any range of two values ​​from these values. The range of H1 / H3 can be 0.2 to 15, 0.5 to 12, 1 to 10, 2 to 9, 3 to 8, 4 to 7, 5 to 6, and all of these ranges, as well as sub-ranges. By adjusting the values ​​of H2, H1 / H2, and H1 / H3 within the aforementioned ranges, the mass and volume proportions of the positive electrode material layer are relatively large. Based on the high energy density and good encapsulation performance of the secondary battery, the protective layer effectively provides a good oleophobic effect, reducing the contact between the positive electrode tabs, positive electrode current collector, and electrolyte in the secondary battery. This mitigates the corrosion of the positive electrode tabs and current collector by the electrolyte, lowering the probability of the positive electrode tabs breaking or detaching due to corrosion, thereby improving the safety performance of the secondary battery. Simultaneously, the secondary battery has a high energy density. In this application, H1 μm refers to the thickness of the single-sided protective layer, and H2 μm refers to the thickness of the single-sided positive electrode material layer.

[0054] In one or more embodiments, the thickness of the protective layer is H1 μm, where 1 ≤ H1 ≤ 300, and optionally, 5 ≤ H1 ≤ 100. For example, the value of H1 can be 1, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, or a range of any two of these values. The value range of H1 can be 1 to 300, 2 to 200, 5 to 100, 5 to 80, 5 to 60, 5 to 40, 10 to 30, and all of these ranges, as well as sub-ranges.

[0055] In one or more embodiments, the thickness of the positive current collector is H3 μm, where 5 ≤ H3 ≤ 20, and optionally, 6 ≤ H3 ≤ 18. For example, the value of H3 can be 5, 6, 7, 9, 10, 12, 14, 15, 16, 18, 20, or a range of any two of these values. The range of H3 can be 5 to 20, 6 to 18, 8 to 15, 10 to 12, and all of these ranges, as well as sub-ranges.

[0056] In one or more embodiments of this application, the width of the protective layer along the width direction of the positive electrode sheet is L1 mm, where 3 ≤ L1 ≤ 60 mm, and optionally, 3 ≤ L1 ≤ 20 mm. For example, the value of L1 can be 3, 5, 8, 10, 12, 15, 18, 20, 30, 40, 50, 60 mm, or a range consisting of any two of these values. The value range of L1 can be 3 to 60, 3 to 50, 3 to 40, 3 to 30, 3 to 20, 5 to 20, 10 to 20, 12 to 18, and all such ranges and sub-ranges. It is understood that L1 mm refers to the dimension of the protective layer along the width direction of the positive electrode sheet. By adjusting the value of L1 within the above range, the protective layer can achieve a good oleophobic effect, reduce the contact between the positive electrode tab, positive electrode current collector and electrolyte in the secondary battery, reduce the corrosion of the positive electrode tab and positive electrode current collector by the electrolyte, and allow exposed positive electrode current collector outside the protective layer in the tab area along the width direction of the positive electrode sheet for welding the positive electrode tab. This helps to reduce the impact of the presence of the protective layer on the welding effect of the positive electrode tab, thereby improving the safety performance of the secondary battery. At the same time, the secondary battery has a high energy density.

[0057] In one or more embodiments of this application, the porosity of the protective layer is f%, where 0 < f ≤ 20. For example, the value of f can be 0.1, 1, 3, 5, 8, 10, 12, 14, 16, 18, 20, or a range consisting of any two of these values. The range of f can be 0.1 to 20, 0.5 to 18, 1 to 15, 1.5 to 10, 2 to 8, 3 to 6, and all of these ranges, as well as sub-ranges. By controlling the porosity f% of the protective layer within the above range, the porosity of the protective layer is lower, making it difficult for the electrolyte to wet the protective layer. This helps reduce the contact between the positive electrode tab, positive electrode current collector, and electrolyte in the secondary battery, mitigating the corrosion of the positive electrode tab and positive electrode current collector by the electrolyte. It also helps reduce the probability of side reactions between the electrolyte and the protective layer, thereby improving the safety performance of the secondary battery.

[0058] In one or more embodiments of this application, a ceramic layer is disposed between the protective layer and the positive electrode material layer along the width direction of the positive electrode sheet, and the ceramic layer is disposed in the main body region. Disposing of a ceramic layer between the protective layer and the positive electrode material layer helps reduce the probability of side reactions occurring when the oleophobic material in the protective layer comes into contact with the positive electrode active material, thereby improving the electrochemical performance of the secondary battery. Simultaneously, disposing of a ceramic layer at the edge of the positive electrode material layer also helps reduce the possibility of the metal burrs of the negative electrode current collector piercing the separator and coming into contact with the positive electrode material layer or the positive electrode current collector, reducing the risk of short circuits between the positive and negative electrodes, thus improving the safety performance of the secondary battery.

[0059] In one or more embodiments, the thickness of the ceramic layer is H4 μm, where 5 ≤ H4 ≤ H1, and optionally, 20 ≤ H4 ≤ 100. For example, the value of H4 can be 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 150, 200, 250, 300, or a range of any two of these values. The value range of H4 can be 5 to 300, 5 to 100, 20 to 100, 20 to 80, 20 to 60, 30 to 50, and all of these ranges, as well as sub-ranges.

[0060] In one or more embodiments, the ceramic layer comprises ceramic particles and a ceramic layer binder. Based on the mass of the ceramic layer, the mass percentage of ceramic particles is 70% to 95%, and the mass percentage of the ceramic layer binder is 5% to 30%. This application does not impose any particular limitation on the ceramic particles and the ceramic layer binder, as long as they achieve the purpose of this application. For example, the ceramic particles may include, but are not limited to, at least one of silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The ceramic layer binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride.

