Electrochemical apparatus and electronic apparatus

Abstract

An electrochemical apparatus, comprising a positive electrode and an electrolyte, the positive electrode comprising a positive electrode current collector, and a positive electrode mixture layer arranged on the surface of the positive electrode current collector. The positive electrode current collector comprises a first region having the positive electrode mixture layer disposed on two surfaces thereof, a second region having the positive electrode mixture layer disposed on only one surface thereof, and a third region, neither surface of which is provided with the positive electrode mixture layer. An average width of the first region is a centimeters, an average width of the third region is b centimeters, and an extension rate of the positive electrode current collector is x, x being equal to a / b-1. The electrolyte comprises a specific type and a specific content (y%) of a lithium salt additive A, and y≥x. The electrochemical apparatus has significantly improved safety.

Description

Electrochemical devices and electronic devices Technical Field

[0001] The present application relates to the field of energy storage, and in particular to an electrochemical device and an electronic device. Background Art

[0002] With the widespread use of electrochemical devices (e.g., lithium-ion batteries) in electronic products such as mobile phones, laptops, tablets, Bluetooth headsets, and wearable watches, people are increasingly demanding higher performance, such as energy density. When the energy density of lithium-ion batteries increases to above 700Wh / L, the voltage of the electrochemical device will increase to above 4.48V, making the lithium salt in the electrolyte more unstable in the high voltage window. It is easy to react with trace amounts of water and decompose, corroding the positive electrode. The dissolved elements easily accumulate to form purple spots, black spots, or lithium precipitation, seriously affecting the safety of lithium-ion batteries.

[0003] In view of the above, there is a real need to provide an electrochemical device with improved safety.

[0004] Summary of the Invention

[0005] The present application attempts to solve at least one problem existing in the related art to at least some extent by providing an electrochemical device.

[0006] According to one aspect of the present application, the present application provides an electrochemical device, which includes a positive electrode and an electrolyte, wherein the positive electrode includes a positive electrode collector and a positive electrode mixture layer arranged on the surface of the positive electrode collector, the positive electrode collector includes a first region, a second region and a third region, the positive electrode mixture layers are respectively arranged on the two surfaces of the first region, the positive electrode mixture layer is arranged on one surface of the second region, and the positive electrode mixture layer is not arranged on both sides of the third region; the average width of the first region is a cm, the average width of the third region is b cm, the elongation of the positive electrode collector is x, x=a / b-1; the electrolyte includes a lithium salt additive A, the lithium salt additive A is selected from at least one of lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorophosphate, lithium difluorobis(oxalatophosphate), lithium bis(fluorosulfonyl imide) and lithium bis(trifluoromethanesulfonyl imide), based on the mass of the electrolyte, the content of the lithium salt additive A is y%, and y is in the range of 0.1 to 3; and y≥x.

[0007] According to an embodiment of the present application, x≤0.8.

[0008] According to an embodiment of the present application, x≤0.4.

[0009] According to an embodiment of the present application, 0.1≤y≤1.5.

[0010] According to an embodiment of the present application, the positive electrode current collector is aluminum foil, and the aluminum foil includes at least one of Mg, Ti, Cr, Cu, Fe, Mn, Ni, Si, V, Zn or Gr elements.

[0011] According to an embodiment of the present application, the content of aluminum in the positive electrode current collector is greater than or equal to 60%.

[0012] According to an embodiment of the present application, the aluminum foil includes Si element, and the mass percentage of Si element is less than or equal to 0.5% based on the total mass of the positive electrode current collector.

[0013] According to an embodiment of the present application, the electrolyte further contains LiPF6, and the content of LiPF6 is z% based on the mass of the electrolyte, and z is 5 to 20.

[0014] According to an embodiment of the present application, 0.01≤y / z≤0.5.

[0015] According to an embodiment of the present application, the charge termination voltage of the electrochemical device is 4.41V to 4.5V.

[0016] According to an embodiment of the present application, the third region is arranged at the tail end, and the area of ​​the third region accounts for 15% to 30% of the area of ​​the outermost positive electrode current collector.

[0017] According to another aspect of the present application, an electronic device includes the electrochemical device according to the present application.

[0018] This application can alleviate the sensitivity of the chemical system in the electrochemical device to water by optimizing the structural parameters of the positive electrode and combining it with a specific type and content of lithium salt, thereby significantly improving the safety of the electrochemical device.

[0019] Additional aspects and advantages of the present application will be described, shown, or explained in part in the following description through implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The following briefly describes the drawings necessary to describe the embodiments of the present application or the prior art to facilitate the description of the embodiments of the present application. Obviously, the drawings described below only represent some of the embodiments of the present application. Those skilled in the art can, without requiring creative effort, derive drawings for other embodiments based on the structures illustrated in these drawings.

[0021] Figure 1 shows a photograph of the unfolded wound cell.

[0022] FIG2A shows a schematic front view of a positive electrode according to an embodiment.

