Electrolyte and battery

Abstract

The application relates to the technical field of lithium ion batteries, in particular to an electrolyte and a battery. The electrolyte comprises an additive, a lithium salt, an organic solvent and an ionic liquid, wherein the additive comprises at least one of compounds with the structures shown in the following formula; in the formula, R1 is selected from any one of a methyl group, a propyl group, an ethyl group, an allyl group, a vinyl group, a propenyl group, an allyl group, an isopropenyl group, an ethynyl group, a propargyl group, a propynyl group, a 2-propenyl group, a 2-propynyl group, an isopropyl group, an ethynyl group and a tert-butyl group. The electrolyte of the application avoids capacity loss caused by three-step phase transition conversion of a traditional liquid electrolyte, can effectively prevent dissolution of polysulfides, and improves the fire-retardant effect, ionic conductivity and thermal chemical stability of the electrolyte.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to an electrolyte and a battery. BACKGROUND

[0002] With the increasing attention to sustainable energy and low-carbon society in the world, electric vehicles are developing rapidly. The requirement for higher energy density has become a major problem to be solved for electric vehicles. Lithium is one of the most suitable negative materials for batteries due to its high energy density and long cycle life. Among them, lithium-sulfur batteries are widely used because of their large storage capacity, high theoretical weight energy density (2600 Wh / kg), environmental friendliness and other advantages. However, lithium-sulfur batteries are still limited in practical and commercial applications by many factors, including the formation of Li dendrites caused by the uneven deposition on the surface of lithium negative electrode, which not only reduces the cycle performance and coulombic efficiency, but also causes safety problems; the shuttle effect of soluble polysulfides during the cycle process, which leads to rapid loss of active material and poor cycle performance.

[0003] The performance and polysulfide solubility of lithium-sulfur batteries are significantly dependent on the electrolyte, which includes polymer solid electrolyte, semi-solid electrolyte and liquid electrolyte, etc. Among them, the full solid polymer electrolyte has the following problems: high crystallinity, low ion mobility, very low ion conductivity at room temperature; more side reactions between electrolyte / electrode interface; dissolution of polysulfides in electrolyte system, leading to uncontrollable shuttle effect. For semi-solid electrolyte, 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME) are currently used as the main solvent, doped with lithium bis(trifluoromethanesulfonylimide) (LiTFSI), and additives and initiators are added in turn to make it polymerize within a certain time, or polymerization is directly achieved by high lithium salt method. High salt concentration electrolyte helps to slow down the diffusion rate of polysulfides, but the ion effects in the solvent will cause the precipitation of lithium salt, which seriously affects the practical application of the electrolyte. These ether-based electrolytes are highly flammable and volatile, which poses a serious fire hazard. 1,3-dioxolane (DOL) will decompose on the surface of the negative active metal lithium, leading to the formation of alkoxy groups, which has a great impact on long-term performance.

[0004] Liquid electrolyte, the sulfur positive electrode will undergo a relatively complex multiphase electrochemical process, the first step is "solid to liquid", elemental sulfur is converted into high-valence polysulfide ions (S n 2- , 6≤n≤8); the second step is "liquid to solid", S n 2- (6≤n≤8) will be further reduced to insoluble low-valence S4 2-(Li2S4); these two steps correspond to the first discharge plateau, about 25% of the discharge capacity. The third step "solid to solid", corresponding to the second discharge plateau, S 4- Final reduction to S 2- , about 75% of the discharge capacity. But this step because of the reduction of insoluble Li2S4 and Li2S2, the reaction is slow, resulting in the remaining capacity can not be fully converted, ultimately leading to the rapid capacity decay. Therefore, in the first step, the local high concentration of lithium salt liquid electrolyte system of lithium-sulfur battery in polysulfide is easily dissolved in the liquid electrolyte, so that it becomes a high concentration of polysulfide electrolyte, and then reacts with lithium metal on the lithium negative electrode side to become a lithium polysulfide electrolyte, reducing the availability of active material sulfur, ultimately leading to a sharp decrease in battery life. Therefore, this method is more suitable for lithium metal batteries, not for lithium-sulfur batteries.