[0061] Figure 5 This is a schematic diagram of the positive electrode 100 along its length X and width Y directions, representing another embodiment of this application. Figure 6 This is a schematic diagram of the positive electrode 100 along its width Y and thickness Z directions, representing another embodiment of this application. Figure 5 and Figure 6 As shown, the positive electrode 100 includes a positive current collector 110, a positive electrode material layer 120, a protective layer 130, and a ceramic layer 140. The positive electrode material layer 120, the protective layer 130, and the ceramic layer 140 are all located on two surfaces of the positive current collector 110. Along the width Y direction of the positive current collector, the positive current collector 110 includes a main body region 111 and a tab region 112. The main body region 111 is located in the middle region along the width Y direction of the positive current collector, and the tab region 112 is located on both sides of the main body region 111 along the width Y direction of the positive current collector. The positive electrode material layer 120 and the ceramic layer 140 are disposed in the main body region 111, the protective layer 130 is disposed in the tab region 112, and the ceramic layer 140 is located between the positive electrode material layer 120 and the protective layer 130.

[0062] Figure 7 This is a schematic diagram of the positive electrode 100 along its length X and width Y directions, according to another embodiment of this application. Figure 8 This is a schematic diagram of the positive electrode 100 along its width Y and thickness Z directions, representing another embodiment of this application. Figure 7 and Figure 8As shown, the positive electrode 100 includes a positive current collector 110, a positive electrode material layer 120, a protective layer 130, and a ceramic layer 140. The positive electrode material layer 120, the protective layer 130, and the ceramic layer 140 are all located on two surfaces of the positive current collector 110. Along the width Y direction of the positive current collector, the positive current collector 110 includes a main body region 111 and a tab region 112. The tab region 112 is located on one side of the main body region 111 along the width Y direction of the positive current collector. The positive electrode material layer 120 and the ceramic layer 140 are disposed in the main body region 111, the protective layer 130 is disposed in the tab region 112, and the ceramic layer 140 is located between the positive electrode material layer 120 and the protective layer 130.

[0063] In one or more embodiments, the positive electrode tab is an aluminum tab, and after the secondary battery is stored at high temperature, the mass content of aluminum in the electrolyte is less than 100 ppm based on the mass of the electrolyte. The above-mentioned high temperature storage conditions can be a temperature ≥ 60°C and a time ≥ 30 days.

[0064] In one or more embodiments, the preparation of the positive electrode sheet may include, but is not limited to, the following steps: (1) providing a positive electrode material layer slurry and a protective layer slurry, wherein the positive electrode material layer slurry includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder, and the protective layer slurry includes an oleophobic material, a dispersant, and a binder; (2) coating the positive electrode material layer slurry in the middle region of one surface of the positive electrode current collector along the width direction of the positive electrode current collector, and then coating the protective layer slurry on at least one side of the surface of the positive electrode current collector of the positive electrode material layer slurry, and drying and cold pressing to form a positive electrode sheet with a positive electrode material layer and a protective layer on one side; (3) optionally, repeating step (2) on the other surface of the positive electrode current collector to obtain a positive electrode sheet with a positive electrode material layer and a protective layer on both sides; (4) cutting and die-cutting the tabs to obtain the positive electrode sheet. This application does not have any particular limitation on the solid content of the positive electrode material layer slurry and the protective layer slurry, as long as the purpose of this application can be achieved. For example, the solid content of the cathode material layer slurry can be 50wt% to 80wt%, and the solid content of the protective layer slurry can be 50wt% to 80wt%.

[0065] In one or more embodiments, the preparation of the positive electrode sheet may include, but is not limited to, the following steps: (1) providing a positive electrode material layer slurry, a protective layer slurry and a ceramic layer slurry, wherein the positive electrode material layer slurry includes a positive electrode active material, a positive electrode conductive agent and a positive electrode binder, the protective layer slurry includes an oleophobic material, a dispersant and a binder, and the ceramic layer slurry includes ceramic particles and a ceramic layer binder; (2) along the width direction of the positive electrode current collector, coating the positive electrode material layer slurry in the middle area of ​​one surface of the positive electrode current collector, and then coating the ceramic layer slurry and the protective layer slurry on at least one side of the surface of the positive electrode current collector, wherein the ceramic layer slurry is located between the positive electrode material layer slurry and the protective layer slurry, and then drying and cold pressing to form a positive electrode sheet with a positive electrode material layer, a ceramic layer and a protective layer on one side; (3) optionally, repeating step (2) on the other surface of the positive electrode current collector to obtain a positive electrode sheet with a positive electrode material layer, a ceramic layer and a protective layer on both sides; (4) obtaining the positive electrode sheet by cutting and die-cutting the tabs. This application does not impose any particular restrictions on the solid content of the cathode material layer slurry, the protective layer slurry, and the ceramic layer slurry, as long as the purpose of this application can be achieved. For example, the solid content of the cathode material layer slurry can be 50wt% to 80wt%, the solid content of the protective layer slurry can be 50wt% to 80wt%, and the solid content of the ceramic layer slurry can be 40wt% to 80wt%.

[0066] In this application, the thickness H1 of the protective layer can be controlled by adjusting the single-sided coating quality and the cold-pressing pressure of the protective layer. For example, when other conditions remain unchanged, increasing the single-sided coating quality of the protective layer increases H1; decreasing the single-sided coating quality decreases H1. When other conditions remain unchanged, increasing the cold-pressing pressure of the protective layer decreases H1; decreasing the cold-pressing pressure increases H1.

