Secondary battery and electric device

Abstract

This application discloses a secondary battery and an electrical device, belonging to the field of battery technology. This application controls the particle size D of silicon-carbon material. v 99. The mass fraction of silicon in the negative electrode active material, as well as the fluoroethylene carbonate, cyclic carbonate additives, and sulfur-containing additives, satisfy the following relationship: 7.70≤(x·a) / [(y+z)·b]≤62.86; significantly improving the high-temperature cycle, high-temperature storage, and gas generation performance of the secondary battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to a secondary battery and an electrical device. Background Technology

[0002] Unlike primary batteries (non-rechargeable batteries), secondary batteries release energy during discharge and can be recharged to restore their charge, thus possessing the characteristic of reusability. Secondary batteries are widely used in portable electronic devices, electric vehicles, energy storage systems, and many other fields.

[0003] With the rapid development of electric vehicles, energy storage systems, and other fields, higher requirements are being placed on the safety and long-cycle stability of batteries. Especially in high-temperature environments, battery storage performance and cycle stability have become key factors restricting their application. Existing rechargeable batteries often suffer from performance degradation and shortened cycle life under high-temperature conditions.

[0004] Therefore, this application is submitted. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a secondary battery and power device, wherein the secondary battery has excellent high-temperature cycling and high-temperature storage performance.

[0006] To achieve the above objectives, a first aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a negative current collector and a negative active material layer on at least one surface of the negative current collector. The negative active material layer includes a negative active material, which includes silicon-carbon material and graphite.

[0007] The electrolyte includes fluoroethylene carbonate, cyclic carbonate additives, and sulfur-containing additives;

[0008] The secondary battery satisfies: 7.70≤(x·a) / [(y+z)·b]≤62.86;

[0009] Wherein, x% is the mass percentage of the fluoroethylene carbonate in the electrolyte;

[0010] y% represents the mass percentage of the cyclic carbonate additive in the electrolyte;

[0011] z% is the mass percentage of the sulfur-containing additive in the electrolyte;

[0012] aμm is the particle size D of the silicon-carbon material. v 99;

[0013] b% is the mass fraction of silicon in the negative electrode active material.

[0014] As an implementation scheme of this application, at least one of the following (Ⅰ) to (Ⅴ) is satisfied:

[0015] (Ⅰ) 2≤x≤20;

[0016] (II) 0.5 ≤ y ≤ 3.5;

[0017] (Ⅲ) 0.2≤z≤3;

[0018] (Ⅳ) 18≤a≤30;

[0019] (V)1.5≤b≤10.

[0020] As an embodiment of this application, the secondary battery satisfies: -3≤x-2b≤4.

[0021] As an embodiment of this application, the cyclic carbonate additive includes at least one of vinylene carbonate and vinyl ethylene carbonate; and / or

[0022] The sulfur-containing additive includes at least one of vinyl sulfate, 1,3-propane sulfonyl lactone, propenyl-1,3-sulfonyl lactone, ethylene sulfite, methanedisulfonate, and 1,4-butyl sulfonyl lactone.

[0023] As an embodiment of this application, the silicon-carbon material accounts for 3% to 20% of the mass percentage of the negative electrode active material.

[0024] As an embodiment of this application, the silicon-carbon material comprises a porous carbon material and elemental silicon distributed in the pores of the porous carbon material.

[0025] As an embodiment of this application, the mass fraction of silicon in the silicon-carbon material is 45% to 65%.

[0026] As an embodiment of this application, the positive electrode sheet includes a positive current collector and a positive active material layer on at least one surface of the positive current collector, the positive active material layer including a positive active material, the positive active material including lithium iron phosphate with carbon coated on its surface;

[0027] The lithium iron phosphate includes Li p Fe 1-q A q At least one of the compounds of PO4, wherein A includes at least one of Ti, Mg, and Mn, 0.9 ≤ p ≤ 1.1, and 0 ≤ q ≤ 0.2.

[0028] As an implementation scheme of this application, at least one of the following (VI) to (VIII) is satisfied:

[0029] (VI) The particle size D of the positive electrode active material v 99 is 3-10 μm;

[0030] (VII) The specific surface area of ​​the positive electrode active material is 5–12 m². 2 / g;

[0031] (VIII) The compacted density of the positive electrode active material under 40 kN pressure is 2–3.5 g / cm³. 3 .

