Electrolytes, secondary batteries and electrical equipment

Abstract

This application discloses an electrolyte, a secondary battery, and an electrical device. The electrolyte comprises a non-aqueous organic solvent, a lithium salt, and a first additive. The first additive of this application contains both isocyanate groups and silicon-oxygen bonds. The isocyanate groups can remove trace amounts of water from the electrolyte and inhibit the generation of hydrofluoric acid, reducing the continuous decomposition of the electrolyte and reducing the damage of hydrofluoric acid to the interfacial film and corrosion of the silicon-carbon anode, inhibiting the dissolution of transition metal ions, improving the structural stability of the interfacial film, the cycle stability of the battery, and reducing its high-temperature gas generation performance. The silicon-oxygen bonds can form an effective network structure on the surface of the silicon-carbon anode, enhancing the flexibility of the interfacial film, thereby inhibiting the volume expansion of the silicon-carbon anode and also improving the cycle stability to a certain extent. Therefore, the electrolyte of this application can improve the performance of the secondary battery.

Description

Technical Field

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

[0002] Currently, lithium-ion batteries are one of the power sources for electric vehicles. Some lithium-ion batteries use silicon-carbon anodes and fluorinated electrolytes. Upon initial contact with the silicon-carbon anode, the fluorinated electrolyte forms an inorganic or organic composite solid interface film (SEI film) on the surface of the anode, which is crucial for the cycle stability of lithium-ion batteries. However, current fluorinated electrolytes contain trace amounts of water, which can generate hydrogen fluoride (HF). This HF reduces the structural stability of the SEI film, causing the unstable SEI film to gradually thicken locally during cycling, resulting in poor film quality and a continuous decrease in the cycle capacity of the lithium-ion battery, leading to poor cycle stability. Furthermore, HF can cause corrosion of the positive and negative electrodes, dissolution of transition metal ions, and continuous decomposition of the electrolyte, resulting in severe gas generation during high-temperature storage. Summary of the Invention

[0003] This application provides an electrolyte, a secondary battery, and an electrical device, which solves the problems of poor cycle stability and severe gas generation during high-temperature storage in secondary batteries using silicon-carbon as the negative electrode active material.

[0004] This application provides an electrolyte comprising: a non-aqueous organic solvent, a lithium salt, and a first additive.

[0005] In some embodiments, the first additive comprises a compound having the structure shown in Formula I:

[0006]

[0007] R1 and R2 are independently selected from fluorine atom (-F), chlorine atom (-Cl), bromine atom (-Br) or hydrogen atom (-H).

[0008] R3 is selected from alkylene groups having 3 to 9 carbon atoms, substituted or unsubstituted with a first substituent; the first substituent includes methyl, ethyl, cyano, or halogen groups.

[0009] R4, R5, and R6 are each independently selected from alkyl groups having 1 to 3 carbon atoms, either substituted or unsubstituted with a second substituent, the second substituent including a halogen group.

[0010] In some embodiments, the first additive comprises one or more of the compounds shown in the following structural formulas:

[0011]

[0012]

[0013] In some embodiments, the mass percentage of the first additive may be 0.1 wt% to 3.0 wt% based on the total mass of the electrolyte.

[0014] In some embodiments, the electrolyte further includes a negative electrode film-forming additive, which includes one or more of fluoroethylene carbonate, sulfate compounds, or sulfite compounds.

[0015] Optionally, the sulfate ester compound includes one or more of ethylene sulfate, 1,3-propanediol cyclosulfate, dimethyl sulfate, methyl ethyl sulfate, dipropyl sulfate, or diisopropyl sulfate.

[0016] Sulfite compounds include one or both of vinyl sulfite or vinyl vinyl sulfite.

[0017] In some embodiments, the negative electrode film-forming additives include fluoroethylene carbonate and ethylene sulfate.

[0018] In some embodiments, the mass percentage of fluoroethylene carbonate is 5.0 wt% to 18.0 wt%, and the mass percentage of ethylene sulfate is 0.2 wt% to 3.0 wt%, based on the total mass of the electrolyte.

[0019] Based on the total mass of the electrolyte, the mass percentage of fluoroethylene carbonate is W1% and the mass percentage of ethylene sulfate is W2%. Then the relationship between W1 and W2 satisfies W1 = (1.67~36) × W2.

[0020] In some embodiments, based on the total mass of the electrolyte, the mass percentage of the first additive is M1 and the mass percentage of the negative electrode film-forming additive is M2, then the relationship between M1 and M2 satisfies 0.65≤M1+0.1×M2≤5.1.

[0021] Based on the total mass of the electrolyte, the mass percentage of the first additive is M1, the mass percentage of the negative electrode film-forming additive is M2, and the mass percentage of the lithium salt is M3. Then the relationship between M1, M2, and M3 satisfies M3 < M1 + 1.8 × M2.

[0022] In some embodiments, the non-aqueous organic solvent includes any two or more of ethylene carbonate (EC), propylene carbonate (PC), butenyl carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), 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), and sulfolane (TMS).

[0023] In some embodiments, the lithium salt comprises lithium hexafluorophosphate (LiPF6) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and the mass percentage of the lithium salt is 10 wt% to 15 wt% based on the total mass of the electrolyte.

[0024] This application provides a secondary battery comprising: a positive electrode, a separator, a negative electrode, and an electrolyte as described in any of the above embodiments.

[0025] The negative electrode includes a negative electrode active material, which includes SiO₂. x Silicon-carbon composite materials formed by combining graphite and graphite, where 1 ≤ x ≤ 2.