[0067] In this application, the thickness H2 of the cathode material layer can be controlled by adjusting the single-sided coating quality and the cold-pressing pressure of the cathode material layer. For example, when other conditions remain unchanged, increasing the single-sided coating quality of the cathode material layer increases H2; decreasing the single-sided coating quality decreases H2. When other conditions remain unchanged, increasing the cold-pressing pressure of the cathode material layer decreases H2; decreasing the cold-pressing pressure increases H2.

[0068] In this application, the porosity of the protective layer can be controlled by adjusting the cold-pressing pressure. For example, when other conditions remain unchanged, increasing the cold-pressing pressure of the protective layer decreases its porosity, while decreasing the cold-pressing pressure increases its porosity. The composition of the protective layer also affects its porosity.

[0069] In this application, the positive electrode sheet includes a positive current collector, a positive electrode material layer, and a protective layer, wherein the positive electrode material layer and the protective layer are both located on at least one surface of the positive current collector. The phrase "the positive electrode material layer and the protective layer are both located on at least one surface of the positive current collector" means that the positive electrode material layer and the protective layer can be disposed on one surface of the positive current collector along its own thickness direction, or on two surfaces of the positive current collector along its own thickness direction.

[0070] The positive current collector in this application contains aluminum. For example, the positive current collector may contain aluminum foil, aluminum alloy foil, or composite current collector (such as aluminum-carbon composite current collector).

[0071] The cathode material layer of this application includes a cathode active material, which comprises a substance capable of reversibly inserting and extracting active ions such as lithium ions. The cathode material layer can be one or more layers, and each layer in a multilayer cathode material layer can contain the same or different cathode active materials. This application does not impose any particular limitation on the cathode active material, as long as it can achieve the purpose of this application. For example, the cathode active material can include, but is not limited to, at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate. The aforementioned lithium nickel cobalt manganese oxide can include LiNi... 0.95 Co 0.03 Mn 0.02 O2 (Ni95), LiNi 0.91 Co 0.03 Mn 0.06 O2 (Ni91), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622), LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523) or LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 At least one of O2 (NCM111).

[0072] The positive electrode material layer of this application also includes a positive electrode conductive agent and a positive electrode binder. This application does not impose any particular limitations on the positive electrode conductive agent and positive electrode binder in the positive electrode material layer, as long as they can achieve the purpose of this application. For example, the positive electrode conductive agent may include, but is not limited to, at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. The conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers; specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. For example, the positive electrode binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, positive electrode conductive agent, and positive electrode binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved. For example, the mass ratio of the positive electrode active material, positive electrode conductive agent, and positive electrode binder in the positive electrode material layer may be (95 to 98):(0.5 to 2.5):(1.5 to 3.4).

[0073] Optionally, the positive electrode may further include a conductive layer located between the positive current collector and the positive electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, which can be a conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer; for example, the conductive agent may be at least one of the aforementioned positive electrode conductive agents, and the binder may be at least one of the aforementioned positive electrode binders.

[0074] In this application, the secondary battery includes a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The phrase "the negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector" means that the negative electrode material layer can be disposed on one surface of the negative current collector along its thickness direction, or on two surfaces of the negative current collector along its thickness direction. It should be noted that the term "surface" here can refer to the entire surface area of ​​the negative current collector or only a portion thereof; this application does not impose any particular limitation, as long as the purpose of this application is achieved.

[0075] This application does not impose any particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collector. For example, the composite current collector may be lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, or titanium copper composite current collector, etc.

[0076] The negative electrode material layer of this application includes a negative electrode active material. This application does not impose any particular limitation on the negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material may include, but is not limited to, natural graphite, artificial graphite, mesophase microcarbon spheres, hard carbon, soft carbon, silicon, silicon-carbon composites, SiOx (0 < x < 2), Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithiated TiO2-Li4Ti5O. 12 At least one of Li-Al alloy or metallic lithium.

[0077] The negative electrode material layer of this application may further include a negative electrode conductive agent and a negative electrode binder, or the negative electrode material layer may further include a negative electrode binder, a negative electrode conductive agent, and a thickener. This application does not particularly limit the types of negative electrode binders and negative electrode conductive agents, as long as they can achieve the purpose of this application. For example, the negative electrode conductive agent may be at least one of the aforementioned positive electrode conductive agents, and the negative electrode binder may be at least one of the aforementioned positive electrode binders. This application does not particularly limit the types of thickeners, as long as they can achieve the purpose of this application. For example, the thickener may include, but is not limited to, at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, or lithium carboxymethyl cellulose. This application does not particularly limit the mass ratio of the negative electrode active material, negative electrode conductive agent, negative electrode binder, and thickener in the negative electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved. For example, the mass ratio of the negative electrode active material, negative electrode conductive agent, thickener, and negative electrode binder in the negative electrode material layer may be (96 to 98): (0.5 to 2): (0 to 1.5): (1.0 to 1.9).

[0078] This application does not impose any particular restrictions on the thickness of the negative electrode material layer and the thickness of the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided negative electrode material layer can be from 30 μm to 130 μm, and the thickness of the negative electrode current collector can be from 6 μm to 10 μm.

