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, a negative electrode, a separator, and an electrolyte. The negative electrode includes a negative active material layer comprising a negative active material, wherein the mass of the negative active material is ag. The separator is disposed between the positive and negative electrode. The electrolyte includes ethylene carbonate, and based on the mass of the electrolyte, the mass content of ethylene carbonate is b%, with a b / a ratio of 1.6 to 6.4. By controlling the mass percentage of ethylene carbonate in the electrolyte to the mass of the negative active material, the cycle performance, thickness expansion rate during high-temperature storage, capacity retention rate after storage, and safety performance of the electrochemical device can be improved.

Description

[0001] This application is a divisional application of the invention patent with application number 202111011717.2 filed on August 31, 2021, entitled "Electrochemical Device and Electronic Device". Technical Field

[0002] This application relates to the field of electrochemical energy storage, and more particularly to electrochemical devices and electronic devices. Background Technology

[0003] Currently, electrochemical devices (such as lithium-ion batteries) are widely used in electric vehicles, consumer electronics, and energy storage devices. With their advantages such as high energy density and no memory effect, lithium-ion batteries have gradually become the mainstream batteries in these fields. However, some applications involve harsher environments or industrial settings. Therefore, improving the high-temperature storage and cycle performance, charging performance, and safety performance of lithium-ion batteries while maintaining their high energy density is a pressing issue for the industry. Summary of the Invention

[0004] Embodiments of this application provide an electrochemical device comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode includes a negative active material layer comprising a negative active material, wherein the mass of the negative active material is ag. The separator is disposed between the positive and negative electrode. The electrolyte comprises ethylene carbonate, and based on the mass of the electrolyte, the mass content of ethylene carbonate is b%, with a b / a ratio of 1.6 to 6.4.

[0005] In some embodiments of this application, b is 1 to 25.

[0006] In some embodiments of this application, the electrolyte further includes at least one of vinylene carbonate or fluoroethylene carbonate, wherein the mass content of vinylene carbonate is m% and the mass content of fluoroethylene carbonate is n% based on the mass of the electrolyte, m+n=c, and 0.01≤c<2.

[0007] In some embodiments of this application, the electrolyte further includes at least one of vinylene carbonate or fluoroethylene carbonate, wherein the mass content of vinylene carbonate is m% and the mass content of fluoroethylene carbonate is n% based on the mass of the electrolyte. In some embodiments, 0 ≤ m < 2; in some embodiments, 0 ≤ n < 2.

[0008] In some embodiments of this application, c / a is 0.001 to 0.36.

[0009] In some embodiments of this application, based on the mass of the electrolyte, m < n.

[0010] In some embodiments of this application, the electrolyte further includes compounds containing sulfur-oxygen double bonds, including at least one of 1,3-propanesulfonyl lactone (PS), 1,4-butanesulfonyl lactone (BS), vinyl sulfate (DTD), methylene disulfonate (MMDS), 1,3-propane disulfonic anhydride (PA), 2-methylbutanesulfonyl lactone, or propenyl-1,3-sulfonyl lactone (PES).

[0011] In some embodiments of this application, based on the mass of the electrolyte, the mass content of the compound containing sulfur-oxygen double bonds is d%, and the positive electrode includes a positive active material layer, the positive active material layer includes a positive active material, the mass of the positive active material is eg, and d / e is 0.1 to 0.6.

[0012] In some embodiments of this application, the positive electrode active material contains cobalt.

[0013] In some embodiments of this application, the electrolyte further includes a lithium salt, which includes at least one of lithium difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium tetrafluoroborate (LiBF4), lithium tetraborate (B4Li2O7), lithium borate (Li3BO3), or lithium trifluoromethanesulfonate (CF3LiO3S). The mass content of the lithium salt is from 0.01% to 3% based on the mass of the electrolyte.

[0014] In some embodiments of this application, the electrolyte further includes a polynitrile compound, which includes at least one selected from succinic anionyl (SN), adiponitrile (ADN), 1,2-bis(cyanoethoxy)ethane (DENE), 1,4-dicyano-2-butene, 1,3,6-hexanetrionitrile (HTCN), or 1,2,3-tris(2-cyanoethoxy)propane, and the mass content of the polynitrile compound is from 0.01% to 6% based on the mass of the electrolyte.

[0015] Some embodiments of this application provide an electronic device, including the electrochemical device described above.

[0016] In the embodiments of this application, by controlling the ratio of the mass percentage of ethylene carbonate in the electrolyte to the mass of the negative electrode active material, the cycle performance, thickness expansion rate during high-temperature storage, capacity retention rate after storage, and safety performance of the electrochemical device can be improved. Detailed Implementation

[0017] 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.