[0026] The mass percentage of the first additive in the electrolyte and the SiO content in the negative electrode active material x The mass percentage ratio is (0.01~0.2):1.

[0027] This application provides an electrical device that includes the secondary battery described in the above embodiments, with the secondary battery serving as the power supply for the electrical device.

[0028] The electrolyte in this application embodiment has at least the following technical effects:

[0029] The electrolyte of this application incorporates a first additive containing both isocyanate groups (NCO) and silicon-oxygen bonds (Si-O). The isocyanate groups (NCO) remove trace amounts of water from the electrolyte, thereby inhibiting the generation of hydrogen fluoride (HF), reducing the continuous decomposition of the electrolyte, and decreasing the damage of HF to the SEI film and corrosion of the silicon-carbon anode. It also inhibits the dissolution of transition metal ions, improving the structural stability of the SEI film, the cycle stability of the battery, and its high-temperature gas generation performance. The silicon-oxygen bonds (Si-O) form an effective network structure on the surface of the silicon-carbon anode, enhancing the flexibility of the SEI film, thereby inhibiting the volume expansion of the silicon-carbon anode and also improving cycle stability to some extent. Therefore, the electrolyte of this application can improve the performance of secondary batteries. Detailed Implementation

[0030] The technical solution of this application will be clearly and completely described below with reference to various embodiments. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0031] This application provides an electrolyte, a secondary battery, and an electrical device. These are described in detail below. It should be noted that the order of description of the following embodiments is not intended to limit the preferred order of the embodiments.

[0032] In this specification, the range of values ​​indicated by “~” represents the range containing the minimum and maximum values ​​recorded before and after “~”, respectively.

[0033] In this application, "substitution" means that the hydrogen atom in the substituent is replaced by the substituent. "Substituted or unsubstituted" means that the hydrogen atom in the defined group may be completely or partially substituted, or may not be substituted.

[0034] This application provides an electrolyte for lithium-ion batteries. The electrolyte comprises a non-aqueous organic solvent, a lithium salt, and a first additive.

[0035] The first additive includes a compound having the structure shown in Formula I:

[0036]

[0037] R1 and R2 are independently selected from fluorine (-F), chlorine (-Cl), bromine (-Br), or hydrogen (-H) atoms, respectively. R1 and R2 can be highly electronegative groups, preferably fluorine atoms. This carbon-fluorine bond (CF bond) is easily broken. When the CF bond breaks, a LiF-rich film can be formed on the surface of the silicon-carbon anode. This film, as a component of the SEI film, can enhance its mechanical strength. Chlorine (-Cl) or bromine (-Br) atoms can also play a similar role, but their effect is not as strong as that of fluorine atoms.

[0038] R3 is selected from alkylene groups having 3 to 9 carbon atoms, either substituted or unsubstituted with a first substituent. The first substituent includes methyl, ethyl, cyano, or halogen groups (-F, -Cl, or -Br). R3 serves to link the carbon-fluorine bond (CF bond) to the silicon-oxygen bond (Si-O). The number of carbon atoms in the R3 group should not be too high; if it is too high (e.g., more than 9 carbon atoms), the viscosity of the first additive increases and its reactivity decreases. Therefore, the number of carbon atoms in R3 is preferably in the range of 3 to 9. In this application, 3 to 9 carbon atoms refers to a number of carbon atoms that can be 3, 4, 5, 6, 7, 8, or 9.

[0039] R4, R5, and R6 are each independently selected from alkyl groups having 1 to 3 carbon atoms, either substituted or unsubstituted with a second substituent, including halogen groups (-F, -Cl, or -Br). The alkyl group with 1 to 3 carbon atoms can include methyl, ethyl, or propyl (n-propyl or isopropyl), meaning the number of carbon atoms can be 1, 2, or 3. The Si-O bond can form an effective network structure on the surface of the silicon-carbon anode, enhancing the flexibility of the SEI film and suppressing the volume expansion of silicon. The number of carbon atoms in R4, R5, and R6 cannot be too high, otherwise it will affect the network formation performance of the Si-O bond. Therefore, R4, R5, and R6 are preferably methyl (-CH3) or ethyl (-CH2CH3).

[0040] The first additive mentioned above contains isocyanate groups (NCO), which can remove trace amounts of water from the electrolyte, thereby inhibiting the generation of hydrofluoric acid (HF). The possible mechanism is as follows: Fluorinated electrolytes often contain fluorides (e.g., LiPF6), which decompose into Lewis acids (e.g., PF5) upon contact with trace amounts of water. These Lewis acids react with the negative electrode film-forming additives (e.g., fluoroethylene carbonate or FEC), causing the additives to decompose and produce hydrofluoric acid (HF). Hydrofluoric acid (HF) attacks the positive and / or negative electrode materials, thereby damaging the structure of the positive and negative electrodes of the lithium-ion battery. In severe cases, it may lead to the release of transition metal elements contained in the positive electrode, thus altering the electrolyte composition. Furthermore, due to the decomposition of the negative electrode film-forming additives, a stable SEI film cannot be formed on the negative electrode surface, exacerbating the corrosion of the negative electrode by hydrofluoric acid. The isocyanate group (NCO) in this application can react with trace amounts of water and hydrofluoric acid in the electrolyte (as shown in reaction equations 1 and 2 below), thus eliminating the trace amounts of water and hydrofluoric acid in the electrolyte. Therefore, it suppresses the side reactions caused by the trace amounts of water and hydrofluoric acid in the electrolyte, improves the stability of the electrolyte, inhibits the damage of hydrofluoric acid to the interfacial film and the corrosion of the positive and negative electrodes, and inhibits the dissolution of metal ions.