[0032] As an embodiment of this application, the electrolyte further includes a lithium salt, which includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium perchlorate, lithium difluorophosphate, lithium tetrafluorophosphate, lithium bis(oxalate)difluorophosphate, lithium bis(oxalate)borate, and lithium difluorooxalateborate.

[0033] As an embodiment of this application, the lithium salt includes lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide;

[0034] The lithium hexafluorophosphate accounts for 6.0% to 14.5% of the electrolyte by mass.

[0035] The lithium difluorosulfonylimide accounts for 0.5% to 3.5% of the mass of the electrolyte.

[0036] A second aspect of this application provides an electrical device comprising the aforementioned secondary battery.

[0037] The beneficial effects of this invention are as follows: This application controls the particle size D of the silicon-carbon material. v 99. The mass fraction of silicon in the negative electrode active material, as well as the fluoroethylene carbonate, cyclic carbonate additives, and sulfur-containing additives, satisfy the following relationship: 7.70≤(x·a) / [(y+z)·b]≤62.86. The fluoroethylene carbonate can tightly couple with silicon in the negative electrode active material. On the one hand, this ensures the film quality of fluoroethylene carbonate on the surface of the silicon-based negative electrode sheet and suppresses the volume expansion of the silicon-based material; on the other hand, it avoids the deterioration of storage and gas generation performance caused by fluoroethylene carbonate. Cyclic carbonate additives can further improve the film stability of the negative electrode, alleviate the volume expansion and contraction problems during charging and discharging, while sulfur-containing additives reduce side reactions at high temperatures and improve the gas generation risk associated with the use of fluoroethylene carbonate. The particle size D of the silicon-carbon material... v The addition of 99% silicon can reduce the van der Waals forces during lithium-ion insertion, reduce the lithium-ion insertion / extraction path, increase the contact area with the electrolyte, improve the structural stability of the negative electrode active material and negative electrode sheet, and significantly improve the high-temperature cycling, high-temperature storage and gas generation performance of secondary batteries. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0039] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0040] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0041] The inventors of this application have discovered that lithium iron phosphate (LiFePO4)-graphite batteries are widely favored in the power battery market due to their advantages such as long lifespan, high safety, low cost, and environmental sustainability. However, as end-users increasingly demand longer driving ranges and faster charging capabilities, the shortcomings of lithium iron phosphate batteries in terms of energy density and kinetic performance are becoming increasingly apparent. Compared to the theoretical specific capacity of 372 mAh / g for graphite anodes, silicon anodes offer a significant improvement in theoretical specific capacity. Simultaneously, the lithium intercalation potential of silicon anodes (0.4V vs. LiFePO4) is significantly lower. + / Li) is slightly higher than graphite (0.05V vs. Li). + The presence of lithium iron phosphate (LiFePO4) cathodes can reduce the risk of lithium plating on the negative electrode during charging (especially fast charging), thereby mitigating the loss of active lithium during battery cycling and avoiding the risk of internal short circuits induced by lithium dendrites. Therefore, matching the lithium iron phosphate cathode with a silicon-based anode can further improve the range and safety of lithium iron phosphate batteries.

[0042] However, the volume expansion of silicon anodes during charging and discharging, along with the accompanying structural collapse and interface damage, will worsen the electron conduction between active particles, accelerate electrolyte consumption and side reactions at the electrode / electrolyte interface, causing irreversible capacity loss and posing safety risks such as gas generation.

[0043] Therefore, based on the above problems, this application provides a secondary battery, including a positive electrode, a negative electrode and an electrolyte. The negative electrode includes a negative current collector and a negative active material layer on at least one surface of the negative current collector. The negative active material layer includes a negative active material, which includes silicon-carbon material and graphite.

[0044] The electrolyte includes fluoroethylene carbonate, cyclic carbonate additives, and sulfur-containing additives;

[0045] The secondary battery satisfies: 7.70≤(x·a) / [(y+z)·b]≤62.86, for example, it can be 7.7, 8, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 62, 62.86 or any two of these values.

[0046] Wherein, x% is the mass percentage of the fluoroethylene carbonate in the electrolyte;

[0047] y% represents the mass percentage of the cyclic carbonate additive in the electrolyte;

[0048] z% is the mass percentage of the sulfur-containing additive in the electrolyte;

[0049] aμm is the particle size D of the silicon-carbon material. v 99;

[0050] b% is the mass fraction of silicon in the negative electrode active material.