[0018] To maintain good high-temperature performance of electrochemical devices, it is necessary to use solvents or additives with good stability in the electrolyte to form a protective effect at the electrode interface and suppress side reactions between the electrolyte and the active material. However, different solvents and additives interact with the active material in different ways, and the content relationship between the solvent or additive and the electrode active material is one of the important parameters affecting the above-mentioned effect. When the electrolyte components cannot form a good protective interface on the electrode surface, it will affect the charge-discharge performance and storage performance of the electrochemical device to varying degrees, thereby affecting the normal operation of the electrochemical device and bringing safety hazards such as lithium plating during charging. The embodiments of this application improve the overall performance of the electrochemical device by better matching the electrolyte solvent and additive with the positive and negative electrode active materials, while maintaining the high energy density of the electrochemical device. This results in good performance in high-temperature storage, cycle performance, charging performance, and safety performance.

[0019] Embodiments of this application provide an electrochemical device comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the separator is disposed between the positive and negative electrode. In some embodiments, the negative electrode may include a negative active material layer comprising a negative active material, wherein the mass of the negative active material is ag. In some embodiments, the negative active material may include at least one of graphite, hard carbon, silicon, silicon suboxide, or organosilicon.

[0020] In some embodiments, the electrolyte may include ethylene carbonate (EC). In some embodiments, the mass content of ethylene carbonate is b%, and the b / a ratio is 1.6 to 6.4, based on the mass of the electrolyte. In some embodiments, as the b / a ratio increases, the overall performance of the electrochemical device, such as cycle performance, high-temperature storage performance at different states of charge (SOC), capacity retention after storage, and safety performance, is significantly improved. This may be because, within a certain range of b / a, EC provides good protection for the negative electrode interface and has a good passivation effect on the lithium plating interface of the negative electrode. When the b / a ratio is too small, the electrolyte (EC) cannot protect the negative electrode interface, thus adversely affecting the cycle performance of the electrochemical device under both room temperature and high temperature conditions. Furthermore, due to poor passivation of the lithium plating interface at the negative electrode, the storage and safety performance of the electrochemical device under low charge conditions is affected. Conversely, when b / a > 6.4, the cycle performance of the electrochemical device cannot be further improved, and the storage and safety performance under high charge conditions are affected. This may be because when b / a is too large, the interfacial kinetics between the electrolyte and the negative electrode deteriorate, causing lithium plating during charging. This leads to violent reactions during high-temperature storage, triggering thermal failure of the electrochemical device. In some embodiments, b / a can be 2 to 6, 2.5 to 5.5, 3 to 5, or 3.5 to 4.5, etc.

[0021] In some embodiments, the negative electrode sheet may further include a negative current collector, which may be at least one of copper foil, nickel foil, or carbon-based current collector. In some embodiments, the negative electrode active material layer may be located on one or both sides of the negative electrode current collector. In some embodiments, the negative electrode active material layer may further include a conductive agent and a binder. In some embodiments, the conductive agent in the negative electrode 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 electrode 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, conductive agent, and binder in the negative electrode active material layer may be (78–98.5):(0.1–10):(0.1–10). It should be understood that the above description is merely an example, and any other suitable materials and mass ratios may be used.

[0022] In some embodiments, b is 1 to 25, that is, the mass content of ethylene carbonate (EC) is 1% to 25% based on the mass of the electrolyte. When the mass content of EC is too low, the protective effect of EC on the negative electrode interface and the passivation effect on the negative electrode lithium plating interface are relatively limited; while when the mass content of EC is too high, side reactions occur in the electrolyte during the cycling of the electrochemical device, resulting in electrolyte consumption and expansion of the electrochemical device. In some embodiments, b can be 5 to 20, 8 to 18, 10 to 15, or 13, etc.

[0023] In some embodiments, 'a' is less than or equal to 15, meaning the mass of the negative electrode active material is less than or equal to 15g. Since the EC and negative electrode active material can only achieve a synergistic effect and better interfacial performance when the b / a ratio is in the range of 1.6 to 6.4, excessively high EC concentrations can lead to side reactions in the electrolyte during cycling if the mass of the negative electrode active material is too large. In some embodiments, 'a' can be 1 to 15, 2 to 14, 3 to 12, or 4 to 10, etc.

[0024] In some embodiments, the electrolyte may further include at least one of vinylene carbonate (VC) or fluoroethylene carbonate (FEC). In some embodiments, the mass content of vinylene carbonate is m%, where 0 ≤ m < 2, and the mass content of fluoroethylene carbonate is n%, where 0 ≤ n < 2, m + n = c, and 0.01 ≤ c < 2, based on the mass of the electrolyte. VC and / or FEC can effectively repair the solid electrolyte membrane at the electrode interface damaged during cycling and storage, thereby suppressing side reactions of the electrolyte at the electrode interface and inhibiting lithium deposition at the negative electrode interface. However, if the content of VC and FEC is too high, the negative electrode interface impedance will be too large, resulting in excessive residues after formation, which can easily lead to oxidation and gas generation at the positive electrode interface, thus affecting the charging performance after storage and also causing storage gas generation and safety hazards. In some embodiments, c can be 0.05 to 1.8, 0.1 to 1.5, 0.2 to 1.3, 0.3 to 1, or 0.5 to 0.8, etc.

