Electrode assembly, secondary battery, and electronic device

Abstract

A secondary battery and an electronic device. The secondary battery comprises an electrode assembly. A positive electrode sheet in the electrode assembly comprises a positive electrode current collector, and bottom coatings and positive electrode active material layers arranged in sequence in directions away from the positive electrode current collector. The bottom coatings include a first bottom coating and a second bottom coating respectively arranged on two sides of the positive electrode current collector. The first bottom coating faces a winding center, and the second bottom coating faces away from the winding center. The thickness of the first coating is A μm, and the thickness of the second bottom coating is B μm, wherein 2.1≤B≤4.5 and 1.02≤A / B≤1.80. By arranging bottom coatings having different thicknesses on two sides of a positive electrode sheet, the risk of short circuits of secondary batteries caused by internal friction and external impact is reduced, and the overall energy density of the positive electrode sheet is further indirectly improved, thereby improving both the energy density and the safety performance of secondary batteries.

Description

Electrode assembly, secondary battery and electronic device TECHNICAL FIELD

[0001] The present application belongs to the technical field of energy storage, and particularly relates to an electrode assembly, a secondary battery and an electronic device. BACKGROUND

[0002] Secondary batteries are widely used in various electronic devices due to their high energy density. With the rapid development of electronic devices, the requirements for secondary batteries are also increasing. However, in the current process of installing a secondary battery in a mobile phone or a notebook computer, the size of the screws and other accessories used is relatively small. If the accessories are lost on the surface of the secondary battery, they often cannot be identified, so that in the process of packaging or charging and discharging, the screws are forced to be pressed into the inside of the battery cell, which easily triggers a short circuit of the secondary battery and causes failure, resulting in fire or explosion, and thus seriously affecting the safety performance of the secondary battery.

[0003] SUMMARY

[0004] Therefore, the present application provides a secondary battery and an electronic device. By setting different thicknesses of the undercoat layers on both sides of the positive electrode sheet in the electrode assembly, the risk of short circuit of the secondary battery caused by internal friction and external impact is reduced, and the safety performance of the secondary battery and the electronic device is improved.

[0005] In a first aspect, the present application provides a secondary battery, which comprises a packaging bag and an electrolyte contained in the packaging bag. The secondary battery further comprises an electrode assembly, which is contained in the packaging bag. The electrode assembly comprises a winding unit formed by stacking a positive electrode sheet, a separator and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and an undercoat layer and a positive electrode active material layer arranged in sequence away from the positive electrode current collector; the undercoat layer comprises a first undercoat layer and a second undercoat layer arranged on both sides of the positive electrode current collector respectively, the first undercoat layer faces the winding center, and the second undercoat layer faces away from the winding center; the thickness of the first undercoat layer is A μm, the thickness of the second undercoat layer is B μm, 2.1≤B≤4.5, and 1.02≤A / B≤1.80. By setting different thicknesses of the undercoat layers on both sides of the positive electrode sheet, the thickness of the second undercoat layer is within a certain range, the risk of short circuit of the secondary battery caused by internal friction and external impact is reduced, and the overall energy density of the positive electrode sheet is indirectly improved, so that the energy density and safety performance of the secondary battery are improved.

[0006] In some embodiments, the electrode assembly satisfies at least one of the following conditions: (1) 1.125≤A / B≤1.5; (2) 2.6≤A≤5.8; (3) 2.5≤B≤4.5. Controlling the first undercoat layer and the second undercoat layer in the electrode assembly to satisfy at least one of the above conditions is conducive to further improving the energy density and safety performance of the secondary battery.

[0007] In some embodiments, the undercoat layer comprises a conductive agent; a mass content of the conductive agent in the first undercoat layer is M% based on a mass of the first undercoat layer; a mass content of the conductive agent in the second undercoat layer is N% based on a mass of the second undercoat layer; 0.5≤M-N≤3.5. By controlling the conductive agent content of the first undercoat layer and the second undercoat layer to satisfy the above relationship, in combination with the thickness of the undercoat layer, the probability of contact of the debris or exposed current collector when subjected to extrusion impact can be further reduced, and the safety performance and charge-discharge performance of the secondary battery can be improved.

[0008] In some embodiments, the electrode assembly satisfies at least one of the following conditions: (1) 2≤M≤7; (2) 1≤N≤6; (3) 0.9≤M-N≤3.2. By controlling the conductive agent content of the first undercoat layer and the second undercoat layer in the electrode assembly to satisfy at least one of the above conditions, the charge-discharge performance and safety performance of the secondary battery can be further improved.

[0009] In some embodiments, the conductive agent is selected from carbon black and / or carbon nanotubes.

