Electrolyte, secondary battery, and electric device

Abstract

This application provides an electrolyte, a secondary battery, and an electrical device. The electrolyte includes a lithium salt, a non-aqueous solvent, a first additive, and a second additive. The first additive is a compound having a structure as shown in Formula I: Formula I, where n is a natural number, 1 ≤ n ≤ 8. The second additive includes at least one of lithium difluorooxalatoborate, lithium difluorodioxalatophosphate, and lithium fluorosulfonate. Adding a trithiocarbonate additive containing cyano and trimethoxysilane substituents, as well as a lithium salt-based second additive, to the electrolyte can significantly improve the interfacial stability and reduce the interfacial internal resistance of the secondary battery, thereby enhancing the electrochemical performance of the secondary battery at high voltages.

Description

Technical Field

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

[0002] Secondary batteries are widely used in electric vehicles, portable consumer electronics, and energy storage grids. Increasing the upper voltage limit to achieve greater capacity (lithium insertion / extraction) allows for higher energy density within a given material system. However, high voltage conditions exacerbate problems such as electrolyte oxidation and decomposition, irreversible cation mixing and surface structure transformation to rock salt phase in the cathode material, cathode particle breakage, and transition metal ion dissolution, placing higher demands on the electrolyte. Specifically, at high voltages, electrolyte decomposition is accompanied by the generation of Lewis acidic byproducts such as HF, PF5, and POF3. These substances can attack the passivation layer at the cathode / electrolyte interface (CEI), intensifying transition metal dissolution, and the loss of active material inevitably reduces battery capacity. Simultaneously, dissolved transition metal ions diffuse in the electrolyte and deposit on the anode surface, disrupting the solid electrolyte interphase (SEI), exacerbating side reactions at the anode interface, and thus deteriorating electrochemical performance.

[0003] The electrochemical and safety performance of secondary batteries is closely related to the SEI and CEI formed on the electrode surface, while the properties and formation mechanism of the electrode / electrolyte interface mainly depend on the electrolyte composition. Among these, electrolyte additives play an important role in regulating the interfacial film-forming properties. Summary of the Invention

[0004] The purpose of this application is to overcome the shortcomings of the prior art and provide an electrolyte, a secondary battery, and an electrical device.

[0005] To achieve the above objectives, in a first aspect, this application provides an electrolyte comprising a lithium salt, a non-aqueous solvent, a first additive, and a second additive. The first additive is a compound having the structure shown in Formula I: Formula I In Equation I, n is a natural number, 1 ≤ n ≤ 8; The second additive includes at least one of lithium difluorooxalate borate, lithium difluorodioxalate phosphate, and lithium fluorosulfonate.

[0006] As an implementation scheme of this application, the value of n satisfies: 2≤n≤6.

[0007] As an embodiment of this application, the first additive includes cyanomethyl[3-(trimethoxysilyl)propyl]trithiocarbonate.

[0008] As an embodiment of this application, based on the total mass of the electrolyte, the mass percentage of the first additive is 0.1% to 2.0%.

[0009] As an embodiment of this application, based on the total mass of the electrolyte, the mass percentage of the second additive is 0.1% to 3.0%.

[0010] As an embodiment of this application, the mass percentage ratio of the first additive to the second additive is 1:(0.2~5).

[0011] A second aspect of this application provides a secondary battery, the secondary battery comprising a positive electrode, a separator, a negative electrode, and the electrolyte described in the first aspect of this application.

[0012] As an embodiment of this application, the positive electrode includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector. The positive active layer includes a positive active material, which includes a first positive active material and a second positive active material. The first positive active material includes a lithium phosphate. The second positive active material includes at least one of lithium transition metal oxide or its modified compound.

[0013] As an embodiment of this application, the lithium transition metal oxide comprises secondary particles composed of primary particles.

[0014] As an embodiment of this application, the lithium phosphate comprises secondary particles composed of primary particles.

[0015] As an embodiment of this application, the negative electrode sheet includes a negative electrode active material, the negative electrode active material comprises secondary particles composed of primary particles, and the primary particles of the negative electrode active material include a negative electrode active core and a carbon coating layer located on the surface of the negative electrode active core.

[0016] A third aspect of this application provides an electrical device that includes the secondary battery described in the second aspect of this application.