[0025] In some embodiments, c / a is 0.001 to 0.36. While c / a is 0.001 to 0.36, if the ratio c / a is too high, the relative content of VC and / or FEC is too high, affecting the negative electrode interface performance and consequently impacting post-storage charging performance, also leading to gas generation and safety hazards during storage. In some embodiments, c / a can be 0.05 to 0.3, 0.1 to 0.25, or 0.15 to 0.2, etc.

[0026] In some embodiments, based on the foregoing, when m < n, the electrolyte forms a low-impedance interfacial composite protective film on the electrode surface during formation and cycling, which improves the cycling performance of the electrochemical device to a certain extent, and the storage characteristics of the electrochemical device under high temperature conditions are also significantly improved.

[0027] In some embodiments, the electrolyte may further comprise a compound containing a sulfur-oxygen double bond, such as a sulfonate ester. In some embodiments, the compound containing a sulfur-oxygen double bond may include at least one of 1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, vinyl sulfate, methylene disulfonate, 1,3-propane disulfonic anhydride, 2-methylbutanesulfonyl lactone, or propenyl-1,3-sulfonyl lactone.

[0028] In some embodiments, the mass content of the sulfur-oxygen double bond-containing compound is d%, based on the mass of the electrolyte. In some embodiments, the positive electrode includes a positive active material layer, which includes positive active material, the mass of which is eg, and the d / e ratio is 0.1 to 0.6. Based on the condition that EC and negative active material satisfy b / a of 1.6 to 6.4, when the d / e ratio is too small, the effect of the sulfur-oxygen double bond-containing compound in stabilizing the CEI and SEI films is relatively limited, and its improvement on various performance aspects of the lithium-ion battery is not significant; when the d / e ratio is too large, it has a significant impact on the storage and safety performance of the electrochemical device at low SOC. In some embodiments, d / e can be 0.2 to 0.5 or 0.3 to 0.4, etc.

[0029] In some embodiments, the positive electrode active material contains cobalt. In some embodiments, the positive electrode active material may include at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium nickel manganese oxide, or lithium iron phosphate. In some embodiments, the Dv50 and Dv90 of the positive electrode active material satisfy: 0.4 micrometers ≤ D V 50 ≤ 20 micrometers, 2 micrometers ≤ D V90 ≤ 40 micrometers; where Dv50 and Dv90 refer to the particle size corresponding to 50% and 90% of the volume distribution from left to right, respectively. In some embodiments, the positive electrode active material includes element A, which includes at least one of Mg, Ti, Cr, B, Fe, Zr, Y, Na, or S, and the mass content of element A is less than 0.5% based on the mass of the positive electrode active material. In some embodiments, the positive electrode sheet may also include a positive electrode current collector, which may be Al foil, or other current collectors commonly used in the art. In some embodiments, the thickness of the positive electrode current collector may be from 1 μm to 200 μm. In some embodiments, the positive electrode active material layer may be located on one or both sides of the positive electrode current collector. In some embodiments, the positive electrode active material layer may include a conductive agent and a binder. In some embodiments, the conductive agent in the positive electrode 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 positive electrode 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 positive electrode active material, conductive agent, and binder in the positive electrode active material layer may be (78–99):(0.1–10):(0.1–10). In some embodiments, the thickness of the positive electrode active material layer may be from 10 μm to 200 μm. It should be understood that the above description is merely an example, and the positive electrode active material layer may use any other suitable material, thickness, and mass ratio.

[0030] In some embodiments, the electrolyte further includes a lithium salt, which includes at least one of lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium tetraborate, lithium borate, or lithium trifluoromethanesulfonate. Adding lithium salts to the electrolyte can significantly improve the cycle performance, high-temperature storage performance, and safety performance of the electrochemical device. This is because lithium salts form low-resistivity SEI and CEI films on the surfaces of the positive and negative electrodes, improving charging performance and reducing side reactions caused by lithium plating. Simultaneously, the stable protective film can effectively reduce side reactions between the electrolyte and the positive and negative electrodes during storage and cycling, while also reducing the consumption of active lithium and improving capacity retention.

[0031] In some embodiments, the lithium salt content is from 0.01% to 3% by mass, based on the mass of the electrolyte. Too low a lithium salt content limits its effect on improving charging performance; while too high a lithium salt content results in excessively high electrolyte viscosity, affecting the kinetic performance of the electrochemical device. In some embodiments, the lithium salt content can be from 0.05% to 2.5%, 0.1% to 2%, 0.5% to 1.8%, or 1% to 1.5%, etc., based on the mass of the electrolyte.