[0079] Optionally, the negative electrode may further include a conductive layer located between the negative electrode current collector and the negative electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer; it can be a conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a conductive binder. This application does not impose any particular limitation on the conductive agent and the conductive binder in the conductive layer; for example, the conductive agent may be at least one of the aforementioned positive electrode conductive agents, and the conductive binder may be at least one of the aforementioned positive electrode binders.

[0080] In this application, the secondary battery includes a separator. This application does not impose any particular limitation on the separator, as long as it achieves the purpose of this application. For example, the material of the separator may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of separator may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0081] In some embodiments, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used.

[0082] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic substances.

[0083] In some embodiments, the inorganic layer comprises ceramic particles and an inorganic layer binder. This application does not particularly limit the ceramic particles; for example, the ceramic particles may include at least one selected from silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not particularly limit the inorganic layer binder; for example, the inorganic layer binder may be at least one of the above-mentioned positive electrode binders. In some embodiments, the polymer layer comprises a polymer, and the polymer material may include, but is not limited to, at least one selected from polyamide, polyacrylonitrile, acrylate polymers, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0084] In this application, there is no particular limitation on the thickness of the separator, as long as it can achieve the purpose of this application. For example, the thickness of the separator can be from 3 μm to 20 μm.

[0085] The secondary battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0086] The secondary battery described in this application is not particularly limited and may include any device in which an electrochemical reaction occurs. In some embodiments, the secondary battery may include, but is not limited to, lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0087] The preparation process of the secondary battery in this application is well known to those skilled in the art, and this application has no particular limitations. For example, the preparation process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, negative electrode, and separator in sequence, and winding, folding, etc., as needed to obtain a wound electrode assembly; placing the electrode assembly into the housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery; or stacking the positive electrode, separator, negative electrode, and separator in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into the housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery.

[0088] The second aspect of this application provides an electronic device that includes the secondary battery provided in the first aspect of this application. The electronic device of this application has a long service life and good performance.

[0089] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. In some embodiments, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0090] Example

[0091] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0092] Test methods and equipment:

[0093] S-value, L1-value, H1-value, H2-value, H3-value test

[0094] The lithium-ion battery was discharged at a constant current of 0.2C to 3.0V, and the positive electrode was removed after disassembly. The positive electrode was cleaned with dimethyl carbonate (DMC) and dried at 60℃ to obtain a positive electrode sample. The dimensions of the positive electrode tab and the protective layer were measured using a micrometer, yielding the width L1 mm of the protective layer along the width direction of the positive electrode and the sum of the areas a mm of the positive electrode tab. 2 The sum of the areas of the protective layers (b mm) 2 The value of S can be calculated using the following formula: S = b / a.

[0095] The cross-section of the positive electrode sample along the thickness direction was polished with argon ions. Then, the cross-section of the positive electrode sample was observed using a scanning electron microscope (SEM), and the thickness of the protective layer, the thickness of the positive electrode material layer, and the thickness of the positive electrode current collector at five locations were measured. The average values ​​were calculated as the thickness H1 of the protective layer, the thickness H2 of the positive electrode material layer, and the thickness H3 of the positive electrode current collector.

[0096] Contact angle θ test between protective layer and electrolyte

[0097] The lithium-ion battery was discharged at a constant current of 0.2C to 3.0V, and the positive electrode was removed after disassembly. The positive electrode was cleaned with dimethyl carbonate (DMC) and dried at 60°C to obtain a positive electrode sample. Electrolytes were prepared according to the methods used in the embodiments and comparative examples. The electrolyte was dropped onto the protective layer using the seat-drop method, and the contact angle θ between the protective layer and the electrolyte was measured using an optical contact angle meter.

[0098] Porosity f% test of protective layer

[0099] The lithium-ion battery was discharged at a constant current of 0.2C to 3.0V, and the positive electrode was removed after disassembly. The positive electrode was cleaned with dimethyl carbonate (DMC) and dried at 60℃ to obtain a positive electrode sample. The area of ​​the positive electrode with the protective layer was cut out, and the porosity (f%) of the protective layer was tested using a porosimetry analyzer via mercury intrusion porosimetry. The specific porosity test was conducted according to the national standard GB / T 21650.1-2008, "Determination of Pore Size Distribution and Porosity of Solid Materials by Mercury Intrusion Porosimetry and Gas Adsorption Methods - Part I: Mercury Intrusion Porosimetry".

[0100] High-temperature storage test

[0101] The lithium-ion batteries of each embodiment and comparative example were charged at 25°C with a constant current of 1C to the upper limit cutoff voltage of 4.3V, and then charged at a constant voltage of 4.3V to 0.05C. They were then placed in a 60°C oven for 30 days before being removed. The lithium-ion batteries were discharged at a constant current of 0.2C to 3.0V and disassembled. The electrolyte was collected by centrifugation, compression, or other methods to obtain electrolyte samples. The positive electrode was removed from the disassembled lithium-ion battery, cleaned with dimethyl carbonate (DMC), and dried at 60°C to obtain positive electrode samples.

[0102] (1) Observation and testing of corrosion of positive electrode tab

[0103] The positive electrode sample was placed under an optical microscope to observe the location of the positive electrode tab and confirm whether there were any corrosion pits on the positive electrode tab. The protective layer on the surface of the positive electrode tab has low porosity, a large contact angle with the electrolyte, and a smooth and flat surface. If there are corrosion pits on the positive electrode tab, they can be clearly observed under an optical microscope.

