Secondary battery and electronic equipment

Abstract

The invention provides a secondary battery and an electronic device. The secondary battery comprises a positive electrode, a negative electrode, a diaphragm and an electrolyte, the positive electrode comprises a current collector, and a bottom coating and a positive electrode active material layer which are sequentially arranged on the current collector; the bottom coating comprises a polymer A and conductive particles which are uniformly distributed, the polymer A is non-conductive, the average particle size of the polymer A is less than 2m, and the melting point of the polymer A is 75-138 DEG C; the electrolyte comprises cyclic carbonate, a component A and a component B, the component A comprises at least one of lithium difluorophosphate, lithium fluorosulfonate and lithium tetrafluoroborate; and the component B comprises at least one of 1, 3-propane sultone, 1, 4-butane sultone, 1, 3-propene sultone, methylene methanedisulfonate, ethylene sulfate, propylene sulfate, methyl ethylene sulfate and cyclic carbonate ethylene sulfate, and the component B comprises at least one of 1, 3-propane sultone, 1, 4-butane sultone, 1, 3-propene sultone, methylene methanedisulfonate, ethylene sulfate, propylene sulfate, methyl ethylene sulfate and cyclic carbonate ethylene sulfate. The secondary battery provided by the invention has excellent low-temperature internal resistance, high-temperature cycle performance and safety characteristic.

Description

Technical Field

[0001] This invention belongs to the field of new energy technology, specifically relating to a secondary battery and electronic device. Background Technology

[0002] With the continuous development of consumer batteries, electric vehicles, and energy storage, the requirements for the energy density and safety performance of lithium-ion batteries are becoming increasingly stringent. There are four modes of lithium-ion battery failure: short circuit between the positive and negative current collectors, short circuit between the positive and negative active materials, short circuit between the negative active material and the positive current collector, and short circuit between the positive active material and the negative current collector. Typically, the most dangerous safety failure mode for lithium batteries is a short circuit between a fully charged negative active material and the positive current collector. This short circuit generates a large amount of heat instantaneously, leading to thermal runaway and fire. To avoid internal short circuits, researchers have made improvements to various aspects of lithium-ion batteries to enhance their overall safety performance.

[0003] Currently, the most common technologies mainly focus on two aspects: one is to coat the surface of the current collector with a safety coating, and the other is to add flame retardants to the electrode material or to the electrolyte. The former requires adding an extra layer of safety coating between the electrode material and the current collector foil to improve the current collector itself, while the latter only requires adding flame retardants during material feeding. Currently, the industry's solutions for safety coatings mainly include: (1) coating the surface of the positive electrode aluminum foil with a lithium iron phosphate safety coating; (2) coating the surface of the positive electrode aluminum foil with an alumina safety coating. Both methods can avoid direct contact between the empty aluminum foil and the negative electrode active material, thereby improving the safety performance of the battery. However, both methods have problems such as reduced battery specific capacity, decreased high-temperature long-cycle performance, increased internal resistance, and easy peeling of the safety coating.

[0004] Therefore, a new type of safe primer needs to be developed to solve the problems existing in the above methods. Summary of the Invention

[0005] One aspect of the present invention provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode comprises a current collector, and a base coating and a positive electrode active material layer sequentially disposed on the current collector; the base coating comprises a uniformly distributed polymer A and conductive particles, wherein the polymer A is non-conductive, has an average particle size of less than 2µm, and has a melting point of 75°C to 138°C; the electrolyte comprises a cyclic carbonate, and further comprises at least component A and component B for inhibiting the swelling of the polymer A; component A comprises at least one of lithium difluorophosphate, lithium fluorosulfonate, and lithium tetrafluoroborate; component B comprises at least one of 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, 1,3-propenesulfonate lactone, methanedisulfonate, vinyl sulfate, propylene sulfate, methyl vinyl sulfate, and cyclovinyl carbonate. According to this configuration, the low-temperature internal resistance, high-temperature cycling, and safety characteristics of the secondary battery can be significantly improved.

[0006] In some embodiments, polymer A includes at least one of the following: homopolymers of non-fluorinated alkenyl monomers, homopolymers of olefin monomers, homopolymers of unsaturated nitrile monomers, homopolymers of acrylate monomers, and homopolymers of ester monomers.

[0007] In some embodiments, polymer A includes polyethylene and / or polypropylene.

[0008] In some embodiments, the melting point of polymer A is 85°C to 128°C.

[0009] In some embodiments, the average particle size of polymer A is 0.2µm to 2µm, preferably 0.5µm to 1.5µm; In some embodiments, the mass ratio of the conductive particles to the polymer A is 2:98 to 20:80.

[0010] In some embodiments, based on 100 parts by mass of the electrolyte, the content of component A is 0.01 to 5 parts by mass, preferably 0.1 to 3 parts by mass.

[0011] In some embodiments, based on 100 parts by mass of the electrolyte, the content of component B is 0.01 to 5.8 parts by mass, preferably 0.1 to 2.9 parts by mass.

[0012] In some embodiments, the cyclic vinyl carbonate in component B includes at least one of the following structural formulas I to V; .

[0013] In some embodiments, the supporting electrolyte contained in the electrolyte includes a lithium salt, which includes lithium hexafluorophosphate.

[0014] In some embodiments, the electrolyte further includes additive C, which is used to inhibit the decomposition of the lithium hexafluorophosphate in the presence of component A and / or component B. Additive C includes at least one of succinic anion, adiponitrile, ethylene glycol bis(propionitrile) ether, 1,3,6-hexanetrionitrile, glycerol trionitrile, lithium 1-ethyl-3-methylimidazolium tetrafluoroborate, lithium 1-butyl-3-methylimidazolium hexafluorophosphate, and lithium 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonylimide).

[0015] In some embodiments, based on 100 parts by mass of the electrolyte, the content of succinic anion is 0.7-3.5 parts by mass, and / or the content of adiponitrile is 0.6-4 parts by mass, and / or the content of ethylene glycol bis(propionitrile) ether is 0.2-2 parts by mass, and / or the content of 1,3,6-hexanetrionitrile is 0.1-3 parts by mass, and / or the content of glycerol trionitrile is 0.1-4 parts by mass, and / or the content of lithium 1-ethyl-3-methylimidazolium tetrafluoroborate is 0.01-2 parts by mass, and / or the content of lithium 1-butyl-3-methylimidazolium hexafluorophosphate is 0.01-2 parts by mass, and / or the content of lithium 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide is 0.01-2 parts by mass.

[0016] In some embodiments, the thickness of the base coating is 0.1µm to 5µm.

[0017] In some embodiments, the base coating layer further includes a binder and / or inorganic particles.

[0018] In some embodiments, the total mass ratio of the conductive particles and polymer A to the mass ratio of the binder is 99.9:0.1 to 95:5.

[0019] In some embodiments, the inorganic particles are made of at least one of lithium iron phosphate, lithium manganese iron phosphate, silicon dioxide, aluminum oxide, boehmite, and barium sulfate.

