Electrochemical devices and electronic devices

Abstract

Embodiments of this application provide an electrochemical device and an electronic device. The electrochemical device includes a positive electrode and an electrolyte. The positive electrode includes a positive active material layer, which includes a positive active material. The positive active material includes a lithium composite oxide, which contains lithium and at least one element selected from cobalt, nickel, manganese, and aluminum. The surface layer of the positive active material has a rock salt phase structure and includes tungsten. Based on the total mass of the positive active material, the mass percentage of tungsten is B, where 0.01% ≤ B ≤ 1%. The electrolyte includes an ionic liquid of formula R. Based on the total mass of the electrolyte, the mass percentage of cations in the ionic liquid of formula R is A, where 0.05% ≤ A ≤ 5%, and P1 = A / B, where 0.08 ≤ P1 ≤ 300. By employing the electrochemical device of this application, high-temperature storage performance can be significantly improved.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage, specifically to electrochemical devices and electronic devices. Background Technology

[0002] High-nickel cathode active materials can improve the energy density of electrochemical devices and reduce costs. However, with the increase of nickel content, the cathode active material itself will undergo a phase transition during the use of the electrochemical device, resulting in volume changes and structural damage, which will affect the high-temperature storage performance. Therefore, it is desirable to improve the stability of the cathode active material in the electrochemical device and improve the storage performance of the electrochemical device. Summary of the Invention

[0003] This application provides an electrochemical device comprising a positive electrode and an electrolyte. The positive electrode includes a positive active material layer, which in turn includes a positive active material. The positive active material comprises a lithium composite oxide, which contains lithium and at least one element selected from cobalt, nickel, manganese, and aluminum. The surface layer of the positive active material has a rock salt phase structure and includes tungsten. Based on the total mass of the positive active material, the mass percentage of tungsten is B, where 0.01% ≤ B ≤ 1%.

[0004] The electrolyte includes the ionic liquid represented by formula R:

[0005]

[0006] Among them, R 1 Alkyl groups selected from C1 to C8; R 2 To R 6 Each is independently selected from hydrogen and fluorine atoms, anion X - The electrolyte is selected from chloride ions, bromide ions, iodide ions, cyanide ions, hexafluorophosphate ions, perchlorate ions, tetrafluoroborate ions, bis(trifluoromethanesulfonyl)imide ions, and sulfonate ions; based on the total mass of the electrolyte, the mass percentage of cations in the ionic liquid represented by formula R is A, 0.05% ≤ A ≤ 5%, P1 = A / B, 0.08 ≤ P1 ≤ 300.

[0007] In some embodiments, in the positive electrode active material, the mass percentage of nickel to the total mass of metal elements other than lithium is p, where 50% ≤ p < 100%, and P2 = A / p, 0.001 ≤ P2 ≤ 0.08. In some embodiments, 0.01 ≤ P2 ≤ 0.06. In some embodiments, 0.5 ≤ P1 ≤ 50.

[0008] In some embodiments, the electrolyte further includes cyclic esters, including at least one of ethylene carbonate (EC), vinylene carbonate (VC), or fluoroethylene carbonate (FEC), wherein the mass percentage of the cyclic esters is C based on the total mass of the electrolyte, 1% ≤ C ≤ 15%, and P3 = A / C, 0.005 ≤ P3 ≤ 3.

[0009] In some embodiments, the cation of the ionic liquid represented by formula R is selected from the following structures:

[0010]

[0011] In some embodiments, the sulfonate group is selected from the following structures:

[0012]

[0013] In some embodiments, the electrolyte further includes a cyclic sulfur-oxygen double bond compound, which includes at least one of 1,3-propanesulfonate lactone (PS), vinyl sulfate (DTD), or methanedisulfonate methylene ester (MMDS); the mass percentage of the cyclic sulfur-oxygen double bond compound is S based on the total mass of the electrolyte, wherein 1% ≤ S ≤ 10%. In some embodiments, P4 = A / S, 0.01 ≤ P4 ≤ 1.

[0014] Embodiments of this application also provide an electronic device, including the electrochemical device described above.