[0041] The following reaction equation 1 is the reaction between isocyanate groups (NCO) and trace amounts of water in the electrolyte.

[0042]

[0043] The value of n varies depending on the aggregation conditions; for example, it can be 5 to 20.

[0044] The following reaction equation 2 is the reaction between the isocyanate group (NCO) and hydrofluoric acid in the electrolyte.

[0045]

[0046] Wherein, the x group represents the part of the first additive's structural formula other than the isocyanate group (NCO).

[0047] Optionally, in some embodiments of this application, the structural formula of the first additive includes any one or more of the compounds shown in the following structural formulas. It should be noted that only the structural formula of the first additive is shown below, and its conformation is not limited.

[0048]

[0049]

[0050] Optionally, in some embodiments of this application, based on the total mass of the electrolyte, the mass percentage of the first additive is 0.1 wt% to 3.0 wt%, the mass percentage of the negative electrode film-forming additive is 5.0 wt% to 21.0 wt%, the mass percentage of the lithium salt is 10.0 wt% to 15.0 wt%, and the mass percentage of the non-aqueous organic solvent is the balance. The sum of the mass percentages of the first additive, the negative electrode film-forming additive, the lithium salt, and the non-aqueous organic solvent is 100 wt%.

[0051] In some embodiments of this application, the mass percentage of the first additive, based on the total mass of the electrolyte, can be 0.2 wt% to 2.8 wt%, 0.5 wt% to 2.5 wt%, 0.8 wt% to 2.0 wt%, 1.0 wt% to 1.8 wt%, or 1.2 wt% to 1.6 wt%. If the mass percentage of the first additive is less than 0.1 wt%, the electrolyte contains too few isocyanate groups (NCO), carbon-fluorine bonds (CF bonds), and silicon-oxygen bonds (Si-O), which cannot fully cover the negative electrode surface, or the passivation film formed on the negative electrode surface is too thin and easily cracked, failing to inhibit the expansion of the silicon-carbon negative electrode, resulting in poor final performance of the lithium-ion battery. If the mass percentage of the first additive is greater than 3.0 wt%, the passivation film formed on the negative electrode surface may be too thick, increasing the impedance and polarization of the lithium-ion battery, which will also affect the performance of the lithium-ion battery.

[0052] In some embodiments of this application, the electrolyte further includes a negative electrode film-forming additive. The negative electrode film-forming additive includes one or more of fluoroethylene carbonate (FEC), sulfate compounds, or sulfite compounds. Optionally, the sulfate compounds include one or more of ethylene sulfate (DTD), 1,3-propanediol cyclosulfate, dimethyl sulfate, methyl ethyl sulfate, dipropyl sulfate, or diisopropyl sulfate. Optionally, the sulfite compounds include one or both of vinyl sulfite (ES) or vinyl vinyl sulfite.

[0053] In some embodiments of this application, if the negative electrode film-forming additive is a single-component additive, then the mass percentage of each negative electrode film-forming additive added individually, based on the total mass of the electrolyte, can be any value from 5.0 to 21.0 wt%, or 6.0 to 20.0 wt%, or 10.0 to 18.0 wt%, or 11.0 to 16.5 wt%, or even 12.5 to 15.0 wt%.

[0054] In some embodiments of this application, if the negative electrode film-forming additive is a two-component additive, that is, the negative electrode film-forming additive contains two components: the first component is fluoroethylene carbonate (FEC), and the second component is any one of a sulfate ester compound or a sulfite ester compound. Therefore, based on the total mass of the electrolyte, the mass percentage of the first component (fluoroethylene carbonate, FEC) is 5.0–18.0 wt%, and the mass percentage of the second component (sulfate ester compound or sulfite ester compound) is 0.2–3.0 wt%. The mass percentage of the first component must be greater than the mass percentage of the second component. The first component (fluoroethylene carbonate, FEC) can enrich the interface film (SEI film) formed on the surface of the silicon-carbon negative electrode with LiF and polyethylene oxide-like compounds (PEO) to ensure the cycle performance of the silicon-carbon negative electrode. Furthermore, the higher the silicon content in the silicon-carbon negative electrode, the greater the amount of the first component added. The second component (sulfate or sulfite compounds) can introduce inorganic components into the formed interfacial film to improve ionic conductivity. However, excessive addition can affect the strength and toughness of the interfacial film, thus impacting the battery's cycle performance. The mass percentage of the first component (fluoroethylene carbonate, FEC) can be 7.0–16.0 wt%, 10.0–15.0 wt%, or 12.0–13.0 wt%. The mass percentage of the second component (sulfate or sulfite compounds) can be 0.3–2.9 wt%, 0.5–2.8 wt%, 0.8–2.5 wt%, or 1.0–2.0 wt%. For single-component or two-component anode film-forming additives, if the mass percentage of the anode film-forming additive is less than 5.0 wt%, a stable SEI film cannot be formed, making the silicon-carbon anode susceptible to corrosion and reducing cycle performance and lifespan. If the mass percentage of the negative electrode film-forming additive is greater than 21.0 wt%, the SEI film will be too thick, affecting conductivity and the transport speed of ions and electrons, which will also affect the performance of the secondary battery.

[0055] In some embodiments of this application, based on the total mass of the electrolyte, the mass percentage of fluoroethylene carbonate is W1% and the mass percentage of ethylene sulfate is W2%, then the relationship between W1 and W2 satisfies W1 = (1.67~36) × W2. The relationship between W1 and W2 can also satisfy W1 = (6~10) × W2.