[0104] (2) Tensile strength test of positive electrode tab

[0105] The empty foil area of ​​the positive electrode tab is cut off from the positive electrode sample to obtain the positive electrode tab sample. The positive electrode tab sample is then cut into test samples with a width of 10 mm (if the width of the positive electrode tab sample is less than 10 mm, no cutting is required). A tensile testing machine is used to perform a tensile test on the test sample along the width direction of the positive electrode sheet (i.e., the length direction of the positive electrode tab or its extension direction) at a tensile speed of 10 mm / min until the test sample breaks. By recording the force and deformation during the loading process, a stress-strain curve is plotted to obtain the tensile strength of the positive electrode tab, in MPa.

[0106] (3) Test of aluminum content in electrolyte

[0107] The mass content of aluminum in the electrolyte sample after high-temperature storage was obtained by inductively coupled plasma optical emission spectrometry (ICP-OES). The specific testing was conducted according to SJ / T 11995-2025, "Test Method for Metal Impurity Content in Lithium-ion Battery Electrolytes".

[0108] Drop test after high temperature storage

[0109] The lithium-ion batteries of each embodiment and comparative example were charged at 25°C with a constant current of 1C to the upper limit cutoff voltage of 4.3V, and then charged at a constant voltage of 4.3V to 0.05C. They were then placed in a 60°C oven for 30 days before being removed for use. At 20°C ± 5°C, the lithium-ion batteries were dropped freely from a height of 1.0 meter onto a hard surface, with one drop at each of the four corners of the battery, for a total of 7 rounds of testing. A lithium-ion battery that did not catch fire, explode, or emit smoke was considered to have passed the test. Twenty lithium-ion batteries were tested for each embodiment or comparative example, and the number of passing batteries was recorded. The drop test pass rate (%) = (number of passing batteries / total number of tests) × 100%.

[0110] The safety performance of lithium-ion batteries is evaluated by assessing the corrosion of the positive electrode tab, its tensile strength, the aluminum content in the electrolyte, and the drop test pass rate after high-temperature storage. Lower corrosion, higher tensile strength, lower aluminum content in the electrolyte, and higher drop test pass rate indicate better battery safety. Conversely, higher corrosion, lower tensile strength, higher aluminum content in the electrolyte, and lower drop test pass rate indicate poorer battery safety.

[0111] Example 1-1

[0112] <Preparation of the positive electrode>

[0113] Lithium nickel cobalt manganese oxide (LiNi) is used as the positive electrode active material.0.8 Co 0.1 Mn 0.1 O2), positive electrode conductive agent conductive carbon nanotubes (CNTs), positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight = 7 × 10⁻⁶). 6 The mixture was prepared by mixing at a mass ratio of 95:2.5:2.5, adding N-methylpyrrolidone (NMP) as a solvent, and stirring under vacuum to obtain a positive electrode material slurry with a solid content of 75wt% and a uniform system.

[0114] The oleophobic material is polytetrafluoroethylene (PTFE, weight average molecular weight = 5 × 10⁻⁶). 6 Dispersant: sodium carboxymethyl cellulose; Binder: polyvinylidene fluoride (PVDF, weight average molecular weight = 7 × 10⁻⁶). 6 The mixture was prepared by mixing at a mass ratio of 95:2.5:2.5, adding N-methylpyrrolidone (NMP) as a solvent, and stirring under vacuum to obtain a protective layer slurry with a solid content of 68wt% and a uniform system.

[0115] The positive electrode material layer slurry and the protective layer slurry are uniformly coated on one surface of a 10μm thick aluminum foil for the positive electrode current collector. The foil is then dried at 90℃ to obtain a positive electrode sheet with a single-sided coating of the positive electrode material layer and the protective layer. The above steps are then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of the positive electrode material layer and the protective layer. After drying at 90℃, the sheet is cold-pressed, then cut and die-cut to obtain a positive electrode sheet with a main body size of 74mm × 867mm for later use (structural reference). Figure 3 and Figure 4 But not with Figure 3 and Figure 4 (For limitations), along the width direction of the positive current collector, there are 17 positive electrode tabs on one side of the positive electrode sheet. Each positive electrode tab has dimensions of 18mm in height and 15mm in width (the height refers to the tab's dimension along the width of the positive electrode sheet, and the width refers to the tab's dimension along the length of the positive electrode sheet). The compaction density of the positive electrode material layer is 3.7 g / cm³. 3 The compaction density of the protective layer is 1.2 g / cm³. 3 The porosity f% of the protective layer, the width L1 of the protective layer, and the area ratio S of the protective layer covering the tab region are shown in Table 1. The thickness H1 of the protective layer, the thickness H2 of the positive electrode material layer, and the thickness H3 of the positive electrode current collector are shown in Table 2.

[0116] <Preparation of Negative Electrode Sheets>

[0117] The negative electrode active material is hard carbon, the negative electrode conductive agent is acetylene black, and the negative electrode binder is styrene-butadiene rubber (SBR, weight average molecular weight = 5 × 10⁻⁶). 6Thickener carboxymethyl cellulose (CMC) was mixed in a mass ratio of 96:2:1:1, and then deionized water was added as a solvent. The mixture was stirred under vacuum to obtain a homogeneous negative electrode slurry with a solid content of 70 wt%. The negative electrode slurry was uniformly coated onto one surface of an 8 μm thick copper foil current collector and dried at 90°C to obtain a negative electrode sheet with a single-sided negative electrode material layer. The above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided negative electrode material layer. After drying at 90°C, the sheet was cold-pressed, then cut and die-cut to obtain a negative electrode sheet with a size of 78 mm × 875 mm for later use. The thickness of the single-sided negative electrode material layer was 130 μm, and the compaction density of the negative electrode material layer was 1.7 g / cm³. 3 Along the width direction of the negative current collector, there are 19 negative electrode tabs on one side of the negative electrode sheet. The dimensions of each negative electrode tab are 18mm in height and 15mm in width (the above height is the dimension of the negative electrode tab in the width direction of the negative electrode sheet, and the width is the dimension of the negative electrode tab in the length direction of the negative electrode sheet).