[0015] This application, by controlling the mass content of tungsten in the positive electrode active material, can control the content of the rock salt phase in the positive electrode active material, thereby enhancing the structural stability of the positive electrode active material. Furthermore, by controlling the mass percentage A of the cations in the ionic liquid (as shown in formula R), the ionic liquid can be fully dissolved in the electrolyte and complexed with the tungsten on the positive electrode surface, improving the high-temperature storage performance of the electrochemical device. Detailed Implementation

[0016] The following embodiments are intended to enable those skilled in the art to fully understand this application, but do not limit this application in any way.

[0017] This application provides an electrochemical device comprising a positive electrode and an electrolyte. In some embodiments, the positive electrode comprises a positive active material layer, which includes a positive active material. In some embodiments, the positive active material comprises a lithium composite oxide, which contains lithium and at least one element selected from cobalt, nickel, manganese, and aluminum. In some embodiments, the surface layer of the positive active material has a rock salt phase structure and includes tungsten. In some embodiments, the mass percentage of tungsten, B, is based on the total mass of the positive active material, and is 0.01% ≤ B ≤ 1%. In some embodiments, B can be 0.01%, 0.1%, 0.5%, 1.0%, or any value between any two of the above ranges. By controlling the mass percentage of tungsten within the above range, the content of the rock salt phase in the positive active material can be controlled, which has a certain beneficial effect on the structural stability of the positive active material. When the B value is higher than 1%, the excessively high rock salt phase content will lead to a decrease in positive electrode capacity; when the B value is lower than 0.01%, the rock salt phase content will be too low, failing to stabilize the structure of the positive active material.

[0018] In some embodiments, the electrolyte comprises an ionic liquid of formula R:

[0019]

[0020] Among them, R 1 Alkyl groups selected from C1 to C8; R 2 To R 6 Each is independently selected from hydrogen and fluorine atoms, anion X - The ionic liquid is selected from chloride ions, bromide ions, iodide ions, cyanide ions, hexafluorophosphate ions, perchlorate ions, tetrafluoroborate ions, bis(trifluoromethanesulfonyl)imide ions, and sulfonate ions. This ionic liquid can participate in the formation of an interfacial film, stabilizing the positive electrode surface structure and preventing damage, while inhibiting the decomposition of the electrolyte on the positive electrode surface, thus significantly improving the high-temperature storage gas generation problem of electrochemical devices. In some embodiments, based on the total mass of the electrolyte, the mass percentage of cations in the ionic liquid represented by formula R is A, 0.05% ≤ A ≤ 5%. In some embodiments, the value of A can be 0.05%, 0.1%, 0.5%, 1%, 3%, 5%, or any value between any two of the above ranges. By controlling the mass percentage A of cations in the ionic liquid represented by formula R within the above range, the ionic liquid can be fully dissolved in the electrolyte and complexed with the tungsten element on the positive electrode surface, improving the high-temperature storage performance of the electrochemical device.

[0021] In some embodiments, the ratio P1 = A / B of the mass percentage A of the ionic liquid of formula R in the electrolyte and the mass percentage B of tungsten in the positive electrode active material satisfies the relationship: 0.08 ≤ P1 ≤ 300. When P1 falls within the above range, the ionic liquid of formula R can stably complex at the surface of the rock salt phase, reducing interfacial reactions caused by surface Li; when P1 is less than 0.08, a complete capping layer cannot be formed, leading to continuous consumption of active Li; when P1 is greater than 300, it leads to increased polarization. In some embodiments, 0.08 ≤ P1 ≤ 300. In some embodiments, 0.5 ≤ P1 ≤ 50.

[0022] In some embodiments, in the positive electrode active material, the mass percentage of nickel in the total mass of metal elements other than lithium is p, where 50% ≤ p < 100%. In some embodiments, the value of p can be 50%, 60%, 70%, 80%, 90%, or any value between any two of the above ranges. By controlling p within the above range, the ionic liquid represented by formula R can stably cover the microcrack surface of the positive electrode active material layer through its own bidentate structure formed by its anions and cations, preventing side reactions between the interface and the electrolyte.