[0017] Compared with the prior art, the beneficial effects of this application are: The electrolyte described in this application contains trithiocarbonate additives with cyano and trimethoxysilane substituents, as well as a lithium salt second additive. The synergistic effect of the two can significantly improve the interface stability of the secondary battery and reduce the interface internal resistance, thereby improving the cycle and storage performance under high voltage and reducing the risk of high-temperature gas generation. Detailed Implementation

[0018] 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.

[0019] 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.

[0020] 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.

[0021] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0022] In a first aspect, this application provides an electrolyte comprising a lithium salt, a non-aqueous solvent, a first additive, and a second additive. The first additive is a compound having the structure shown in Formula I: Formula I In Equation I, n is a natural number, 1 ≤ n ≤ 8; The second additive includes at least one of lithium difluorooxalate borate (LiODFB), lithium difluorodioxalate phosphate (LiODFP), and lithium fluorosulfonate (LiFS).

[0023] The compound shown in Formula I is a trithiocarbonate additive containing cyano and trimethoxysilane substituents. The trithiocarbonate functional group increases the sulfur-containing compound content in the SEI film-forming component of the negative electrode, which helps reduce the chemical reactivity of the SEI and thus improves interfacial stability. The cyano functional group on the compound of Formula I can combine with the outer layer of transition metal ions of the positive electrode active material through coordination, lowering the energy barrier of the film-forming reaction at the positive electrode interface, thereby promoting the formation of a passivation layer on the surface of the positive electrode, improving the interfacial stability of the electrolyte in the positive electrode active layer, and effectively inhibiting the dissolution of transition metal ions in the positive electrode active layer. The trimethoxysilane functional group in the compound of Formula I can adsorb trace amounts of H2O and HF in the electrolyte, effectively mitigating the corrosion of the positive and negative electrode interfaces by acidic substances by inhibiting lithium salt hydrolysis and reducing the generation of acidic byproducts, thus significantly improving the interfacial stability of the secondary battery.

[0024] This application also adds a second additive with a larger anionic radius to the electrolyte. The second additive with a larger anionic radius has a higher degree of dissociation, allowing the anions to migrate to the positive electrode interface under the influence of the electric field during charging. There, they preferentially lose electrons and oxidize, forming a stable and homogeneous passivation layer on the positive electrode surface. This suppresses subsequent electrolyte side reactions, thereby reducing battery self-discharge, improving storage capacity retention, and lowering the risk of gas generation at high temperatures. Furthermore, the second additive can interact with the first additive, adjusting the film-forming reaction pathway of the first additive and reducing the Li2S content in the film-forming component of the first additive. Li2S has poor ion conductivity, and an increase in its content will worsen battery impedance, causing battery kinetic losses. Moreover, the second additive itself contains lithium salt, which can alleviate the consumption of active lithium during film formation, thereby improving cycle and storage life.

[0025] The synergistic effect of the first and second additives enables the development of long-life, high-voltage, and safe rechargeable batteries.

[0026] In some embodiments of this application, the value of n satisfies: 2 ≤ n ≤ 6. The value of n achieves a balance between "rapid film formation" and "stable film formation." The molecular migration rate meets the requirements of fast charging, while the stability of the molecular structure is sufficient to resist oxidation attacks under high voltage. The interfacial adsorption of cyano groups, the water-suppressing and acid-removing effects of trimethoxysilane, and the excellent film-forming effect of trithiocarbonate can work synergistically to form an SEI film with both high ion conductivity and high stability. This not only facilitates rapid ion transport during fast charging but also suppresses interfacial side reactions, improving the reversibility of high-voltage fast charging. Specifically, the value of n can be any one of 2, 3, 4, 5, or 6, or a range formed by any two values.

[0027] In some embodiments of this application, the first additive comprises cyanomethyl[3-(trimethoxysilyl)propyl]trithiocarbonate.

[0028] In some embodiments of this application, the mass percentage of the first additive is 0.1% to 2.0% based on the total mass of the electrolyte. Within this range, the mass percentage of the first additive in the electrolyte can form a stable and dense interfacial passivation film on the positive and negative electrode surfaces, thus improving interfacial stability. Specifically, the mass percentage of the first additive in the electrolyte can be any one of 0.1%, 0.2%, 0.5%, 1.0%, 1.5%, or 2.0%, or a range formed by any two of these values.

[0029] In some embodiments of this application, the mass percentage of the second additive is 0.1% to 3.0% based on the total mass of the electrolyte. The second additive can directly participate in the film-forming reaction, thereby further improving battery life. The specific mass percentage of the second additive in the electrolyte can be any one of 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, and 3.0%, or a range formed by any two values.