[0023] FIG2B shows a schematic top view of a positive electrode according to an embodiment.

[0024] FIG3A shows a schematic diagram of a wound cell with an embedded tab structure.

[0025] FIG3B shows a schematic diagram of a wound cell with a non-embedded tab structure.

[0026] FIG3C shows a schematic diagram of a wound battery cell with a multi-electrode structure.

[0027] Figure 4 shows the normal interface photo of the positive electrode after cycling.

[0028] Figure 5 shows the abnormal interface photo of the positive electrode after cycling. DETAILED DESCRIPTION

[0029] The embodiments of the present application will be described in detail below. The embodiments of the present application should not be interpreted as limiting the present application.

[0030] In the detailed description and claims, a list of items linked by the term "at least one of" can mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A can contain a single element or multiple elements. Item B can contain a single element or multiple elements. Item C can contain a single element or multiple elements.

[0031] Electrochemical devices (e.g., lithium-ion batteries) have been widely used in various fields due to their superior performance, and people have increasingly higher requirements for their performance, such as energy density and safety. In high-voltage, high-energy-density electrochemical devices, trace amounts of water and impurity elements will affect the performance of the electrochemical device, deteriorate the health of the electrochemical device interface after cycling, and affect the safety of the electrochemical device. Specifically, when the energy density of the electrochemical device is increased to above 700Wh / L, the voltage of the electrochemical device increases accordingly (e.g., ≥4.48V). In the high voltage window, the lithium salt in the electrolyte is more unstable and easily reacts with trace amounts of water to decompose, corroding the positive electrode (e.g., aluminum foil), resulting in the dissolution of elements (e.g., Si) in the positive electrode. The dissolved elements are easily enriched at the positive electrode-diaphragm-negative electrode interface, resulting in the inability to insert and extract lithium ions normally at the enriched position. The dissolved elements then accumulate to form purple spots, black spots, or lithium precipitation, which creates a huge safety risk. This phenomenon is most pronounced at the tail of the positive electrode (corresponding to the outermost coil of the wound cell). This is because, in a wound cell design, the outermost coil is typically the single-sided coated positive electrode sheet, exposing a large area of ​​the positive current collector to the free electrolyte. Corrosion products easily accumulate at the interface between the separator and the positive electrode, corresponding to the two folds at the tail of the wound cell. Figure 1 shows a photograph of an unfolded wound cell, with the box inside representing the two folds at the tail of the wound cell.

[0032] In order to solve the above problems, the present application provides an electrochemical device comprising a positive electrode and an electrolyte, wherein the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer arranged on the surface of the positive electrode current collector, the positive electrode current collector sequentially comprising a first region, a second region and a third region along the length direction, the positive electrode mixture layer being arranged on both surfaces of the first region, the positive electrode mixture layer being arranged on one surface of the second region, and the positive electrode mixture layer being not arranged on both surfaces of the third region; the average width of the first region is a centimeter, the average width of the third region is b centimeters, the elongation of the positive electrode current collector is x, x=a / b-1; the electrolyte includes a lithium salt additive A, the lithium salt additive A is selected from at least one of lithium tetrafluoroborate (LiBF4), lithium bis(oxalatoborate) (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium difluorophosphate (LiDFP), lithium difluorobis(oxalatophosphate) (LiDODFP), lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), based on the mass of the electrolyte, the content of the lithium salt additive A is y%, and y is in the range of 0.1 to 3; and y≥x.

[0033] LiBF4, LiBOB, LiDFOB, LiDFP, LiDODFP, LiFSI and LiTFSI all have high ionic conductivity and a wide electrochemical window, which can achieve high voltage output. The addition of these lithium salt additives has advantages over traditional LiPF6. For example, taking LiTFSI as an example:

[0034] 1) Better thermal stability: LiTFSI is more stable than LiPF6 and is not easily decomposed to produce gas, which reduces the safety risks of electrochemical devices.

[0035] 2) Lower water sensitivity, reducing the dissolution of metal ions: The incoming electrolyte and the battery cell preparation process contain trace amounts of water (100-200ppm). Lithium salts are easily decomposed by water at high temperatures, generating byproducts or gases, which affect the performance of electrochemical devices. Compared with pure LiPF6, the products after the hydrolysis reaction of LiTFSI are less corrosive and less likely to further corrode the metal positive electrode current collector and positive electrode active material, reducing the dissolution of metal ions. The specific reaction is as follows:

[0036] The hydrolysis reaction of LiPF6 is as follows: LiPF6+H2O→LiF+HF+H2PO4 - .

[0037] In this reaction, water molecules react with LiPF6 to generate lithium fluoride, hydrofluoric acid and hydrogen phosphate ions, among which hydrofluoric acid is highly corrosive.

[0038] The hydrolysis reaction of LiTFSI is as follows: LiTFSI+H2O→LiOH+TFSI - +H + .