[0023] In some embodiments, P2 = A / p, 0.001 ≤ P2 ≤ 0.08. When the P2 value is within the above range, it effectively improves the exposure of microcracks in the positive electrode active material layer. When P2 is greater than 0.08, it leads to an increase in interfacial impedance. When P2 is less than 0.001, the microcracks cannot be completely covered. In some embodiments, 0.01 ≤ P2 ≤ 0.06.

[0024] In some embodiments, 0.05% ≤ A ≤ 3%. When A is greater than 3%, it is easy to cause the positive electrode impedance to deteriorate. When 0.05% ≤ A ≤ 3%, the increase in positive electrode impedance can be reduced while improving high-temperature storage performance.

[0025] In some embodiments, the electrolyte further includes cyclic esters, including at least one selected from ethylene carbonate, vinylene carbonate, or fluoroethylene carbonate. In some embodiments, the mass percentage of the cyclic ester is C, based on the total mass of the electrolyte, where 1% ≤ C ≤ 15%. In some embodiments, C can be 1%, 5%, 8%, 10%, 15%, or any value between any two of the above ranges. The aforementioned cyclic carbonates have a high dielectric constant and high polarity, which can promote the dissociation of lithium salts, thereby improving conductivity. In some embodiments, P3 = A / C, 0.005 ≤ P3 ≤ 3, and when P3 is within the above range, the kinetic performance of the electrochemical device can be further improved. In some embodiments, 0.005 ≤ P3 ≤ 0.5.

[0026] In some embodiments, the cation of the ionic liquid represented by formula R is selected from the following structures:

[0027]

[0028] In some embodiments, the sulfonate anion is selected from the following structures:

[0029]

[0030] In some embodiments, the electrolyte further includes a cyclic sulfur-oxygen double bond compound, which includes at least one selected from 1,3-propanesulfonate lactone (PS), vinyl sulfate (DTD), or methane disulfonate methylene ester (MMDS). Adding a cyclic sulfur-oxygen double bond compound to the electrolyte can further improve the high-temperature storage performance of the electrochemical device.

[0031] In some embodiments, the mass percentage of the cyclic sulfur-oxygen double bond compound is S, based on the total mass of the electrolyte, where 1% ≤ S ≤ 10%. In some embodiments, P4 = A / S, and 0.01 ≤ P4 ≤ 1. When P4 is within the above range, the high-temperature storage performance of the electrochemical device can be improved. In some embodiments, 0.05 ≤ P4 ≤ 1.

[0032] In some embodiments, the positive electrode sheet further includes a positive current collector, with the positive active material layer located on one or both sides of the positive current collector. In some embodiments, the positive current collector can be aluminum foil, or other positive current collectors commonly used in the art. In some embodiments, the thickness of the positive current collector can be from 1 μm to 50 μm.

[0033] In some embodiments, the positive electrode active material layer may further include a conductive agent and a binder. The conductive agent in the positive electrode active material layer may include at least one of conductive carbon black, sheet graphite, graphene, or carbon nanotubes. In some embodiments, the binder in the positive electrode active material layer may include at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, a styrene-acrylate copolymer, a styrene-butadiene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. In some embodiments, the mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode active material layer is (80-99):(0.1-10):(0.1-10), but this is only an example, and any other suitable mass ratio may be used.

[0034] In some embodiments, the electrochemical device further includes a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative active material layer, the negative active material layer being located on one or both sides of the negative current collector. In some embodiments, the negative active material layer includes a negative active material, which may include graphite, silicon-based materials, etc. In some embodiments, the silicon-based material includes at least one of silicon, silicon-oxygen materials, silicon-carbon materials, or silicon-oxygen-carbon materials. In some embodiments, the negative current collector may be at least one of copper foil, nickel foil, or carbon-based current collector. The negative active material layer may also include a conductive agent, a binder, and a thickener (e.g., sodium carboxymethyl cellulose). In some embodiments, the conductive agent in the negative active material layer may include at least one of conductive carbon black, Ketjen black, sheet graphite, graphene, carbon nanotubes, or carbon fibers. In some embodiments, the binder in the negative active material layer may include at least one of carboxymethyl cellulose (CMC), polyacrylic acid, polyvinylpyrrolidone, polyaniline, polyimide, polyamide-imide, polysiloxane, styrene-butadiene rubber, epoxy resin, polyester resin, polyurethane resin, or polyfluorene. In some embodiments, the mass ratio of the negative electrode active material, thickener, and binder in the negative electrode active material layer can be (97 to 98): (0.2 to 0.6): (1.8 to 2.4). It should be understood that this is merely an example, and any other suitable mass ratio may be used.