[0051] The applicant of this application has discovered that the volume average particle size of the negative electrode active material, the mass fraction of silicon in the negative electrode active material, and the use of fluoroethylene carbonate, cyclic carbonate additives, and sulfur-containing additives can affect the performance of the secondary battery. This application controls the particle size D of the silicon-carbon material... v 99. The mass fraction of silicon in the negative electrode active material, as well as the content of fluoroethylene carbonate, cyclic carbonate additives, and sulfur-containing additives, satisfy the following relationship: 7.70≤(x·a) / [(y+z)·b]≤62.86. In this embodiment, fluoroethylene carbonate can be tightly coupled with silicon in the negative electrode active material. On the one hand, this ensures the film quality of fluoroethylene carbonate on the surface of the negative electrode sheet and suppresses the volume expansion of silicon-carbon materials; on the other hand, it avoids the deterioration of storage and gas generation performance caused by fluoroethylene carbonate. Cyclic carbonate additives can further improve the film stability of the negative electrode and alleviate the volume expansion and contraction problems during charge and discharge. At the same time, sulfur-containing additives reduce side reactions generated at high temperatures and improve the gas generation risk caused by the use of fluoroethylene carbonate. The particle size D of the silicon-carbon material... vThe addition of 99% silicon can reduce the van der Waals forces during lithium-ion insertion, reduce the lithium-ion insertion / extraction path, increase the contact area with the electrolyte, improve the structural stability of the negative electrode active material and negative electrode sheet, and significantly improve the high-temperature cycling, high-temperature storage and gas generation performance of secondary batteries.

[0052] Among them, the particle size D of silicon-carbon material v 99 represents the particle size corresponding to a cumulative volume percentage of 99% for the silicon-carbon material. It should be noted that the particle size D of the silicon-carbon material in this invention... v The test for 99% follows GB-T19077-2016 and is performed using a particle size analysis laser diffractometer.

[0053] In one embodiment, 2≤x≤20, for example, can be a range of 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 or any two of these values. By controlling the content of fluoroethylene carbonate within this range, it is possible to promote the formation of a stable SEI film on the negative electrode, reduce the generation of hydrofluoric acid, alleviate the volume expansion of the negative electrode active material, and tightly couple with silicon in the negative electrode active material, thereby further improving the high-temperature cycling, high-temperature storage and gas generation performance of the secondary battery.

[0054] In one embodiment, 0.5≤y≤3.5, for example, can be a range of 0.5, 0.8, 1, 1.5, 2, 2.5, 3, 3.5 or any two of these values. By controlling the content of cyclic carbonate additives within this range, the stability of the film formation at the negative electrode interface can be improved, effectively preventing direct reaction between the electrolyte and the negative electrode active material, thereby slowing down the decomposition of the electrolyte. At the same time, it reduces the side reactions between the electrolyte and the negative electrode active material, alleviates the volume expansion and contraction problems during charging and discharging, and further improves the high-temperature cycling, high-temperature storage and gas generation performance of the secondary battery.

[0055] In one embodiment, 0.2≤z≤3, for example, can be a range of 0.2, 0.5, 0.8, 1, 1.5, 2, 2.5, 3 or any two of these values. By controlling the content of sulfur-containing additives within this range, it is possible to reduce the shedding of active materials and the decomposition of electrolyte, reduce side reactions generated at high temperatures, and improve the risk of gas generation caused by the use of fluoroethylene carbonate, thereby further improving the high-temperature cycling, high-temperature storage and gas generation performance of secondary batteries.

[0056] In one embodiment, 18 ≤ a ≤ 30, for example, it can be a range of 18, 20, 22, 24, 25, 26, 28, 30, or any two of these values, by controlling the particle size D of the silicon-carbon material. vWithin this range, 99% can improve the wettability of the electrolyte, reduce electrode impedance, improve lithium plating, increase the migration rate of lithium ions, and further improve the high-temperature cycling, high-temperature storage and gas generation performance of secondary batteries.

[0057] In one embodiment, 1.5 ≤ b ≤ 10, for example, it can also be a range of 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, or any two of these values. By controlling the mass fraction of silicon in the negative electrode active material within this range, during battery operation, silicon reacts with the byproduct F in the electrolyte. – By combining these materials, the HF is prevented from damaging the interface, thereby mitigating the side reactions between the negative electrode interface and the electrolyte solvent and suppressing gas production. At the same time, the addition of silicon-carbon materials is also beneficial to improving the energy density of the secondary battery.