[0039] In this reaction, LiTFSI is decomposed into Li + and TFSI - Two parts, releasing H at the same time + ions and OH - ions. No hydrofluoric acid is generated in this reaction, thus reducing the corrosion to the positive electrode current collector in the electrochemical device.

[0040] Adding a certain amount of LiTFSI to the electrolyte can alleviate the corrosion of the electrolyte on the positive electrode current collector while ensuring the original excellent conductivity / rate performance of LiPF6, especially alleviate the precipitation of silicon elements in the positive electrode current collector (for example, aluminum foil), and avoid the formation of purple spots, black spots or lithium precipitation at the interface.

[0041] In addition, as shown in FIG2A , the positive electrode current collector includes a first region 201 in which positive electrode mixture layers are respectively provided on both surfaces, a second region 202 in which a positive electrode mixture layer is provided on one surface, and a third region 203 in which no positive electrode mixture layer is provided on both sides. Due to the different settings of the positive electrode mixture layers, the thicknesses of the three regions are different. The positive electrode current collector undergoes links such as cold pressing of the pole piece, hot pressing of the battery cell, and hot pressing of the formation during the preparation of the battery cell, resulting in the extension of the positive electrode current collector in the XY direction of the plane (as shown in FIG2B ). Since the ratio of the overpressure of the cold pressing roller in the thicker area (i.e., the first area) of the positive electrode is different from the overpressure of the cold pressing roller in the thinner area (i.e., the third area), the elongation rate of the positive electrode current collector can reflect the pressure used during the cold pressing process.

[0042] The testing method for the elongation of the positive electrode current collector is as follows: the positive electrode obtained by disassembling the electrochemical device is measured, and the average width of the first area where the positive electrode mixture layer is set on both surfaces of the positive electrode current collector is measured (three points are dispersed in the first area for testing and the average value is taken as the average width of the positive electrode current collector in the first area), which is recorded as a cm (as shown in Figure 2B); the average width of the third area where no positive electrode mixture layer is set on both surfaces of the positive electrode current collector is measured (three points are dispersed in the third area for testing and the average value is taken as the average width of the positive electrode current collector in the first area), which is recorded as b cm (as shown in Figure 2B); the elongation x of the positive electrode current collector is calculated by the following formula: x = a / b-1.

[0043] The positive electrode current collector is at risk of local microcracks after extension. The microcracks will expose more grain boundary defect locations, making it easier for corrosion reactions to occur, accelerating the precipitation of impurity elements in the positive electrode current collector. The greater the elongation of the positive electrode current collector, the greater the pressure on the positive electrode during cold pressing. The overpressure of the thicker first region causes the third region (i.e., the pure current collector region) with a thinner width than the first region to extend more, indicating that more microcracks may be generated inside the positive electrode current collector after cold pressing. The more microcracks there are, the more defects are exposed. In the presence of hydrofluoric acid, the greater the corrosion of the positive electrode current collector caused by the defective locations, the greater the dissolution concentration of impurity elements (e.g., Si), and the more serious the interface problems caused by the accumulation of dissolution products at the interface of the two folded positive electrodes at the tail of the wound battery cell.

[0044] The present application adds a certain amount of lithium salt additive A to the electrolyte to reduce the risk of corrosion and dissolution of the positive electrode current collector caused by hydrofluoric acid generated by the reaction of LiPF6 with water. Furthermore, when the content y of lithium salt additive A is greater than or equal to the elongation x of the positive electrode current collector, the effect of lithium salt additive A on improving corrosion and dissolution can offset the worsening effect of ductile cracks in the positive electrode current collector after manufacturing on corrosion and dissolution of the positive electrode current collector, thereby preventing abnormalities at the tail two-fold positive electrode interface after cycling of the electrochemical device, thereby significantly improving the safety of the electrochemical device.

[0045] In some embodiments, x≤0.8. In some embodiments, x≤0.4. In some embodiments, x is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or a value within a range formed by any two of the foregoing values.

[0046] In some embodiments, 0.1≤y≤1.5. In some embodiments, 0.5≤y≤1. In some embodiments, y is 0.1, 0.5, 1, 1.5, 2, 2.5, 3, or a value within a range formed by any two of the foregoing values.

[0047] In some embodiments, the positive electrode current collector is aluminum foil, and the aluminum foil includes at least one of Mg, Ti, Cr, Cu, Fe, Mn, Ni, Si, V, Zn or Gr.

[0048] In some embodiments, the aluminum content of the positive electrode current collector is greater than or equal to 60%. In some embodiments, the aluminum content of the positive electrode current collector is greater than or equal to 70%. In some embodiments, the aluminum content of the positive electrode current collector is greater than or equal to 80%. In some embodiments, the aluminum content of the positive electrode current collector is greater than or equal to 90%.