[0035] In some embodiments, the electrochemical device further includes a separator, wherein the positive electrode and the negative electrode are separated by the separator disposed therebetween.

[0036] In some embodiments, the separator comprises at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, polyethylene comprises at least one selected from high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene. Polyethylene and polypropylene, in particular, are effective in preventing short circuits and can improve battery stability through a turn-off effect. In some embodiments, the thickness of the separator is in the range of about 3 μm to 20 μm.

[0037] In some embodiments, the surface of the separator may further include a porous layer disposed on at least one surface of the separator. The porous layer comprises inorganic particles and a binder. The inorganic particles are selected from at least one of alumina (Al₂O₃), silicon oxide (SiO₂), magnesium oxide (MgO), titanium oxide (TiO₂), hafnium dioxide (HfO₂), tin oxide (SnO₂), cerium dioxide (CeO₂), nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO₂), yttrium oxide (Y₂O₃), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. In some embodiments, the pores of the separator have a diameter in the range of about 0.01 μm to 1 μm. The binder for the porous layer is selected from at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The porous layer on the surface of the separator can improve the separator's heat resistance, oxidation resistance, and electrolyte wetting properties, and enhance the adhesion between the separator and the electrode.

[0038] In some embodiments, the electrochemical device includes a lithium-ion battery, but this application is not limited thereto. In some embodiments, the electrolyte further includes at least one of a fluoroether, a fluoroethylene carbonate, or an ether nitrile. In some embodiments, the electrolyte further includes a lithium salt, including lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate, wherein the concentration of the lithium salt is from 1 mol / L to 2 mol / L, and the mass ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is from 0.06 to 5. In some embodiments, the electrolyte may also include a non-aqueous solvent. The non-aqueous solvent may be a carbonate compound, a carboxylic acid ester compound, an ether compound, other organic solvents, or a combination thereof.

[0039] Carbonate compounds can be chain carbonate compounds, fluorocarbonate compounds, or combinations thereof.

[0040] Examples of chain carbonate compounds are diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), and combinations thereof. Examples of carboxylic acid ester compounds are methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerate lactone, mevalonate lactone, caprolactone, methyl formate, or combinations thereof.

[0041] Examples of ether compounds are dibutyl ether, tetraethylene dimethyl ether, diethylene dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, or combinations thereof.

[0042] Examples of other organic solvents include dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphate esters or combinations thereof.

[0043] Embodiments of this application also provide electronic devices including the electrochemical devices described above. The electronic devices in the embodiments of this application are not particularly limited and can be any electronic device known in the prior art. In some embodiments, the electronic device may include, but is not limited to, laptops, pen-based 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, drones, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0044] The following are some specific embodiments and comparative examples to better illustrate this application, wherein a lithium-ion battery is used as an example.

[0045] Example 1-1

[0046] Preparation of the negative electrode sheet: Artificial graphite (negative electrode active material), conductive carbon black (Super P), sodium carboxymethyl cellulose (CMC) (thickener), styrene-butadiene rubber (SBR) (binder), and lithium polyacrylate were mixed in a mass ratio of 92:1.5:0.5:1:5. Deionized water was then added as a solvent to prepare a negative electrode slurry with a solid content of 54%. The mixture was stirred under vacuum until a homogeneous negative electrode slurry was formed. Super P (conductive agent) and SBR (binder) were mixed in a mass ratio of 9:1. Deionized water was then added as a solvent to prepare a conductive layer slurry with a solid content of 10%. The conductive layer slurry and the negative electrode slurry were sequentially and uniformly coated onto one surface of an 8 μm thick copper foil current collector for the negative electrode. The coating was then dried at 85°C to obtain a single-sided coated negative electrode with a conductive layer thickness of 2 μm and a negative electrode active material layer thickness of 100 μm. The above steps were then repeated on the other surface of the negative electrode to obtain a double-sided coated negative electrode. After coating, the negative electrode is cold-pressed and cut into 90.7mm×393mm sizes for later use.