[0118] <Isolation membrane>

[0119] A 7μm thick porous polyethylene polymer film (manufacturer: Hunan Zhongli New Materials Co., Ltd.) was used as the separator.

[0120] <Preparation of Electrolyte>

[0121] In an argon-atmospheric glove box with a water content of less than 10 ppm, non-aqueous solvents ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC) were uniformly mixed at a mass ratio of 30:40:30 to obtain a base solvent. Then, lithium hexafluorophosphate (LiPF6) and lithium difluorosulfonyl imide (LiFSI), a compound containing fluorosulfonyl groups, were added to the base solvent and dissolved and mixed uniformly to obtain the electrolyte. The concentration of LiPF6 in the electrolyte was 0.5 mol / L, and the concentration of LiFSI in the electrolyte (c mol / L) is shown in Table 1. The remainder was the base solvent.

[0122] <Preparation of Lithium-ion Batteries>

[0123] The prepared positive electrode, separator, negative electrode, and separator are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. The electrode assembly is then wound to form an electrode assembly. This assembly is placed in an aluminum-plastic film and dehydrated at 80°C. A prepared electrolyte is then injected, followed by vacuum sealing, settling, formation, degassing, and edge trimming to obtain a lithium-ion battery. The formation upper limit voltage is 4.15V, the formation temperature is 70°C, and the formation settling time is 2 hours.

[0124] Examples 1-2 to 1-22

[0125] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as in Example 1-1. Among them, in Example 1-15, the porosity f of the protective layer is controlled by adjusting the compaction density of the protective layer; in Examples 1-20 to 1-22, the dimensions of the positive and negative electrode tabs are adjusted so that L1 and S are as shown in Table 1: in Example 1-20, the dimensions of the positive and negative electrode tabs are 3.3 mm in height and 15 mm in width; in Example 1-21, the dimensions of the positive and negative electrode tabs are 22.2 mm in height and 15 mm in width; and in Example 1-22, the dimensions of the positive and negative electrode tabs are 66.7 mm in height and 15 mm in width.

[0126] Examples 2-1 to 2-9

[0127] Except for adjusting the corresponding preparation parameters according to Table 2, the rest is the same as in Examples 1-1. Specifically, the thickness H1 of the protective layer is controlled by adjusting the single-sided coating quality of the protective layer, and the thickness H2 of the positive electrode material layer is controlled by adjusting the single-sided coating quality of the positive electrode material layer; when the thickness of the positive electrode material layer changes, the thickness of the negative electrode material layer changes accordingly, keeping the N / P ratio constant.

[0128] Example 2-10

[0129] Except for the preparation method used in <Preparation of the positive electrode sheet>, the rest is the same as in Example 1-1.

[0130] <Preparation of the positive electrode>

[0131] Lithium nickel cobalt manganese oxide (LiNi) is used as the positive electrode active material. 0.8 Co 0.1 Mn 0.1 O2), positive electrode conductive agent conductive carbon nanotubes (CNTs), positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight = 7 × 10⁻⁶). 6 The mixture was prepared by mixing at a mass ratio of 95:2.5:2.5, adding N-methylpyrrolidone (NMP) as a solvent, and stirring under vacuum to obtain a positive electrode material slurry with a solid content of 75wt% and a uniform system.

[0132] Alumina ceramic particles and polyvinylidene fluoride (PVDF) ceramic layer binder were mixed at a mass ratio of 75:25. N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was stirred under vacuum to obtain a ceramic layer slurry with a solid content of 65 wt% and a homogeneous system.

[0133] The oleophobic material is polytetrafluoroethylene (PTFE, weight average molecular weight = 5 × 10⁻⁶). 6 Dispersant: sodium carboxymethyl cellulose; Binder: polyvinylidene fluoride (PVDF, weight average molecular weight = 7 × 10⁻⁶).6 The mixture was prepared by mixing at a mass ratio of 95:2.5:2.5, adding N-methylpyrrolidone (NMP) as a solvent, and stirring under vacuum to obtain a protective layer slurry with a solid content of 68wt% and a uniform system.

[0134] The positive electrode material layer slurry, ceramic layer slurry, and protective layer slurry are uniformly coated on one surface of a 10μm thick aluminum foil used for positive electrode current collectors. The foil is then dried at 90℃ to obtain a single-sided positive electrode sheet coated with the positive electrode material layer, ceramic layer, and protective layer. The above steps are then repeated on the other surface of the aluminum foil to obtain a double-sided positive electrode sheet coated with the same material layer, ceramic layer, and protective layer. After drying at 90℃, the sheet is cold-pressed, then cut and die-cut to obtain a positive electrode sheet with a main body size of 74mm × 867mm for later use (structural reference). Figure 7 and Figure 8 But not with Figure 7 and Figure 8 (For limitations), along the width direction of the positive current collector, there are 17 positive electrode tabs on one side of the positive electrode sheet. Each positive electrode tab has dimensions of 18mm in height and 15mm in width (the height refers to the tab's dimension along the width of the positive electrode sheet, and the width refers to the tab's dimension along the length of the positive electrode sheet). The compaction density of the positive electrode material layer is 3.7 g / cm³. 3 The compaction density of the protective layer is 1.2 g / cm³. 3 The compaction density of the ceramic layer is 2.5 g / cm³. 3 The thickness H4 of the ceramic layer is 20 μm, and the width of the ceramic layer along the width direction of the positive electrode current collector is 3 mm. The porosity f% of the protective layer, the width L1 of the protective layer, and the area ratio S of the protective layer covering the tab region are the same as in Example 1-1. The thickness H1 of the protective layer, the thickness H2 of the positive electrode material layer, and the thickness H3 of the positive electrode current collector are shown in Table 2.