[0058] In one embodiment, the secondary battery satisfies: -3≤x-2b≤4, for example, it can be a range of -3, -2, -1, 0, 1, 2, 3, 4 or any two of these values. By controlling x-2b within this range, the fluoroethylene carbonate can more effectively couple with the silicon element in the negative electrode active material, promoting the formation of a more uniform and dense SEI film, effectively suppressing the volume expansion and interface passivation of the negative electrode active material, avoiding the deterioration of storage and gas generation performance by the fluoroethylene carbonate, and further improving the high-temperature cycling, high-temperature storage and gas generation performance of the secondary battery.

[0059] In one embodiment, the cyclic carbonate additive includes at least one of vinylene carbonate and vinyl ethylene carbonate. The above additive can improve film-forming stability.

[0060] The sulfur-containing additive includes at least one selected from vinyl sulfate, 1,3-propanesulfonyl lactone, propenyl-1,3-sulfonyl lactone, ethylene sulfite, methanedisulfonate, and 1,4-butylsulfonyl lactone. These additives can inhibit the gas production from electrolyte decomposition.

[0061] In one embodiment, the silicon-carbon material accounts for 3% to 20% of the mass percentage of the negative electrode active material. For example, it can be 3%, 4%, 5%, 6%, 8%, 10%, 12%, 14%, 15%, 16%, 18%, 20%, or any two of these values. By controlling the content of silicon-carbon material in the negative electrode active material within this range, the energy density of the secondary battery can be effectively improved.

[0062] In one embodiment, the graphite includes at least one of natural graphite and artificial graphite.

[0063] In one embodiment, the graphite constitutes 80% to 97% of the negative electrode active material by mass, for example, 80%, 82%, 85%, 86%, 88%, 90%, 92%, 94%, 95%, 97%, or any two of these values. A graphite content within this range can improve the cycle stability of the negative electrode and enhance the long-cycle performance of the battery.

[0064] In one embodiment, the silicon-carbon material comprises a porous carbon material and elemental silicon distributed within the pores of the porous carbon material. This can increase the energy density of the negative electrode active material while suppressing the expansion of the negative electrode material.

[0065] In one embodiment, the mass fraction of silicon in the silicon-carbon material is 45% to 65%, for example, it can be 45%, 46%, 48%, 50%, 52%, 55%, 56%, 58%, 60%, 62%, 64%, 65% or any two of these values. By controlling the mass fraction of silicon in the silicon-carbon material within this range, the stability of the silicon-carbon material can be taken into account, and the expansion of the silicon-carbon material during cycling can be suppressed.

[0066] In one embodiment, the positive electrode includes a positive current collector and a positive active material layer on at least one surface of the positive current collector, the positive active material layer comprising a positive active material, the positive active material comprising lithium iron phosphate with a carbon-coated surface. In some embodiments, the carbon content accounts for 1% to 3.5% of the mass percentage of the positive active material, for example, it can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, or any two of these values, thus balancing the stability and conductivity of the positive active material.

[0067] In one embodiment, the lithium iron phosphate includes Li p Fe 1-q A q At least one of the compounds of PO4, wherein A includes at least one of Ti, Mg, and Mn, 0.9≤p≤1.1, 0≤q≤0.2. When A is incorporated into lithium iron phosphate, the conductivity of the positive electrode active material can be further improved, the crystal structure of lithium iron phosphate can be effectively stabilized, the stability of lithium iron phosphate can be improved, and the resistance to lithium ion insertion and extraction can be reduced.

[0068] In one embodiment, the particle size D of the positive electrode active material v 99 represents a particle size of 3–10 μm, for example, it can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or any two of these values. The particle size D of the positive electrode active material... vWhen 99% of the lithium ions are within this range, it is beneficial for the uniform distribution and transport of lithium ions, thereby improving the cycle stability and consistency of the battery.

[0069] Among them, the particle size D of the positive electrode active material v 99 represents the particle size corresponding to a cumulative volume percentage of 99% for the positive electrode active material. It should be noted that the Dv99 of the positive electrode active material of this invention was tested with reference to GB-T 19077-2016 using a particle size analysis laser diffractometer.