[0020] Another aspect of the present invention provides an electronic device comprising a secondary battery as described in any of the technical solutions. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the positive electrode structure in one embodiment of the present invention; 1-Current collector; 2-Underlayer coating; 3-Positive electrode active material layer. Detailed Implementation

[0023] The invention will now be described in more detail to allow for a clearer understanding. In this context, it will be understood that the terms or words used in this specification and claims should not be construed as having the meanings defined in common dictionaries, and it will be further understood that, based on the principle that the inventors may appropriately define the meanings of terms or words to best interpret the invention, these terms or words should be interpreted as having meanings consistent with the technical concept of the invention and the context of related art.

[0024] This invention provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a current collector and a base coating and a positive electrode active material layer sequentially disposed on the current collector. The base coating comprises a uniformly distributed polymer A and conductive particles. The polymer A is non-conductive, has an average particle size of less than 2 µm, and has a melting point of 75°C to 138°C. The electrolyte comprises a cyclic carbonate and further includes at least component A and component B for inhibiting the swelling of the polymer A. Component A includes at least one of lithium difluorophosphate, lithium fluorosulfonate, and lithium tetrafluoroborate. Component B includes at least one of 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, 1,3-propenesulfonate lactone, methanedisulfonate, vinyl sulfate, propylene sulfate, methyl vinyl sulfate, and cyclovinyl carbonate. This secondary battery exhibits excellent low-temperature internal resistance, high-temperature cycling, and safety characteristics.

[0025] The components of the secondary battery of the present invention will be described below.

[0026] positive electrode

[0027] The positive electrode has a current collector, an undercoating layer formed on the current collector, and a positive electrode active material layer formed on the undercoating layer.

[0028] In some embodiments, the base coating comprises polymer A with a melting point of 75°C to 138°C, conductive particles, a binder, and optional inorganic particles.

[0029] Examples of positive current collectors include sheets or foils containing materials such as stainless steel, aluminum, and titanium. Among these, sheets or foils containing aluminum are preferred. The thickness of the sheets and foils is not particularly limited, but from the viewpoint of ensuring the strength and processability required for use as a current collector, 1 to 50 µm is preferred, 2 to 20 µm is more preferred, and 5 to 15 µm is even more preferred. The undercoating is formed on one or both sides of the positive current collector in the thickness direction.

[0030] By using a non-conductive polymer A with an average particle size of less than 2µm and a melting point of 75~138℃ in the undercoat, the small particle size of the polymer allows conductive particles to be easily and uniformly distributed within the undercoat, thus forming a conductive network that serves as an electron transport path in a substantially uniform manner throughout the entire undercoat. More importantly, in the event of thermal runaway, polymer A can melt and cover the conductive particles in time, blocking the current flow within the undercoat. This not only blocks electron conduction between the positive electrode active material layer and the positive electrode current collector but also between the negative electrode active material layer and the positive electrode current collector, preventing internal short circuits caused by thermal runaway and significantly improving battery safety performance. If the melting point of polymer A is too low, it will affect the battery's high-temperature performance, for example, melting during charging and discharging will increase internal resistance; if the melting point of polymer A is too high, it will affect battery safety performance, for example, if the internal temperature of the battery reaches above 138℃, the separator substrate will shrink, causing an internal short circuit, at which point the polymer cannot play its blocking role before it melts.

[0031] Polymer A can be exemplified by particles of a non-conductive thermoplastic resin. Polymer A includes at least one of the following: homopolymers of non-fluorinated alkenyl monomers, homopolymers of olefin monomers, homopolymers of unsaturated nitrile monomers, homopolymers of acrylic acid monomers, and homopolymers of ester monomers. Preferably, polymer A includes polyethylene or polypropylene. For example, polymer A may include at least one of polyethylene wax, oxidized polyethylene wax, polypropylene wax, oxidized polypropylene wax, Fischer-Tropsch wax, paraffin wax, modified paraffin wax, high-density polyethylene (HDPE), and oxidized high-density polyethylene.

[0032] Polymer A can be used alone, or two or more can be used in combination as needed.

[0033] From the perspective of improving battery safety, polymer A has a melting point of 75°C to 138°C, preferably 85°C to 128°C.

[0034] The average particle size of polymer A is less than 2 µm, preferably 0.2 to 2 µm, and more preferably 0.5 to 1.5 µm. Within this range, the positive electrode active material layer can be formed more uniformly on the positive electrode current collector, improving the positive electrode's resistance to delamination. If the average particle size of polymer A is too large, the surface of the undercoat layer will be relatively rough, making it difficult for the active material layer to spread uniformly; therefore, a particle size of less than 2 µm is more suitable.

[0035] Examples of conductive particles include carbon particles such as graphite, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; metal particles such as nickel particles; metal carbides such as WC, B4C, ZrC, NbC, MoC, TiC, and TaC; metal nitrides such as TiN, ZrN, and TaN; and metal silicides such as WSi2 and MoSi2. Among these, carbon particles and metal particles are preferred, with carbon particles being more preferred. A single conductive particle can be used, or two or more can be used in combination as needed. It should be noted that conductive particles with PTC functionality can be used, such as alkaline earth titanate metal salts like barium titanate, barium strontium titanate, and barium lead titanate, as well as solid solutions formed by dissolving dissimilar metals in alkaline earth titanate metal salts.

[0036] When using carbon particles as conductive particles, from the viewpoint of further improving battery performance, the average particle size of the primary particles constituting the carbon particles is preferably 10 to 500 nm, more preferably 15 to 200 nm, and even more preferably 20 to 100 nm.

[0037] As conductive particles, acetylene black with a structure in which primary particles are connected to a certain extent is particularly preferred. In terms of the degree of connection of primary particles (the degree of development of the structure), for example, acetylene black with a shape factor of 5 to 50, calculated by dividing the average length of the chain formed by the connection of primary particles by the average particle size of the primary particles, is preferred.

[0038] The mass ratio of conductive particles to polymer A is preferably 2:98 to 20:80, more preferably 3:97 to 15:85, and even more preferably 5:95 to 10:90. Within this range, on the one hand, the electron movement path within the undercoat layer can be sufficiently ensured, tending to improve the output characteristics of the battery; on the other hand, after polymer A melts, it can fully cover the conductive particles, avoiding internal short circuits caused by thermal runaway.

[0039] The average particle size of the conductive particles and polymer A can be set, for example, to the following value: the arithmetic average of the long side lengths of all particles in a transmission electron microscope image covering a 10µm x 10µm area in the center of the base coating.

[0040] Examples of binders include carboxymethyl cellulose, sodium carboxymethyl cellulose and other carboxymethyl cellulose derivatives, polyvinyl alcohol, polyvinylpyrrolidone, water-soluble alginate derivatives, gelatin, carrageenan, glucomannan, pectin, gellan gum, gellan gum, polyacrylic acid, and polyacrylic acid derivatives. Among these, carboxymethyl cellulose derivatives, polyvinyl alcohol, polyvinylpyrrolidone, and polyacrylic acid are particularly preferred, and carboxymethyl cellulose derivatives are even more preferred. Furthermore, the ratio of the conductive particles and polymer A mixed particles to the binder is not particularly limited. Preferably, the mass ratio of the total mass of conductive particles and polymer A to the mass of the binder is 99.9:0.1 to 95:5, more preferably 99.5:0.5 to 98:3, and even more preferably 99.5:0.5 to 98:2. Within the above range, on the one hand, the conductive particles are sufficiently dispersed, which can adequately ensure the electron movement path within the undercoat layer, tending to improve battery characteristics; on the other hand, the viscosity of the resulting dispersion does not easily increase, making it less prone to film detachment.