[0056] In some embodiments of this application, the mass percentage of the first additive is M1 and the mass percentage of the negative electrode film-forming additive is M2, based on the total mass of the electrolyte. Then the relationship between M1 and M2 satisfies 0.65≤M1+0.1×M2≤5.1.

[0057] Based on the total mass of the electrolyte, the mass percentage of the first additive is M1, the mass percentage of the negative electrode film-forming additive is M2, and the mass percentage of the lithium salt is M3. Then the relationship between M1, M2, and M3 satisfies M3 < M1 + 1.8 × M2.

[0058] For example, the negative electrode film-forming additive contains 10.0–15.0 wt% fluoroethylene carbonate (FEC) and 1.0–2.0 wt% ethylene sulfate (DTD) based on the total mass of the electrolyte. Further, the negative electrode film-forming additive contains 10.0 wt% fluoroethylene carbonate (FEC) and 1.0 wt% ethylene sulfate (DTD) based on the total mass of the electrolyte.

[0059] In some embodiments of this application, the non-aqueous organic solvent includes any two or more of ethylene carbonate (EC), propylene carbonate (PC), butenyl carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), 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), and sulfolane (TMS).

[0060] In some embodiments of this application, lithium salts are used as electrolytes, including one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(oxaloyl)borate (LiBOB), lithium difluorooxaloylborate (LiDFOB), lithium difluorodioxaloyl phosphate (LiDFOP), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0061] The lithium salt can be a two-component lithium salt, including lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6), with the lithium salt accounting for 10 wt% to 15 wt% of the total mass of the electrolyte. For example, in a two-component electrolyte lithium salt, the content of lithium bis(fluorosulfonyl)imide (LiFSI) accounts for 5.0 to 7.5 wt% of the total mass of the electrolyte, and the content of lithium hexafluorophosphate (LiPF6) accounts for 5.0 to 7.5 wt% of the total mass of the electrolyte, together accounting for 10.0 to 15.0 wt% of the total mass of the electrolyte. For example, in a two-component electrolyte lithium salt, the content of lithium bis(fluorosulfonyl)imide (LiFSI) accounts for 7.5 wt% of the total mass of the electrolyte, and the content of lithium hexafluorophosphate (LiPF6) accounts for 5.0 wt% of the total mass of the electrolyte, together accounting for 12.5 wt% of the total mass of the electrolyte. In the preferred embodiments of this application, the use of a two-component electrolyte lithium salt has the following beneficial effects: Lithium hexafluorophosphate (LiPF6) in the non-aqueous organic solvents of this application exhibits a moderate ion transport number, a moderate dissociation constant, good antioxidant properties, and good passivation capability for negative electrode current collectors (such as aluminum foil). Lithium bis(fluorosulfonyl)imide (LiFSI) has high dissociation degree, thermal stability, and insensitivity to water, thus improving the stability of the electrolyte and the lithium-ion transport number. The synergistic effect of the two can improve the stability, conductivity, and other properties of the lithium-ion battery electrolyte. The mass percentage of lithium salt, based on the total mass of the electrolyte, is preferably in the range of 10.0–15.0 wt%. If the total lithium salt content is too low, it will affect the conductivity of the electrolyte. If the total lithium salt content is too high, it will increase the viscosity of the electrolyte, affecting not only the conductivity of the electrolyte but also the cycle performance of the lithium-ion battery.

[0062] This application also provides a method for preparing a first additive, the synthesis steps of which are shown in the following reaction process:

[0063]

[0064] In this case, R7 has two fewer carbon atoms than R3 in Formula I. Therefore, when R3 is selected from an alkylene group having 3 to 9 carbon atoms that has been substituted or unsubstituted with a first substituent, R7 is selected from an alkylene group having 1 to 7 carbon atoms that has been substituted or unsubstituted with a first substituent.

[0065] Specifically, taking fluorosiloxane isocyanates as an example, their preparation methods include:

[0066]

[0067] In the above reaction process, 2,2-difluoroglutaric acid was used as the starting material, and lithium aluminum hydride was used as the catalyst. The reaction was carried out at room temperature to obtain product 1, with a molar ratio of 2,2-difluoroglutaric acid to lithium aluminum hydride of 1:1. Then, using alumina as the catalyst, product 1 underwent dehydration at 300°C to generate product 2. Then, using chloroplatinic acid as the catalyst, product 2 underwent a hydrosilylation reaction with trimethoxysilane at 60°C to obtain product 3. Then, product 3 reacted with diphenylphosphoryl azide (DPPA) at 13°C to 17°C under the catalysis of diisopropylethylamine to obtain product 4, which is an acyl azide product. Then, the target product (fluorosiloxane isocyanate) was obtained by Curtius rearrangement reaction at 90°C.

[0068] This application also provides a method for preparing an electrolyte, comprising: mixing multiple components constituting a non-aqueous organic solvent uniformly in a proportion under an inert gas environment and at room temperature to obtain a non-aqueous organic solvent; gradually adding a lithium salt to the non-aqueous organic solvent and mixing uniformly, ensuring that the electrolyte temperature rise does not exceed 2°C during the addition process, otherwise the electrolyte composition will be damaged; then adding a first additive and mixing uniformly to obtain the electrolyte. Optionally, a negative electrode film-forming additive may also be added when adding the first additive.