[0010] In some embodiments, the undercoat layer further comprises inorganic particles; the inorganic particles satisfy at least one of the following conditions: (1) the average particle size of the inorganic particles is Wμm, 0.5≤W≤1.4; (2) the inorganic particles are selected from at least one of boehmite, aluminum oxide, or lithium iron phosphate; (3) a mass content of the inorganic particles is H% based on a mass of the undercoat layer, 85≤H≤95. By controlling the inorganic particles to satisfy at least one of the above conditions, the safety performance and charge-discharge performance of the secondary battery can be further improved.

[0011] In some embodiments, the undercoat layer further comprises a binder selected from at least one of polyacrylic acid, polyacrylate, styrene-acrylic emulsion, or styrene-butadiene rubber.

[0012] In some embodiments, the thickness of the single-layer positive active material layer is Cμm, 30≤C≤50. Under this condition, the positive active material layer can better cooperate with the undercoat layer, and the safety performance and charge-discharge performance of the secondary battery can be improved.

[0013] In a second aspect, the present application provides an electronic device comprising the secondary battery of the first aspect.

[0014] The application sets asymmetric undercoating layers on both sides of the positive electrode current collector, the thickness of the second undercoating layer is within a certain range, and by adjusting the thickness ratio of the first undercoating layer and the second undercoating layer, when the electrode assembly is subjected to external extrusion or impact, the first undercoating layer towards the winding center can buffer external impact, reduce the damage of the pole piece and external short circuit caused by impact, and the second undercoating layer away from the winding center can resist the internal friction of the electrode assembly, reduce the thinning of the coating caused by friction. The two undercoating layers cooperate to reduce the probability of debris generation and reduce or avoid the exposure of the positive electrode current collector, which is beneficial to improve the safety performance of the secondary battery under the condition of screw extrusion impact; at the same time, the above design can also reduce the amount of undercoating layer material that does not provide energy density, indirectly improving the overall energy density of the positive electrode pole piece, thereby achieving the improvement of the energy density and safety performance of the secondary battery. On this basis, the application further adjusts the content relationship of the conductive agent in the first undercoating layer and the second undercoating layer, which can reduce the contact probability of debris or exposed current collector when subjected to extrusion impact, further reduce the risk of short circuit, and thus improve the charge-discharge performance and safety performance of the secondary battery. BRIEF DESCRIPTION OF DRAWINGS

[0015] Fig. 1 is a schematic diagram of the positive electrode pole piece in the electrode assembly provided by the specific embodiment of the application;

[0016] Fig. 2 is an SEM diagram of the cutting surface of the positive electrode pole piece in the lithium ion battery of the application examples 1-7. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical scheme and advantages of the application more clear and understandable, the application will be further described in detail below in combination with examples. It should be understood that the specific examples described herein are only used to explain the application and not to limit the application.

[0018] In the face of safety problems caused by screw extrusion of secondary batteries, the prior art generally adopts means such as increasing the strength of battery materials, optimizing electrode structure such as setting undercoating layer. When setting the undercoating layer, in order to keep the structure and performance of the current collector on both sides stable and consistent, reduce performance fluctuations, generally symmetric structure is used on both sides of the current collector.

[0019] However, the inventors found during the research that the morphology of the coating on the two sides of the positive electrode tab, which faces and faces away from the winding center, is slightly different after the electrode assembly with a winding structure is subjected to screw extrusion. The fracture surface of the coating on the side facing away from the winding center is relatively flat, showing a morphology similar to that of a cut, while the fracture surface on the side facing the winding center shows friction marks. The inventors speculate that this phenomenon may be due to the different ways in which the positive electrode tab on the two sides facing and facing away from the winding center is subjected to external force during extrusion cutting. The coating on the side facing away from the winding center mainly bears impact force, showing a motion pattern close to dynamic friction, while the force on the side facing the winding center is close to static friction. The upper limit of the friction coefficient of the static friction on the side facing the winding center is greater than that of the dynamic friction on the side facing away from the winding center. Therefore, there is a feasibility of optimizing the coating on the side facing away from the winding center.