[0049] In some embodiments, the aluminum foil includes Si, and the mass percentage of Si is less than or equal to 0.5% based on the total mass of the positive electrode current collector. In some embodiments, the mass percentage of Si is less than or equal to 0.3% based on the total mass of the positive electrode current collector. In some embodiments, the mass percentage of Si is less than or equal to 0.1% based on the total mass of the positive electrode current collector.

[0050] In some embodiments, the electrolyte further comprises LiPF6, and the content of LiPF6 is z% based on the mass of the electrolyte, and z is 5 to 20. In some embodiments, z is 8 to 15. In some embodiments, z is 10 to 12. In some embodiments, z is 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a value within a range consisting of any two of the foregoing values.

[0051] In some embodiments, 0.01≤y / z≤0.5. In some embodiments, 0.05≤y / z≤0.3. In some embodiments, 0.1≤y / z≤0.2. In some embodiments, y / z is 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, or a value within a range formed by any two of the foregoing values.

[0052] In some embodiments, the electrochemical device has a charge termination voltage of 4.41V to 4.5V.

[0053] In some embodiments, the third region is located at the end of the winding, and the area of ​​the third region accounts for 15% to 30% of the area of ​​the outermost positive current collector. In some embodiments, the area of ​​the third region accounts for 20% to 25% of the area of ​​the outermost positive current collector. The term "outermost" refers to the portion of the battery cell winding structure that faces the outer packaging. The "outermost positive current collector" refers to the outermost positive current collector of the battery cell winding structure.

[0054] In some embodiments, the electrochemical device includes a wound battery cell having an embedded tab structure (i.e., the tab is welded to the blank current collector at the head and tail) (as shown in FIG3A ), a non-embedded tab structure (i.e., the tab is welded to the middle of the pole piece) (as shown in FIG3B ) or a multi-tab structure (i.e., multiple tabs are welded on the pole piece) (as shown in FIG3C ).

[0055] In some embodiments, the positive electrode mixture layer includes a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent.

[0056] In some embodiments, the positive electrode active material may include at least one of lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminum oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel lithium manganese oxide, spinel lithium nickel manganese oxide, or lithium titanate.

[0057] In some embodiments, the positive electrode binder may include a binder polymer such as, but not limited to, at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyolefins, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified SBR rubber, or polyurethane.

[0058] In some embodiments, any conductive material can be used as the positive electrode conductive agent, as long as it does not cause chemical changes. Examples of positive electrode conductive agents include, but are not limited to, carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.; metal-based materials such as metal powders or metal fibers including copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0059] The material, composition and manufacturing method of the negative electrode used in the electrochemical device of the present application may include any technology disclosed in the prior art.

[0060] In some embodiments, the negative electrode active material is any material that can electrochemically occlude and release metal ions such as lithium ions. In some embodiments, the negative electrode active material includes a carbonaceous material, a silicon-carbon material, an alloy material, or a lithium-containing metal composite oxide material. In some embodiments, the negative electrode active material includes one or more of the above.

[0061] In some embodiments, the negative electrode can be manufactured by any method known in the art. In some embodiments, the negative electrode can be formed by adding a binder and a solvent to the negative electrode active material and, if necessary, adding a thickener, a conductive material, a filler, etc. to form a slurry, coating the slurry on a current collector, drying it, and then pressing it.

[0062] In some embodiments, when the negative electrode includes an alloy material, the negative electrode active material layer may be formed by evaporation, sputtering, plating, or the like.

[0063] In some embodiments, when the negative electrode includes lithium metal, the negative electrode active material layer is formed using, for example, a spherical, twisted conductive skeleton and metal particles dispersed in the conductive skeleton. The spherical, twisted conductive skeleton may have a porosity of about 5% to about 85%, and a protective layer may also be provided on the lithium metal negative electrode active material layer.

[0064] The solvents that can be used in the electrolyte of the embodiments of the present application include, but are not limited to: cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, cyclic ethers, chain ethers, phosphorus-containing organic solvents, sulfur-containing organic solvents, and aromatic fluorine-containing solvents.

[0065] In some embodiments, cyclic carbonates include, but are not limited to, ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), and butylene carbonate. In some embodiments, cyclic carbonates have 3-6 carbon atoms.

[0066] In some embodiments, the linear carbonates include, but are not limited to, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl n-propyl carbonate, ethyl n-propyl carbonate, di-n-propyl carbonate and the like, and fluorine-substituted linear carbonates, such as bis(fluoromethyl) carbonate, bis(difluoromethyl) carbonate, bis(trifluoromethyl) carbonate, bis(2-fluoroethyl) carbonate, bis(2,2-difluoroethyl) carbonate, bis(2,2,2-trifluoroethyl) carbonate, 2-fluoroethyl methyl carbonate, 2,2-difluoroethyl methyl carbonate and 2,2,2-trifluoroethyl methyl carbonate.

[0067] In some embodiments, the cyclic carboxylic acid ester includes, but is not limited to, γ-butyrolactone and γ-valerolactone. In some embodiments, some of the hydrogen atoms of the cyclic carboxylic acid ester may be substituted with fluorine.