[0005] In view of this, the present application is proposed. SUMMARY

[0006] The first object of the present application is to provide an electrolyte that can avoid the capacity loss caused by the three-step phase transition of the traditional liquid electrolyte, effectively prevent the dissolution of polysulfide, and improve the flame retardant effect, ionic conductivity and thermal chemical stability of the electrolyte.

[0007] The second object of the present application is to provide a battery with excellent battery capacity, coulombic efficiency and cycle performance.

[0008] In order to achieve the above object of the present application, the following technical solutions are adopted:

[0009] The present application provides an electrolyte comprising an additive, a lithium salt, an organic solvent and an ionic liquid, the additive comprising at least one of the compounds having the structure shown below:

[0010]

[0011] In the formula, R1 is selected from any one of methyl, propyl, ethyl, allyl, vinyl, propenyl, allyl, isopropenyl, ethynyl, propargyl, propynyl, 2-propenyl, 2-propynyl, isopropyl, ethynyl and tert-butyl.

[0012] Further, the mass of the additive accounts for 5% to 50% of the total mass of the electrolyte.

[0013] Further, the ionic liquid comprises a fluorine-containing sulfonamide anion.

[0014] Preferably, the ionic liquid comprises at least one of 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide, and 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide salt, 1-methyl-1-propylpiperidinium bis(fluorosulfonyl)imide, 1-methyl-1-pentylpyrrolidinium bis(trifluoromethylsulfonyl)imide, and 1-methyl-1-n-octylpyrrolidinium bis(trifluoromethylsulfonyl)imide.

[0015] Further, the volume ratio of the organic solvent to the ionic liquid is 1:0.5-4.

[0016] Further, the lithium salt comprises a fluorine-containing sulfonamide type lithium salt.

[0017] Preferably, the lithium salt comprises at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(nonafluorobutylsulfonyl)imide, lithium (fluorosulfonyl)(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, potassium bis(fluorosulfonyl)imide, magnesium bis(trifluoromethylsulfonyl)imide, zinc bis(trifluoromethylsulfonyl)imide, and calcium bis(trifluoromethylsulfonyl)imide.

[0018] Further, in the electrolyte, the concentration of the lithium salt is 0.5-8 mol / L.

[0019] Further, the organic solvent comprises a dimethyl ether compound.

[0020] Preferably, the organic solvent comprises at least one of ethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, triethylene glycol dimethyl ether, diethylene glycol dimethyl ether, o-dimethyl ether, m-dimethyl ether, isosorbide dimethyl ether, tetraethylene glycol monomethyl ether, and triethylene glycol butyl methyl ether.

[0021] The application also provides a battery comprising the electrolyte as described above.

[0022] Further, the battery further comprises a positive electrode sheet; the positive electrode active material in the positive electrode sheet comprises at least one of elemental sulfur, sulfur-carbon composite material, and sulfur-metal composite material.

[0023] Further, the battery further comprises a negative electrode sheet; the negative electrode active material in the negative electrode sheet comprises copper-lithium composite material and / or lithium foil.

[0024] Compared with the prior art, the application has the following beneficial effects:

[0025] The electrolyte of the present application, by adding a dithioether compound containing a sulfur atom as an additive of the electrolyte, the additive reacts with sulfur to generate a trisulfide, and then is directly reduced to the final product, avoiding the capacity loss caused by the three-step phase conversion of the traditional liquid electrolyte; can adjust the deposition form of Li2S, relieve the passivation of the electrode, and promote the redox kinetics of sulfur; the allyl group in the additive is cross-linked on the lithium negative electrode surface by free radical polymerization to form a uniform film, thereby realizing dendrite-free lithium negative electrode and inhibiting the diffusion and reduction of dissolved lithium.