[0030] In some embodiments of this application, the mass percentage ratio of the first additive to the second additive is 1:(0.2~5). Within this range, the prepared secondary battery can possess both low internal resistance and long cycle life. Specifically, the mass percentage ratio of the first additive to the second additive can be any ratio or a range formed by any two of the following: 1:0.2, 1:1, 1:2, 1:3, 1:4, 1:5.

[0031] In some embodiments of this application, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonylimide (LiFSI). Lithium hexafluorophosphate exhibits moderate ion transference number, moderate dissociation constant, good antioxidant properties, and good aluminum foil passivation capability in commonly used non-aqueous organic solvents, and can be matched with various positive and negative electrode materials, making it the most important lithium salt in secondary batteries. Lithium bisfluorosulfonylimide has a larger anionic structure and weaker cation-anion interaction, resulting in higher dissociation in the electrolyte, which helps to improve ionic conductivity.

[0032] In some embodiments of this application, the mass percentage of the lithium salt is 8.0% to 21.5% based on the total mass of the electrolyte. Maintaining the mass percentage of lithium salt in the electrolyte within this suitable range allows the prepared secondary battery to possess both high ionic conductivity and long cycle life. Specifically, the mass percentage of lithium salt in the electrolyte can be any one of 8.0%, 12.8%, 20.0%, and 21.5%, or a range formed by any two of these values.

[0033] In this application, no particular limitation is made to non-aqueous solvents; all commonly used non-aqueous solvents in the art can be used to prepare the electrolyte in this application. Exemplarily, the non-aqueous solvents include, but are not limited to, at least two of the following: dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), butenyl carbonate (BC), methyl methyl carbonate (MPC), diphenyl carbonate (DPhC), methyl formate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), γ-butyrolactone (γ-GBL), acetonitrile (AN), sulfolane (TMS), ethylene carbonate (EC), propylene carbonate (PC), and fluoroethylene carbonate (FEC).

[0034] In this application, other additives can also be added to the electrolyte according to performance requirements, as long as they can meet the corresponding performance requirements and do not affect the cycle performance of the secondary battery. For example, additives such as ethylene sulfate (DTD) and lithium difluorophosphate (LiPOF2) can also be added to the electrolyte.

[0035] A second aspect of this application provides a secondary battery, the secondary battery comprising a positive electrode, a separator, a negative electrode, and the electrolyte described in the first aspect of this application. The positive electrode comprises a positive current collector and a positive active layer disposed on at least one surface of the positive current collector. The positive active layer comprises a positive active material, the positive active material comprising a first positive active material and a second positive active material, the first active material and the second active material being physically mixed and dispersed in the positive active layer. The first positive active material comprises a lithium phosphate; the second positive active material comprises at least one of a lithium transition metal oxide or a modified compound thereof.

[0036] In some embodiments of this application, commonly used lithium-containing phosphates can be used to prepare secondary batteries. These lithium-containing phosphates include, but are not limited to, at least one of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites. The addition of lithium-containing phosphates can improve the cycle stability of the secondary battery.

[0037] In some embodiments of this application, the lithium transition metal oxide includes, but is not limited to, at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, and lithium nickel manganese cobalt oxide. Modification of the lithium transition metal oxide can be achieved through carbon coating, elemental doping, etc. The addition of lithium transition metal oxides can give the secondary battery a higher capacity.

[0038] In some embodiments of this application, the lithium transition metal oxide comprises secondary particles composed of primary particles.

[0039] In some embodiments of this application, the lithium phosphate comprises secondary particles consisting of primary particles.

[0040] In some embodiments of this application, the positive electrode active material includes secondary particles composed of primary lithium transition metal oxide particles, secondary particles composed of primary lithium phosphate particles, primary lithium transition metal oxide particles, and primary lithium phosphate particles. Primary particles have a smaller particle size, larger specific surface area, shorter lithium-ion migration pathways, and better kinetic performance; secondary particles have a larger particle size, higher compaction density, and larger capacity. Secondary particles can combine the advantages of both large and small particle sizes. Furthermore, the secondary particle positive electrode active material formed using a granulation process can increase the isotropy of the positive electrode active material, improving the battery's initial efficiency and kinetic performance.