[0041] In this invention, the binder preferably has a weight-average molecular weight greater than or equal to 1000 Da. From the viewpoint of the dispersibility of conductive particles, the weight-average molecular weight of the binder is more preferably greater than or equal to 5000 Da, further preferably greater than or equal to 10000 Da, and particularly preferably greater than or equal to 50000 Da.

[0042] As optional inorganic particles, the material of the inorganic particles may include at least one selected from lithium iron phosphate, lithium manganese iron phosphate, silicon dioxide, aluminum oxide, boehmite, and barium sulfate. The inclusion of these inorganic particles in the undercoat can further prevent internal short circuits caused by thermal runaway.

[0043] The positive electrode active material layer is formed on one or both sides of the base coating. It contains positive electrode active material and may further contain conductive materials, binder materials, etc. as needed.

[0044] As the positive electrode active material, commonly used substances in this field can be used, such as lithium-containing complex metal oxides, olivine-type lithium salts, chalcogenides, and manganese dioxide. Lithium-containing complex metal oxides are metal oxides containing lithium and transition metals, or metal oxides in which a portion of the transition metal is replaced by an alternative element. Examples of alternative elements include Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, V, and B, with Mn, Al, Co, Ni, and Mg being preferred. One alternative element can be used alone, or two or more can be used in combination as needed.

[0045] Among them, lithium-containing composite metal oxides are preferred. Examples of lithium-containing composite metal oxides include, for instance, Li. x CoO2, Lix NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1 1-y O z Li x Co y M 1 1-y O z , of which M 1 It indicates that at least one element is selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Ni, Cu, Zn, Al, Cr, Pb, Sb, V, and B. Li x Ni 1-y M 2 y O z Li x Ni 1-y M 2 y O z In the middle, M 2 It indicates that at least one element is selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Cu, Zn, Al, Cr, Pb, Sb, V, and B. Li x Mn2O4, Li x Mn 2-y M 3 y O4, Li x Mn 2-y M 3 y In O4, M 3 The element is selected from the group consisting of Na, Mg, Sc, Y, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, V, and B. In the above chemical formula, x, y, and z can satisfy, for example, 0 < x ≤ 1.2, y = 0~0.9, and z = 2.0~2.3. Furthermore, the value of x, representing the molar ratio of lithium, increases or decreases with charge and discharge.

[0046] In addition, examples of olivine-type lithium salts include LiFePO4. Examples of chalcogenides include titanium disulfide and molybdenum disulfide. Furthermore, other positive electrode active materials include Li2MPO4F and Li2MPO4F, where M represents at least one element selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, V, and B.

[0047] The positive electrode active material can be used alone, or two or more can be used in combination as needed.

[0048] Conductive materials that can be used in the positive electrode active material layer include, for example, carbon black, graphite, carbon fiber, and metal fiber. Examples of carbon black include acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked carbon black. Examples of graphite include, for example, natural graphite and synthetic graphite. A single conductive material can be used, or two or more materials can be used in combination as needed.

[0049] Examples of binder materials that can be used in the positive electrode active material layer include polyethylene, polypropylene, polyvinyl acetate, polymethyl methacrylate, nitrocellulose, fluoropolymers, and rubber particles.

[0050] Examples of fluoropolymers include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and vinylidene fluoride-hexafluoropropylene copolymer.

[0051] Examples of rubber particles include styrene-butadiene rubber particles and acrylonitrile rubber particles.

[0052] Among them, fluorine-containing binder materials are preferred if the oxidation resistance of the positive electrode active material layer is to be improved.

[0053] One type of adhesive can be used alone, or two or more can be used in combination as needed.

[0054] The positive electrode active material layer can be formed, for example, by coating a positive electrode active material slurry onto a base layer, drying it, and then calendering it as needed. The positive electrode active material slurry can be prepared by adding the positive electrode active material, binder, conductive material, etc., together with a dispersion medium and mixing them. The dispersion medium includes, for example, one or more of N-methyl-2-pyrrolidone (NMP), tetrahydrofuran, dimethylformamide, etc. It should be noted that the dispersion medium is preferably selected from those that do not dissolve the polymer A particles contained in the base layer. Some polymer A particles are poorly soluble in organic solvents and water; when using such polymer A particles, the type of dispersion medium is not limited.

[0055] Furthermore, in some embodiments, from the viewpoint of balancing battery characteristics and safety, the thickness of the undercoat is preferably 0.1µm to 5µm, more preferably 0.5µm to 3µm, even more preferably 0.8µm to 2.6µm, and particularly preferably 1.2µm to 2µm.

[0056] Furthermore, from the viewpoint of balancing discharge capacity and discharge rate, the thickness of the positive electrode active material layer is preferably 30µm to 200µm, more preferably 50µm to 180µm, and even more preferably 70µm to 150µm.

[0057] Figure 1 This is a schematic diagram of the positive electrode structure in one embodiment, such as... Figure 1 As shown, it includes a current collector 1, a base coating 2 formed on the current collector 1, and a positive electrode active material layer formed on the base coating 2. Figure 1 In this embodiment, the base coating 2 is formed on one surface of the current collector 1, but in other embodiments, the base coating can be formed on both opposite surfaces of the current collector.

[0058] negative electrode

[0059] The negative electrode of the present invention includes a negative current collector and a negative electrode mixture layer formed on the current collector, wherein the negative electrode mixture layer includes a negative electrode active material.

[0060] In this specification, "negative electrode composite layer" refers to the portion of the negative electrode constituent elements other than the negative electrode current collector, and includes a negative electrode active material and a negative electrode conductive agent composition, and may include additives such as conductive aids as needed. The negative electrode active material is a material capable of absorbing and desorbing lithium. In this specification, substances that do not absorb and desorb lithium, such as the conductive agent composition, are not included in the negative electrode active material.

[0061] The negative electrode current collector can be copper foil, iron foil, or stainless steel foil. The negative electrode active material can be a material that can reversibly insert / deintercalate lithium ions, lithium metal, lithium metal alloy, a material capable of doping and dedoping lithium, and / or a transition metal oxide.

[0062] Materials that can reversibly insert / deintercalate lithium ions include carbon materials. Carbon materials are any suitable carbon-based anode active material commonly used in rechargeable lithium-ion batteries. Examples of carbon materials include crystalline carbon, amorphous carbon, and combinations thereof. Crystalline carbon can be amorphous natural graphite and / or synthetic graphite, such as flake, sheet, spherical, and / or fibrous natural graphite and / or synthetic graphite. Amorphous carbon can be soft carbon, hard carbon, mesophase pitch carbonization products, calcined coke, etc.

[0063] Lithium metal alloys include, for example, lithium, and also include one or more metals selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn.