[0135] Comparative Example 1

[0136] Except for the preparation method used in <Preparation of the positive electrode sheet>, the rest is the same as in Example 1-1.

[0137] <Preparation of the positive electrode>

[0138] Lithium nickel cobalt manganese oxide (LiNi) is used as the positive electrode active material. 0.8 Co 0.1 Mn 0.1 O2), positive electrode conductive agent conductive carbon nanotubes (CNTs), positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight = 7 × 10⁻⁶). 6The materials were mixed at a mass ratio of 95:2.5:2.5, with N-methylpyrrolidone (NMP) added as a solvent. The mixture was stirred under vacuum to obtain a homogeneous positive electrode material slurry with a solid content of 75 wt%. The positive electrode material slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil used as a positive electrode current collector. The foil was then dried at 90°C to obtain a positive electrode sheet with a single-sided coating. This process was repeated on the other surface of the foil to obtain a positive electrode sheet with a double-sided coating. After drying at 90°C, the sheet was cold-pressed, then cut and die-cut to obtain a positive electrode sheet with a main body size of 74 mm × 867 mm. Along the width of the positive electrode current collector, one side of the positive electrode sheet had 17 positive electrode tabs, each with dimensions of 18 mm in height and 15 mm in width (the height is the dimension of the negative electrode tab in the width direction of the negative electrode sheet, and the width is the dimension of the negative electrode tab in the length direction of the negative electrode sheet). The compaction density of the positive electrode material layer is 3.7 g / cm³. 3 The thickness H2 of the positive electrode material layer and the thickness H3 of the positive electrode current collector are the same as in Example 1-1.

[0139] Comparative Examples 2 to 3

[0140] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0141] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Tables 1 and 2.

[0142] Table 1

[0143] Note: " / " in Table 1 indicates that there is no corresponding parameter. Taking Example 1-1 as an example, the drop test pass rate after high temperature storage is "20 / 20", which means that 20 out of 20 lithium-ion batteries passed the drop test. Other similar expressions follow the same principle.

[0144] As can be seen from Examples 1-1 to 1-22 and Comparative Examples 1 to 3, this application, by designing the structure of the positive electrode and controlling the values ​​of S and θ within the range of this application, shows that after high-temperature storage, the positive electrode tab of the lithium-ion battery is not corroded, the tensile strength of the positive electrode tab is greater, the mass content of aluminum in the electrolyte is lower, and the drop test pass rate of the lithium-ion battery is higher, indicating that the lithium-ion battery of this application has good safety performance. However, Comparative Example 1 did not have a protective layer on its positive electrode tab, and the values ​​of S or θ of the positive electrode tabs in Comparative Examples 2 and 3 were not within the range of this application. After high-temperature storage, the positive electrode tabs of the lithium-ion batteries in Comparative Examples 1 to 3 were severely corroded, the tensile strength of the positive electrode tab was lower, the mass content of aluminum in the electrolyte was higher, and the drop test pass rate of the lithium-ion batteries was lower, indicating poor safety performance of the lithium-ion batteries.

[0145] The S-value affects the safety performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-4 and Comparative Examples 1 to 2, when the S-value is too small, such as in Comparative Examples 1 and 2, the positive electrode tab of the lithium-ion battery suffers severe corrosion after high-temperature storage, resulting in lower tensile strength, higher aluminum content in the electrolyte, and a lower drop test pass rate, indicating poor safety performance. When the S-value is within the range specified in this application, the positive electrode tab is not corroded after high-temperature storage, exhibiting higher tensile strength, lower aluminum content in the electrolyte, and a higher drop test pass rate, indicating good safety performance of the lithium-ion battery in this application.

[0146] The type and mass percentage (Ws%) of the oleophobic material, the type and mass percentage (Wf%) of the dispersant, the type and mass percentage (Wf%) of the binder, the porosity (f%) of the protective layer, and the type and concentration of the fluorosulfonyl group compound in the electrolyte all affect the safety performance of the lithium-ion battery. As can be seen from Examples 1-1 to 1-22, when the type and mass percentage (Ws%) of the oleophobic material, the type and mass percentage (Wf%) of the dispersant, the type and mass percentage (Wf%) of the binder, the porosity (f%) of the protective layer, and the type and concentration of the fluorosulfonyl group compound in the electrolyte are within the scope of this application, the positive electrode tab of the lithium-ion battery is not corroded after high-temperature storage, the tensile strength of the positive electrode tab is high, the mass content of aluminum in the electrolyte is low, and the drop test pass rate of the lithium-ion battery is high, indicating that the lithium-ion battery of this application has good safety performance.