[0041] The first additive in the electrolyte of this application contains cyano, trimethoxysilane, and trithiocarbonate functional groups. Among them, the cyano group, as a strong field ligand, can coordinate with transition metal ions such as Ni, Mn, Co, and Fe in the positive electrode active material, and adsorb at the positive electrode interface under the action of coordination bonds, effectively promoting the formation of a passivation layer at the positive electrode / electrolyte interface, and significantly suppressing side reactions and the dissolution of transition metal ions at the positive electrode interface.

[0042] In some embodiments of this application, the surface of the lithium phosphate is coated with a lithium transition metal oxide. Coating the surface of the lithium phosphate with the lithium transition metal oxide helps to improve the stability of the lithium phosphate. The first additive contains a cyano group, which, as a strong field ligand, can coordinate with transition metal ions such as Ni, Mn, Co, and Fe in the positive electrode active material. The lithium transition metal oxide is located on the surface of the lithium phosphate, which helps to reduce side reactions at the positive electrode interface and the dissolution of transition metal ions.

[0043] In some embodiments of this application, the positive electrode active layer further includes a conductive agent and a binder. This application does not limit the types of conductive agents and binders in the positive electrode active layer. The conductive agent includes, but is not limited to, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene, and carbon nanofibers. The binder includes, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0044] In some embodiments of this application, the negative electrode sheet includes a negative current collector and a negative active layer disposed on at least one side surface of the negative current collector, wherein the negative active layer includes a negative active material.

[0045] In some embodiments of this application, the negative electrode sheet includes a negative electrode active material, which comprises secondary particles composed of primary particles. Each primary particle of the negative electrode active material includes a negative electrode active core and a carbon coating layer on the surface of the negative electrode active core. The carbon coating layer reduces the coverage area of ​​the dense film formed by the first additive on the surface of the negative electrode active material. It is understood that, based on the carbon coating layer covering the primary particles, on the one hand, it can improve the structural stability of the material used for the negative electrode active core, suppress particle breakage caused by volume expansion / contraction of the negative electrode active material during charging and discharging, reduce side reactions at the fresh interface, alleviate electrolyte consumption and active lithium loss, and improve the lifespan of the secondary battery. On the other hand, the carbon layer on the surface of the primary particles has good conductivity, constructing ample electron pathways on the surface of the negative electrode active material, which can improve the electronic conductivity of the negative electrode active material, improve the kinetic performance of the secondary battery, and simultaneously reduce the non-uniformity of the interface electron distribution, reduce the risk of interface lithium plating, and improve the safety of the secondary battery.

[0046] This application does not specifically limit the type of material used for the negative electrode active core; any negative electrode active core known in the art for use in batteries can be used. As an example, the material used for the negative electrode active core may include at least one of the following: artificial graphite (AG), natural graphite (NG), soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials; other conventional materials that can be used as negative electrode active materials in batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0047] In some embodiments, the electrolyte system of this application is used, and combined with the above-mentioned positive and negative active materials, so that the positive and negative electrodes are matched with each other, which helps to improve the overall lifespan, fast charging capability and safety of the secondary battery.

[0048] In some embodiments of this application, the negative electrode active layer further includes a conductive agent, a dispersant, and a binder. This application does not limit the types of conductive agents, dispersants, and binders in the negative electrode active layer. The conductive agents include, but are not limited to, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene, and carbon nanofibers. The dispersants include, but are not limited to, sodium carboxymethyl cellulose (CMC). The binders include, but are not limited to, polyacrylic acid (PAA).

[0049] In some embodiments of this application, there are no restrictions on the type of solvent used to form the positive electrode slurry and / or negative electrode slurry, as long as it is a solvent that can dissolve or disperse the positive electrode active material, the negative electrode active material, the conductive agent, the binder, or the dispersant.

[0050] Commonly used positive and negative current collectors in this field can be used to prepare the secondary battery described in this application. The negative current collector can be copper foil or carbon-coated copper foil. The positive current collector can be made of metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum; carbon materials such as carbon cloth and carbon paper; or composite materials formed by polymers and metal layers. In some embodiments, aluminum foil is used as the positive current collector.

[0051] This application does not limit the type of diaphragm; any known porous diaphragm with good chemical and mechanical stability can be selected.