[0032] In some embodiments, the electrolyte may further include a polynitrile compound, including at least one selected from succinic anhydride, adiponitrile, 1,2-bis(cyanoethoxy)ethane, 1,4-dicyano-2-butene, 1,3,6-hexanetrionitrile, or 1,2,3-tris(2-cyanoethoxy)propane. Adding a polynitrile compound to the electrolyte can significantly improve the high-temperature storage performance and safety performance of the electrochemical device. This is because the polynitrile compound can effectively complex with the positive electrode active material, reducing the oxidation activity of the positive electrode active material, reducing side reactions, and simultaneously inhibiting the deposition of the positive electrode transition metal onto the negative electrode after dissolution, thus preventing damage to the SEI film, thereby reducing gas production and improving capacity retention. Furthermore, when EC is used in combination with a polynitrile compound, EC can inhibit the decomposition of the polynitrile compound at the negative electrode interface, reducing the increase in negative electrode impedance, allowing more polynitrile compound to effectively act on the positive electrode; while the passivation of the positive electrode by the polynitrile compound can reduce the oxidation of EC at the positive electrode interface, allowing more EC to be used for the protection of the negative electrode interface, thereby maximizing the advantages of both substances and achieving optimal electrical performance.

[0033] In some embodiments, the mass content of the polynitrile compound is 0.01% to 6% based on the mass of the electrolyte. When the mass content of the polynitrile compound is too low, its improving effect is relatively limited; while when the mass content is too high, its improving effect no longer increases further, and it is also detrimental to improving the energy density of the electrochemical device. In some embodiments, the mass content of the polynitrile compound can be 0.05% to 5%, 0.1% to 4%, 0.5% to 3%, or 1% to 2%, etc., based on the mass of the electrolyte.

[0034] In some embodiments, the electrolyte may further include a PO bond-containing compound, which comprises at least one of tris(trimethylsilane) phosphate (TMSP), tris(trimethylsilyl) phosphite (TTSPi), triallyl phosphate, triargyl phosphate, triallyl phosphite, or triargyl phosphite.

[0035] 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.

[0036] Carbonate compounds can be chain carbonate compounds, cyclic carbonate compounds, fluorocarbonate compounds, or combinations thereof.

[0037] 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 cyclic carbonate compounds are ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), vinyl ethylene carbonate (VEC), or combinations thereof. Examples of fluorinated carbonate compounds are fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or combinations thereof.

[0038] 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, mevalonic acid lactone, caprolactone, methyl formate, or combinations thereof.

[0039] 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.

[0040] 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.

[0041] In some embodiments, the separator may include at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, polyethylene includes 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 5 μm to 500 μm.

[0042] In some embodiments, the surface of the separator may further include a porous layer disposed on at least one surface of the substrate 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.

[0043] In some embodiments of this application, the electrode assembly of the electrochemical device is a wound electrode assembly, a stacked electrode assembly, or a folded electrode assembly. In some embodiments, the positive electrode and / or negative electrode of the electrochemical device can be a multilayer structure formed by winding or stacking, or a single-layer structure consisting of a single-layer positive electrode, a separator, and a single-layer negative electrode stacked together. In some embodiments, the electrochemical device includes a lithium-ion battery, but this application is not limited thereto.

[0044] In some embodiments of this application, taking a lithium-ion battery as an example, the positive electrode, separator, and negative electrode are sequentially wound or stacked into an electrode assembly, then encapsulated in, for example, an aluminum-plastic film, and injected with electrolyte. The assembly is then formed and encapsulated to produce a lithium-ion battery. The prepared lithium-ion battery is then subjected to performance testing.

[0045] Those skilled in the art will understand that the methods for preparing the electrochemical devices (e.g., lithium-ion batteries) described above are merely examples. Other methods commonly used in the art can be employed without departing from the disclosure of this application.

[0046] Embodiments of this application also provide electronic devices including the aforementioned electrochemical apparatus. The electronic devices in these embodiments 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, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0047] The following are some specific embodiments and comparative examples to better illustrate this application, wherein a lithium-ion battery is used as an example.

[0048] Example 1-1

[0049] Preparation of the positive electrode: A 10μm aluminum foil was used as the positive electrode current collector, and the positive electrode active material LiNi was... 0.5 Co 0.2 Mn 0.3 O2, conductive carbon black (conductive agent), and polyvinylidene fluoride (PVDF) (binder) are dissolved in an N-methylpyrrolidone (NMP) solution at a mass ratio of 97:1.4:1.6 to form a positive electrode slurry. This slurry is then coated onto both sides of the positive electrode current collector to a thickness of 80 μm on each side, resulting in the positive electrode active material layer. After drying at 85°C, cold pressing, and cutting, the positive electrode sheet is obtained.