[0069] This application also provides a secondary battery, comprising: a positive electrode, a separator, a negative electrode, and an electrolyte. The separator is located between the positive and negative electrode. The positive electrode, separator, and negative electrode can be stacked or wound to form a battery intermediate, and after injecting the electrolyte and undergoing post-processing steps, a secondary battery is obtained.

[0070] The positive electrode sheet includes a positive current collector and a layer of positive active material disposed on one or both surfaces of the positive current collector. The positive active material layer includes positive active material. The positive active material layer can be one or more layers. Each layer in a multilayer positive active material layer can contain the same or different positive active materials. The positive active material includes any material capable of reversibly inserting and deintercalating active ions such as lithium ions.

[0071] In some embodiments, the positive electrode active material includes a substance containing lithium and at least one transition metal. In some embodiments of this application, the positive electrode active material includes a substance with the general formula Li. a Ni 1-x-y Co x M y O2 compounds, wherein: 0.95≤x≤1.2, 0≤x≤0.5, 0≤y≤0.5, and 0≤x+y≤1, and M includes Mn and / or Al. Optionally, the positive electrode active material is LiNi. 0.8 Mn 0.1 Co 0.1 O2(NMC811).

[0072] The negative electrode includes a negative current collector and a layer of negative active material formed on one or both surfaces of the negative current collector, the negative active material layer containing negative active material. The negative active material layer can be one or more layers, and each layer in a multilayer negative active material layer can contain the same or different negative active materials. The negative active material includes any material capable of reversibly inserting and de-intercalating active ions such as lithium ions. In some embodiments, the rechargeable capacity of the negative active material is greater than the discharge capacity of the positive active material to prevent unintentional deposition of lithium metal on the negative electrode during charging. The negative active material includes SiO₂. x Silicon-carbon composite materials formed by combining graphite, 1≤x≤2. The mass percentage of the first additive in the electrolyte and the mass percentage of SiO in the negative electrode active material. x The mass percentage ratio is (0.01~0.2):1, or any value or range of any two values ​​from 0.02:1, 0.05:1, 0.10:1, 0.12:1, 0.15:1, 0.16:1, 0.18:1. The SiO₂ content in the negative electrode active material... x The mass percentage is 0.5% to 15%, or any value or range of any two values ​​from 1%, 2%, 5%, 8%, 10%, 12%, to 14%.

[0073] This application also provides an electrical device including the aforementioned secondary battery, which serves as the power supply for the device. The electrical devices described in this application include, but are not limited to, backup power supplies, motors, electric vehicles, electric motorcycles, electric bicycles, bicycles, power tools, and large household batteries.

[0074] The following description is based on specific embodiments.

[0075] Example 1

[0076] This embodiment provides an electrolyte composed of lithium salt, non-aqueous organic solvent, negative electrode film-forming additive, and a first additive.

[0077] The first additive is a substance with the following structural formula: fluorosiloxane isocyanate. The mass percentage of the first additive is 0.5 wt% based on the total mass of the electrolyte.

[0078]

[0079] The lithium salt is lithium hexafluorophosphate (LiPF6). The lithium salt accounts for 12.5 wt% of the total electrolyte mass.

[0080] The negative electrode film-forming additive is fluoroethylene carbonate (FEC). The mass percentage of the negative electrode film-forming additive is 10 wt% based on the total mass of the electrolyte.

[0081] The non-aqueous organic solvent is a mixture of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), with a mass ratio of EC / PC / EMC / DEC of 1:2:5:2. The non-aqueous organic solvent is added in the balance, such that the sum of the mass percentages of lithium salt, non-aqueous organic solvent, negative electrode film-forming additive, and the first additive is 100 wt%.

[0082] The electrolyte preparation method provided in this embodiment includes:

[0083] At room temperature, in a glove box filled with argon (H2O < 1 ppm, O2 < 1 ppm), EC, PC, EMC, and DEC were mixed thoroughly in a mass ratio of 1:2:5:2. Molecular sieves were used to remove water, resulting in a mixed solvent. Lithium salt LiPF6, equivalent to 12.5 wt% of the total electrolyte mass, was added sequentially to the obtained mixed solvent. The mixture was continuously stirred and cooled, ensuring the electrolyte temperature did not rise by more than 2°C. Lithium salt LiPF6 was then added until a colorless, transparent liquid was obtained. Then, 10 wt% of fluoroethylene carbonate (FEC) and 0.5 wt% of fluorosiloxane isocyanate (the first additive) were added separately, and the mixture was stirred until homogeneous to obtain the electrolyte of this embodiment.

[0084] This embodiment also provides a method for preparing a secondary battery, which includes the following steps:

[0085] The 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 tab welding, a bare cell is obtained. The bare cell is placed in an outer aluminum-plastic film package and baked in an oven at 85±10℃ for 24 hours to obtain a battery intermediate. The electrolyte prepared above (15g) is injected into the dried battery intermediate, and after standing, formation, and capacity testing, a secondary battery (i.e., a lithium-ion pouch battery) is obtained.

[0086] The preparation method of the positive electrode includes: using LiNi as the positive electrode active material. 0.8 Mn 0.1 Co 0.1 O2 (NMC811), conductive agent acetylene black (Super P), and binder polyvinylidene fluoride (PVDF) are mixed evenly in a mass ratio of NMC811:Super P:PVDF = 94:3:3, and then evenly dispersed in 1-methyl-2-pyrrolidone (NMP) to form a uniform black slurry. The mixed black slurry is then coated on both sides of an aluminum foil (as the positive electrode current collector), and after baking, rolling, and cutting, the positive electrode sheet is obtained.