[0020] On this basis, the inventors propose a secondary battery, which includes a packaging bag and an electrolyte contained in the packaging bag. The secondary battery also includes an electrode assembly contained in the packaging bag. The electrode assembly includes a winding unit formed by stacking a positive electrode tab, a separator, and a negative electrode tab. Referring to the schematic diagram shown in FIG. 1, the positive electrode tab includes a positive current collector and a bottom coating and a positive active material layer arranged in sequence away from the positive current collector. The bottom coating includes a first bottom coating and a second bottom coating arranged on both sides of the positive current collector, respectively. In the electrode assembly, the first bottom coating faces the winding center, and the second bottom coating faces away from the winding center. The thickness of the first bottom coating is A μm, and the thickness of the second bottom coating is B μm, 2.1 ≤ B ≤ 4.5, and 1.02 ≤ A / B ≤ 1.80. By arranging the first bottom coating and the second bottom coating with asymmetric design on both sides of the positive current collector, and adjusting the thickness of the second bottom coating within a certain range, and regulating the thickness relationship between the two bottom coatings, when the electrode assembly is subjected to external extrusion or impact, the first bottom coating facing the winding center can buffer the external impact, reducing the damage of the tab and the external short circuit caused by the impact. The second bottom coating facing away from the winding center can resist the internal friction of the electrode assembly, reducing the thinning of the coating caused by friction. The two bottom coatings with different thicknesses cooperate to reduce the probability of debris generation and reduce or avoid the exposure of the positive current collector. At the same time, the above design can also reduce the amount of bottom coating material that does not provide energy density, indirectly improving the overall energy density of the positive electrode tab, thereby achieving the improvement of the energy density and safety performance of the secondary battery.

[0021] Positive electrode tab

[0022] In some embodiments of the present application, 1.02≤A / B≤1.80, preferably 1.125≤A / B≤1.5, for example, the value of A / B can be 1.02, 1.07, 1.13, 1.125, 1.15, 1.24, 1.26, 1.33, 1.41, 1.44, 1.5, 1.61, 1.68, 1.73, 1.75, 1.80 or a value within a range consisting of any two of these values. By regulating the thicknesses of the first and second undercoating layers to satisfy the above relationship, the overall energy density can be improved on the basis of ensuring the safety performance of the secondary battery, achieving a balance between improving the energy density and safety performance of the secondary battery.

[0023] In some embodiments, 2.6≤A≤5.8, preferably 3.7≤A≤5.2, for example, A can be 2.6, 2.7, 3.0, 3.4, 3.6, 4.0, 4.3, 5.1, 5.1, 5.6, 5.8 or a value within a range consisting of any two of these values. By regulating the thickness of the first undercoating layer to satisfy the above range, the first undercoating layer can better resist internal friction when subjected to extrusion impact, and after cooperating with the second undercoating layer, the energy density can be improved while the safety performance of the secondary battery is improved.

[0024] In some embodiments, 2.1≤B≤4.5, preferably 2.9≤B≤4.0, for example, B can be 2.1, 2.3, 2.5, 2.7, 2.9, 3.4, 3.4, 3.7, 4.0, 4.3, 4.5 or a value within a range consisting of any two of these values. By regulating the thickness of the second undercoating layer to satisfy the above range, the second undercoating layer can better resist external impact when subjected to extrusion impact, and cooperating with the first undercoating layer is conducive to balancing the safety performance and energy density of the secondary battery.

[0025] In some embodiments, the undercoating layer comprises a conductive agent; the mass content of the conductive agent in the first undercoating layer is M% based on the mass of the first undercoating layer; the mass content of the conductive agent in the second undercoating layer is N% based on the mass of the second undercoating layer; 0.5≤M-N≤3.5. In some preferred embodiments, 0.9≤M-N≤3.2. In some more preferred embodiments, 1.5≤M-N≤2.5. For example, the value of M-N can be 0.5, 0.6, 0.9, 1.1, 1.2, 1.5, 1.8, 2.0, 2.3, 2.5, 2.6, 2.8, 3.1, 3.3, 3.5 or a value within a range consisting of any two of these values. By regulating the content relationship of the conductive agent in the first and second undercoating layers, cooperating with the thickness relationship of the undercoating layer, the contact probability of debris or exposed current collector when subjected to extrusion impact can be reduced on the basis of ensuring excellent charge and discharge performance of the secondary battery, further reducing the risk of short circuit, thereby balancing the charge and discharge performance and safety performance of the secondary battery.

[0026] In some embodiments, 2≤M≤7, preferably 3.7≤M≤6.5. For example, M is selected from 2, 2.2, 2.5, 2.8, 3.4, 3.6, 4.1, 4.5, 4.8, 5.3, 5.7, 6.1, 6.3, 6.8, 7, or a range defined by any two of these values. Controlling the mass content of the conductive agent in the first undercoat layer to meet the above range, in combination with the conductive agent of the second undercoat layer, can improve the charge-discharge performance of the secondary battery, reduce the full charge time, and further improve the safety performance of the secondary battery.

[0027] In some embodiments, 1≤N≤6, preferably 2.5≤N≤4.5. For example, N is selected from 1, 1.1, 1.7, 2.1, 2.4, 2.9, 2.9, 3.5, 3.8, 4.1, 4.5, 5.0, 5.6, 5.7, 6, or a range defined by any two of these values. Controlling the content of the conductive agent in the second undercoat layer to meet the above range, in combination with the conductive agent of the first undercoat layer, can improve the safety performance and charge-discharge performance of the secondary battery.