[0047] Preparation of the positive electrode: Nickel sulfate (NiSO4), cobalt sulfate (CoSO4), and manganese sulfate (MnSO4) were used as nickel, cobalt, and manganese sources, respectively, and a 3 mol / L mixed solution was prepared at an elemental mass ratio of 0.8:0.1:0.1. A sodium hydroxide solution with a concentration of approximately 3 mol / L was prepared using sodium hydroxide as a precipitant, and an ammonia solution with a concentration of approximately 2 mol / L was prepared using ammonia as a complexing agent. All three solutions were simultaneously added dropwise to a reaction vessel, controlling the pH at approximately 11.5 and the reaction vessel temperature at approximately 50°C. After stirring, the mixture was filtered, washed with water, and dried to obtain a lithium nickel cobalt manganese oxide precursor. The lithium nickel cobalt manganese oxide precursor was mixed and ground with WO3 powder, controlling the amount of WO3 powder to ensure that the mass fraction of tungsten (W) in the positive electrode active material was 0.02%. Then, it was calcined at 700°C in an oxygen atmosphere for 12 h to obtain a lithium nickel cobalt manganese oxide positive electrode active material (Li1Ni) with a rock salt phase surface. 0.8 Co 0.1 Mn 0.1 O2), in which the surface layer is doped with tungsten, and nickel accounts for 80% of the total mass of the metal elements other than lithium.

[0048] The positive electrode active material (Li1Ni) 0.8 Co 0.1 Mn 0.1 O2), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) binder were mixed in a mass ratio of 97:1.4:1.6. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75%. The mixture was stirred under vacuum until a homogeneous positive electrode slurry was formed. The positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil current collector and dried at 85°C to obtain a single-sided coated positive electrode with a 110 μm thick active material layer. The above steps were then repeated on the other surface of the positive electrode to obtain a double-sided coated positive electrode. After coating, the positive electrode was cold-pressed and cut into 89.5 mm × 388 mm dimensions for later use.

[0049] Preparation of the separator membrane: A 5 μm thick polyethylene (PE) porous membrane (provided by Celgard) was used.

[0050] Electrolyte preparation: In an argon-atmospheric glove box with a water content of less than 10 ppm, dimethyl carbonate (DMC) and propylene carbonate (PC) were mixed at a mass ratio of 6:4. Then, lithium salt (LiPF6) at a mass percentage of WI was added to the base solvent, dissolved, and mixed thoroughly. Subsequently, the cationic formula R-5 of the compound represented by general formula (R) at a mass percentage of Wr was added. -The electrolyte was obtained by stirring Rc-1 until homogeneous. Based on the mass of the electrolyte, the mass percentage of lithium salt LiPF6 was 12.5%, the mass percentage of the cation Wr of the compound represented by general formula (R) was 0.5%, and the remainder was the base solvent.

[0051] Preparation of lithium-ion batteries: The positive electrode, separator, and negative electrode prepared above are stacked in sequence, with the separator positioned between the positive and negative electrode to provide isolation. The electrodes are then wound to obtain the electrode assembly. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging shell and dried in an 85°C vacuum oven for 12 hours to remove moisture. The prepared electrolyte is then injected. The lithium-ion battery is obtained through vacuum sealing, settling, formation (0.02C constant current charging to 3.3V), shaping, and capacity testing.

[0052] In Comparative Example 1-1, the difference from Example 1-1 is that the compound shown in Formula R was not added to the electrolyte; otherwise, it is the same as Example 1-1. Furthermore, the relevant preparation and performance parameters for Examples 1-2 to 1-25 and Comparative Examples 1-1 to 1-6 are shown in Table 1; except for the differences shown, they are the same as in Example 1-1.

[0053] In addition, the relevant parameters are measured using the following method in this application.

[0054] High-temperature storage performance test:

[0055] The lithium-ion battery was placed in a constant temperature environment of 25℃ and allowed to stand for 5 minutes to reach a constant temperature. It was then charged at a constant current of 0.2C to 4.3V, followed by constant voltage charging to a current of 0.05C. The thickness of the lithium-ion battery was recorded as the initial thickness. The lithium-ion battery was then transferred to an 80℃ high-temperature furnace for storage for 24 hours, and the thickness after 24 hours was recorded as the storage thickness. The thickness expansion rate of the lithium-ion battery was calculated and used as an indicator to evaluate the gas production during high-temperature storage.

