Composite additive, electrolyte and lithium ion battery

Abstract

The application provides a composite additive, an electrolyte and a lithium ion battery, and relates to the technical field of lithium batteries.The composite additive comprises a compound shown in structural formula I and a compound shown in structural formula II, the mass ratio of the compound shown in the structural formula I to the compound shown in the structural formula II is 1:5-5:1;in the compound shown in the structural formula I, R1 is a halogenated hydrocarbon;in the compound shown in the structural formula II, R2 is selected from one or two of methoxy and halogen elements.The composite additive of the application can not only form a thin and stable interface film on the surface of a high-nickel NCM positive electrode and a negative electrode graphite, but also can remove by-products on the surface of the positive and negative electrode materials, greatly improve the ion conductivity of the interface film, and avoid thermal decomposition of lithium salt in the electrolyte due to active hydrogen at high temperature, and has excellent high-temperature cycle and high-temperature storage performance.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, specifically to a composite additive, an electrolyte, and a lithium-ion battery. Background Technology

[0002] In recent years, lithium-ion batteries have received widespread attention and become a new research hotspot in areas such as hybrid vehicles and mobile device power supplies due to their advantages such as high energy density, environmental friendliness and economy.

[0003] However, under high temperature conditions, the lithium salt in the electrolyte of lithium-ion batteries will undergo thermal decomposition due to the active hydrogen. The decomposition products will trigger side reactions, damage the electrode material structure, reduce the lithium-ion migration rate, and reduce the battery capacity. In addition, the decomposition will produce a large amount of gas, causing the battery to expand, thus affecting the performance, life and safety of lithium-ion batteries. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] To address the shortcomings of existing technologies, this invention provides a composite additive, electrolyte, and lithium-ion battery, which solves the technical problem that the electrolyte in lithium-ion batteries undergoes thermal decomposition under high-temperature conditions, producing byproducts and a large amount of gas.

[0006] (II) Technical Solution

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] On one hand, the present invention provides a composite additive comprising compounds represented by structural formulas I and II.

[0009]

[0010] The mass ratio of the compound shown in structural formula I to the compound shown in structural formula II is 1:5-5:1;

[0011] In the compound represented by structural formula I, R1 is a haloalkane; in the compound represented by structural formula II, R2 is selected from one or both of methoxy groups and halogen elements.

[0012] In a preferred embodiment, the compound represented by structural formula I is selected from compound 1.

[0013]

[0014] As a preferred embodiment, to further maximize the synergistic effect of Formula I and Formula II, the compound shown in Formula II is selected from one or both of Compound 2 and Compound 3.

[0015]

[0016] On the other hand, the present invention provides an electrolyte comprising a lithium salt, an organic solvent, and the aforementioned composite additive, wherein the amount of the composite additive added is 0.1-3 wt% of the total mass of the electrolyte.

[0017] In a preferred embodiment, the organic solvent includes any one or more of cyclic carbonates, chain carbonates, and carboxylic acid esters.

[0018] In a preferred embodiment, the cyclic carbonate is selected from one or more of ethylene carbonate, propylene carbonate, butene carbonate, and γ-butyrolactone.

[0019] The chain carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate.

[0020] The carboxylic acid ester is selected from one or more of methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, and ethyl butyrate.

[0021] In a preferred embodiment, the electrolyte further includes conventional additives, the amount of which is 0.5-3 wt% of the total mass of the electrolyte. The conventional additives are selected from any one or more of lithium difluorosulfonylimide, LiBF4, lithium bis(oxalato)borate, lithium difluorooxalato)borate, lithium difluorooxalatophosphate, lithium difluorooxalatophosphate, lithium bis(trifluoromethyl)sulfonylimide, sulfone, tris(trimethylsilane)phosphite, vinyl sulfate, and methylene disulfonate.

[0022] On the other hand, the present invention provides a lithium-ion battery, including a negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode, and the electrolyte.

[0023] In a preferred embodiment, the positive electrode sheet includes a positive electrode material, wherein the positive electrode material is LiNi. (1-x-y) Co x Mn y O2, where 0≤X≤1, 0≤Y≤1, the negative electrode sheet includes a negative electrode material, which is graphite.