[0052] In some embodiments of this application, the diaphragm material may be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The diaphragm may be a single-layer film or a multi-layer composite film, without particular limitation. When the diaphragm is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0053] In some embodiments of this application, the preparation of the secondary battery includes: stacking the positive electrode, separator, and negative electrode in sequence, so that the separator is positioned between the positive and negative electrode to act as a separator, then winding it into a square bare cell, installing it into a battery casing, then baking it at 65~95°C to remove water, injecting electrolyte, sealing it, and then undergoing processes such as standing, hot and cold pressing, formation, clamping, and capacity testing to obtain the secondary battery.

[0054] In some embodiments of this application, the secondary battery may include an outer packaging, which 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-type soft pack. The material of the soft pack can be plastic, such as one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate, or an aluminum-plastic film, such as an aluminum-plastic film formed by a composite of a PA layer, an aluminum layer, and a PP layer. The shape of the secondary battery is not particularly limited; it can be cylindrical, square, or any other arbitrary shape.

[0055] A third aspect of this application provides an electrical device, which includes the secondary battery described in the second aspect of this application. The electrical device can be an application device such as a vehicle, mobile phone, portable device, laptop computer, ship, spacecraft, electric toy, and power tool. Vehicles can be new energy vehicles, including pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles; spacecraft include airplanes, rockets, space shuttles, and spacecraft; electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers. This application does not impose special limitations on the above-described device.

[0056] The following are specific embodiments of this application, and the technical solutions of this application are further described in conjunction with the embodiments. However, this application is not limited to these embodiments. Unless otherwise specified, the reagents, methods, and equipment used in this application are all conventional reagents, methods, and equipment in this technical field.

[0057] Example 1 This embodiment provides a secondary battery, the preparation method of which includes the following steps: Preparation of positive electrode sheet The positive electrode active material, conductive agent acetylene black (ACET), and binder polyvinylidene fluoride (PVDF) were mixed evenly at a mass ratio of positive electrode active material:ACET:PVDF = 94:3:3. This mixture was then uniformly dispersed in 1-methyl-2-pyrrolidone (NMP) to prepare a uniform black slurry. The mixed black slurry was coated onto both sides of the positive electrode current collector aluminum foil, and after baking, rolling, and cutting, the positive electrode sheet was obtained. The compacted density of the positive electrode active material layer was 3.45 g / cm³. 3 ; In this embodiment, the positive electrode active material is LiFePO4 and Li(Ni) 0.8 Mn 0.1 Co 0.1The mixed positive electrode active material is obtained by mixing O2 (abbreviated as NMC811) in a mass ratio of 7:3, wherein the secondary particles of LiFePO4 have a particle size Dv50 of 1.5μm, and Li(Ni) 0.8 Mn 0.1 Co 0.1 The particle size Dv50 of the secondary particles of O2 is 10.6 μm. The particle size of the secondary particles of the positive electrode active material was obtained by particle size analysis laser diffraction.

[0058] Preparation of negative electrode sheet Artificial graphite (AG), a negative electrode active material, acetylene black (ACET), carbon nanotubes (CNT), and binder SBR were mixed evenly in a mass ratio of AG:ACET:CNT:SBR = 94:2:1:3, and then uniformly dispersed in deionized water to form a uniform black slurry. This slurry was then coated onto both sides of a copper foil, followed by baking and rolling (compacted density 1.65 g / cm³). 3 The negative electrode sheet is obtained after slitting and cutting. In this embodiment, the negative electrode active material is a secondary particle formed from primary particles, and the primary particles of the negative electrode active material are artificial graphite with amorphous carbon coated on the surface.

[0059] Preparation of electrolyte S1: At room temperature (25°C), in a glove box filled with argon (H2O<1ppm, O2<1ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed evenly at a mass ratio of 40:60, and water is removed using a 4Å molecular sieve to obtain a mixed solvent. S2: Add 12.80% of lithium salt LiPF6 based on the total weight of the electrolyte to the mixed solvent. During the addition of lithium salt, stir continuously and use dry ice to cool down to ensure that the electrolyte temperature rise does not exceed 2°C, and finally obtain a colorless and transparent liquid. S3: Add electrolyte additives to the transparent liquid obtained in step S2 to obtain an electrolyte. The composition and mass percentage of the electrolyte additives in the electrolyte are detailed in Table 1.