[0070] In one embodiment, the specific surface area of ​​the positive electrode active material is 5–12 m². 2 / g, for example, could be 5m 2 / g、6m 2 / g、8m 2 / g, 10m 2 / g、12m 2 / g or any two of these values, when the specific surface area of ​​the positive electrode active material is within this range, can increase the contact area with the electrolyte, improve the wettability of the electrolyte, and increase the lithium ion transport rate.

[0071] It should be noted that in this invention, a specific surface area and pore size distribution measuring instrument is used to test the specific surface area of ​​the negative electrode active material, and the testing standard is GB / T 19587-2017 Gas Adsorption BET Method.

[0072] In one embodiment, the compacted density of the positive electrode active material under a pressure of 40 kN is 2–3.5 g / cm³. 3 For example, it could be 2g / cm 3 2.2g / cm 3 2.5g / cm 3 2.6g / cm 3 2.8g / cm 3 3.2g / cm 3 3.5g / cm 3 The positive electrode active material has a high compaction density and a high interparticle filling degree, which not only improves the processing performance of the positive electrode active material but also effectively increases the energy density of the battery. The compaction density can be measured according to GB / T 24533-2009.

[0073] In one embodiment, the electrolyte further includes a lithium salt, which includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium perchlorate, lithium difluorophosphate, lithium tetrafluorophosphate, lithium difluorophosphate bis(oxalate)difluorophosphate, lithium di(oxalate)borate, and lithium difluorooxalateborate.

[0074] In one embodiment, the lithium salt comprises lithium hexafluorophosphate and lithium difluorosulfonylimide.

[0075] The lithium hexafluorophosphate accounts for 6.0% to 14.5% of the electrolyte by mass, for example, it can be 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 14.5% or any two of these values.

[0076] In one embodiment, the lithium bis(fluorosulfonyl)imide accounts for 0.5% to 3.5% of the electrolyte by mass, for example, it can be 0.5%, 0.8%, 1%, 1.5%, 2.5%, 3%, 3.5% or any two of these values.

[0077] This application uses a combination of lithium hexafluorophosphate and lithium difluorosulfonylimide as a lithium salt, which can improve the lithium ion transference number and antioxidant performance, as well as have good aluminum foil passivation ability. It has good compatibility with the positive and negative electrode active materials system of this application, improves stability, repairs the electrode / electrolyte interface, improves ionic conductivity, and promotes the conduction of lithium ions in the bulk phase.

[0078] In one embodiment, the electrolyte further includes an organic solvent, said organic solvent being at least one of ethylene carbonate (EC), propylene carbonate (PC), butenyl carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), diphenyl carbonate (DPhC), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), and γ-butyrolactone (γ-GBL).

[0079] In one embodiment, the positive current collector includes metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper.

[0080] In one embodiment, the positive electrode active material layer further includes a conductive agent and a binder.

[0081] In this application, the negative electrode current collector includes copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or composite current collectors.

[0082] In one embodiment, the negative electrode active material layer further includes a conductive agent and a negative electrode binder.

[0083] In one embodiment, the conductive agent includes at least one of carbon materials such as acetylene black, needle coke, carbon nanotubes, and graphene.

[0084] In one embodiment, the negative electrode binder includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, styrene-butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymer or its hydride, ethylene-propylene-diene terpolymer, styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer, syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, propylene-α-olefin copolymer, polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymer.

[0085] In the secondary battery mentioned in this application, a separator is usually provided between the positive and negative electrodes to prevent short circuits. The separator includes at least one of polypropylene separator and polyethylene separator.

[0086] In one embodiment, the secondary battery may include an outer packaging that can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0087] In one embodiment, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0088] This application does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0089] One embodiment of this application provides an electrical device including the secondary battery described above.

[0090] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0091] The present application is further illustrated below with specific embodiments:

[0092] Example 1

[0093] A method for preparing a secondary battery includes the following steps:

[0094] (1) Preparation of positive electrode sheet

[0095] Preparation of positive electrode active material: Mixed solvent A was prepared by mixing ethanol and water at a volume ratio of 1:3; lithium hydroxide, iron phosphate, diammonium hydrogen phosphate, and titanium dioxide were weighed according to a Li:Fe:P:Ti molar ratio of 1:1:1:0.05. Under nitrogen protection, lithium hydroxide, iron phosphate, diammonium hydrogen phosphate, and titanium dioxide were added to the mixed solution and stirred evenly. Ammonia solution was added to adjust the pH to 9.8. Hydrothermal reaction was carried out at 180℃ for 5 hours. After natural cooling to room temperature, the mixture was filtered, washed (washed 3 times with distilled water, then washed 3 times with ethanol), and dried (vacuum dried at 80℃) to obtain the precursor.