[0024] In a preferred embodiment, the charging voltage of the lithium-ion battery is not higher than 4.8V.

[0025] (III) Beneficial Effects

[0026] This invention provides a composite additive, an electrolyte, and a lithium-ion battery. Compared with the prior art, it has the following advantages:

[0027] The composite additive of this invention can not only form a thin and stable interfacial film on the graphite surface of the high-nickel NCM cathode and anode, but also remove by-products on the surface of the cathode and anode materials, greatly improving the ionic conductivity of the interfacial film. Moreover, it can prevent the lithium salt in the electrolyte from thermally decomposing due to active hydrogen at high temperatures. Therefore, the lithium-ion battery prepared by this invention has a high lithium-ion migration rate, high battery capacity, low battery expansion rate, high safety, and excellent high-temperature cycling and high-temperature storage performance under high-temperature conditions. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] This application provides a composite additive, an electrolyte, and a lithium-ion battery, which solves the technical problem of lithium-ion battery electrolytes decomposing to produce byproducts and a large amount of gas under high-temperature conditions.

[0030] The technical solution in this application is to solve the above-mentioned technical problems, and the general idea is as follows:

[0031] This application provides an electrolyte comprising lithium salt, organic solvent, composite additive, and conventional additive.

[0032] The composite additive includes compounds represented by structural formulas I and II. The compound represented by structural formula I contains a cyano group. Cyanides have a high dielectric constant and low viscosity. The strongly electronegative cyano group can form CN-Co bonds with the surface of NCM metal oxides, inhibiting direct contact between the electrolyte and the positive electrode, thereby improving the stability of the positive electrode interface. At the same time, the cyano group has strong electrophilic activity and can react with water in the electrolyte and active hydrogen in the positive and negative electrode materials, reducing lithium salt decomposition caused by active hydrogen under high temperature conditions.

[0033] The compound shown in Structural Formula II contains CF bonds, which are easily broken, readily generating LiF (the main component of the SEI film) and exhibiting good thermal stability. The compound also contains silicon groups, which, as electron-donating groups, have a high electron cloud density, reducing oxidation potential and facilitating film formation on the positive electrode. This prevents direct contact between the electrode and electrolyte at high temperatures and also delays the high-temperature oxidative decomposition reaction of the electrolyte. Therefore, the synergistic effect of the compounds shown in Structural Formula I and Structural Formula II in the composite additive not only facilitates the formation of thinner and denser CEI and SEI protective films on the surfaces of the NCM cathode and graphite anode, but also removes residual lithium compounds from the positive and negative electrode surfaces, improving the ionic conductivity of the interfacial film. Furthermore, it prevents the thermal decomposition of lithium salts in the electrolyte due to active hydrogen at high temperatures. Consequently, the lithium-ion battery of this invention exhibits high lithium-ion migration rate, high battery capacity retention, low battery expansion rate, high safety and recovery rate under high-temperature conditions, and excellent high-temperature cycling and high-temperature storage performance.

[0034] The CEI and SEI protective films generated at the positive and negative electrodes are dynamic and will break and regenerate. The components of the broken films become lithium compounds remaining on the surface of the positive and negative electrodes. The compounds represented by structural formulas I and II in the composite additives of this application can work together to remove the lithium compounds remaining on the surface of the positive and negative electrodes, thereby reducing the impedance of the battery.

[0035] The process of CEI and SEI protective films breaking down and regenerating consumes lithium ions and solvents. The compounds represented by structural formulas I and II in the composite additives of this application work synergistically to generate thinner and denser CEI and SEI protective films on the surfaces of NCM cathodes and graphite anodes, which are less prone to breaking down, thereby reducing the consumption of active lithium.

[0036] In a further preferred embodiment, the above-mentioned conventional additives include any one or more of lithium difluorosulfonylimide, LiBF4, lithium bis(oxalato)borate, lithium difluorooxalato)borate, lithium difluorooxalatophosphate, lithium bis(trifluoromethyl)sulfonylimide, sulfone, tris(trimethylsilane)phosphite, vinyl sulfate, methylene disulfonate, vinylene carbonate, methane disulfonate, and fluorovinyl carbonate.