[0068] In some embodiments, the chain carboxylic acid esters include, but are not limited to: methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, sec-butyl acetate, isobutyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, isopropyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, methyl isobutyrate, ethyl isobutyrate, methyl valerate, ethyl valerate, methyl pivalate, and ethyl pivalate. In some embodiments, some of the hydrogen atoms of the chain carboxylic acid esters may be substituted with fluorine. In some embodiments, fluorine-substituted chain carboxylic acid esters include, but are not limited to: methyl trifluoroacetate, ethyl trifluoroacetate, propyl trifluoroacetate, butyl trifluoroacetate, and 2,2,2-trifluoroethyl trifluoroacetate.

[0069] In some embodiments, cyclic ethers include, but are not limited to, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 2-methyl 1,3-dioxolane, 4-methyl 1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, and dimethoxypropane.

[0070] In some embodiments, chain ethers include, but are not limited to, dimethoxymethane, 1,1-dimethoxyethane, 1,2-dimethoxyethane, diethoxymethane, 1,1-diethoxyethane, 1,2-diethoxyethane, ethoxymethoxymethane, 1,1-ethoxymethoxyethane, and 1,2-ethoxymethoxyethane.

[0071] In some embodiments, the phosphorus-containing organic solvent includes, but is not limited to, trimethyl phosphate, triethyl phosphate, dimethyl ethyl phosphate, diethyl methyl phosphate, ethylidene methyl phosphate, ethylidene ethyl phosphate, triphenyl phosphate, trimethyl phosphite, triethyl phosphite, triphenyl phosphite, tris(2,2,2-trifluoroethyl) phosphate, and tris(2,2,3,3,3-pentafluoropropyl) phosphate.

[0072] In some embodiments, the sulfur-containing organic solvent includes, but is not limited to: sulfolane, 2-methylsulfolane, 3-methylsulfolane, dimethyl sulfone, diethyl sulfone, ethyl methyl sulfone, methylpropyl sulfone, dimethyl sulfoxide, methyl methanesulfonate, ethyl methanesulfonate, methyl ethanesulfonate, ethyl ethanesulfonate, dimethyl sulfate, diethyl sulfate, and dibutyl sulfate. In some embodiments, some of the hydrogen atoms of the sulfur-containing organic solvent may be substituted with fluorine.

[0073] In some embodiments, the aromatic fluorine-containing solvent includes, but is not limited to, fluorobenzene, difluorobenzene, trifluorobenzene, tetrafluorobenzene, pentafluorobenzene, hexafluorobenzene, and trifluoromethylbenzene.

[0074] In some embodiments, the solvent used in the electrolyte of the present application includes one or more of the above. In some embodiments, the solvent used in the electrolyte of the present application includes cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, and combinations thereof. In some embodiments, the solvent used in the electrolyte of the present application includes an organic solvent selected from the group consisting of the following substances: ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl propionate, propyl propionate, n-propyl acetate, ethyl acetate, and combinations thereof. In some embodiments, the solvent used in the electrolyte of the present application includes: ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl propionate, propyl propionate, γ-butyrolactone, or a combination thereof.

[0075] The material and shape of the diaphragm used in the electrochemical device of the present application are not particularly limited, and it may be any technology disclosed in the prior art. In some embodiments, the diaphragm includes a polymer or inorganic substance formed from a material that is stable to the electrolyte of the present application. In some embodiments, the diaphragm may include a substrate layer and a surface treatment layer. In some embodiments, the substrate layer is a non-woven fabric, a film or a composite film having a porous structure. In some embodiments, the material of the substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate and polyimide. In some embodiments, the material of the substrate layer includes at least one of 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.

[0076] In some embodiments, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic layer.

[0077] In some embodiments, the inorganic layer includes inorganic particles and a binder, wherein the inorganic particles are selected from 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, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene alkoxide, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0078] In some embodiments, the polymer layer comprises a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene alkoxide, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0079] In some embodiments, the electrochemical device of the present application includes, but is not limited to, all types of primary batteries, secondary batteries, or capacitors. In some embodiments, the electrochemical device is a lithium secondary battery. In some embodiments, the lithium secondary battery includes, but is not limited to, a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery. In some embodiments, the electrochemical device is a sodium ion battery.

[0080] The present application further provides an electronic device, which includes the electrochemical device according to the present application.

[0081] The electronic devices or devices of the present application are not particularly limited. In some embodiments, the electronic devices of the present application include, but are not limited to, laptop computers, pen-type computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD televisions, portable cleaners, portable CD players, mini-discs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries and lithium-ion capacitors, etc.