[0087] The preparation method of the negative electrode includes: SiO2 x Graphite, conductive agent acetylene black (Super P), and binder SBR are mixed evenly in a mass ratio of graphite:Super P:SBR = 9.4:84.6:3:3, and then uniformly dispersed in deionized water to form a uniform black slurry. This black slurry is coated onto both sides of a copper foil (as the negative electrode current collector), and then baked, rolled, and cut to obtain the negative electrode sheet. The negative electrode active material is SiO₂. x It is composed of graphite and belongs to silicon-carbon composite materials. SiO₂ is one of them. x The mass ratio of SiO to graphite is 9.4:84.6. In this negative electrode active material, SiO... x The mass percentage of the first additive in the electrolyte is 10%. The mass percentage of the first additive in the electrolyte is 0.5 wt%. Therefore, the mass percentage of the first additive in the electrolyte is related to the mass percentage of SiO in the negative electrode active material. x The mass percentage is 0.05:1.

[0088] The component allocations of the various embodiments and comparative examples of this application are shown in Table 1, and the proportional relationships of the added amounts are shown in Table 2. Battery performance tests were conducted on the secondary batteries obtained from the various embodiments and comparative examples of this application, and the test results are shown in Table 3. The battery performance testing method is as follows:

[0089] 1. Initial DCR test at room temperature: At 25±2℃, the obtained soft pack battery was charged to 4.25V at 1C, then discharged at 1C capacity for 30 minutes. After adjusting to 50% SOC, it was pulsed discharged at 5C constant current for 10 seconds and then charged for 10 seconds. The DCR was calculated as (voltage before pulse discharge - voltage after pulse discharge) / discharge current × 100%.

[0090] 2. Room Temperature Cycling Performance Test: At 25±2℃, the obtained pouch cells were subjected to charge-discharge cycle tests at a 1C / 1C charge-discharge rate within the range of 2.8~4.25V. The discharge specific capacity of the battery in the first cycle and the discharge specific capacity after 1000 cycles were recorded. Capacity retention rate after 1000 cycles = discharge specific capacity after 1000 cycles / discharge specific capacity in the first cycle × 100%.

[0091] 3. High-Temperature Storage Performance: The obtained pouch batteries were placed at 60±2℃ and charged / discharged at a 1C / 1C rate within the range of 2.8–4.25V. The discharge specific capacity of the batteries in the first week was recorded. Afterward, the batteries were stored at 60±2℃ for 120 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 120 days / discharge specific capacity in the first week × 100%.

[0092] 4. High-Temperature Gas Generation Test: The obtained pouch cells were charged at a constant current rate of 1C to 4.25V at 25±2℃, and then charged at a constant voltage of 4.25V until the current dropped below 0.05C, bringing them to a fully charged state of 4.25V. The volume of the fully charged battery before storage was measured using the water displacement method and recorded as V0. The fully charged battery was then placed in an oven at 60±2℃ for 50 days. After 50 days, the battery was removed, and its volume after storage was immediately measured and recorded as V1. Volume expansion rate = (V1-V0) / V0×100%.

[0093] Examples 2 to 5, Comparative Example 1

[0094] The above embodiments and comparative examples provide an electrolyte and a secondary battery. The differences between the above embodiments and comparative examples and Example 1 are: the amount of the first additive added is different, while the rest are basically the same as Example 1. From Example 1 to Example 5, the amount of the first additive added varies. Comparative Example 1 did not add the first additive. The amount of the first additive added in Examples 2 to 4 is all within the range of 0.1 wt% to 3.0 wt%. The amount of the first additive added in Example 5 is 5.0 wt%, which is greater than 3.0 wt%.

[0095] By comparing the experimental results of the above embodiments and comparative examples, it can be seen that adding the first additive shown in Formula I-1 to the silicon-carbon anode system can improve the cycle retention rate and high-temperature storage capacity retention rate of the secondary battery, while reducing high-temperature gas generation. The main reason is that the isocyanate group NCO in the molecular structure of the first additive can remove trace water in the electrolyte and inhibit the generation of hydrogen fluoride (HF), improve the stability of the electrolyte, inhibit the damage of HF to the interface film and corrosion of the positive and negative electrodes, and inhibit the dissolution of transition metal ions in the positive electrode. The Si-O bond can form an effective network structure on the surface of the silicon anode, enhance the flexibility of the SEI film, and inhibit the volume expansion of silicon. In addition, when the CF bond breaks, a LiF-rich SEI film can be generated on the surface of the silicon-carbon anode, which enhances the mechanical strength of the SEI film.

[0096] Furthermore, comparing the experimental results of the above embodiments and comparative examples shows that adding a preferred amount (0.1wt% to 3.0wt%) of the first additive to the silicon-carbon anode system can improve the cycle capacity retention rate and high-temperature storage capacity retention rate of the secondary battery, while reducing high-temperature gas generation. It can also be seen that as the content of the first additive increases, the generated interface film (SEI film) becomes increasingly dense, leading to an increase in the initial DCR, higher storage capacity retention rate, and lower volume expansion rate. However, the room-temperature cycling performance of the secondary battery shows a trend of first increasing and then decreasing with increasing first additive content, mainly because an overly dense interface film leads to increased polarization, affecting the kinetic performance of the secondary battery. Therefore, a higher content of the first additive is not necessarily better; thus, based on the total mass of the electrolyte, the mass percentage of the first additive should preferably be in the range of 0.1wt% to 3.0wt%.