[0037] For further optimization, the content of conventional additives in the electrolyte is 0.5-3 wt%, for example, it can be 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, or 3 wt%.

[0038] To select lithium salts with better compatibility with the aforementioned additives, in a preferred embodiment, the lithium salt in the electrolyte is selected from one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, lithium difluorooxalate phosphate, lithium bis(fluorosulfonyl)imide, or lithium bis(trifluoromethylsulfonyl)imide. The molar concentration of the lithium salt is 0.5-1.5M, for example, 0.5M, 0.75M, 1M, 1.25M, or 1.5M. Limiting the molar concentration of the lithium salt within the above range allows the battery to simultaneously achieve superior electrochemical performance and room-temperature cycle retention.

[0039] In a preferred embodiment, the organic solvent is one or more of a chain carbonate compound, a cyclic carbonate compound, or a carboxylic acid ester compound. The cyclic carbonate compound is selected from one or more of ethylene carbonate, fluoroethylene carbonate, or propylene carbonate; the chain carbonate compound is selected from one or more of dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate; and the carboxylic acid ester compound is selected from propyl acetate and / or ethyl acetate. The above-mentioned additives and this type of organic solvent have better compatibility, thereby enabling the battery to simultaneously achieve superior electrochemical performance and capacity retention.

[0040] In a preferred embodiment, the electrolyte can be prepared by the following method: first, adding lithium salt to an organic solvent, and after the lithium salt is completely dissolved, adding additives to the system to obtain the electrolyte. Therefore, the electrolyte obtained in this application has better performance uniformity.

[0041] The present invention also provides a lithium-ion battery comprising a negative electrode, a positive electrode, and an electrolyte, wherein the positive electrode is made of LiNi. (1-x-y) Co x Mn y O2, the negative electrode material is graphite, and the electrolyte is the aforementioned electrolyte. Based on the reasons stated above, the lithium-ion battery of this application exhibits excellent capacity retention and capacity recovery rates. In a preferred embodiment, the charging voltage of the lithium-ion battery is not higher than 4.8V, preferably 2.8-4.25V.

[0042] The composite additive and electrolyte of the present invention can form a thin and stable interfacial film on the graphite surface of the high-nickel positive electrode and negative electrode, effectively isolating the electrolytes and having functions such as water removal, acid reduction or complexation, which greatly improves the conductivity of the interfacial film.

[0043] The beneficial technical effects of this application will be explained below with reference to specific embodiments and comparative examples.

[0044] Compounds with structural formula I and structural formula II were purchased from the Aladdin Reagent website.

[0045] Example 1

[0046] This embodiment provides a method for preparing a lithium-ion battery, including the following steps:

[0047] S1. Preparation of electrolyte:

[0048] Ethyl carbonate (EC), ethyl propionate (EP), and ethyl methyl carbonate (EMC) were mixed in a mass ratio of EC:EP:EMC = 1:1:2. After mixing, 0.7 mol of lithium hexafluorophosphate (LiPF6) and 0.3 mol of lithium difluorosulfonyl imide (LiFSI) were added. After the lithium salts were completely dissolved, compound 1 (CAS: 27328-86-5) and compound 2 (CAS: 866252-52-0) were added in a mass ratio of 2:1. The amount of compound 1 and compound 2 added was 3% wt of the total mass of the electrolyte.

[0049] S2, Preparation of the positive electrode: The lithium nickel cobalt manganese oxide ternary material LiNi 0.7 C0 0.1 Mn 0.2 Conductive agent Super P, binder PVDF, and carbon nanotubes (CNTs) were mixed uniformly at a mass ratio of 97.5:0.5:1:1 to prepare a lithium-ion battery positive electrode slurry of a certain viscosity. This slurry was then coated onto aluminum foil used for current collectors, with a coating weight of 360 g / m². 2 After drying at 85℃, the material is cold-pressed; then slitting and slicing are performed, followed by vacuum drying at 85℃ for 4 hours to produce a lithium-ion battery positive electrode sheet that meets the requirements.