[0056] Thickness expansion rate = (storage thickness - initial thickness) / initial thickness × 100%.

[0057] Table 1 shows the parameters and evaluation results.

[0058] Table 1

[0059]

[0060]

[0061] Note: " / " in Table 1 indicates that this component is not present.

[0062] Comparing Examples 1-1 to 1-25 and Comparative Example 1-1, it is evident that adding an ionic liquid of formula R to the electrolyte, with 0.05% ≤ A ≤ 5% and 0.08 ≤ P1 ≤ 300, can significantly reduce the high-temperature storage expansion rate of lithium-ion batteries. Comparing Examples 1-1 to 1-25 and Comparative Example 1-2, it is evident that doping the surface layer of the positive electrode active material with W element, with 0.01% ≤ B ≤ 1%, can significantly reduce the high-temperature storage expansion rate of lithium-ion batteries. Comparing Examples 1-1 to 1-6 and Comparative Examples 1-3 to 1-4, it is evident that when 0.05% ≤ A ≤ 5%, the high-temperature storage expansion rate of lithium-ion batteries can be significantly reduced. Comparing Examples 1-1 to 1-11, Examples 1-19 to 1-20 and Comparative Examples 1-5 to 1-6, it is evident that when 0.08 ≤ P1 ≤ 300, the high-temperature storage expansion rate of lithium-ion batteries can be significantly reduced. When evaluating the overall performance of lithium-ion batteries, it is important to focus on their high-temperature storage performance. Generally speaking, a thickness expansion rate greater than 10% during high-temperature storage is unacceptable. Therefore, the high-temperature storage expansion rates of Comparative Examples 1-1 to 1-6 are unacceptable.

[0063] Comparing Examples 1-1 to 1-6, it can be seen that as A increases, the high-temperature storage expansion rate of the lithium-ion battery first decreases, then increases, then decreases again, then increases again, and then decreases again. Comparing Examples 1-7 to 1-11, it can be seen that as B increases, the high-temperature storage expansion rate of the lithium-ion battery first decreases, then increases, and then decreases again.

[0064] As can be seen from Examples 1-3 and Examples 1-12 to 1-18, the high-temperature storage expansion rate of lithium-ion batteries varies with the compound represented by general formula R. Similarly, as can be seen from Examples 1-21 to 1-25, the high-temperature storage expansion rate of lithium-ion batteries varies with the compound represented by general formula R.

[0065] Table 2 shows the relevant preparation and performance parameters. In Examples 2-1 to 2-13, the Ni content was further adjusted based on Example 1-1. Except for the differences shown in Table 2, the other parameters were the same as in Example 1-1.

[0066] Table 2

[0067]

[0068]

[0069] Note: " / " in Table 2 indicates that this component is not present.

[0070] By comparing Examples 1-1, 2-1 to 2-9, it can be seen that when the mass percentage p of nickel in the total mass of metal elements other than lithium satisfies 50% ≤ p < 100%, the lithium-ion battery has a better high-temperature storage expansion rate.

[0071] By comparing Examples 2-1 to 2-10 and Examples 2-12 to 2-13, it can be seen that when P2 satisfies 0.001≤P2≤0.08, the high-temperature storage performance of lithium-ion batteries can be significantly improved.

[0072] Table 3 shows the relevant preparation and performance parameters. In Examples 3-1 to 3-20, cyclic esters were further added based on Examples 1-1. Except for the differences shown in Table 3, the other parameters were the same as in Example 1-1.

[0073] Table 3

[0074]

[0075]

[0076] Comparisons of Examples 3-1 to 3-20 show that adding an appropriate amount of cyclic ester to the electrolyte, ensuring P3 satisfies 0.005 ≤ P3 ≤ 3, significantly improves the high-temperature storage performance of lithium-ion batteries. Comparisons of Examples 3-1 to 3-6 show that as the amount of added cyclic ester increases, the high-temperature storage expansion rate of the lithium-ion battery first decreases and then increases. Comparisons of Examples 3-5, 3-11, and 3-13 show that adding combinations of different types of cyclic esters further improves the high-temperature storage expansion rate of lithium-ion batteries.