[0096] The precursor and glucose were mixed evenly at a mass ratio of 98:2, and then the mixture was sintered at 750°C for 6 hours under a nitrogen atmosphere to obtain the positive electrode active material. The positive electrode active material was lithium iron phosphate (LiFeTi) with a carbon-coated surface. 0.05 The carbon content (PO4) of the positive electrode active material is between 2% by mass, and the specific surface area is 9.8 m². 2 / g,D v 99 has a particle size of 6μm, and its compacted density under 40KN pressure is 2.4g / cm³. 3 ;

[0097] The positive electrode active material, conductive agent acetylene black (Super P), and binder polyvinylidene fluoride (PVDF) were mixed evenly at a mass ratio of LiFePO4:Super P:PVDF = 94:3:3, and then uniformly dispersed in 1-methyl-2-pyrrolidone (NMP) to form a uniform black slurry. The slurry was then coated on both sides of an aluminum foil, and subsequently baked and rolled (compacted to a density of 2.6 g / cm³). 3 After slitting and cutting, positive electrode sheets are obtained.

[0098] (2) Preparation of negative electrode sheet

[0099] Preparation of silicon-carbon materials: Porous carbon materials (specific surface area 1600 m²) 2 / g) is fed into a fluidized bed reactor, nitrogen is introduced to fluidize the porous carbon, the temperature is controlled at 650℃ and the pressure of silane gas is 0.4MPa, silane gas is introduced to carry out chemical vapor deposition, so that elemental silicon is deposited in the pores of the porous carbon material, and the silicon content is controlled at 50wt% to prepare silicon-carbon material.

[0100] Artificial graphite (AG), silicon-carbon material (50% silicon by mass, particle size Dv99 of 22 μm), acetylene black (Super P), carbon nanotubes (CNTs), and binder SBR were mixed uniformly in a mass ratio of AG:silicon-carbon material:Super P:CNT:SBR = 87:7:2:1:3 and dispersed evenly in deionized water to form a uniform black slurry. The slurry was then coated onto both sides of a copper foil, baked, and rolled (compacted to a density of 1.5 g / cm³). 3 After slitting and cutting, negative electrode sheets are obtained.

[0101] (3) Preparation of electrolyte

[0102] At room temperature, in a glove box filled with argon (H2O < 1 ppm, O2 < 1 ppm), non-aqueous organic solvents ethylene carbonate, ethyl methyl carbonate, and ethyl acetate were mixed in a mass ratio of 30:40:30 to form a mixed solvent. Then, the mixture was... Molecular sieves are used to remove water; ready for use.

[0103] Lithium salt (LiPF6+1LiFSI) was added to the mixed solvent, and the mixture was stirred and cooled continuously to obtain a colorless and transparent liquid.

[0104] The electrolyte is obtained by adding 7.0% fluoroethylene carbonate (FEC), 1.5% vinylene carbonate (VC), and 0.5% 1,3-propane sulpholol (sulfur-containing additive, PS) equivalent to the total mass of the electrolyte. LiPF6 and LiFSI account for 12.8% and 1% of the total mass of the electrolyte, respectively.

[0105] (4) Assembly of secondary batteries:

[0106] The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes. After winding, hot pressing and shaping, Hi-pot testing, and tab welding, a bare cell is obtained. The bare cell is then encapsulated in an outer aluminum-plastic film and baked in an oven at 100±5℃ for 48 hours. The electrolyte prepared above (with an injection coefficient of 4.0 g / Ah) is injected into the dried battery. After standing, formation, and capacity testing, a secondary battery is obtained.

[0107] The specific parameters of this embodiment are shown in Table 1.

[0108] Examples 2-6

[0109] The difference between Examples 2-6 and Example 1 is that the content of FEC is adjusted.

[0110] Examples 7-10

[0111] The difference between Examples 7-10 and Example 1 is that the content of VC is adjusted.