[0082] The preparation process of electrochemical devices and electronic devices is well known to those skilled in the art and is not particularly limited in this application. For example, the battery can be prepared by the following method: the positive electrode and the negative electrode are overlapped via a diaphragm, and are wound, folded, and the like as needed to form a bare cell. Stainless steel is selected as the packaging shell, and the bare cell is placed inside the packaging shell. The aluminum tabs of the bare cell and the poles in the sealing assembly are laser welded together to achieve electronic conduction between the poles and the bare cell. The steel shell cover and the shell are connected by laser welding to achieve sealing of the battery. There are through holes on the surface of the steel shell, and the electrolyte is injected into the interior of the cell through the through holes. After sufficient infiltration, the cell is activated under certain current and voltage conditions, and the preparation of the battery is completed.

[0083] The preparation of lithium-ion batteries is described below using lithium-ion batteries as an example and in combination with specific embodiments. Those skilled in the art will understand that the preparation method described in this application is only an example, and any other suitable preparation method is within the scope of this application.

[0084] Example

[0085] The following describes the performance evaluation of the lithium-ion battery according to the embodiments and comparative examples of the present application.

[0086] 1. Preparation of lithium-ion batteries

[0087] 1. Preparation of negative electrode

[0088] Artificial graphite, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) were mixed in a mass ratio of 95:2:3. Deionized water was then added as a solvent to form a slurry with a solid content of 70 wt%, which was then stirred evenly. The slurry was evenly coated on one surface of an 8 μm thick copper foil, dried at 110°C, and cold-pressed to produce a negative electrode sheet with a single-sided negative electrode active material layer of 150 μm. The above coating steps were then repeated on the other surface of the negative electrode sheet to produce a negative electrode sheet coated on both sides with a negative electrode active material layer. The negative electrode sheet was cut into a size of 76 mm x 851 mm and the tabs were welded before use.

[0089] 2. Preparation of positive electrode

[0090] The positive electrode active material lithium cobalt oxide (chemical formula: LiCoO2), the conductive agent conductive carbon black (Super P), and the binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 97:2:1, and N-methylpyrrolidone (NMP) is added as a solvent. The mixture is stirred in a vacuum mixer until the solid content is 75wt% and the system is uniform. The positive electrode slurry is evenly coated on one surface of a positive electrode current collector with a thickness of 10μm and dried at 85℃ for 4h to obtain a positive electrode sheet with a single-sided positive electrode active material layer (thickness 110μm). After that, the above steps are repeated on the other surface of the positive electrode current collector to obtain a positive electrode sheet with a double-sided positive electrode active material layer. After cold pressing, cutting and slitting, the positive electrode sheet with a specification of 74mm×867mm is obtained under vacuum conditions at 85°C for 4 hours. The positive electrode collector in the positive electrode sheet includes a first area (i.e., double-sided coating area), a second area (single-sided coating area) and a third area (uncoated area) in the length direction.

[0091] The elongation of the positive electrode current collector can be controlled by regulating the pressure of the roller during the cold pressing process of the positive electrode sheet preparation process.

[0092] 3. Preparation of electrolyte

[0093] In a dry argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are uniformly mixed in a mass ratio of EC:PC:EMC:DEC=1:3:3:3 to form a base solvent, and then vinylene carbonate and 1,3-propane sultone are added to the base solvent, and then lithium salt LiPF6 and lithium salt additive A (if present) are added, dissolved and mixed evenly to form an electrolyte, wherein, based on the mass of the electrolyte, the mass percentage of vinylene carbonate is 1%, the mass percentage of 1,3-propane sultone is 3%, the mass percentage of LiPF6 is 12.5%, and the remainder is EC, PC, EMC, DEC and lithium salt additive A.

[0094] 5. Preparation of diaphragm

[0095] The diaphragm substrate is a 9μm thick polyethylene (PE) porous membrane, and a 2μm thick heat-resistant layer is coated on one side of the diaphragm substrate (wherein, in the heat-resistant layer slurry, based on the total mass of alumina and binder PVDF, the mass percentage of alumina is 95%, and the mass percentage of PVDF is 5%), and then polyvinylidene fluoride (PVDF) slurry is coated on both sides, and the final diaphragm is obtained after drying.

[0096] 6. Preparation of lithium-ion batteries

[0097] The positive electrode, separator, and negative electrode are stacked in order, with the separator positioned between the positive and negative electrodes to provide isolation, and then wound to form an electrode assembly. The electrode assembly is then placed in an aluminum-plastic film outer packaging. After dehydration at 80°C, the aforementioned electrolyte is injected and packaged. The lithium-ion battery is then produced through a series of processes, including standing, formation, capacity testing, degassing, trimming, shaping, and aging.

[0098] 2. Test Method

[0099] 1. Test method for lithium salt content in electrolyte

[0100] Discharge the lithium-ion battery at 0.2C to 3.0V. Repeat this step three times to prevent voltage rebound.