[0064] Materials capable of doping and dedoping lithium include Si and SiO2. x(0 < x < 2), Si-Q alloy (where Q is selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements other than Si, Group 15 elements, Group 16 elements, transition metals, rare earth elements or combinations thereof), Sn, SnO2, Sn-R alloy (where R is selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements other than Sn, Group 15 elements, Group 16 elements, transition metals, rare earth elements or combinations thereof), etc. At least one of them can be mixed with SiO2.

[0065] The element Q and the element R can further independently be selected from one or more of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn (the element R does not include Sn), In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po.

[0066] The transition metal oxide can be vanadium oxide, lithium vanadium oxide, etc.

[0067] In some exemplary embodiments, the negative electrode active material can be selected from at least one of graphite and Si composites.

[0068] The Si composite includes Si particles and amorphous carbon, and for example, the Si particles can be selected from at least one of Si-C composites, SiO k (0 < k ≤ 2) and Si alloys.

[0069] For example, the Si-C composite can include Si particles and amorphous carbon, the amorphous carbon has a porous structure, and the Si particles are disposed in the porous structure.

[0070] The Si particles can have a median particle size of 10 nm to 200 nm. The median particle size (D50) can be the particle size at a volume ratio of 50% in the cumulative size distribution curve. When the median particle size of the Si particles is within the above range, volume expansion occurring during charging and discharging can be suppressed or reduced, and disconnection of the conduction path due to particle pulverization during charging and discharging can be prevented or reduced.

[0071] Based on the total weight of the Si composite, the amount of Si particles included can be 1 wt% to 60 wt%, for example 3 wt% to 60 wt%.

[0072] The amorphous carbon can be a combination of one or more of soft carbon, hard carbon, mesophase pitch carbonized product, calcined coke.

[0073] The negative electrode active material can further include crystalline carbon.

[0074] When the negative electrode active material includes both Si complex and crystalline carbon, it can be a mixture of Si complex and crystalline carbon, and in some embodiments, the weight ratio of Si complex and crystalline carbon can be 1:99 to 50:50. In some embodiments, the weight ratio of Si complex and crystalline carbon can be 3:97 to 20:80 or 5:95 to 20:80.

[0075] Crystalline carbon can be, for example, graphite, such as natural graphite, artificial graphite, or mixtures thereof.

[0076] Crystalline carbon can have a median particle size of 5µm to 30µm.

[0077] Amorphous carbon precursors may include coal pitch, mesophase pitch, petroleum pitch, coal oil, petroleum heavy oil and / or polymer resins (such as one or more of phenolic resins, furan resins, and polyimide resins).

[0078] In the negative electrode mixture layer, the amount of negative electrode active material included, based on the total weight of the negative electrode mixture layer, can be 95wt%~99wt%.

[0079] In some example embodiments, the negative electrode binder layer may further include a binder and optionally include a conductive material. In the negative electrode binder layer, the amount of binder may be 1 wt% to 5 wt% based on the total weight of the negative electrode binder layer. When the negative electrode binder layer further includes a conductive material, the negative electrode binder layer may include 90 wt% to 98 wt% of the negative electrode active material, 1 wt% to 5 wt% of the binder, and 1 wt% to 5 wt% of the conductive material.

[0080] The binder improves the bonding characteristics between the negative electrode active material particles and the bonding characteristics between the negative electrode active material and the current collector. The binder may include one or more of the following: polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, and polyimide. The binder may also include rubber-based binders and / or polymer resin binders. Rubber-based binders may be selected from one or more of the following: styrene-butadiene rubber (SBR), acrylated styrene-butadiene rubber, acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, and fluororubber. Polymer resin binders may be selected from one or more of the following: polytetrafluoroethylene, ethylene-propylene copolymer, polyethylene oxide, polyvinylpyrrolidone, polyepoxychloropropane, polyphosphazene, polyacrylonitrile, polystyrene, ethylene-propylene-diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, and polyvinyl alcohol.

[0081] Conductive materials provide electrode conductivity (e.g., electrical conductivity), and any suitable conductive material can be used as a conductive material unless it causes undesirable chemical changes (e.g., unless it causes undesirable changes in a rechargeable lithium battery). Conductive materials may include carbon-based materials (such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.), metallic materials (such as metal powders or metal fibers of copper, nickel, aluminum, silver, etc.), conductive polymers (such as polyphenylene derivatives, etc.), or mixtures thereof.

[0082] diaphragm

[0083] The diaphragm can be made of porous sheet material with ion permeability and insulation. Specific examples of porous sheet material include microporous films, woven fabrics, and nonwoven fabrics. The diaphragm material can include olefin resins such as polyethylene and polypropylene, cellulose, etc. The diaphragm can be either a single-layer structure or a multilayer structure. A heat-resistant layer containing a heat-resistant material can be formed on the surface of the diaphragm. Examples of heat-resistant materials include polyamide resins such as aliphatic polyamides and aromatic polyamides (aramids), polyamide-imide resins such as polyimide, and polyimide resins such as polyimide.

[0084] electrolyte

[0085] As an electrolyte, an organic electrolyte in which the supporting electrolyte is dissolved in an organic solvent is typically used. As a supporting electrolyte, lithium salts can be used, for example, in lithium-ion secondary batteries. Examples of lithium salts include LiPF6, LiAsF6, LiBF4, LiSbF6, LiAlCl4, and LiClO4. A single electrolyte can be used, or two or more can be used in combination. Since there is a general tendency for using a supporting electrolyte with a higher degree of dissociation to result in higher lithium-ion conductivity, the lithium-ion conductivity can be adjusted according to the type of supporting electrolyte.

[0086] As for the organic solvent used in the electrolyte, there are no particular limitations as long as it can dissolve and support the electrolyte. For example, in lithium-ion secondary batteries, carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butyl carbonate (BC), and ethyl methyl carbonate (EMC) are preferred; esters such as γ-butyrolactone, methyl formate, ethyl acetate, propyl propionate, and ethyl propionate are preferred; ethers such as 1,2-dimethoxyethane and tetrahydrofuran are preferred; and sulfur-containing compounds such as sulfolane and dimethyl sulfoxide are preferred. Mixtures of these solvents can also be used. Among these, carbonates and carboxylic acid esters are preferred because they have high dielectric constants and wide stable potential ranges. Since there is a general tendency for lower viscosity solvents to have higher lithium-ion conductivity, the lithium-ion conductivity can be adjusted according to the type of solvent.

[0087] The electrolyte of the secondary battery provided by the present invention includes component A and component B. Component A includes at least one of lithium difluorophosphate, lithium fluorosulfonate, and lithium tetrafluoroborate; component B includes at least one of 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, 1,3-propenesulfonate lactone, methylene disulfonate, vinyl sulfate, propylene sulfate, methyl vinyl sulfate, and cyclovinyl carbonate.

[0088] This invention discovered that using polymer A in the undercoat significantly improves the adhesion between the undercoat and the positive electrode current collector. However, under high-temperature conditions, polymer A is prone to swelling in electrolyte systems containing cyclic carbonates (such as ethylene carbonate and propylene carbonate), leading to reduced adhesion and delamination of the undercoat. Through systematic research, this invention found that including components A and B in the electrolyte unexpectedly inhibits the swelling of polymer A in the secondary battery, improving not only the low-temperature internal resistance and high-temperature cycle performance of the secondary battery but also significantly enhancing its safety.