[0112] Examples 11-12

[0113] The difference between Examples 11 and 12 and Example 1 is that the content of VC and the types of sulfur-containing additives (PST, i.e., propylene-1,3-sulfonyl lactone, and MMDS, i.e., methanedisulfonate) are adjusted.

[0114] Examples 13-15

[0115] The difference between Examples 13-15 and Example 1 is that the amount of sulfur-containing additive is adjusted.

[0116] Examples 16-18

[0117] The difference between Examples 16-18 and Example 1 is that the particle size D of the silicon-carbon material is adjusted. v 99.

[0118] Examples 19-22

[0119] The difference between Examples 19-22 and Example 1 is that the mass fraction of silicon in the negative electrode active material is adjusted by adjusting the ratio of silicon-carbon material and artificial graphite.

[0120] Examples 22-27

[0121] The difference between Examples 22-27 and Example 1 is that the type and amount of lithium salt are adjusted.

[0122] Examples 28-30

[0123] Examples 28-30 differ from Example 1 in that the particle size D of the positive electrode active material is changed by ball milling and sieving. v 99, thereby changing the specific surface area of ​​the positive electrode active material and the powder compaction density under 40KN pressure.

[0124] Comparative Examples 1-4

[0125] The preparation process is the same as in Example 1, except that the amount of FEC added, the proportion of silicon carbide added, the content of VC, and the particle size D of silicon carbide are adjusted. v 99. Content of Vitamin C and content of sulfur-containing additives.

[0126] Table 1

[0127]

[0128]

[0129] Table 2

[0130]

[0131] Battery performance test

[0132] Room temperature DCR test: At 25±2℃, the secondary batteries obtained in the examples and comparative examples were charged to 3.65V at 1C, then discharged at 1C capacity for 30 minutes, adjusted to 50% SOC, and then discharged at 5C constant current pulse discharge for 10s and charged for 10s. The DCR was calculated as (voltage before pulse discharge – voltage after pulse discharge) / discharge current * 100%. The results are shown in Table 3.

[0133] Observation of lithium plating at the interface: At 25±2℃, the secondary batteries obtained in the examples and comparative examples were subjected to charge-discharge cycle tests at a step-charge rate (peak 5C, 10%~80% SOC, ≤10min) within the range of 2.5~3.65V. After 10 cycles, the batteries were disassembled and the lithium plating on the negative electrode was observed. The results are recorded in Table 3 (slight lithium plating: lithium plating area on the negative electrode surface accounts for 5%~20%; moderate lithium plating: lithium plating area on the negative electrode surface accounts for 20%~40%).

[0134] Room temperature cycle performance test: At 25±2℃, the secondary batteries obtained in the examples and comparative examples were subjected to charge-discharge cycle tests within the range of 2.5~3.65V at a charge-discharge rate of 1C / 1C. The discharge specific capacity of the battery in the first cycle and the discharge specific capacity after 1000 cycles were recorded. After 1000 fast-charge cycles, the batteries were disassembled to observe the lithium plating on the negative electrode. The capacity retention rate after 1000 cycles = discharge specific capacity after 1000 cycles / discharge specific capacity in the first cycle * 100%. The recorded data are shown in Table 3.

[0135] High-temperature cycling performance test: At 45±2℃, the secondary batteries obtained in the examples and comparative examples were subjected to charge-discharge cycle tests within the range of 2.5~3.65V at a charge-discharge rate of 1C / 1C. The discharge specific capacity of the battery in the first cycle and the discharge specific capacity after 1000 cycles were recorded. The capacity retention rate after 1000 cycles = discharge specific capacity after 1000 cycles / discharge specific capacity in the first cycle * 100%. The recorded data are shown in Table 3.

[0136] High-temperature storage performance: The secondary batteries obtained in the examples and comparative examples were placed at 60±2℃ and subjected to charge-discharge tests at a charge-discharge rate of 1C / 1C within the range of 2.5 to 3.65V. The discharge specific capacity of the batteries in the first week was recorded. Afterwards, the batteries were stored at 60±2℃ for 90 days, and the charge-discharge test was performed again, with the discharge specific capacity recorded. High-temperature storage capacity retention rate = discharge specific capacity after 90 days / discharge specific capacity in the first week * 100%. The recorded data are shown in Table 3.