[0147] Table 2

[0148] The values ​​of H2, H1 / H2, and H1 / H3 affect the safety performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-9, when the values ​​of H2, H1 / H2, and H1 / H3 are within the range of this application, the positive electrode tab of the lithium-ion battery is not corroded after high-temperature storage, the tensile strength of the positive electrode tab is high, the mass content of aluminum in the electrolyte is low, and the drop test pass rate of the lithium-ion battery is high, indicating that the lithium-ion battery of this application has good safety performance. However, as the protective layer thickness H1 increases, although the tensile strength of the positive electrode tab increases and the mass content of aluminum in the electrolyte decreases after high-temperature storage, it leads to a decrease in the energy density of the lithium-ion battery (e.g., Examples 2-4, 2-5, and 2-9). When the difference between H1 and H2 is too large, i.e., H1 / H2 is too small, it also makes the processing of the positive electrode sheet difficult, affecting the processing performance of the lithium-ion battery (e.g., Example 2-6). While increasing the thickness H2 of the cathode material layer is beneficial for increasing the energy density of lithium-ion batteries, it also leads to a decrease in kinetic performance and an increase in the risk of lithium plating, affecting the cycle performance and safety performance of lithium-ion batteries (e.g., in Examples 2-4 and 2-5). Excessive H2 can also affect the processing performance of lithium-ion batteries and increase the processing difficulty.

[0149] The presence of a ceramic layer in the positive electrode affects the safety performance of lithium-ion batteries. As can be seen from Examples 1-1 and 2-10, by placing a ceramic layer between the protective layer and the positive electrode material layer, the positive electrode tab of the lithium-ion battery was not corroded after high-temperature storage, exhibiting high tensile strength, low aluminum content in the electrolyte, and a high drop test pass rate, indicating that the lithium-ion battery of this application has good safety performance.

[0150] It should be noted that, in this document, the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0151] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0152] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery, comprising a positive electrode and an electrolyte, wherein the positive electrode comprises a positive current collector, a positive electrode material layer, and a protective layer, wherein the positive electrode material layer and the protective layer are both located on at least one surface of the positive current collector. Along the width direction of the positive current collector, the positive current collector includes a main body region and a tab region, the positive electrode material layer is disposed in the main body region, and the protective layer is disposed in the tab region; The area ratio of the protective layer covering the tab region is S, where 0.6 ≤ S ≤ 1, and the contact angle between the protective layer and the electrolyte is θ, where 90° < θ ≤ 165°.

2. The secondary battery according to claim 1, wherein, The secondary battery satisfies at least one of the following characteristics: （1）0.8≤S≤1； (2) 120°≤θ≤165°.

3. The secondary battery according to claim 1, wherein, The electrolyte comprises a compound containing a fluorosulfonyl group, wherein the compound containing the fluorosulfonyl group comprises at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, or lithium bis(pentafluoroethanesulfonyl)imide, and the concentration of the compound containing the fluorosulfonyl group in the electrolyte is c mol / L, where 0.2 ≤ c ≤ 2.

4. The secondary battery according to claim 1, wherein, The protective layer includes an oleophobic material, which includes at least one of polytetrafluoroethylene, perfluoropolyether, siloxane, silane coupling agent, nano-silica, or fluorinated silane. Based on the mass of the protective layer, the mass percentage of the oleophobic material is Ws%, 80≤Ws≤99.

5. The secondary battery according to claim 4, wherein, The secondary battery satisfies at least one of the following characteristics: (1) The weight-average molecular weight of the polytetrafluoroethylene is 10. 6 Up to 10 7 ; (2) The weight-average molecular weight of the perfluoropolyether is 5 × 10⁻⁶. 5 Up to 5×10 6 ; (3) The weight-average molecular weight of the siloxane is 2 × 10⁻⁶. 3 Up to 5×10 4 ; (4) The weight-average molecular weight of the silane coupling agent is 150 to 300; (5) The average particle size of the nano-silica is 5 nm to 100 nm; (6) The weight-average molecular weight of the fluorinated silane is 300 to 600; (7) 90≤Ws≤95.

6. The secondary battery according to claim 1, wherein, The protective layer further includes a dispersant and a binder. The dispersant includes at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium dodecyl sulfate, or polyethylene glycol. The binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber, polyvinyl alcohol, polytetrafluoroethylene, epoxy resin, or polyurethane. The polytetrafluoroethylene has a weight-average molecular weight of 10. 5 Up to 5×10 5 ; Based on the quality of the protective layer, the mass percentage of the dispersant is Wf%, 0.5≤Wf≤10, and the mass percentage of the binder is Wn%, 0.5≤Wn≤10.

7. The secondary battery according to any one of claims 1 to 6, wherein, The thickness of the protective layer is H1 μm, the thickness of the positive electrode material layer is H2 μm, and the thickness of the positive electrode current collector is H3 μm, 10≤H2≤3000, 0.1≤H1 / H2≤1, and 0.2≤H1 / H3≤15.

8. The secondary battery according to claim 7, wherein, 50≤H2≤500, and / or, 0.2≤H1 / H2≤0.5, and / or, 1≤H1 / H3≤10.

9. The secondary battery according to any one of claims 1 to 6, wherein, Along the width direction of the positive electrode sheet, the width of the protective layer is L1 mm, where 3 ≤ L1 ≤ 60 mm.

10. The secondary battery according to claim 9, wherein, 3≤L1≤20。 11. The secondary battery according to any one of claims 1 to 6, wherein, The porosity of the protective layer is f%, 0 < f ≤ 20.

12. The secondary battery according to any one of claims 1 to 6, wherein, Along the width direction of the positive electrode sheet, a ceramic layer is disposed between the protective layer and the positive electrode material layer, and the ceramic layer is disposed in the main body area.

13. An electronic device comprising a secondary battery as described in any one of claims 1 to 12.

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