[0089] The cyclic vinyl carbonate includes at least one of the following structural formulas I to V; .

[0090] Based on 100 parts by mass of the electrolyte, the content of component A is 0.01 to 7 parts by mass, preferably 0.1 to 5 parts by mass.

[0091] Based on 100 parts by mass of the electrolyte, the content of component B is 0.01 to 5.8 parts by mass, preferably 0.1 to 2.9 parts by mass.

[0092] When component A and / or component B meet the above range, the swelling of polymer A can be further suppressed, the low-temperature internal resistance and high-temperature cycling performance of the secondary battery can be improved, and the safety can also be significantly improved.

[0093] This invention discovers that when the supporting electrolyte used in the electrolyte contains lithium hexafluorophosphate, components A and B catalyze the decomposition of lithium hexafluorophosphate under high-temperature conditions. Systematic research has shown that when the electrolyte includes additive C, additive C can inhibit the catalytic decomposition of lithium hexafluorophosphate by components A and B at high temperatures, thereby further improving the low-temperature internal resistance and high-temperature cycle performance of the secondary battery, and significantly improving safety. Additive C includes at least one of succinic anionyl, adiponitrile, ethylene glycol bis(propionitrile) ether, 1,3,6-hexanetrionitrile, glycerol trionitrile, lithium 1-ethyl-3-methylimidazolium tetrafluoroborate, lithium 1-butyl-3-methylimidazolium hexafluorophosphate, and lithium 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.

[0094] Based on 100 parts by mass of the electrolyte, the content of succinic anion is 0.7-3.5 parts by mass, and / or the content of adiponitrile is 0.6-4 parts by mass, and / or the content of ethylene glycol bis(propionitrile) ether is 0.2-2 parts by mass, and / or the content of 1,3,6-hexanetrionitrile is 0.1-3 parts by mass, and / or the content of glycerol trionitrile is 0.1-4 parts by mass, and / or the content of lithium 1-ethyl-3-methylimidazolium tetrafluoroborate is 0.01-2 parts by mass, and / or the content of lithium 1-butyl-3-methylimidazolium hexafluorophosphate is 0.01-2 parts by mass, and / or the content of lithium 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide is 0.01-2 parts by mass. When additive C meets the above ranges, it can further improve the low-temperature internal resistance and high-temperature cycle performance of the secondary battery, and significantly improve safety.

[0095] Manufacturing method of secondary batteries

[0096] Secondary batteries can be manufactured by, for example, overlapping the positive and negative electrodes with a separator, winding or folding them according to the battery shape as needed, placing them in a battery container, injecting electrolyte into the battery container, and sealing it. To prevent internal pressure rise, overcharging, and over-discharging from occurring, overcurrent protection components such as fuses and PTC elements, porous metal mesh, and conductive plates can also be added as needed.

[0097] The secondary battery of the present invention can be a wound battery structure or a stacked battery structure.

[0098] The secondary battery can be a lithium secondary battery. The shape of the lithium secondary battery of the present invention is not particularly limited. For example, it can be cylindrical, prismatic, pouch-shaped or coin-shaped.

[0099] The present invention also provides an electronic device comprising the secondary battery described in any of the technical solutions of the present invention.

[0100] The secondary battery of the present invention can be used in electronic devices such as 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, household large-capacity batteries, and lithium-ion capacitors, but is not limited thereto.

[0101] Example

[0102] The following examples and comparative examples illustrate the present invention in more detail, but the present invention is not limited to the following examples.

[0103] 1. Preparation of the positive electrode

[0104] Example 1-1

[0105] Preparation of the base coating

[0106] A composition of acetylene black, polymer A (polyethylene particles, melting point 75°C, average particle size 1µm) (acetylene black to polymer A mass ratio 2:98), and polyacrylic acid were mixed at a solids mass ratio of 95:5 and uniformly dispersed. Water was added to the resulting mixture to prepare a primer slurry. This primer slurry was applied to both opposite sides of a 9µm thick aluminum foil and dried at 60°C to prepare a 2µm thick primer coating.

[0107] Positive electrode production

[0108] The positive electrode is prepared as follows: Acetylene black (as a conductive material) and polyvinylidene fluoride (PVDF) (as a binder) are sequentially added to lithium cobalt oxide (as the positive electrode active material) and mixed to prepare a positive electrode paste. The mass ratio of the positive electrode active material, acetylene black, and binder satisfies: positive electrode active material: acetylene black: binder = 97:1.5:1.5. Furthermore, N-methyl-2-pyrrolidone (as a dispersing solvent) is added to the above positive electrode paste and kneaded to form a slurry. This slurry is uniformly and homogeneously coated onto the surface of a base coating. Then, a drying process is performed, followed by compaction until a predetermined density is reached.

[0109] Examples 1-2

[0110] Preparation of the base coating

[0111] A composition of acetylene black, polymer A (polypropylene particles, melting point 138℃, average particle size 2.0µm) (acetylene black to polymer A mass ratio 20:80), and polyacrylic acid were mixed at a solids mass ratio of 95:5 and uniformly dispersed. Water was added to the resulting mixture to prepare a primer slurry. This primer slurry was applied to both sides of a 9µm thick aluminum foil and dried at 60℃ to prepare a 2µm thick primer coating.

[0112] Positive electrode production

[0113] The positive electrode is prepared as follows: Acetylene black (as a conductive material) and polyvinylidene fluoride (PVDF) (as a binder) are sequentially added to lithium cobalt oxide, which serves as the positive electrode active material, and mixed to prepare a positive electrode paste. The mass ratio of the positive electrode active material, acetylene black, and binder is: positive electrode active material: acetylene black: binder = 97:1.5:1.5. Furthermore, N-methyl-2-pyrrolidone (NMP) is added as a dispersing solvent to the above positive electrode paste for mixing, thereby forming a slurry. This slurry is uniformly and homogeneously coated onto the surface of a base coating. Then, a drying process is performed, followed by compaction until a predetermined density is reached.

[0114] Examples 1-3

[0115] A composition of acetylene black, polymer A (polyethylene particles, melting point 82°C, average particle size 1.2µm) (acetylene black to polymer A mass ratio 5:95), and polyacrylic acid were mixed at a solids mass ratio of 95:5 and uniformly dispersed. Water was added to the resulting mixture to prepare a primer slurry. This primer slurry was applied to both opposite sides of a 9µm thick aluminum foil and dried at 60°C to prepare a 2µm thick primer coating.

[0116] Positive electrode production

[0117] The positive electrode is prepared as follows: Acetylene black (as a conductive material) and polyvinylidene fluoride (PVDF) (as a binder) are sequentially added to lithium cobalt oxide, which serves as the positive electrode active material, and mixed to prepare a positive electrode paste. The mass ratio of the positive electrode active material, acetylene black, and binder is: positive electrode active material: acetylene black: binder = 97:1.5:1.5. Furthermore, N-methyl-2-pyrrolidone (NMP) is added as a dispersing solvent to the above positive electrode paste for mixing, thereby forming a slurry. This slurry is uniformly and homogeneously coated onto the surface of a base coating. Then, a drying process is performed, followed by compaction until a predetermined density is reached.