[0028] In some embodiments, the conductive agent is selected from carbon black and / or carbon nanotubes. For example, the carbon nanotubes are selected from single-walled carbon nanotubes and / or multi-walled carbon nanotubes.

[0029] In some embodiments, the undercoat layer further comprises inorganic particles.

[0030] In some embodiments, the average particle size of the inorganic particles is W pm, 0.5≤W≤1.4, preferably 0.8≤W≤1. For example, W can be 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, or a range defined by any two of these values. By controlling the average particle size of the inorganic particles to meet the above range, in combination with the thickness of the undercoat layer of the present application, the inorganic particles and the conductive agent can be better combined in the undercoat layer, further improving the safety performance and charge-discharge performance of the secondary battery.

[0031] In some embodiments, the inorganic particles are selected from at least one of boehmite, aluminum oxide, or lithium iron phosphate.

[0032] In some embodiments, the mass content of the inorganic particles is H%, 85≤H≤95, based on the mass of the undercoat layer. Preferably, 90≤H≤92. For example, H can be 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, or a range defined by any two of these values. By controlling the mass content of the inorganic particles to meet the above range, the safety performance and charge-discharge performance of the secondary battery can be further improved.

[0033] In some embodiments, the primer layer further comprises a binder selected from at least one of polyacrylic acid, polyacrylate, styrene-acrylic emulsion, or styrene-butadiene rubber.

[0034] In some embodiments, the thickness of the single layer of the positive electrode active material layer is C pm, 30≤C≤50, preferably 43≤C≤45. For example, the value of C can be selected from 30, 31, 33, 34, 35, 37, 39, 40, 42, 42, 45, 47, 48, 50, or a range between any two of these values. The positive electrode active material layer in the present application comprises at least two layers of positive electrode active material layers on both sides of the positive electrode current collector, and the thickness of the single layer of the positive electrode active material layer is controlled to be within the above range, which can better cooperate with the primer layer of the present application, and improve the safety performance and charge-discharge performance of the secondary battery.

[0035] The positive electrode active material in the present application is not particularly limited as long as it can achieve the purpose of the present application. For example, the positive electrode active material is selected from at least one of Li-Ni-Co-Al (NCA), Li-Ni-Co-Mn (NCM), lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMnO2), lithium nickel oxide (LiNiO2), or lithium iron phosphate (LiFePO4).

[0036] The positive electrode current collector in the present application is not particularly limited as long as it can achieve the purpose of the present application. For example, the positive electrode current collector can include an aluminum foil, an aluminum alloy foil, or a composite current collector (such as an aluminum-carbon composite current collector), etc.

[0037] The kind of the positive electrode binder used in the manufacturing of the positive electrode active material layer is not particularly limited, and in the case of the coating method, it is only required to be a material that is soluble or dispersible in the liquid medium used at the time of manufacturing the electrode. Examples of the positive electrode binder can include, but are not limited to, one or more of the following: resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, and the like; rubber-like polymers such as styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), fluororubber, isoprene rubber, polybutadiene rubber, ethylene-propylene rubber, and the like; thermoplastic elastomer-like polymers such as styrene-diene-styrene block copolymer or its hydrogenated product, ethylene-propylene-diene terpolymer (EPDM), styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer or its hydrogenated product, and the like; soft resin-like polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, propylene-a-olefin copolymer, and the like; fluorine-based polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene-ethylene copolymer, and the like; and polymer compositions having ionic conductivity of alkali metal ions (particularly lithium ions), and the like. The above-mentioned positive electrode binder can be used alone or in any combination.

[0038] In the present application, the positive electrode active material layer can further include a positive electrode conductive agent. The present application does not particularly limit the kind of the positive electrode conductive agent in the positive electrode active material layer, as long as the purpose of the present application can be achieved, for example, the positive electrode conductive agent can include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, ketjen black, graphene, metal materials, conductive polymers, and the like. The above-mentioned carbon nanotubes can include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers can include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or nanocarbon fibers. The above-mentioned metal materials can include, but are not limited to, metal powder and / or metal fibers, and specifically, the metal can include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above-mentioned conductive polymers can include, but are not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The present application does not particularly limit the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode active material layer, and a person skilled in the art can select according to the actual needs, as long as the purpose of the present application can be achieved. The present application does not particularly limit the thickness of the positive electrode active material layer, as long as the purpose of the present application can be achieved.