[0097] Examples 6 to 10

[0098] Examples 6-10 provide an electrolyte and a secondary battery. The formulations of the electrolytes provided in Examples 6-10 are shown in Table 1. The amount of negative electrode film-forming additive (fluoroethylene carbonate) added in Examples 6-8 is between 5.0 wt% and 18.0 wt%. No negative electrode film-forming additive was added in Example 9. In Example 10, the amount of negative electrode film-forming additive (fluoroethylene carbonate) added is 25.0 wt%, which is greater than 18.0 wt%. According to the experimental results, an SEI film can be formed even without the addition of a negative electrode film-forming additive, but the generated SEI film is unstable and thin. If the silicon-carbon negative electrode undergoes volume expansion or contraction during cycling, the SEI film may rupture. If the amount of negative electrode film-forming additive added is too high, the resulting SEI will be too thick and too dense, which is not conducive to ion and electron transport and will reduce the conductivity of the secondary battery. Therefore, the amount of negative electrode film-forming additive (fluoroethylene carbonate) added is preferably in the range of 5.0 wt% to 18.0 wt%.

[0099] Examples 11 to 14

[0100] The above embodiments provide an electrolyte and a secondary battery. The difference between the above embodiments and Embodiment 1 is that the components and content of the negative electrode film-forming additive are different; otherwise, they are the same as in Embodiment 3. The negative electrode film-forming additives in Embodiments 11 and 12 use a combination of ethylene sulfate (DTD) and fluoroethylene carbonate (FEC). In Embodiment 11, based on the total mass of the electrolyte, the mass percentage of fluoroethylene carbonate is W1 (10 wt%), and the mass percentage of ethylene sulfate is W2 (1.0 wt%), then the relationship between W1 and W2 satisfies W1 = (1.67–36) × W2. In Embodiment 12, based on the total mass of the electrolyte, the mass percentage of fluoroethylene carbonate is W1 (15 wt%), and the mass percentage of ethylene sulfate is W2 (2.0 wt%), then the relationship between W1 and W2 satisfies W1 = (1.67–36) × W2. The negative electrode film-forming additives in Embodiments 13 and 14 only contain ethylene sulfate (DTD). The experimental results from Examples 1 and 11 to 14 show that using ethylene sulfate (DTD) alone is less effective than combining DTD and fluoroethylene carbonate (FEC). The combined use of FEC and DTD preferentially decomposes on the surface of the silicon-carbon anode to form a stable and resilient SEI film, effectively mitigating the volume expansion of silicon during repeated charge-discharge cycles and preventing electrolyte decomposition. This improves the reversible capacity and cycle performance of the secondary battery, resulting in excellent high-temperature performance for the silicon-carbon anode.

[0101] Examples 15 to 16

[0102] The above embodiments provide an electrolyte and a secondary battery. The difference between the above embodiments and Embodiment 11 is that the composition and content of the lithium salt are different; otherwise, they are the same as in Embodiment 11. Experimental results show that dual-component lithium salts (e.g., simultaneously adding LiPF6 and LiFSI) have better effects. The reasons are as follows: Lithium hexafluorophosphate in the embodiments has a moderate ion transport number, a moderate dissociation constant, good antioxidant properties, and good aluminum foil passivation ability in the non-aqueous organic solvent. Lithium bis(fluorosulfonyl)imide has high dissociation degree and thermal stability, and is insensitive to water, thus improving electrolyte stability and lithium ion transport number; however, lithium bis(fluorosulfonyl)imide is more prone to corroding aluminum foil. The synergistic effect of the two combines their advantages, improving electrolyte stability and conductivity, reducing the initial impedance of the secondary battery, and improving cycle performance, high-temperature performance, and other properties.

[0103] Comparative Examples 2 to 3

[0104] The above comparative examples provide an electrolyte and a secondary battery. Comparative Example 2, compared to Example 11, did not add the first additive, but instead added a mixture of the second and third additives. Comparative Example 3, compared to Example 15, did not add the first additive, but instead added a mixture of the second and third additives.

[0105] The structural formula of the second additive is shown in Formula II:

[0106]

[0107] The structural formula of the third additive is shown in Formula III:

[0108]

[0109] The molar ratio of the second additive to the third additive is 1:1, and the total mass percentage of the second additive and the third additive is 1 wt% based on the total mass of the electrolyte.

[0110] The experimental results show that the first additive in this embodiment contains three functional groups, including an isocyanate group (NCO), a silicon-oxygen bond (Si-O) in a siloxane group, and a carbon-fluorine bond (CF). The resulting secondary battery has a lower initial DCR than the secondary batteries in Comparative Examples 2 and 3, exhibiting better kinetic performance. The possible reasons are as follows: the three groups in the comparative examples are distributed on different molecules. The phenyl group is easily oxidized to form a high-impedance CEI film at the positive electrode, resulting in a large initial DCR, leading to poor kinetics and affecting cycle performance. Therefore, the effects of the two comparative examples are not as good as those in the relevant embodiments of this application.

[0111] Examples 17 to 20

[0112] The above embodiments provide an electrolyte and a secondary battery. The difference between the above embodiments and Embodiment 3 is that the structural formula of the first additive is different, while the rest is the same as in Embodiment 3.