[0026] The electrolyte of the present application uses an ionic liquid as a diluent, which helps to improve the flame retardancy, ion conductivity, migration rate, and good thermal stability and electrochemical stability of the electrolyte; due to the addition of lithium salt and ionic liquid, the concentration of fluorosulfonyl amide type anion in the electrolyte is relatively high, and the donor ability of lithium ion to polysulfide (Li2S n ) is weak, which hinders the dissolution conversion of polysulfide in the step of converting elemental sulfur into high-valence polysulfide ion soluble in the electrolyte, helps the additive to react with sulfur to generate trisulfide, improves the yield of the final reduction product, and thus improves the capacity, coulombic efficiency and cycle performance of the battery. DETAILED DESCRIPTION

[0027] The technical solutions of the present application will be described clearly and completely in combination with the specific embodiments below, but those skilled in the art will understand that the following described embodiments are part of the embodiments of the present application, not all the embodiments, and should not be regarded as limiting the scope of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts are within the scope of protection of the present application. If the specific conditions are not specified in the embodiments, the conventional conditions or the conditions recommended by the manufacturer are used. If the reagents or instruments used are not specified by the manufacturer, they are all conventional products that can be purchased on the market.

[0028] The electrolyte and battery of an embodiment of the present application will be described in detail below.

[0029] In some embodiments of the present application, an electrolyte is provided, comprising an additive, a lithium salt, an organic solvent and an ionic liquid, the additive comprising at least one of the compounds having the structure shown below;

[0030]

[0031] In the formula, R1 is selected from any one of methyl, propyl, ethyl, allyl, vinyl, propenyl, allyl, isopropenyl, ethynyl, propargyl, propynyl, 2-propenyl, 2-propynyl, isopropyl, ethynyl and tert-butyl.

[0032] The electrolyte of the present application adopts a disulfide compound as an additive, the additive reacts with sulfur to generate a trisulfide, and then is directly reduced to a final product, thereby avoiding capacity loss caused by three-step phase conversion of a traditional liquid electrolyte; and can adjust deposition form of Li2S, relieve passivation of an electrode, and promote redox kinetics of sulfur.

[0033] The liquid electrolyte of the present application can effectively prevent dissolution of polysulfides, and improve fire-retardant effect, ionic conductivity, and thermal-chemical stability of the electrolyte.

[0034] In some embodiments of the present application, the mass of the additive accounts for 5% to 50% of the total mass of the electrolyte; typically but not limitedly, for example, the mass of the additive accounts for 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45% or the like of the total mass of the electrolyte. Preferably, the mass of the additive accounts for 5% to 20% of the total mass of the electrolyte.

[0035] In some embodiments of the present application, the ionic liquid comprises a fluorine-containing sulfonamide anion.

[0036] In some embodiments of the present application, the ionic liquid comprises at least one of 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide, and 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide salt, 1-methyl-1-propylpiperidinium bis(fluorosulfonyl)imide, 1-methyl-1-pentylpyrrolidinium bis(trifluoromethylsulfonyl)imide, and 1-methyl-1-n-octylpyrrolidinium bis(trifluoromethylsulfonyl)imide.

[0037] In some embodiments of the present application, the volume ratio of the organic solvent to the ionic liquid is 1:0.5 to 4; typically but not limitedly, for example, the volume ratio of the organic solvent to the ionic liquid is 1:0.5, 1:1, 1:2, 1:3 or 1:4 or the like.

[0038] In some embodiments of the present application, the lithium salt comprises a fluorine-containing sulfonamide lithium salt.

[0039] In some embodiments of the present application, the lithium salt comprises at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(nonafluorobutylsulfonyl)imide, lithium (fluorosulfonyl)(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, potassium bis(fluorosulfonyl)imide, magnesium bis(trifluoromethylsulfonyl)imide, zinc bis(trifluoromethylsulfonyl)imide, and calcium bis(trifluoromethylsulfonyl)imide.

[0040] In some embodiments of the application, the concentration of the lithium salt in the electrolyte is 0.5-8 mol / L. Typically but not exclusively, the concentration of the lithium salt in the electrolyte is 0.5 mol / L, 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L or 8 mol / L, etc.