[0039] Negative electrode sheet

[0040] The negative electrode sheet is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. In the present application, the negative electrode active material layer can be provided on one surface in the thickness direction of the negative electrode current collector, or can be provided on both surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" herein can be the entire area of the negative electrode current collector, or can be a partial area of the negative electrode current collector, and the present application is not particularly limited, as long as the object of the present application can be achieved. The negative electrode current collector is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, it can include, but is not limited to, a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, or a composite current collector (such as a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.), and the like. In the present application, the thickness of the negative electrode current collector, the negative electrode active material layer, and the negative electrode sheet are not particularly limited, as long as the object of the present application can be achieved.

[0041] The negative electrode active material layer of the present application includes a negative electrode active material, which can include, but is not limited to, graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0.5 < x < 1.6), LiSn alloy, LiSnO alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 LiAl alloy, and at least one of metallic lithium.

[0042] The negative electrode active material layer in the present application can further include a negative electrode binder and a negative electrode conductive agent, or the negative electrode active material layer can further include a negative electrode binder, a negative electrode conductive agent, and a thickening agent. The type of the negative electrode binder and the negative electrode conductive agent is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the negative electrode binder can include, but is not limited to, at least one of the above-mentioned positive electrode binders, and the negative electrode conductive agent can include, but is not limited to, at least one of the above-mentioned positive electrode conductive agents. The type of the thickening agent is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the thickening agent can include, but is not limited to, at least one of sodium carboxymethyl cellulose or carboxymethyl cellulose. The mass ratio of the negative electrode active material, the negative electrode conductive agent, the negative electrode binder, and the thickening agent in the negative electrode active material layer is not particularly limited in the present application, and a person skilled in the art can select according to actual needs, as long as the object of the present application can be achieved.

[0043] Separation film

[0044] The separator film is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the material of the separator film can include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, aramid; the type of the separator film can include at least one of woven film, non-woven film, microporous film, composite film, calendered film, spun film. For example, the separator film can include a base layer and a surface treatment layer. The base layer can be a non-woven fabric, a film or a composite film having a porous structure, and the material of the base layer can include at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film can be used. Optionally, a surface treatment layer is provided on at least one surface of the base layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance. For example, the inorganic layer includes inorganic particles and a binder, and the inorganic particles are not particularly limited in the present application, and can include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium sulfate. The binder is not particularly limited in the present application, and can be at least one of the positive electrode binders described above. The polymer layer contains a polymer, and the polymer is not particularly limited in the present application, and can include at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyvinyl ether or polyvinylidene fluoride, poly(vinylidene hexafluoropropylene). In the present application, the thickness of the separator film is not particularly limited, as long as the object of the present application can be achieved, and for example, the thickness of the separator film can be 5 μm to 500 μm.

[0045] Electrolyte solution

[0046] In the present application, the electrolyte includes a lithium salt and a nonaqueous solvent. The lithium salt can include at least one of LiPF6, LiPO2F2, LiNO3, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluoroborate, etc. The present application does not limit the content of the lithium salt in the electrolyte, as long as the purpose of the present application is achieved. The present application does not particularly limit the nonaqueous solvent, as long as the purpose of the present application is achieved. For example, the nonaqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylate compound, an ether compound, or other organic solvents. The carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, a fluorinated carbonate compound. The chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), ethylpropyl carbonate, methyl ethyl carbonate. The cyclic carbonate compound can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate, butylene carbonate, vinyl ethylene carbonate. The fluorinated carbonate compound can include, but is 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-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, trifluoromethyl ethylene carbonate. The carboxylate compound can include, but is not limited to, at least one of methyl formate, methyl acetate, methyl propionate, methyl butyrate, ethyl formate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl formate, propyl acetate, propyl propionate, propyl butyrate, butyl butyrate, butyl propionate, pentyl propionate. The ether compound can include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran. The other organic solvents can include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, 1,3-propane sultone (PS), adiponitrile (ADN).

[0047] The packaging bag of the present application is used to contain the electrode assembly and the electrolyte, and other components known in the art in the secondary battery, and the present application does not limit the above-mentioned other components. The packaging bag of the present application is not particularly limited, and can be the packaging bag known in the art, as long as the purpose of the present application can be achieved.

[0048] In the second aspect, the present application provides an electronic device comprising the secondary battery of the above-mentioned second aspect. The secondary battery of the present application has good energy density, charge-discharge performance and safety performance, so that the electronic device of the present application also has a longer service life and higher safety performance when applied.

[0049] The electronic device of the present application is not particularly limited, and it can be any electronic device known in the prior art. For example, the electronic device can include, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a head-mounted stereo headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, an illuminating appliance, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a household large storage battery and a lithium ion capacitor.

[0050] The scheme of the present application will be described below in combination with the following specific examples. Unless otherwise specified, the raw materials used in the following examples are all from ordinary commercially available products, and the devices or equipment used are all purchased from conventional market sales channels.