[0050] Preparation of the negative electrode sheet: Artificial graphite, conductive carbon black (conductive agent), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) (binder) were dissolved in deionized water at a mass ratio of 96.4:1.5:0.5:1.6 to form a negative electrode slurry with a solid content of 54 wt%. An 8 μm copper foil was used as the negative electrode current collector. The negative electrode slurry was coated onto both sides of the current collector with a thickness of 50 μm on each side, resulting in a negative electrode active material layer. After drying at 85℃, the layer was cut to obtain the negative electrode sheet.

[0051] Preparation of the separator membrane: The separator membrane is made of 7μm thick polyethylene (PE).

[0052] Electrolyte preparation: Under conditions with a water content of less than 10 ppm, LiPF6 was added to a non-aqueous organic solvent (ethylene carbonate (EC): propylene carbonate (PC): ethyl methyl carbonate (EMC): diethyl carbonate (DEC) = 10:30:30:30, mass ratio), with a LiPF6 concentration of 1 mol / L. The mixture was thoroughly mixed to obtain the basic electrolyte. The types and amounts of other additives are shown in the table below, and the content of each substance is a mass percentage calculated based on the mass of the electrolyte.

[0053] Preparation of lithium-ion batteries: Positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. The electrodes are then wound to form an electrode assembly. The electrode assembly is placed in an outer aluminum-plastic film package, and after dehydration at 80°C, the electrolyte is injected and the battery is sealed. Following formation, degassing, and edge trimming processes, a lithium-ion battery is obtained.

[0054] The remaining embodiments and comparative examples are based on Examples 1-1 with parameter changes. The mass of the negative electrode active material and the mass of the positive electrode active material are adjusted by the length of the wound electrode sheet. In the embodiments and comparative examples provided in this application, the mass of the negative electrode active material / mass of the positive electrode active material = 0.55, and the electrolyte mass / mass of the positive electrode active material = 0.7. Furthermore, the content of EC and additives in the electrolyte may differ from that in Example 1. The remaining portion of the electrolyte, excluding EC and additives, consists of the same mass of PC, EMC, and DEC. For specific EC and additive contents, please refer to the table below and related descriptions.

[0055] The test methods for each parameter of this application are described below.

[0056] (1) Cyclic performance test of lithium-ion batteries

[0057] The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 30 minutes to allow it to reach a constant temperature. The battery was then charged at a constant current of 1C to a voltage of 4.2V, followed by constant voltage charging at 4.2V to a current of 0.05C, and finally constant current discharging at 1C to a voltage of 2.8V. This constitutes one charge-discharge cycle. The initial discharge capacity was taken as 100%, and the charge-discharge cycles were repeated until the discharge capacity decreased to 80%. The test was then stopped, and the number of cycles was recorded as an indicator of the lithium-ion battery's cycle performance.

[0058] Meanwhile, the cycle performance of lithium-ion batteries at 45°C was tested. Except for the temperature, the test method was the same as the cycle performance test at 25°C.

[0059] (2) High-temperature storage performance test of lithium-ion batteries (60℃, storage for 1680h)

[0060] A: 100% SOC storage

[0061] The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 30 minutes to allow it to reach a constant temperature. It was then charged at a constant current of 1C to 4.2V, and then charged at a constant voltage to a current of 0.05C. It was then discharged at a constant current of 1C to 2.8V, and the discharge capacity was recorded as the initial capacity of the lithium-ion battery. Afterwards, it was charged at a constant current of 0.5C to 4.2V, and then charged at a constant voltage to a current of 0.05C. The thickness of the lithium-ion battery was measured and recorded using a micrometer. The tested lithium-ion battery was then transferred to a 60°C constant temperature chamber for storage for 1680 hours. During this period, the lithium-ion battery was removed every 168 hours, and its thickness (THK), open circuit voltage (OCV), and impedance (IMP) were measured. The lithium-ion battery was then transferred to a 25°C constant temperature chamber, left to stand for 60 minutes, and then discharged at a constant current of 0.5C to 2.8V. The discharge capacity was recorded as the residual capacity of the lithium-ion battery. The lithium-ion battery was charged at a constant current of 0.5C to 4.2V, then charged at a constant voltage to a current of 0.05C, and finally discharged at a constant current of 0.5C to 2.8V. The discharge capacity was recorded as the recoverable discharge capacity of the lithium-ion battery. The thickness expansion rate of the lithium-ion battery during storage was calculated and used as an indicator to evaluate the gas production of the lithium-ion battery during high-temperature storage. The residual capacity retention rate and recoverable capacity retention rate of the lithium-ion battery after storage were also calculated. The number of lithium-ion batteries that smoked or burned during storage was also recorded.