[0041] The use of the ionic liquid as a diluent in the electrolyte of the application helps to improve the flame retardancy, ionic conductivity, migration rate, and good thermal stability and electrochemical stability of the electrolyte. Due to the addition of the lithium salt and the ionic liquid, the concentration of the fluorosulfonyl amide anion in the electrolyte is relatively high, and the donor ability of lithium ions to polysulfides (Li2S n ) is weak, which hinders the dissolution conversion of polysulfides in the step of converting elemental sulfur into high-valence polysulfide ions that are soluble in the electrolyte, helps the additive to react with sulfur to form trisulfide, improves the final reduction product rate, and thus improves the capacity, coulombic efficiency and cycle performance of the battery.

[0042] In some embodiments of the application, the organic solvent includes a methyl ether compound.

[0043] In some embodiments of the application, the organic solvent includes at least one of ethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, triethylene glycol dimethyl ether, diethylene glycol dimethyl ether, o-dimethyl ether, m-dimethyl ether, isosorbide dimethyl ether, tetraethylene glycol monomethyl ether and triethylene glycol butyl methyl ether.

[0044] In some embodiments of the application, a battery is also provided, which includes the electrolyte as described above.

[0045] In some specific embodiments of the application, the battery includes a lithium-sulfur battery.

[0046] In some embodiments of the application, the battery further includes a positive electrode sheet; the positive electrode active material in the positive electrode sheet includes at least one of elemental sulfur, sulfur-carbon composite material and sulfur-metal composite material.

[0047] In some embodiments of the application, the battery further includes a negative electrode sheet; the negative electrode active material in the negative electrode sheet includes copper-lithium composite material and / or lithium foil.

[0048] Examples 1-6

[0049] The preparation method of the lithium-sulfur battery provided in this embodiment includes the following steps:

[0050] The positive electrode sheet, the negative electrode sheet and the separator are made into a lithium-sulfur battery with two positive electrodes and three negative electrodes by the lamination process, the capacity is 1000 mAh, the electrolyte is injected, and the battery is completed.

[0051] The preparation method of the positive electrode sheet comprises the following steps:

[0052] The positive electrode active material, the conductive agent (Super P), the adhesive PAA and the carbon nanotube (CNT) are mixed uniformly at a mass ratio of 75:10:5:10 to prepare a positive electrode slurry of a lithium-sulfur battery, which is coated on an aluminum foil as a current collector, dried at 70℃, and then cold-pressed; then, after edge cutting, sheet cutting and slitting, the positive electrode slurry is dried at 65℃ under vacuum for 8 hours to obtain the positive electrode sheet.

[0053] The preparation method of the negative electrode sheet comprises the following steps:

[0054] The copper lithium composite tape with a copper foil of 8μm thick and lithium coated on both sides (the thickness of lithium is 20μm) is used; after edge cutting, sheet cutting and slitting, the negative electrode sheet is obtained.

[0055] The preparation method of the electrolyte comprises the following steps:

[0056] The lithium salt is dissolved in an organic solvent, then an ionic liquid is added, and finally an additive is added to obtain the electrolyte.

[0057] The components in the electrolytes of Examples 1-6 and the contents of the components in the electrolytes and the positive electrode active materials are shown in Table 1.

[0058] Table 1

[0059]

[0060] Comparative Example 1

[0061] The preparation method of the lithium-sulfur battery provided in the comparative example is the same as that in Example 1, except that 1-methyl-1-propylpyrrolidinium bis (fluorosulfonyl) imide is replaced by 1,3-dioxolane.

[0062] Test Example 1

[0063] The electrochemical performance of the lithium-sulfur batteries of Examples 1-3 is tested, and the results are shown in Table 2; the electrochemical performance of the lithium-sulfur battery of Comparative Example 1 is tested, and the results are shown in Table 3.

[0064] The method for the normal temperature formation test is as follows: at 25℃, the battery is charged at 0.1C to 2.4V, then charged at 0.2C to 2.8V, then charged at constant voltage of 2.8V to the cutoff current of 0.05C, and then discharged at 0.1C to 1.8V.