[0118] Examples 1-4

[0119] A composition of acetylene black, polymer A (polyethylene particles, melting point 95°C, average particle size 1.1µm) (acetylene black to polymer A mass ratio 10:90), and polyacrylic acid were mixed at a solids mass ratio of 95:5 and uniformly dispersed. Water was added to the resulting mixture to prepare a primer slurry. This primer slurry was applied to both opposite sides of a 9µm thick aluminum foil and dried at 60°C to prepare a 2µm thick primer coating.

[0120] Positive electrode production

[0121] The positive electrode is prepared as follows: Acetylene black (as a conductive material) and polyvinylidene fluoride (PVDF) (as a binder) are sequentially added to lithium cobalt oxide, which serves as the positive electrode active material, and mixed to prepare a positive electrode paste. The mass ratio of the positive electrode active material, acetylene black, and binder is: positive electrode active material: acetylene black: binder = 97:1.5:1.5. Furthermore, N-methyl-2-pyrrolidone (NMP) is added as a dispersing solvent to the above positive electrode paste for mixing, thereby forming a slurry. This slurry is uniformly and homogeneously coated onto the surface of a base coating. Then, a drying process is performed, followed by compaction until a predetermined density is reached.

[0122] Examples 1-5

[0123] A composition of acetylene black, polymer A (polyethylene particles, melting point 116℃, average particle size 1.1µm) (acetylene black to polymer A mass ratio 15:85), and polyacrylic acid were mixed at a solids mass ratio of 95:5 and uniformly dispersed. Water was added to the resulting mixture to prepare a primer slurry. This primer slurry was applied to both sides of a 9µm thick aluminum foil and dried at 60℃ to prepare a 2µm thick primer coating.

[0124] Positive electrode production

[0125] The positive electrode is prepared as follows: Acetylene black (as a conductive material) and polyvinylidene fluoride (PVDF) (as a binder) are sequentially added to lithium cobalt oxide, which serves as the positive electrode active material, and mixed to prepare a positive electrode paste. The mass ratio of the positive electrode active material, acetylene black, and binder is: positive electrode active material: acetylene black: binder = 97:1.5:1.5. Furthermore, N-methyl-2-pyrrolidone (NMP) is added as a dispersing solvent to the above positive electrode paste for mixing, thereby forming a slurry. This slurry is uniformly and homogeneously coated onto the surface of a base coating. Then, a drying process is performed, followed by compaction until a predetermined density is reached.

[0126] Examples 1-6

[0127] A composition of acetylene black, polymer A (polyethylene particles, melting point 126℃, average particle size 1.5µm) (acetylene black to polymer A mass ratio 10:90), and polyacrylic acid were mixed at a solids mass ratio of 95:5 and uniformly dispersed. Water was added to the resulting mixture to prepare a primer slurry. This primer slurry was applied to both opposite sides of a 9µm thick aluminum foil and dried at 60℃ to prepare a 2µm thick primer coating.

[0128] Positive electrode production

[0129] The positive electrode is prepared as follows: Acetylene black (as a conductive material) and polyvinylidene fluoride (PVDF) (as a binder) are sequentially added to lithium cobalt oxide, which serves as the positive electrode active material, and mixed to prepare a positive electrode paste. The mass ratio of the positive electrode active material, acetylene black, and binder is: positive electrode active material: acetylene black: binder = 97:1.5:1.5. Furthermore, N-methyl-2-pyrrolidone (NMP) is added as a dispersing solvent to the above positive electrode paste for mixing, thereby forming a slurry. This slurry is uniformly and homogeneously coated onto the surface of a base coating. Then, a drying process is performed, followed by compaction until a predetermined density is reached.

[0130] Examples 1-7

[0131] A composition of acetylene black, polymer A (polypropylene particles, melting point 132℃, average particle size 1.6µm) (acetylene black to polymer A mass ratio 10:90), and polyacrylic acid were mixed at a solids mass ratio of 95:5 and uniformly dispersed. Water was added to the resulting mixture to prepare a primer slurry. This primer slurry was applied to both sides of a 9µm thick aluminum foil and dried at 60℃ to prepare a 2µm thick primer coating.

[0132] Positive electrode production

[0133] The positive electrode is prepared as follows: Acetylene black (as a conductive material) and polyvinylidene fluoride (PVDF) (as a binder) are sequentially added to lithium cobalt oxide, which serves as the positive electrode active material, and mixed to prepare a positive electrode paste. The mass ratio of the positive electrode active material, acetylene black, and binder is: positive electrode active material: acetylene black: binder = 97:1.5:1.5. Furthermore, N-methyl-2-pyrrolidone (NMP) is added as a dispersing solvent to the above positive electrode paste for mixing, thereby forming a slurry. This slurry is uniformly and homogeneously coated onto the surface of a base coating. Then, a drying process is performed, followed by compaction until a predetermined density is reached.

[0134] Examples 1-8

[0135] A composition of acetylene black, polymer A (polypropylene particles, melting point 132℃, average particle size 1.2µm) (acetylene black to polymer A mass ratio 10:90), lithium iron phosphate, and polyacrylic acid were mixed at a solids mass ratio of 92:5:3 and uniformly dispersed. Water was added to the resulting mixture to prepare a base coating slurry. This base coating slurry was applied to both sides of a 9µm thick aluminum foil and dried at 60℃ to prepare a 2µm thick base coating.

[0136] Positive electrode production

[0137] The positive electrode is prepared as follows: Acetylene black (as a conductive material) and polyvinylidene fluoride (PVDF) (as a binder) are sequentially added to lithium cobalt oxide, which serves as the positive electrode active material, and mixed to prepare a positive electrode paste. The mass ratio of the positive electrode active material, acetylene black, and binder is: positive electrode active material: acetylene black: binder = 97:1.5:1.5. Furthermore, N-methyl-2-pyrrolidone (NMP) is added as a dispersing solvent to the above positive electrode paste for mixing, thereby forming a slurry. This slurry is uniformly and homogeneously coated onto the surface of a base coating. Then, a drying process is performed, followed by compaction until a predetermined density is reached.

[0138] Comparative Example 1-1

[0139] The only difference between Comparative Example 1-1 and Example 1 is that the polyethylene particles used in Comparative Example 1-1 have a melting point of 70°C. The rest of the implementation is the same as in Example 1, and will not be repeated here.

[0140] Comparative Examples 1-2

[0141] The only difference between Comparative Examples 1-2 and Example 2 is that the polypropylene particles used in Comparative Examples 1-2 have a melting point of 145°C and an average particle size of 3µm. The rest of the implementation is the same as in Example 1, and will not be repeated here.

[0142] 2. Construction of secondary batteries

[0143] A slurry for forming the negative electrode composite layer was prepared by mixing natural graphite and silicon carbon (as the negative electrode active material), styrene-butadiene rubber and polyacrylate (as the binder), and carboxymethyl cellulose (as the tackifier) ​​in a mass ratio of 85:10:2:1.5:1.5, and then mixing with deionized water. This slurry was coated onto copper foil, dried, and then pressed to produce a negative electrode sheet.