[0062] Thickness expansion rate = (Thickness after storage - Initial thickness) / Initial thickness × 100%

[0063] Recoverable capacity retention rate = (Recoverable discharge capacity after storage / Initial discharge capacity) x 100%

[0064] B: 50% SOC storage

[0065] The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 30 minutes to allow it to reach a constant temperature. It was then charged at a constant current of 1C to 4.2V, and then charged at a constant voltage to a current of 0.05C. It was then discharged at a constant current of 1C to 2.8V, and the discharge capacity was recorded as the initial capacity of the lithium-ion battery. Afterwards, it was charged at a constant current of 0.5C to 3.65V, and then charged at a constant voltage to a current of 0.05C (50% SOC). The thickness of the lithium-ion battery was measured and recorded using a micrometer. The tested lithium-ion battery was then transferred to a 60°C constant temperature chamber for storage for 1680 hours. During this period, the lithium-ion battery was removed every 168 hours, and its thickness (THK), open circuit voltage (OCV), and impedance (IMP) were measured. The lithium-ion battery was then transferred to a 25°C constant temperature chamber, left to stand for 60 minutes, and then discharged at a constant current of 0.5C to 2.8V. The discharge capacity was recorded as the residual capacity of the lithium-ion battery. The lithium-ion battery was charged at a constant current of 0.5C to 4.2V, then charged at a constant voltage to a current of 0.05C, and finally discharged at a constant current of 0.5C to 2.8V. The discharge capacity was recorded as the recoverable discharge capacity of the lithium-ion battery. The thickness expansion rate of the lithium-ion battery during storage was calculated and used as an indicator to evaluate the gas production of the lithium-ion battery during high-temperature storage. The residual capacity retention rate and recoverable capacity retention rate of the lithium-ion battery after storage were also calculated. The number of lithium-ion batteries that smoked or burned during storage was also recorded.

[0066] Thickness expansion rate = (Thickness after storage - Initial thickness) / Initial thickness × 100%

[0067] C:0% SOC Storage

[0068] The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 30 minutes to allow it to reach a constant temperature. It was then charged at a constant current of 1C to 4.2V, and then charged at a constant voltage to a current of 0.05C. Finally, it was discharged at a constant current of 1C to 2.8V, and the discharge capacity was recorded as the initial capacity of the lithium-ion battery. The thickness of the lithium-ion battery was measured and recorded using a micrometer. The test lithium-ion battery was then transferred to a 60°C constant temperature chamber for storage for 1680 hours. During this period, the lithium-ion battery was removed every 168 hours, and its thickness (THK), open circuit voltage (OCV), and impedance (IMP) were measured. The lithium-ion battery was then transferred back to a 25°C constant temperature chamber and left to stand for 60 minutes. It was then charged at a constant current of 0.5C to 4.2V, and then charged at a constant voltage to a current of 0.05C. Finally, it was discharged at a constant current of 0.5C to 2.8V, and the discharge capacity was recorded as the recoverable discharge capacity of the lithium-ion battery. The thickness expansion rate of lithium-ion batteries during storage was calculated and used as an indicator to evaluate the gas production of lithium-ion batteries during high-temperature storage; the recoverable capacity retention rate of lithium-ion batteries after storage was also calculated. The number of lithium-ion batteries that emitted smoke or burned during storage was also recorded.

[0069] Thickness expansion rate = (Storage process thickness - Initial thickness) / Initial thickness × 100%

[0070] Recoverable capacity retention rate = (Recoverable discharge capacity during storage / Initial discharge capacity) x 100%

[0071] (3) Lithium-ion battery overcharge test

[0072] The lithium-ion battery was discharged to 2.8V at 0.5C at 25℃, then charged to 5.4V at a constant current of 2C, and then charged at a constant voltage for 3 hours. The surface temperature change of the lithium-ion battery was monitored (the passing standard is: the battery does not catch fire, does not burn, and does not explode). Ten lithium-ion batteries were tested for each example, and the pass rate was recorded.

[0073] (4) Hot box test

[0074] The lithium-ion battery was charged at 25°C with a constant current of 0.5C to 4.2V, and then charged at 4.2V with a constant voltage until the current was less than or equal to 0.05C. After full charging, the lithium-ion battery was placed in a high-low temperature chamber, and the temperature was increased to 150°C at a rate of 5°C / minute. This temperature was maintained at 150°C for 1 hour, and the lithium-ion battery was monitored. The passing standard was: the battery did not catch fire or explode. Ten batteries were tested for each example, and the pass rate was recorded.

[0075] Tables 1 and 2 show the parameters and evaluation results for Examples 1-1 to 1-12 and Comparative Examples 1 to 4.