[0051] Example 1

[0052] The lithium ion battery of the present embodiment comprises an electrode assembly, wherein the electrode assembly comprises a winding unit formed by stacking a positive electrode sheet, a separator and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a bottom coating layer and an active material layer arranged in sequence away from the positive electrode current collector; the bottom coating layer comprises a first bottom coating layer and a second bottom coating layer arranged on both sides of the positive electrode current collector, respectively, the first bottom coating layer faces the winding center, and the second bottom coating layer faces away from the winding center; the thickness A of the first bottom coating layer is 4.5 μm, the thickness B of the second bottom coating layer is 4.4 μm, and A / B = 1.023. The thickness C of the single-layer active material layer is 41 μm.

[0053] The undercoat layer comprises a conductive agent, inorganic particles and a binder, the conductive agent is carbon black, the inorganic particles are boehmite, and the binder is polyacrylic acid. The mass content M% of the conductive agent in the first undercoat layer is 5% based on the mass of the first undercoat layer; the mass content N% of the conductive agent in the second undercoat layer is 5% based on the mass of the second undercoat layer; M-N = 0. The average particle size Wμm of the inorganic particles is 1.2μm, and the mass content H% of the inorganic particles is 87% based on the mass of the undercoat layer.

[0054] Preparation of the positive electrode sheet:

[0055] Boehmite, polyacrylic acid and carbon black with a mass ratio of 87:8:5 were mixed in N-methyl pyrrolidone (NMP) to obtain an undercoat slurry, and the undercoat slurry was coated on an aluminum foil, wherein one side of the aluminum foil was taken as the side facing the winding center, and after coating and drying, a first undercoat layer was formed; then the other side of the aluminum foil was taken as the side away from the winding center, and after coating the undercoat slurry and drying, a second undercoat layer was formed.

[0056] Positive active material, polyvinylidene fluoride (PVDF) and carbon black with a mass ratio of 97.6:1.3:1.1 were mixed in N-methyl pyrrolidone (NMP) to obtain a positive electrode slurry, and the positive electrode slurry was coated on the first undercoat layer and the second undercoat layer, and after drying, cold pressing, sheet cutting and slitting, the positive electrode slurry was baked at 85°C under vacuum for 12h to obtain a positive electrode sheet.

[0057] Preparation of the negative electrode sheet: Artificial graphite, Super P, sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) with a mass ratio of 96.4:1.5:0.5:1.6 were mixed in deionized water to obtain a negative electrode slurry, wherein the solid content of the negative electrode slurry was 54wt%; the negative electrode slurry was coated on a copper foil, and the copper foil was dried at 85°C, and then after cold pressing, sheet cutting and slitting, the negative electrode slurry was dried at 80°C under vacuum for 12h to obtain a negative electrode sheet.

[0058] Preparation of the electrolyte: In an argon atmosphere glove box with a water content of less than 10ppm, ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) were mixed in a mass ratio of EC:PC:EMC:DEC = 1:3:3:3, and after sufficient stirring, lithium salt LiPF6 was added, and after uniform mixing, an electrolyte was obtained. The molar concentration of lithium salt LiPF6 was 1.5mol / L based on the mass of the electrolyte.

[0059] Preparation of the separator film: A polyethylene (PE) separator film with a thickness of 7μm was selected.

[0060] Preparation of lithium ion battery: the positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, with the separator between the positive electrode sheet and the negative electrode sheet to play a separating role, and then the electrode assembly is obtained by winding. After welding the tab, the electrode assembly is placed in an aluminum plastic film outer packaging bag, the electrolyte prepared above is injected into the dry electrode assembly, and after vacuum packaging, standing, formation, shaping and other processes, a lithium ion battery is obtained.

[0061] Test method:

[0062] Coating thickness test:

[0063] The positive electrode sheet is cut into a size of 1 cm x 1 cm, and then the positive electrode sheet cutting surface is polished using argon plasma polishing technology (CP) to prepare a CP sample. The CP sample is observed under a scanning electron microscope at a magnification of 3000 times, the thickness is measured at 5 positions on the coating to be tested, and the measured thickness values are arithmetically averaged to obtain the thickness of the coating to be tested.

[0064] Average particle size test:

[0065] The positive electrode sheet is cut into a size of 1 cm x 1 cm, and then the positive electrode sheet cutting surface is polished using argon plasma polishing technology (CP) to prepare a CP sample. The CP sample is observed by scanning electron microscopy, and an area of 8 μm x 8 μm is selected as a sample area. The SEM photo of the sample area is taken by scanning electron microscopy, and then 10 sample particles (e.g. inorganic particles) to be tested are randomly selected from the SEM photo as sample particles using image analysis software, and the areas of these sample particles are counted. Then, assuming that the sample particles are spherical, the particle size R (diameter) of each of these sample particles is calculated by the following formula: R = 2 x (S / π) 1 / 2 ; where S is the area of the sample particle. The particle size R of the above-mentioned sample particles is calculated for 3 SEM photos, and the particle sizes R of the 30 (10 x 3) sample particles obtained are arithmetically averaged to obtain the average particle size of the particles to be tested.