[0060] Preparation of secondary batteries The positive electrode, separator, and negative electrode are stacked in sequence, with the separator width greater than the width of the positive and negative electrode sheets, so that the separator acts as an insulator between the positive and negative electrode sheets. Then, they are wound to obtain a bare battery. The bare battery is then encapsulated in an aluminum-plastic film. After drying the dry cell in an oven for more than 12 hours to remove moisture, electrolyte is injected (injection coefficient of 5.0 g / Ah). Following vacuum sealing, settling, formation, and hot pressing, a secondary battery is obtained. Subsequently, formation and capacity testing are performed to obtain an activated secondary battery. The formation process involves placing the secondary battery at 25±1℃ and letting it stand for 30 minutes. Then, it is charged to 3.75V at a charging rate of 0.05C and left to stand for 5 minutes. During the formation process, the swelling of the secondary battery is observed every hour. If the battery swelling exceeds 100%, it is immediately removed from the cabinet to avoid safety risks.

[0061] Examples 2-21, Comparative Examples 1-3 A series of secondary batteries are provided, and the preparation method is the same as in Example 1. The difference from Example 1 is that the composition and amount of electrolyte additives, the composition of positive electrode active material, etc. are detailed in Table 1.

[0062] Example 22 A secondary battery is provided, and the preparation method is the same as in Example 4 (the preparation method of Example 4 is the same as in Example 1). The difference from Example 4 is that the positive electrode active material is a composite positive electrode active material, which is Li(Ni) coated on the surface of LiFePO4. 0.8 Mn 0.1 Co 0.1 O2 layer, denoted as LiFePO4@NMC811, LiFePO4 and Li(Ni) 0.8 Mn 0.1 Co 0.1 The mass ratio of O2 is the same as in Example 1.

[0063] Example 23 A secondary battery is provided, prepared according to Example 4 (the preparation method of Example 4 is the same as that of Example 1). The difference from Example 4 is that both positive electrode active materials are primary particles; the particle size Dv50 of the primary LiFePO4 particles is 1.5 μm, and the Li(Ni)... 0.8 Mn 0.1 Co 0.1 The particle size Dv50 of the primary particles of O2 is 10.6 μm. The particle size of the primary particles of the positive electrode active material was obtained by measuring the particle size distribution.

[0064] It should also be noted that primary and secondary particles in the positive electrode active material can be identified by particle size analysis laser diffraction.

[0065] Comparative Example 4 A series of secondary batteries are provided, and the preparation method is the same as that in Example 4 (the preparation method of Example 4 is the same as that in Example 1). The difference between Comparative Example 4 and Example 4 is that the first additive is replaced by a mixed additive after mixing the following compounds A and B in a 1:1 molar ratio. The amount of the mixed additive and the first additive added in the electrolyte remains unchanged. Compound A Compound B.

[0066] Table 1 The performance of the secondary batteries obtained in the above embodiments and comparative examples was tested. The specific test items and methods are as follows, and the results are shown in Table 2: (1) Initial DCR: The secondary batteries obtained in the examples and comparative examples were placed at 25±1℃ and charged and discharged for 3 weeks at a charge-discharge rate of 0.33C / 0.33C within a range of 2.5~4.25V. The average discharge capacity of the secondary batteries over the 3 weeks was recorded and used as the nominal capacity.

[0067] At 25±1℃, the secondary batteries obtained from each implementation case and comparative example were charged to 4.25V at 0.33C, then charged at a constant voltage to 0.05C, and then discharged at 0.33C for 90 minutes. After adjusting to 50% SOC, they were pulsed with a constant current of 10C for 10 seconds. The SOC was then adjusted to 50% according to the above method, and then charged at a constant current of 4C for 10 seconds. The DCR was calculated as (voltage before pulse discharge - voltage after pulse discharge) / discharge current.

[0068] (2) Cyclic performance test at 25℃: The secondary batteries obtained in the examples and comparative examples were placed at 25±1℃ and charged and discharged for 3 weeks at a charge-discharge rate of 0.33C / 0.33C within a range of 2.5~4.25V. The average discharge capacity of the secondary batteries over the 3 weeks was recorded and used as the nominal capacity.

[0069] The secondary batteries obtained from the examples and comparative examples were subjected to constant current (1C / 1C) and constant voltage (4.25V constant voltage to current less than 0.05C) charge-discharge cycle tests in the range of 2.5~4.25V, and the discharge specific capacity of the secondary batteries in the first cycle and the discharge specific capacity after 1000 cycles were recorded.

[0070] The capacity retention rate over 1000 cycles = discharge specific capacity over 1000 cycles / discharge specific capacity in the first cycle × 100%. The recorded data is shown in Table 2.