[0076] Table 1

[0077]

[0078]

[0079] Table 2

[0080]

[0081] Comparing Examples 1-1 to 1-12 and Comparative Examples 1 to 4, it can be seen that within a certain range, as the ratio b / a increases, the cycle performance, high-temperature storage performance at different SOCs, capacity retention after storage, and safety performance of lithium-ion batteries are significantly improved. This may be due to the protection of the negative electrode interface by the electrolyte (EC) and its good passivation effect on the lithium plating interface of the negative electrode. However, when b / a is too small, the EC cannot protect the negative electrode interface, thus adversely affecting the cycle performance of the electrochemical device under both room temperature and high temperature conditions. Furthermore, due to the poor passivation effect on the lithium plating interface of the negative electrode, the storage and safety performance of the electrochemical device under low charge state are affected. When b / a > 6.4, the cycle performance cannot be further improved, and the storage and safety performance under high charge state are affected. This may be because when b / a is too large, the interfacial dynamics between the electrolyte and the negative electrode are affected, causing lithium plating during charging, which leads to violent reactions during storage at high temperatures, resulting in thermal failure of the lithium-ion battery.

[0082] Tables 3 and 4 show the parameters and evaluation results for Examples 1-2, 2-1 to 2-11, and Comparative Examples 5 to 6. In each of the examples and comparative examples listed in the tables below, the mass of the negative electrode active material is 5.5 g.

[0083] Table 3

[0084]

[0085] Table 4

[0086]

[0087]

[0088] Comparisons of Comparative Examples 5 and 6, Examples 1-2, and Examples 2-1 to 2-11 show that adding specific amounts of solvent EC and carbonate VC or FEC to the electrolyte significantly improves the cycle performance, high-temperature storage performance, post-storage capacity retention, and safety performance of lithium-ion batteries compared to the cases where no additives VC and FEC are added, or only one of them is added. This is mainly due to the fact that VC and FEC can effectively repair the SEI damaged at the negative electrode interface during storage and suppress side reactions and negative electrode lithium deposition. Specifically, when the VC content is lower than that of FEC, the high-temperature storage performance and capacity retention during storage are even better. This may be because, at this concentration, the electrolyte forms a composite interface protective film with low impedance on the electrode surface, reducing the consumption of active lithium after storage. Furthermore, when the content of VC and FEC is too high, the negative electrode interface impedance is too large, resulting in excessive residue after formation. This easily leads to oxidation and gas generation at the positive electrode interface, thus affecting post-storage charging performance and also causing storage gas generation and safety hazards.

[0089] Tables 5 and 6 show the parameters and evaluation results for Examples 2-6, 3-1 to 3-12, and Comparative Examples 5 and 7. In the examples and comparative examples listed below, the amounts of positive and negative electrode active materials are the same as in Examples 2-6.

[0090] Table 5

[0091]

[0092]

[0093] Table 6

[0094]

[0095] A comparison of Examples 2-6, 3-1 to 3-12, and Comparative Examples 5 and 7 shows that adding specific amounts of solvent EC, carbonate, and sulfur-oxygen double-bonded compounds to the electrolyte significantly improves the cycling, storage, and safety performance of lithium-ion batteries under different states compared to adding only EC and one of carbonate or sulfur-oxygen double-bonded compounds. A comparison of Examples 3-1 and 2-6 shows that in this system, a low mass ratio of sulfur-oxygen double-bonded compounds to positive electrode active materials does not significantly improve the performance of lithium-ion batteries. A comparison of Examples 3-9, 3-1 to 3-7, and 3-10 to 3-12 shows that an excessively high mass ratio of sulfur-oxygen double-bonded compounds to positive electrode active materials significantly affects the storage and safety performance of the battery at high SOC.

[0096] Tables 7 and 8 show the parameters and evaluation results for Examples 2-6, 4-1 to 4-13, and Comparative Example 5. In the examples and comparative examples listed below, the amounts of positive and negative electrode active materials are consistent with those in Examples 2-6.

[0097] Table 7

[0098]

[0099] Table 8

[0100]

[0101]

[0102] The comparisons of Examples 2-6, 4-1 to 4-13, and Comparative Example 5 show that adding specific amounts of solvent EC and lithium salt to the electrolyte significantly improves the cycle performance, high-temperature storage performance, and safety performance of lithium-ion batteries compared to the case without lithium salt. This is mainly due to the formation of low-resistance SEI and CEI films on the positive and negative electrode surfaces by the lithium salt, which improves charging performance and reduces side reactions caused by lithium plating. Simultaneously, the stable protective films effectively reduce side reactions between the electrolyte and the positive and negative electrodes during storage and cycling, while also reducing the consumption of active lithium and improving capacity retention.

[0103] Tables 9 and 10 show the parameters and evaluation results for Examples 2-6, 5-1 to 5-13 and Comparative Examples 5 and 8. In the examples and comparative examples listed below, the content of the positive and negative electrode active materials is consistent with that in Examples 2-6.