[0050] S3. Preparation of negative electrode sheet: Artificial graphite, conductive agent Super P, thickener CMC, and binder SBR (styrene-butadiene rubber latex) are mixed in a mass ratio of 95:1.5:1.0:2.5 to form a slurry. The mixture is coated on both sides of copper foil, dried, and rolled to obtain a negative electrode sheet. Then, it is dried in a vacuum at 85°C for 4 hours to produce a lithium-ion battery negative electrode sheet that meets the requirements.

[0051] S4. Preparation of lithium-ion battery: The positive electrode, negative electrode and separator prepared according to the above process are stacked to form a lithium-ion battery with a thickness of 0.5 mm, a width of 8 mm and a length of 10 mm, with a capacity of 3 Ah. The battery is then vacuum baked at 85 °C for 48 hours and injected with the above electrolyte to complete the battery manufacturing.

[0052] Examples 2 to 11 and Comparative Examples 1 to 6 were prepared by changing the specific proportions and types of substances in the electrolyte, while keeping other aspects the same as in Example 1. The electrolyte formulations are shown in Table 1.

[0053] Table 1. Electrolyte composition and battery performance of the examples and comparative examples.

[0054]

[0055]

[0056] The composite additive of this invention can improve the overall high-temperature performance of the battery, which is mainly manifested in two major electrical properties: high-temperature cycling performance and high-temperature storage performance. Table 1 compares the 28-day storage retention rate at 60°C and the 800-cycle retention rate at 45°C of the batteries prepared in Examples 1-11 with those in Comparative Examples 1-6. It can be seen that the batteries prepared in Examples 1-11 have higher 28-day storage retention rates at 60°C and 800-cycle retention rates at 45°C. This is because the introduction of the composite additive makes the positive and negative electrode films more dense and stable, reduces the interfacial impedance of the positive and negative electrodes, and simultaneously removes by-products from the surfaces of the positive and negative electrodes, improving the ionic conductivity of the interfacial film and significantly improving the high-temperature performance of the battery.

[0057] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0058] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention. Any aspects of the present invention not described in detail are well-known to those skilled in the art.

Claims

1. A complex additive characterized in that, The composite additive comprises compounds shown in structural formula I and structural formula II, The mass ratio of the compound shown in structural formula I to the compound shown in structural formula II is 1:5-5:

1. In the compound shown in structural formula I, R1 is halogenated hydrocarbon. The compound shown in structural formula II is compound 2, 2. The composite additive of claim 1, wherein The compound shown in structural formula I is selected from compound 1, 3. An electrolyte, characterized by The electrolyte comprises a lithium salt, an organic solvent and the composite additive according to any one of claims 1-2, and the amount of the composite additive is 0.1-3wt% of the total mass of the electrolyte.

4. The electrolyte of claim 3, wherein The organic solvent comprises any one or more of cyclic carbonates, chain carbonates and carboxylic acid esters.

5. The electrolyte of claim 4, wherein The cyclic carbonates are selected from one or more of ethylene carbonate, propylene carbonate, butylene carbonate and γ-butyrolactone. The chain carbonates are selected from one or more of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate and ethyl propyl carbonate. The carboxylic acid esters are selected from one or more of methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate and ethyl butyrate.

6. The electrolyte of claim 3, wherein The electrolyte further comprises a conventional additive, and the amount of the conventional additive is 0.5-3wt% of the total mass of the electrolyte, and the conventional additive is selected from any one or more of lithium bisfluorosulfonylimide, LiBF4, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluoro(oxalato)phosphate, lithium bis(trifluoromethylsulfonyl)imide, benzophenone, tris(trimethylsilyl)phosphite, vinyl sulfite and methyl methylene disulfonate.

7. A lithium-ion battery, characterized by The lithium ion battery comprises a negative electrode sheet, a positive electrode sheet, a separator arranged between the negative electrode sheet and the positive electrode sheet and the electrolyte according to any one of claims 3-6.

8. The lithium-ion battery of claim 7, wherein, The positive electrode sheet includes a positive electrode material, the positive electrode material being LiNi (1-x-y) Co x Mn y O2, wherein 0≤X≤1, 0≤Y≤1, the negative electrode sheet includes a negative electrode material, the negative electrode material being graphite.

9. The lithium-ion battery of claim 7, wherein the lithium-ion battery is a lithium-ion battery. The charging voltage of the lithium ion battery is not higher than 4.8V.

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