[0071] (3) 45℃ cycle performance test: The secondary batteries obtained in the examples and comparative examples were placed at 25±1℃ and charged and discharged for 3 weeks at a charge-discharge rate of 0.33C / 0.33C within a range of 2.5~4.25V. The average discharge capacity of the secondary batteries over the 3 weeks was recorded and used as the nominal capacity.

[0072] The secondary batteries obtained in the examples and comparative examples were placed in an oven at 45±1℃ and left to stand for 30 minutes. Then, constant current (1C / 1C) and constant voltage (4.25V constant voltage to current less than 0.05C) charge-discharge cycle tests were performed in the range of 2.8~4.25V, and the discharge specific capacity of the secondary batteries in the first cycle and the discharge specific capacity after 1000 cycles were recorded.

[0073] The capacity retention rate over 1000 cycles = discharge specific capacity over 1000 cycles / discharge specific capacity in the first cycle × 100%. The recorded data is shown in Table 2.

[0074] (4) Gas production test during storage at 70℃: The secondary batteries obtained in the examples and comparative examples were placed at 25±1℃ and charged and discharged for 3 weeks at a charge-discharge rate of 0.33C / 0.33C within a range of 2.5~4.25V. The average discharge capacity of the secondary batteries over the 3 weeks was recorded and used as the nominal capacity.

[0075] Charge the battery to 4.25V at a constant current rate of 0.33C, then maintain a constant voltage of 4.25V until the current drops below 0.05C to bring it to a fully charged state of 4.25V. After that, store it at 60±1℃.

[0076] The volume of the fully charged secondary battery before storage was measured and recorded as V0. The fully charged secondary battery was then placed in an oven at 70±1℃ for 30 days. After that, the battery was removed and its volume after storage was immediately measured and recorded as V1. The volume expansion rate = (V1–V0) / V0×100%, and the results are shown in Table 2.

[0077] Table 2 The results above show that: This application improves the composition of the electrolyte by adding trithiocarbonate additives containing cyano and trimethoxysilane substituents, as well as lithium salt second additives, which can significantly improve the interfacial stability and reduce the interfacial internal resistance of the secondary battery, thereby enhancing the cycle stability of the secondary battery under high voltage.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application 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 this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. An electrolyte, characterized in that, The electrolyte includes lithium salt, non-aqueous solvent, first additive, and second additive; The first additive is a compound having the structure shown in Formula I: Formula I In Equation I, n is a natural number, 1 ≤ n ≤ 8; The second additive includes at least one of lithium difluorooxalate borate, lithium difluorodioxalate phosphate, and lithium fluorosulfonate.

2. The electrolyte according to claim 1, characterized in that, The value of n satisfies: 2≤n≤6.

3. The electrolyte according to claim 1, characterized in that, The first additive includes cyanomethyl[3-(trimethoxysilyl)propyl] trithiocarbonate.

4. The electrolyte according to claim 1, characterized in that, Based on the total mass of the electrolyte, the mass percentage of the first additive is 0.1% to 2.0%; And / or, based on the total mass of the electrolyte, the mass percentage of the second additive is 0.1% to 3.0%.

5. The electrolyte according to claim 1, characterized in that, The mass percentage ratio of the first additive to the second additive is 1:(0.2~5).

6. A secondary battery, characterized in that, The secondary battery includes a positive electrode, a separator, a negative electrode, and the electrolyte as described in any one of claims 1 to 5.

7. The secondary battery according to claim 6, characterized in that, The positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector. The positive active layer includes a positive active material, and the positive active material includes a first positive active material and a second positive active material. The first positive electrode active material includes lithium phosphate; The second positive electrode active material includes at least one of lithium transition metal oxides or modified compounds thereof.

8. The secondary battery according to claim 7, characterized in that, It satisfies at least one of the following characteristics: (1) The lithium transition metal oxide comprises secondary particles composed of primary particles; (2) The lithium phosphate contains secondary particles composed of primary particles.

9. The secondary battery according to claim 6, characterized in that, The negative electrode sheet includes a negative electrode active material, which comprises secondary particles composed of primary particles. The primary particles of the negative electrode active material include a negative electrode active core and a carbon coating layer located on the surface of the negative electrode active core.

10. An electrical appliance, characterized in that, The electrical equipment includes a secondary battery as described in any one of claims 6 to 9.

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