[0101] The lithium-ion battery was disassembled in a dry room (water content <2%), and the bare cells were removed. The cells were quickly placed in a centrifuge tube and centrifuged at 3200 rpm for 20-30 minutes. The centrifuge was then opened, the tube removed, and the electrolyte solution obtained by centrifugation was transferred to a sample injection bottle using a syringe. The sample was injected into a Thermo5 AQUION ion chromatograph (flow rate range: 0.00-5.00 mL / min, maximum pressure: 35 MPa (5000 psi), detection limit ≤ 0.1%; COV < 5%) to determine the mass percentage of different lithium salt species in the total electrolyte.

[0102] 2. Test method for trace doping element content in aluminum foil

[0103] Discharge the lithium-ion battery at 0.2C to 3.0V. Repeat this step three times to prevent voltage rebound.

[0104] The lithium-ion battery was disassembled in a dry room (water content <2%), the positive electrode was removed, and the blank foil (positive electrode current collector without positive electrode active material coating) on ​​the positive electrode was cut. The aluminum foil sample was dissolved in an aqua regia solution composed of concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO3) in a volume ratio of 3:1 to obtain a completely dissolved ion solution. The solution was injected into an ICP plasma emission spectrometer PE 7000DV instrument for analysis, and the mass percentage of different elemental substances in the total substrate was determined.

[0105] 3. Method for judging the positive electrode interface condition after electrochemical device cycling

[0106] (1) Place the lithium-ion battery in a 25-degree environment for 10 minutes.

[0107] (2) Lithium-ion battery charging: 2.6C constant current charging to 4.2V, then constant voltage charging to 2.35C; 2.35C constant current charging to 4.25V, then constant voltage charging to 2C; 2C constant current charging to 4.3V, then constant voltage charging to 1.5C; 1.5C constant current charging to 4.38V, then constant voltage charging to 1.2C; 1.2C constant current charging to 4.45V, then constant voltage charging to 1C; 1C constant current charging to 4.48V, then constant voltage charging to 0.1C.

[0108] (3) Let it stand for 10 minutes.

[0109] (4) Lithium-ion battery discharge: Discharge to 3V at a constant current of 0.2C for the initial cycle and every 50 cycles (1, 51, 101, 151…1001) in the cycle, and discharge to 3V at a constant current of 1C for the remaining cycles.

[0110] (5) Let it stand for 10 minutes.

[0111] (6) Repeat the above steps (2)-(5) 1001 times.

[0112] (7) After the cycle is completed, fully charge the lithium-ion battery according to step (2) again, then disassemble the battery cell and take a photo of the interface of the fully charged positive electrode. At the same time, observe the interface of the front and back of the folded positive electrode with the naked eye, and judge whether the interface is normal according to the following standards:

[0113] Normal interface: The main area of ​​the positive / negative interface at the folded end of the positive electrode is golden yellow, and the area where the positive electrode extends beyond the width of the negative electrode is purple (as shown in Figure 4). The lithium deposition area in the normal interface is less than 2%;

[0114] Abnormal interface: Abnormal purple spots, black spots or lithium deposition appear on the front / back interface of the folded positive electrode tail (as shown in Figure 5). The area of ​​the abnormal interface can be calculated by taking a picture under a polarizing microscope and circling the abnormal interface using image processing software. The total area of ​​the abnormal interface is obtained by adding up the areas of all abnormal interfaces. If the total area of ​​the abnormal interface accounts for no more than 5% and greater than or equal to 2% of the total area of ​​the active material layer on the front and back sides of the folded positive electrode tail, it is a mild abnormality. If the total area of ​​the abnormal interface accounts for more than 5% of the total area of ​​the active material layer on the front and back sides of the folded positive electrode tail, it is a severe abnormality.

[0115] Lithium deposition area = total area of ​​abnormal interface / total area of ​​active material layer on both sides of the positive electrode tail folded in half.

[0116] 3. Test Results

[0117] Table 1 shows the effects of the elongation of the positive electrode current collector in lithium-ion batteries and the content of lithium salt additive A in the electrolyte on the interface of the two folds at the tail of the positive electrode after lithium-ion battery cycling.

[0118] Table 1

[0119] " / " means it is not added or does not exist.

[0120] The contents of various elements in the positive electrode current collector aluminum foil used in the examples and comparative examples in Table 1 are shown in the following table:

[0121] The results show that when the electrolyte does not contain lithium salt additive A or the content of lithium salt additive A in the electrolyte (y%) is less than the elongation of the positive electrode current collector (x), the positive electrode tail folds in two and an abnormal interface appears after lithium-ion battery cycling, posing a serious safety hazard to the lithium-ion battery. When the electrolyte contains 0.1%-3% lithium salt additive A and its content is greater than or equal to the elongation of the positive electrode current collector, the interface of the positive electrode tail folds in two after lithium-ion battery cycling is normal, significantly improving the safety of the lithium-ion battery.