[0137] High-temperature gas generation test: The secondary batteries obtained in the examples and comparative examples were charged at a constant current rate of 1C to 3.65V at 25±2℃, and then charged at a constant voltage of 3.65V until the current equals 0.05C, so that it is in a fully charged state of 3.65V. The volume of the fully charged battery before storage was measured and recorded as V0; then the fully charged battery was placed in an oven at 60±2℃. After 14 days, the battery was taken out and its volume after storage was immediately measured and recorded as V1. The volume expansion rate = (V1 – V0) / V*100%, and the results are shown in Table 3.

[0138] Table 3

[0139]

[0140] As can be seen from Table 1, by controlling the volume average particle size of the negative electrode active material, the mass fraction of silicon in the negative electrode active material, and the fluoroethylene carbonate, cyclic carbonate additives, and sulfur-containing additives to satisfy the following relationship: 7.70≤(x·a) / [(y+z)·b]≤62.86, the present invention can significantly improve the high-temperature cycling, high-temperature storage, and gas generation performance of the secondary battery.

[0141] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that, The negative electrode sheet includes a negative current collector and a negative active material layer on at least one surface of the negative current collector, wherein the negative active material layer includes a negative active material, and the negative active material includes silicon carbide and graphite. The electrolyte includes fluoroethylene carbonate, cyclic carbonate additives, and sulfur-containing additives; The secondary battery satisfies: 7.70 ≤ (x·a) / [(y+z)·b] ≤ 62.86; Wherein, x% is the mass percentage of the fluoroethylene carbonate in the electrolyte, and 2≤x≤20; y% represents the mass percentage of the cyclic carbonate additive in the electrolyte, where 0.5 ≤ y ≤ 3.

5. z% is the mass percentage of the sulfur-containing additive in the electrolyte, where 0.2 ≤ z ≤ 3; a μm is the particle size D of the silicon-carbon material v 99, 18 ≤ a ≤ 30; b% is the mass fraction of silicon in the negative electrode active material, where 1.5 ≤ b ≤ 10.

2. The secondary battery according to claim 1, characterized in that, The secondary battery satisfies: -3≤x-2b≤4.

3. The secondary battery according to claim 1, characterized in that, The cyclic carbonate additives include at least one of vinylene carbonate and vinyl ethylene carbonate; and / or The sulfur-containing additive includes at least one of vinyl sulfate, 1,3-propane sulfonyl lactone, propenyl-1,3-sulfonyl lactone, ethylene sulfite, methanedisulfonate, and 1,4-butyl sulfonyl lactone.

4. The secondary battery according to claim 1, characterized in that, The silicon-carbon material accounts for 3% to 20% of the mass percentage of the negative electrode active material.

5. The secondary battery according to claim 4, characterized in that, The silicon-carbon material comprises porous carbon material and elemental silicon distributed in the pores of the porous carbon material.

6. The secondary battery according to claim 5, characterized in that, The silicon-carbon material contains 45% to 65% silicon by mass.

7. The secondary battery according to claim 1, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer on at least one surface of the positive current collector, the positive active material layer including a positive active material, the positive active material including lithium iron phosphate with carbon coated on its surface; The lithium iron phosphate includes Li p Fe 1-q A q At least one of the compounds of PO4, wherein A includes at least one of Ti, Mg, and Mn, 0.9 ≤ p ≤ 1.1, and 0 ≤ q ≤ 0.

2.

8. The secondary battery according to claim 7, characterized in that, Satisfy at least one of the following conditions (VI) to (VIII): (VI) The particle size D of the positive electrode active material v 99 is 3~10μm; (VII) The specific surface area of ​​the positive electrode active material is 5~12m². 2 / g; (VIII) The compacted density of the positive electrode active material under 40 kN pressure is 2~3.5 g / cm³. 3 .

9. The secondary battery according to claim 1, characterized in that, The electrolyte further includes lithium salts, which include at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium perchlorate, lithium difluorophosphate, lithium tetrafluorophosphate, lithium bis(oxalate)difluorophosphate, lithium bis(oxalate)borate, and lithium difluorooxalateborate.

10. The secondary battery according to claim 9, characterized in that, The lithium salt includes lithium hexafluorophosphate and lithium difluorosulfonylimide; The lithium hexafluorophosphate accounts for 6.0% to 14.5% of the mass of the electrolyte; The lithium difluorosulfonylimide accounts for 0.5% to 3.5% of the mass of the electrolyte.

11. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 10.

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