[0077] Table 4 shows the relevant preparation and performance parameters. In Examples 4-1 to 4-27, cyclic esters and / or cyclic sulfur-oxygen double bond compounds were added to the basis of Example 1-1. Except for the differences shown in Table 4, the other parameters were the same as in Example 1-1.

[0078] Table 4

[0079]

[0080]

[0081]

[0082] Comparisons of Examples 1-1 and 4-1 to 4-14 show that adding an appropriate amount of cyclic sulfur-oxygen double bond compound to the electrolyte can significantly improve the high-temperature storage performance of lithium-ion batteries. Comparisons of Examples 4-1 to 4-3 and 4-26 to 4-27 show that by ensuring P4 satisfies 0.01 ≤ P4 ≤ 1, the high-temperature storage performance of lithium-ion batteries is even better. Comparisons of Examples 4-10 and 4-15 to 4-25 show that when cyclic esters and cyclic sulfur-oxygen double bond compounds are used in combination, the high-temperature storage performance of lithium-ion batteries is better than that of using either cyclic ester or cyclic sulfur-oxygen double bond compound alone.

[0083] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of disclosure in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by any combination of the above-described technical features or their equivalents. For example, technical solutions formed by substituting the above-described features with technical features having similar functions disclosed in this application.

Claims

1. An electrochemical device comprising: A positive electrode sheet and an electrolyte, wherein the positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, the positive electrode active material includes a lithium composite oxide, the lithium composite oxide contains lithium and at least one element selected from cobalt, nickel, manganese and aluminum, the surface layer of the positive electrode active material has a rock salt phase structure and includes tungsten; based on the total mass of the positive electrode active material, the mass percentage of tungsten is B, 0.01% ≤ B ≤ 1%; The electrolyte includes an ionic liquid represented by formula R: Among them, R 1 Alkyl groups selected from C1 to C8; R 2 To R 6 Each is independently selected from hydrogen and fluorine atoms, anion X - Selected from chloride ions, bromide ions, iodide ions, cyanide ions, hexafluorophosphate ions, perchlorate ions, tetrafluoroborate ions, bis(trifluoromethanesulfonyl)imide ions, and sulfonate ions; Based on the total mass of the electrolyte, the mass percentage of cations in the ionic liquid represented by formula R is A, 0.05% ≤ A ≤ 5%, P1 = A / B, 0.08 ≤ P1 ≤ 300.

2. The electrochemical device according to claim 1, wherein, In the positive electrode active material, the mass percentage of nickel in the total mass of metal elements other than lithium is p, where 50% ≤ p < 100%, and P2 = A / p, where 0.001 ≤ P2 ≤ 0.

08.

3. The electrochemical device according to claim 1, wherein, 0.01≤P2≤0.06。 4. The electrochemical device according to claim 1, wherein, 0.5≤P1≤50。 5. The electrochemical device according to claim 1, wherein, The electrolyte also includes cyclic esters, which include at least one of ethylene carbonate, vinylene carbonate, or fluoroethylene carbonate. Based on the total mass of the electrolyte, the mass percentage of the cyclic ester is C, where 1% ≤ C ≤ 15%, and P3 = A / C, where 0.005 ≤ P3 ≤ 3.

6. The electrochemical device according to claim 1, wherein, The cation of the ionic liquid represented by formula R is selected from the following structures:

7. The electrochemical device according to claim 1, wherein, The sulfonate group is selected from the following structures:

8. The electrochemical device according to claim 1, wherein, The electrolyte also includes cyclic sulfur-oxygen double bond compounds, which include at least one of 1,3-propanesulfonate lactone, vinyl sulfate, or methane disulfonate. Based on the total mass of the electrolyte, the mass percentage of the cyclic sulfur-oxygen double bond compound is S, where 1% ≤ S ≤ 10%.

9. The electrochemical device according to claim 8, wherein, P4 = A / S, 0.01 ≤ P4 ≤ 1.

10. An electronic device comprising an electrochemical device according to any one of claims 1 to 9.

Images