[0144] A polyethylene film with a double-sided ceramic coating is used as the separator.

[0145] Preparation of electrolyte: In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC) were mixed uniformly at a mass ratio of 1:0.8:2. Based on 100 parts by mass of the total electrolyte, 10 parts by mass of lithium LiPF6 were added and dissolved in the organic solvent. Then, 5 parts by mass of fluoroethylene carbonate, component A, component B, and optional additive C were added to obtain the electrolyte. The partial composition of the electrolytes in each example and comparative example is shown in Table 1.

[0146] In Table 1, component A is abbreviated as: lithium difluorophosphate (A1), lithium fluorosulfonate (A2), and lithium tetrafluoroborate (A3).

[0147] The abbreviations for component B in Table 1 are: 1,3-propanesulfonate lactone (B1), 1,4-butanesulfonate lactone (B2), 1,3-propenesulfonate lactone (B3), methylene disulfonate (B4), vinyl sulfate (B5), propylene sulfate (B6), methyl vinyl sulfate (B7), cyclic vinyl carbonate (B8), structural formula II (B9), structural formula III (B10), structural formula IV (B11), and structural formula V (B12).

[0148] The abbreviations for additive C in Table 1 are: succinic anion (C1), adiponitrile (C2), ethylene glycol bis(propionitrile) ether (C3), 1,3,6-hexanetrionitrile (C4), glyceryl trionitrile (C5), lithium 1-ethyl-3-methylimidazolium tetrafluoroborate (C6), lithium 1-butyl-3-methylimidazolium hexafluorophosphate (C7), and lithium 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonylimide) (C8).

[0149] Table 1

[0150] Note: The numbers in parentheses corresponding to the relevant components in Table 1 represent the mass fractions of the relevant components based on 100 parts by mass of electrolyte.

[0151] Fabrication of wound-type battery cells: The negative electrode sheet obtained above is wound together with the positive electrode sheet and the separator to form a battery cell. Fabrication of stacked-type battery cells: The negative electrode sheet obtained above is stacked together with the positive electrode sheet and the separator to form a battery cell.

[0152] Preparation of secondary batteries: After the prepared cells are hot-pressed and shaped, they are packaged in aluminum-plastic film, baked to remove moisture, and then injected with electrolyte to obtain secondary batteries.

[0153] The relevant performance of the assembled secondary battery was tested using the following methods: (1) High-temperature cycling characteristics The secondary batteries prepared in the examples and comparative examples were left to stand at 25°C for 24 hours, and then charged and discharged at a rate of 4.5V and 0.1C. The initial capacity C0 was measured. Then, the charge-discharge cycle was repeated at 60°C, charging to 4.5V and discharging to 3.0V at a rate of 0.1C, and the capacity C1 after 500 cycles was measured. High-temperature cycling characteristics were evaluated using the capacity retention rate shown as ΔC = C1 / C0 × 100 (%). A higher value indicates less capacity reduction and better cycling characteristics. The A++, A+, A, B, C, D, and E values ​​in the "High-Temperature Cycling" column represent: A++: above 95%; A+: 90% or higher but less than 95%; A: Above 85% but below 90%; B: Above 80% but below 85%; C: 75% or higher but less than 80%; D: 70% or higher but less than 75%; E: Below 70%.

[0154] (2) Low-temperature internal resistance characteristics

[0155] To evaluate the low-temperature characteristics of the secondary batteries fabricated in the examples and comparative examples, the IV resistance was measured as follows. At -10°C, the batteries were charged to 50% of their State of Charge (SOC) at 1C (C is a value expressed as rated capacity (mA) / 1 hour (h)). Then, using 50% SOC as the center, the batteries were charged and discharged for 15 seconds at 0.5C, 1.0C, 1.5C, and 2.0C, respectively. The battery voltage after 15 seconds was plotted against the current value for each case (charging side and discharging side), and the slope of the plot was used as the IV resistance (Ω) (IV resistance during charging and IV resistance during discharging). The obtained IV resistance values ​​(Ω) were evaluated using the following criteria. A smaller IV resistance value indicates a smaller internal resistance and better low-temperature characteristics. The A++, A+, A, B, C, D, and E values ​​in the "Low-Temperature Internal Resistance" column represent: A++: IV resistance is below 2Ω; A+: IV resistance exceeds 2Ω but is below 4Ω; A: IV resistance exceeds 4Ω but is below 6Ω; B: IV resistance exceeds 6Ω but is below 8Ω; C: IV resistance exceeds 8Ω but is below 10Ω; D: IV resistance exceeds 10Ω but is below 20Ω; E: IV resistance exceeds 20Ω.

[0156] (3) Safety test

[0157] Safety test for heat suppression during internal short circuit (forced internal short circuit test): The secondary batteries prepared in the examples and comparative examples were subjected to CC-CV charging at a constant current of 0.2C (upper limit cell voltage 4.5V) and CC-discharged to 3.0V at a constant current of 0.2C. This 0.2C charge-discharge cycle was repeated three times. Then, under an atmosphere of 25°C, they were charged to 4.5V at a charge rate of 0.2C using constant voltage-constant current (CC-CV) (termination condition: 0.02C). Then, near the center of the secondary battery, a 3mm diameter, 10cm long iron nail was driven through it at a speed of 5m / min, forcibly short-circuiting it. Ten secondary batteries prepared in the same manner were subjected to this forced short-circuiting, and the following criteria were used to evaluate them based on the number of test specimens that did not rupture or ignite. The more test specimens that did not rupture or ignite, the better the heat release suppression performance of the secondary battery during internal short circuit. The A++, A+, A, B, C, D, and E values ​​in the "Safety" column represent: A++: The number of test subjects that neither cracked nor caught fire was 10; A+: The number of test subjects that did not crack or catch fire was 9; A: The number of test subjects that did not crack or catch fire was 8; B: The number of test subjects that did not crack or catch fire was 7; C: The number of test subjects that did not crack or catch fire was 5 to 6; D: The number of test subjects that did not crack or catch fire was 2 to 4; E: The number of test subjects that neither cracked nor caught fire is 0 to 1.

[0158] Examples 2-1 to 2-8

[0159] Examples 2-1 to 2-8 used the positive electrode and electrolyte E1-1 prepared in Examples 1-1 to 1-8 respectively to prepare secondary batteries. The battery performance test results are shown in Table 2.

[0160] Comparative Examples 2-1 to 2-6

[0161] Comparative Examples 2-1 to 2-2 used the positive electrode and electrolyte prepared in Comparative Examples 1-1 to 1-2 respectively to prepare secondary batteries; Comparative Examples 2-3 to 2-4 used the positive electrode and electrolyte prepared in Comparative Examples 1-1 to 1-2 respectively to prepare secondary batteries; Comparative Examples 2-5 to 2-6 used the positive electrode prepared in Example 1-1 and the electrolytes E0-1 and E0-2, respectively, to fabricate secondary batteries. The battery performance test results are shown in Table 2.