[0104] Table 9

[0105]

[0106] Table 10

[0107]

[0108] The comparisons of Examples 2-6, 5-1 to 5-13, and Comparative Examples 5 and 8 show that adding specific amounts of the solvent EC and polynitrile compounds to the electrolyte significantly improves the high-temperature storage performance and safety performance of lithium-ion batteries compared to the cases where neither EC nor polynitrile compounds are added, or only one of them is added. This is mainly due to the fact that polynitrile compounds can effectively complex with the positive electrode active material, reducing the oxidation activity of the positive electrode material, reducing side reactions, and simultaneously inhibiting the deposition of the positive electrode transition metal onto the negative electrode, thus reducing gas generation and improving capacity retention. Furthermore, the combination of EC and polynitrile compounds has the advantage that EC can inhibit the decomposition of polynitrile compounds at the negative electrode interface, reducing the increase in negative electrode impedance, allowing more polynitrile compounds to effectively act on the positive electrode; while the passivation of the positive electrode by polynitrile compounds can reduce the oxidation of EC at the positive electrode interface, allowing more EC to be used for the protection of the negative electrode interface, thereby maximizing the advantages of both substances and achieving optimal electrical performance.

[0109] 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: Positive electrode sheet; A negative electrode sheet, wherein the negative electrode sheet includes a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material, wherein the mass of the negative electrode active material is ag; A separator is disposed between the positive electrode and the negative electrode; An electrolyte comprising ethylene carbonate, wherein the mass content of ethylene carbonate is b% based on the mass of the electrolyte, b / a is 1.6 to 5.5, and b is 1 to 25; The electrolyte further includes a polynitrile compound, which includes at least one selected from succinic anhydride, adiponitrile, 1,2-bis(cyanoethoxy)ethane, 1,4-dicyano-2-butene, 1,3,6-hexanetrionitrile, or 1,2,3-tris(2-cyanoethoxy)propane, and the mass content of the polynitrile compound is from 0.01% to 6% based on the mass of the electrolyte.

2. The electrochemical device according to claim 1, wherein, b / a ranges from 2 to 5.

5.

3. The electrochemical device according to claim 1, wherein, b is between 5 and 20, or b / a is between 2.5 and 5.

5.

4. The electrochemical device according to claim 1, wherein, The mass content of the polynitrile compound is 1% to 6%.

5. The electrochemical device according to claim 1, wherein, The mass content of the polynitrile compound is 2% to 6%.

6. The electrochemical device according to claim 1, wherein, The electrolyte further includes at least one of vinylene carbonate or fluoroethylene carbonate, and based on the mass of the electrolyte, the mass content of vinylene carbonate is m%, the mass content of fluoroethylene carbonate is n%, m + n = c, and 0.01 ≤ c < 2.

7. The electrochemical device according to claim 6, wherein, Based on the mass of the electrolyte, the mass content of vinylene carbonate is m%, the mass content of fluoroethylene carbonate is n%, and the electrochemical device satisfies at least one of the following relationships: i) 0 ≤ m < 2; ii) 0 ≤ n < 2; iii) m < n; iv) c / a is between 0.001 and 0.

36.

8. The electrochemical device according to claim 1, wherein, The electrolyte also includes compounds containing sulfur-oxygen double bonds, including at least one of 1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, vinyl sulfate, methylene disulfonate, 1,3-propane disulfonic anhydride, 2-methylbutanesulfonyl lactone, or propenyl-1,3-sulfonyl lactone.

9. The electrochemical device according to claim 8, wherein, Based on the mass of the electrolyte, the mass content of the compound containing sulfur-oxygen double bonds is d%, and the positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, the mass of the positive electrode active material is eg, and the d / e ratio is 0.1 to 0.

6.

10. The electrochemical device according to claim 9, wherein, The positive electrode active material contains cobalt.

11. The electrochemical device according to claim 1, wherein, The electrolyte further includes at least one of lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium tetraborate, lithium borate, or lithium trifluoromethanesulfonate. Based on the mass of the electrolyte, the mass content of at least one of lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium tetraborate, lithium borate, or lithium trifluoromethanesulfonate is from 0.01% to 3%.

12. The electrochemical device according to claim 11, wherein, Based on the mass of the electrolyte, the mass content of at least one of lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium tetraborate, lithium borate, or lithium trifluoromethanesulfonate is 0.5% to 3%.

13. The electrochemical device according to claim 11, wherein, Based on the mass of the electrolyte, the mass content of at least one of lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium tetraborate, lithium borate, or lithium trifluoromethanesulfonate is from 0.1% to 2%.

14. An electronic device comprising an electrochemical device according to any one of claims 1 to 13.

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