[0122] When the elongation (x) of the positive electrode current collector is less than or equal to 0.8, and particularly less than or equal to 0.4, the interface at the positive electrode tail where the battery folds after cycling can be further improved, thereby further enhancing the safety of the battery. When the content of lithium salt additive A in the electrolyte is between 0.1% and 1.5%, this helps further enhance the safety of the battery.

[0123] When the electrolyte also contains 5%-20% LiPF6, the safety of the lithium-ion battery can be further improved.

[0124] When the content ratio (y / z) of lithium salt additive A to LiPF6 in the electrolyte is 0.01-0.5, the interface of the two folds at the tail of the positive electrode after the lithium-ion battery cycle can be further improved, thereby improving the safety of the lithium-ion battery.

[0125] Table 2 shows the composition of the positive electrode current collector and the interface of the positive electrode tail fold after lithium-ion battery cycling in Example 16. Except for the composition of the aluminum foil, the other conditions of Example 16 were the same as those of Example 4.

[0126] Table 2

[0127] The results show that the improvement effect is particularly obvious when the positive electrode current collector is an aluminum foil containing a doping element (at least one of Mg, Ti, Cr, Cu, Fe, Mn, Ni, Si, V, Zn or Gr) and when the mass percentage of Si element in the aluminum foil is less than or equal to 0.5%.

[0128] In addition, when the first region is arranged at the tail end of the positive electrode current collector and the area of ​​the first region accounts for 15% to 30% of the area of ​​the outermost circle positive electrode current collector, it can not only improve the energy density of the electrochemical device, but also avoid excessive contact between the blank positive electrode current collector and the electrolyte, thereby reducing the risk of corrosion of the positive electrode current collector, thereby further improving the interface of the two folds of the positive electrode tail after the lithium-ion battery cycle, thereby further improving the safety of the lithium-ion battery.

[0129] References throughout this specification to "an embodiment," "part of an embodiment," "one embodiment," "another example," "an example," "a specific example," or "a portion of an example" mean that at least one embodiment or example in this application includes the specific features, structures, materials, or characteristics described in that embodiment or example. Therefore, descriptions appearing throughout this specification, such as, "in some embodiments," "in an embodiment," "in one embodiment," "in another example," "in an example," "in a specific example," or "an example," are not necessarily references to the same embodiment or example in this application. In addition, the specific features, structures, materials, or characteristics described herein may be combined in any suitable manner in one or more embodiments or examples.

[0130] Although illustrative embodiments have been shown and described, those skilled in the art should understand that the above embodiments should not be construed as limitations on the present application, and that changes, substitutions, and modifications may be made to the embodiments without departing from the spirit, principles, and scope of the present application.

Claims

1. An electrochemical device comprising a positive electrode and an electrolyte, wherein: The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer arranged on the surface of the positive electrode current collector, the positive electrode current collector comprises a first region, a second region and a third region, the positive electrode mixture layers are arranged on both surfaces of the first region, the positive electrode mixture layer is arranged on one surface of the second region, and the positive electrode mixture layer is not arranged on both surfaces of the third region; the average width of the first region is a centimeters, the average width of the third region is b centimeters, and the elongation of the positive electrode current collector is x, x=a / b-1; The electrolyte comprises a lithium salt additive A, wherein the lithium salt additive A is selected from at least one of lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorophosphate, lithium difluorobis(oxalatophosphate), lithium bis(fluorosulfonyl imide) and lithium bis(trifluoromethanesulfonyl imide), and the content of the lithium salt additive A is y% based on the mass of the electrolyte, and y is in the range of 0.1 to 3; and Where y ≥ x. The electrochemical device according to claim 1 , wherein x≤0.

8. The electrochemical device according to claim 1 , wherein x≤0.

4. The electrochemical device according to claim 1 , wherein 0.1≤y≤1.

5.

5. The electrochemical device according to claim 1, wherein the positive electrode current collector is an aluminum foil, and the aluminum foil includes at least one of Mg, Ti, Cr, Cu, Fe, Mn, Ni, Si, V, Zn or Gr elements. 6 . The electrochemical device according to claim 5 , wherein the content of aluminum in the positive electrode current collector is greater than or equal to 60%. 7 . The electrochemical device according to claim 5 , wherein the aluminum foil comprises Si element, and the mass percentage of the Si element is less than or equal to 0.5% based on the total mass of the positive electrode current collector.

8. The electrochemical device according to claim 1, wherein the electrolyte further comprises LiPF6, and the content of LiPF6 is z% based on the mass of the electrolyte, and z is 5 to 20. The electrochemical device according to claim 1 , wherein 0.01≤y / z≤0.

5.

10. The electrochemical device according to claim 1, wherein a charge termination voltage of the electrochemical device is 4.41V to 4.5V.

11. The electrochemical device according to claim 1, wherein the third region is disposed at the tail end, and the area of ​​the third region accounts for 15% to 30% of the area of ​​the outermost positive electrode current collector.

12. An electronic device comprising the electrochemical device according to any one of claims 1 to 11.

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