[0066] Volume energy density test:

[0067] The lithium ion battery of each example or comparative example is placed in a thermostat at 25°C for 30 minutes, charged at a constant current of 0.5C to 4.5V, then charged at a constant voltage of 4.5V to 0.025C, left standing for 5 minutes, then discharged at a constant current of 0.2C to 3.0V, the discharge capacity E and discharge voltage platform U of the lithium ion battery are recorded, and the length, width and height of the lithium ion battery at 50% state of charge are tested to obtain the volume V of the lithium ion battery, and the energy density = E*U / V.

[0068] Full charge time test:

[0069] The lithium ion battery of each example or comparative example was placed in a thermostat at 25°C for 30 minutes, charged to 4.5V at a constant current of 1C, and then charged to 0.2C at a constant voltage of 4.5V. The total time of the charging was recorded as the full charging time.

[0070] Screw extrusion test:

[0071] The lithium ion battery of each example or comparative example was placed in a thermostat at 25°C for 30 minutes, charged to 100% SOC (charged to 4.5V at a constant current of 1C, and then charged to 0.2C at a constant voltage of 4.5V), and then placed on a pressure electrode plate and a test screw was placed on the surface of the lithium ion battery to be tested. Then, the pressure device was used to apply pressure perpendicular to the electrode plate, and an extrusion pressure of 13±1kN was applied between the two pressure electrode plates, including: 20N of pre-load force was used as the pressure starting recording point, the extrusion speed was 15kN / min, and the test was stopped when the pressure reached 13kN. If the lithium ion battery did not catch fire or explode, it was considered to have passed the screw extrusion test. Ten lithium ion batteries were tested in parallel according to the above steps each time, and the passing rate was recorded as P, for example, 4P / 10 indicated that four out of ten tested lithium ion batteries passed the screw extrusion test.

[0072] The lithium ion batteries of each example and comparative example in Table 1 differed from Example 1-1 only in that the thickness A (μm) of the first undercoat layer, the thickness B (μm) of the second undercoat layer, and the thickness ratio of the two were adjusted according to Table 1. The specific adjustment conditions and performance test results are shown in Table 1. FIG. 2 is an SEM image of the cut surface of the positive electrode sheet of the lithium ion battery of Example 1-7, as shown in FIG. 2, the thickness relationship between the first undercoat layer facing the winding center (located below the current collector) and the second undercoat layer facing away from the winding center (located above the current collector) in the positive electrode sheet conforms to the above range.

[0073] Table 1

[0074] As can be seen from Table 1, when the undercoat layer is designed symmetrically, although increasing the thickness of the undercoat layer can also obtain a higher screw extrusion test passing rate, an excessively thick undercoat layer will reduce the energy density and affect the performance of the secondary battery. By making the thickness B of the second undercoat layer satisfy 2.1≤B≤4.5, and adjusting the thickness of the first undercoat layer and the second undercoat layer to satisfy 1.02≤A / B≤1.80, the lithium ion battery can have a higher volumetric energy density, while improving the screw extrusion test passing rate, and achieving the improvement of the energy density and safety performance of the lithium ion battery. Especially, when 1.125≤A / B≤1.5 is satisfied, the energy density and safety performance of the lithium ion battery can be further improved.

[0075] Particularly, on the basis of A / B satisfying the above range, further regulating the thickness of the first undercoat layer to satisfy 2.6≤A≤5.8, preferably 3.7≤A≤5.2, can make the lithium ion battery exhibit higher energy density and safety performance. On the other hand, regulating the thickness of the second undercoat layer to satisfy 2.5≤B≤4.5, preferably 2.9≤B≤4.0, can better cooperate with the first undercoat layer, further improving the energy density and safety performance of the lithium ion battery.

[0076] The lithium ion batteries of each example in Table 2 differ from Examples 1-7 only in that the thickness A (μm) of the first undercoat layer, the thickness B (μm) of the second undercoat layer, A / B, the content M (%) of the conductive agent in the first undercoat layer, the content N (%) of the conductive agent in the second undercoat layer, and / or M-N (%) are adjusted according to Table 2, and the specific adjustment conditions and performance test results are shown in Table 2 below.