[0113] The structural formula of the first additive in Example 3 is as follows:

[0114]

[0115] The structural formula of the first additive in Example 17 is:

[0116]

[0117] The structural formula of the first additive in Example 18 is:

[0118]

[0119] The structural formula of the first additive in Example 19 is:

[0120]

[0121] The structural formula of the additive in Example 20 is:

[0122]

[0123] The experimental results show that, compared with the comparative example, the addition of the first additive shown in formulas I-2 to I-7 to the silicon-carbon anode system in this embodiment can improve the cycle retention rate and high-temperature storage capacity retention rate of the secondary battery, while reducing high-temperature gas generation. The main reason is that the isocyanate group NCO in the molecular structure of the first additive can remove trace water in the electrolyte and inhibit the generation of hydrogen fluoride (HF), improve the stability of the electrolyte, inhibit the damage of HF to the interface film and corrosion of the positive and negative electrodes, and inhibit the dissolution of transition metal ions in the positive electrode. The Si-O bond can form an effective network structure on the surface of the silicon anode, enhance the flexibility of the SEI film, and inhibit the volume expansion of silicon. In addition, when the CF bond breaks, a LiF-rich SEI film can be generated on the surface of the silicon-carbon anode, enhancing the mechanical strength of the SEI film.

[0124] Examples 21 to 22

[0125] The above embodiments provide an electrolyte and a secondary battery. The difference between the above embodiments and Embodiment 1 is that the mass percentage of the first additive in the electrolyte and the mass percentage of SiO in the negative electrode active material are different. x The mass percentage ratio is different, but the rest is the same as in Example 1.

[0126] In Example 21, the negative electrode active material is composed of SiO2. x It is composed of graphite and belongs to silicon-carbon composite materials. SiO₂ is one of them. x The mass ratio of SiO to graphite is 9.4:84.6. In this negative electrode active material, SiO... x The mass percentage of the first additive in the electrolyte is 10%. The mass percentage of the first additive in the electrolyte is 0.1 wt%. Therefore, the mass percentage of the first additive in the electrolyte is related to the mass percentage of SiO in the negative electrode active material. x The mass percentage is 0.01:1.

[0127] In Example 22, the negative electrode active material is composed of SiO2. x It is composed of graphite and belongs to silicon-carbon composite materials. SiO₂ is one of them. x The mass ratio of SiO to graphite is 13:74. In this negative electrode active material, SiO... x The mass percentage of the first additive in the electrolyte is 15%. The mass percentage of the first additive in the electrolyte is 3.0 wt%. Therefore, the mass percentage of the first additive in the electrolyte is related to the mass percentage of SiO in the negative electrode active material. xThe mass percentage is 0.2:1.

[0128] In the above embodiments, as the silicon content in the negative electrode active material increases, the mass percentage of the first additive in the electrolyte also increases, thereby helping to improve the relevant performance of the battery.

[0129] Table 1. Electrolyte formulations for each embodiment and comparative example.

[0130]

[0131]

[0132]

[0133] Table 2. Component addition relationships for each embodiment and comparative example.

[0134]

[0135]

[0136] Table 3 Performance table of the secondary batteries in each embodiment and comparative example

[0137]

[0138]

[0139] The present application has been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of the present application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present application. Therefore, the content of this specification should not be construed as a limitation of the present application.

Claims

1. An electrolyte, characterized by, The electrolyte comprises: a non-aqueous organic solvent, a lithium salt, and a first additive; the first additive comprises a compound having a structure shown in Formula I: Formula I; wherein R1, R2 are independently selected from a fluorine atom (-F), a chlorine atom (-Cl), a bromine atom (-Br), or a hydrogen atom (-H); R3 is selected from an alkylene group having 3 to 9 carbon atoms, which is substituted with a first substituent or unsubstituted; the first substituent comprises a methyl group, an ethyl group, a cyano group, or a halogen group; R4, R5, R6 are independently selected from an alkyl group having 1 to 3 carbon atoms, which is substituted with a second substituent or unsubstituted; the second substituent comprises a halogen group; the electrolyte further comprises a negative electrode film-forming additive; the negative electrode film-forming additive comprises fluoroethylene carbonate and ethylene sulfate; a mass percentage of the first additive is M1%, a mass percentage of the negative electrode film-forming additive is M2%, and a mass percentage of the lithium salt is M3% based on a total mass of the electrolyte; a relationship between M1 and M2 satisfies 0.65≤M1+0.1×M2≤5.1; a relationship among M1, M2, and M3 satisfies M3 2. The electrolyte according to claim 1, characterized in that, a structural formula of the first additive comprises any one or several of the following compounds: Formula I-1; Formula I-2; Formula I-3; Formula I-4; Formula I-5; Formula I-6; Formula I-7.

3. The electrolyte of claim 1, wherein a mass percentage of fluoroethylene carbonate is W1%, and a mass percentage of ethylene sulfate is W2% based on a total mass of the electrolyte; a relationship between W1 and W2 satisfies W1= (1.67-36)×W2. the non-aqueous organic solvent comprises any two or more of the following: ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), 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), and tetramethyl sulfone (TMS).

4. The electrolyte of claim 1, wherein the lithium salt comprises lithium hexafluorophosphate (LiPF6) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI); a mass percentage of the lithium salt is 10wt%-15wt% based on a total mass of the electrolyte.

5. The electrolyte of claim 1, wherein The application further provides a secondary battery comprising the electrolyte.

6. A secondary battery characterized by comprising: The application further provides a secondary battery comprising the electrolyte as a power supply for the power-consuming device. The positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte solution according to any one of claims 1 to 5; the negative electrode sheet includes a negative electrode active material, and the negative electrode active material includes SiO x A silicon-carbon composite material in which silicon is compounded with graphite, 1≤x≤2; The ratio of the mass percentage of the first additive in the electrolyte to the mass percentage of SiO x in the negative electrode active material is (0.01-0.2):

1.

7. An electric device, characterized by ​

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