[0162] Table 2

[0163] Examples 3-1 to 3-50

[0164] Examples 3-1 to 3-50 used the positive electrodes prepared in Examples 1-1 to 1-8 and electrolytes E1-2 to E1-50 to fabricate secondary batteries. The battery performance test results are shown in Table 3.

[0165] Table 3

[0166] Examples 4-1 to 4-12

[0167] Examples 4-1 to 4-12 used the positive electrodes prepared in Examples 1-6 and 1-7, and the electrolytes E2-1 to E2-9, respectively, to prepare secondary batteries. The battery performance test results are shown in Table 4.

[0168] Table 4

[0169] Examples 5-1 to 5-20

[0170] Examples 5-1 to 5-20 used the positive electrodes prepared in Examples 1-6 and 1-7, respectively, and electrolytes E2-10 to E2-24 to fabricate secondary batteries. The battery performance test results are shown in Table 5.

[0171] Table 5

[0172] As shown in Tables 1-5, this invention utilizes polymer A with a melting point of 75℃~138℃ and an average particle size of less than 2 micrometers in the undercoat layer. This allows conductive particles to be easily and uniformly distributed within the undercoat layer, thus forming a conductive network that serves as an electron movement path in the entire undercoat layer. When the battery experiences thermal runaway, the non-conductive polymer A can melt in time and cover the conductive particles, blocking the current flow within the undercoat layer. This not only blocks electron conduction between the positive electrode active material layer and the positive electrode current collector but also blocks electron conduction between the negative electrode active material layer and the positive electrode current collector, preventing internal short circuits caused by thermal runaway and significantly improving battery safety performance. If the melting point of polymer A is below 75℃, it affects the battery's high-temperature performance. During charging and discharging, when the battery heats up internally, polymer A melts, leading to increased internal resistance and poor high-temperature cycling. If the melting point of polymer A is high, it affects battery safety performance. For example, when the internal temperature of the battery reaches above 138℃, the separator substrate shrinks, causing a rapid internal short circuit. At this time, the polymer has not yet melted and cannot play a blocking role, resulting in a safety accident. Using polymer A in the base coating can significantly improve the adhesion between the base coating and the positive electrode current collector. However, this invention found that under high temperature conditions, polymer A is prone to swelling in electrolyte systems containing cyclic carbonates (such as ethylene carbonate, propylene carbonate, etc.). Through systematic research, it was found that when the electrolyte of this secondary battery contains components A and B as described in this invention, the swelling of polymer A is unexpectedly suppressed. This not only improves the low-temperature internal resistance and high-temperature cycle performance of the secondary battery, but also significantly improves its safety.

[0173] This invention also discovered that the presence of components A and B catalyzes the decomposition of lithium hexafluorophosphate at high temperatures. Systematic research revealed that when the electrolyte also includes the additive C described in this invention, additive C can inhibit the catalytic decomposition of lithium hexafluorophosphate by components A and B at high temperatures, thereby further improving the low-temperature internal resistance and high-temperature cycle performance of the secondary battery, and significantly improving safety.

[0174] It should be noted that the present invention is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments that have the same structure and perform the same effects as the technical concept within the scope of the present invention are included within the scope of the present invention. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of the present invention, are also included within the scope of the present invention.

Claims

1. A secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that: The positive electrode includes a current collector, and a base coating and a positive electrode active material layer sequentially disposed on the current collector; The base coating comprises a uniformly distributed polymer A and conductive particles. The polymer A is non-conductive, has an average particle size of less than 2µm, and has a melting point of 75℃~138℃. The electrolyte comprises a cyclic carbonate and further comprises at least component A and component B for inhibiting the swelling of polymer A; component A comprises at least one of lithium difluorophosphate, lithium fluorosulfonate, and lithium tetrafluoroborate; component B comprises at least one of 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, 1,3-propenesulfonate lactone, methylene disulfonate, vinyl sulfate, propylene sulfate, methyl vinyl sulfate, and cyclic vinyl carbonate.

2. The secondary battery according to claim 1, characterized in that: The polymer A comprises at least one of the following: homopolymers of non-fluorinated alkenyl monomers, homopolymers of olefin monomers, homopolymers of unsaturated nitrile monomers, homopolymers of olefinic monomers, and homopolymers of ester monomers; preferably, the polymer A comprises polyethylene and / or polypropylene. And / or, the melting point of polymer A is 85°C to 128°C; And / or, the average particle size of polymer A is 0.2µm to 2µm, preferably 0.5µm to 1.5µm; And / or, the mass ratio of the conductive particles to the polymer A is 2:98 to 20:

80.

3. The secondary battery according to claim 1, characterized in that: Based on 100 parts by mass of the electrolyte, the content of component A is 0.01 to 5 parts by mass, preferably 0.1 to 3 parts by mass; And / or, based on 100 parts by mass of the electrolyte, the content of component B is 0.01 to 5.8 parts by mass, preferably 0.1 to 2.9 parts by mass.

4. The secondary battery according to claim 1, characterized in that: The cyclic vinyl carbonate includes at least one of the following structural formulas I to V; ； 。 5. The secondary battery according to claim 1, characterized in that, The supporting electrolyte contained in the electrolyte includes a lithium salt, which includes lithium hexafluorophosphate.

6. The secondary battery according to claim 5, characterized in that: The electrolyte further includes additive C, which is used to inhibit the decomposition of the lithium hexafluorophosphate in the presence of component A and / or component B. Additive C includes at least one of succinic anion, adiponitrile, ethylene glycol bis(propionitrile) ether, 1,3,6-hexanetrionitrile, glycerol trionitrile, lithium 1-ethyl-3-methylimidazolium tetrafluoroborate, lithium 1-butyl-3-methylimidazolium hexafluorophosphate, and lithium 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonylimide).

7. The secondary battery according to claim 6, characterized in that: Based on 100 parts by mass of the electrolyte, the content of succinic anion is 0.7-3.5 parts by mass, and / or the content of adiponitrile is 0.6-4 parts by mass, and / or the content of ethylene glycol bis(propionitrile) ether is 0.2-2 parts by mass, and / or the content of 1,3,6-hexanetrionitrile is 0.1-3 parts by mass, and / or the content of glycerol trionitrile is 0.1-4 parts by mass, and / or the content of lithium 1-ethyl-3-methylimidazolium tetrafluoroborate is 0.01-2 parts by mass, and / or the content of lithium 1-butyl-3-methylimidazolium hexafluorophosphate is 0.01-2 parts by mass, and / or the content of lithium 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide is 0.01-2 parts by mass.

8. The secondary battery according to claim 1, characterized in that: The thickness of the base coating is 0.1µm to 5µm; And / or, the undercoat may further include a binder and / or inorganic particles.

9. The secondary battery according to claim 8, characterized in that: The total mass ratio of the conductive particles and polymer A to the mass ratio of the binder is 99.9:0.1 to 95:5; And / or, the inorganic particles are made of at least one of lithium iron phosphate, lithium manganese iron phosphate, silicon dioxide, aluminum oxide, boehmite, and barium sulfate.

10. An electronic device, characterized in that: Includes the secondary battery as described in any one of claims 1 to 9.

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