[0077] Table 2

[0078] As can be seen from Table 2, further regulating the content of the conductive agent in the undercoat layer to satisfy 0.5≤M-N≤3.5 can reduce the full charging time of the lithium ion battery, improve the charge-discharge performance, and at the same time is also conducive to obtaining a higher screw extrusion test pass rate and improving the safety performance. In particular, controlling the content of the conductive agent to satisfy 0.9≤M-N≤3.2 can make the lithium ion battery have excellent safety performance and charge-discharge performance. More preferably, when satisfying 1.5≤M-N≤2.5, the safety performance and charge-discharge performance of the lithium ion battery can be further improved.

[0079] Particularly, regulating the content of the conductive agent in the first undercoat layer to satisfy 2≤M≤7, preferably 3.7≤M≤6.5, is conducive to further improving the safety performance and charge-discharge performance of the lithium ion battery. Particularly, regulating the content of the conductive agent in the second undercoat layer to satisfy 1≤N≤6, preferably 2.5≤N≤4.5, can make the lithium ion battery exhibit more excellent safety performance and charge-discharge performance.

[0080] The lithium ion batteries of each example in Table 3 differ from Examples 2-12 only in that the average particle size W (μm) of the inorganic particles, the mass content H (%) of the inorganic particles, and / or the thickness C (μm) of the single-layer active material layer are adjusted according to Table 3, and the specific adjustment conditions and performance test results are shown in Table 3 below.

[0081] Table 3

[0082] As can be seen from Table 3, the average particle size of the inorganic particles is regulated to satisfy 0.5≤W≤1.4, preferably 0.8≤W≤1, which can promote the better cooperation of the inorganic particles and the conductive agent in the base coating, and further improve the safety performance and charge-discharge performance of the lithium ion battery. In particular, the mass content of the inorganic particles in the base coating is regulated to satisfy 85≤H≤95, preferably 90≤H≤92, which can more significantly improve the safety performance and charge-discharge performance of the lithium ion battery. In particular, the single-layer thickness of the positive electrode active material layer is regulated to satisfy 30≤C≤50, preferably 43≤C≤45, which can make the lithium ion battery exhibit more excellent safety performance and charge-discharge performance.

[0083] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the principles of the present application shall be included in the protection scope of the present application.

Claims

1. A secondary battery comprising a packaging bag, and an electrolyte and an electrode assembly accommodated in the packaging bag, the electrode assembly comprising a wound unit formed by winding a positive electrode sheet, a separator, and a negative electrode sheet in a stacked manner, characterized in that, The positive electrode tab comprises a positive current collector and, sequentially arranged away from the positive current collector, a base coating layer and a positive active material layer; The base coating layer comprises a first base coating layer and a second base coating layer arranged on both sides of the positive current collector respectively, the first base coating layer faces the winding center, and the second base coating layer faces away from the winding center; The thickness of the first base coating layer is A μm, the thickness of the second base coating layer is B μm, 2.1≤B≤4.5, and 1.02≤A / B≤1.

80.

2. The secondary battery according to claim 1, characterized by The electrode assembly satisfies at least one of the following conditions: (1) 1.125≤A / B≤1.5; (2)2.6≤A≤5.8； (3)2.5≤B≤4.5。 3. The secondary battery according to claim 1 or 2, characterized by The base coating layer comprises a conductive agent; The mass content of the conductive agent in the first base coating layer is M% based on the mass of the first base coating layer, and the mass content of the conductive agent in the second base coating layer is N% based on the mass of the second base coating layer; 0.5≤M-N≤3.

5.

4. The secondary battery according to claim 3, characterized by The electrode assembly satisfies at least one of the following conditions: (1)2≤M≤7； (2)1≤N≤6； (3) 0.9≤M-N≤3.

2.

5. The secondary battery according to claim 3, characterized by The conductive agent is selected from carbon black and / or carbon nanotubes.

6. The secondary battery according to claim 1 or 2, characterized by The base coating layer further comprises inorganic particles; the inorganic particles satisfy at least one of the following conditions: (1) the average particle size of the inorganic particles is W μm, and 0.5≤W≤1.4; (2) the inorganic particles are selected from at least one of boehmite, aluminum oxide or lithium iron phosphate; (3) the mass content of the inorganic particles is H% based on the mass of the base coating layer, and 85≤H≤95.

7. The secondary battery according to claim 6, characterized by The base coating layer further comprises a binder selected from at least one of polyacrylic acid, polyacrylate, styrene-acrylic emulsion or butadiene-styrene rubber.

8. The secondary battery according to claim 1 or 2, characterized by The thickness of a single layer of the positive active material layer is C μm, and 30≤C≤50.

9. The secondary battery according to claim 1 or 2, characterized by The electrode assembly satisfies at least one of the following conditions: (1) 1.125≤A / B≤1.25; (2)3.7≤A≤5.2； (3)2.9≤B≤4.0。 10. An electronic device, comprising: The secondary battery comprises any one of claims 1 to 9.

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