[0065] The method of normal temperature cycle test is as follows: charging at 0.3C to 2.8V at 25℃, charging at 2.8V to the cut-off current of 0.05C, and then discharging at 0.5C to 1.8V; the discharge capacity is recorded as C1, repeating the charging and discharging steps 300 times to obtain the discharge capacity CN of the Nth week, and the capacity retention rate = CN / C1*100%.

[0066] The cycle test capacity retention rate of the soft package lithium-sulfur battery is less than 80%, the coulomb efficiency is less than 90%, and the battery stops the test.

[0067] Table 2

[0068] Example 1 Example 2 Example 3 Initial efficiency (%) 95.45 96.23 94.67 Capacity retention at 50th cycle (%) 99.98 99.36 99.67 Coulomb efficiency at 50th cycle (%) 99.46 99.65 99.29 Capacity retention at 100th cycle (%) 95.73 97.89 96.78 Coulomb efficiency at 100th cycle (%) 98.64 97.98 98.75

[0069] Table 3

[0070] Initial efficiency (%) Capacity retention at 10th cycle (%) Coulomb efficiency at 10th cycle (%) Comparative Example 1 91.32 40.67 85.32

[0071] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A battery, characterized by, The battery comprises an electrolyte and a positive electrode sheet; the positive electrode active material in the positive electrode sheet comprises at least one of elemental sulfur, sulfur-carbon composite material and sulfur-metal composite material; The electrolyte comprises an additive, a lithium salt, an organic solvent and an ionic liquid, the additive comprises at least one of compounds with the following structure: ； In the formula, R1 is selected from any one of methyl, propyl, ethyl, allyl, vinyl, propenyl, allyl, isopropenyl, ethynyl, propargyl, propynyl, 2-propenyl, 2-propynyl, isopropyl, ethynyl and tert-butyl; the mass of the additive accounts for 5% to 50% of the total mass of the electrolyte; The ionic liquid comprises fluorine-containing sulfonamide anions; the ionic liquid comprises at least one of 1-methyl-1-propyl pyrrolidinium bis (fluorosulfonyl) imide, 1-butyl-1-methyl pyrrolidinium bis (fluorosulfonyl) imide and 1-methyl-1-propyl pyrrolidine bis (trifluoromethylsulfonyl) imide salt, 1-methyl-1-piperidinium bis (fluorosulfonyl) imide, 1-methyl-1-pentyl pyrrolidinium bis (trifluoromethylsulfonyl) imide and 1-methyl-1-n-octyl pyrrolidinium bis (trifluoromethylsulfonyl) imide; The lithium salt comprises fluorine-containing sulfonamide type lithium salt; The organic solvent comprises a methyl ether compound, and the volume ratio of the organic solvent to the ionic liquid is 1:0.5-4.

2. The battery of claim 1, wherein, The lithium salt comprises at least one of lithium bisfluorosulfonylimide, lithium bis-trifluoromethylsulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium bis (nonafluorobutylsulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium bis (pentafluoroethylsulfonyl) imide, potassium bisfluorosulfonylimide, magnesium bis (trifluoromethylsulfonyl) imide, zinc bis (trifluoromethylsulfonyl) imide and calcium bis (trifluoromethylsulfonyl) imide.

3. The battery of claim 1, wherein, The concentration of the lithium salt in the electrolyte is 0.5-8 mol / L.

4. The battery of claim 1, wherein, The organic solvent comprises at least one of ethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, triethylene glycol dimethyl ether, diethylene glycol dimethyl ether, o-dimethyl ether, m-dimethyl ether, isosorbide dimethyl ether, tetraethylene glycol monomethyl ether and triethylene glycol butyl methyl ether.

5. The battery of claim 1, wherein, The battery further comprises a negative electrode sheet; the negative electrode active material in the negative electrode sheet comprises copper-lithium composite material and / or lithium foil.

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