Electrolyte for lithium-sulfur battery and lithium-sulfur battery

Abstract

The invention relates to the technical field of batteries, and particularly provides an electrolyte for a lithium-sulfur battery and the lithium-sulfur battery. The technical problems that in the prior art, polysulfide generated in the charging and discharging process of a sulfur positive electrode forms a shuttle effect in electrolyte, so that active substances of the positive electrode of the lithium-sulfur battery are lost, and the capacity of the lithium-sulfur battery is rapidly attenuated are solved. Therefore, the electrolyte provided by the invention comprises an additive, and the additive is a chain compound or a cyclic compound containing a carbon-sulfur bond. According to the electrolyte disclosed by the invention, a liquid-phase intermediate species polysulfide can be efficiently captured by using a small amount of additive, the additive is applied to the lithium-sulfur battery and can catalyze the conversion of the polysulfide, the reaction kinetics of the polysulfide is remarkably improved, the aggregation and diffusion of the polysulfide are slowed down, and the lithium-sulfur battery performance is improved. Stable circulation of the lithium-sulfur battery can be realized under the conditions of high sulfur load and low electrolyte consumption, and the service life of the lithium-sulfur battery is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular provides an electrolyte for a lithium-sulfur battery and a lithium-sulfur battery. Background Art

[0002] Lithium-sulfur batteries are considered to be one of the most promising candidates for the next generation of energy storage and power batteries due to their ultra-high theoretical energy density (2600Wh / kg), low cost and environmental friendliness, and have attracted widespread attention from academia and industry.

[0003] However, the low sulfur loading, fast capacity decay rate and low coulombic efficiency in the sulfur cathode limit the practical application of lithium-sulfur batteries. This is mainly due to the fact that the intermediate polysulfide produced by the sulfur cathode during the charge and discharge process is easily soluble in the electrolyte and forms a shuttle effect, resulting in irreversible loss of active substances and rapid decay of the capacity of lithium-sulfur batteries. In addition, the complex multiphase and multi-electron redox reactions of the sulfur cathode lead to slow reaction kinetics, further deteriorating the rate performance, discharge specific capacity and cycle performance of lithium-sulfur batteries.

[0004] Accordingly, the art needs a new technical solution to solve the above problems. Summary of the invention

[0005] The present invention aims to solve the above technical problems, that is, to solve the technical problems in the prior art that polysulfides generated by the sulfur positive electrode during the charging and discharging process form a shuttle effect in the electrolyte, resulting in the loss of active materials in the positive electrode of the lithium-sulfur battery and the rapid attenuation of the capacity of the lithium-sulfur battery.

[0006] In a first aspect, the present invention provides an electrolyte for a lithium-sulfur battery, wherein the electrolyte comprises an additive, wherein the additive is a chain compound or a cyclic compound comprising a carbon-sulfur bond, and the structural formula thereof is shown in Formula 1, Formula 2 or Formula 3:

[0007]

[0008]

[0009]

[0010] Wherein, R1, R2, R3 and R4 in Formula 1 and Formula 2 are independently selected from H, C1-C12 alkyl or benzyl, and n in Formula 1 satisfies 1≤n≤10.

[0011] In the preferred technical solution of the above electrolyte, R1, R2, R3 and R4 are independently selected from H, methyl or benzyl, and n in Formula 1 satisfies n=1.

[0012] In the preferred technical solution of the above electrolyte, the additive is ethylene trithiocarbonate, vinylene trithiocarbonate, S,S-dibenzyl trithiocarbonate or dimethyl trithiocarbonate.

[0013] In the preferred technical solution of the above electrolyte, the weight average molecular weight of the additive is less than 900.

[0014] In the preferred technical solution of the above electrolyte, the content of the additive in the electrolyte is 0.005 mol / L to 0.2 mol / L.

[0015] In the preferred technical solution of the above electrolyte, the content of the additive in the electrolyte is 0.05 mol / L to 0.15 mol / L.

[0016] In the case of adopting the above technical scheme, by adding a compound containing a carbon-sulfur bond characteristic functional group such as ethylene trithiocarbonate or dimethyl trithiocarbonate as an additive in the electrolyte, the sulfur-carbon double bond and the carbon-sulfur single bond co-existing in the additive can play a synergistic role in efficiently capturing the intermediate polysulfide in the electrolyte, improving the reaction kinetics of the polysulfide, and promoting the conversion of the polysulfide to the final discharge product during the charge and discharge process, so as to reduce the content of polysulfide in the electrolyte, improve the utilization rate of the active substance sulfur in the positive electrode of the lithium-sulfur battery, thereby improving the stable cycle of the lithium-sulfur battery and increasing the life of the lithium-sulfur battery.

[0017] In the preferred technical solution of the above electrolyte, the electrolyte further comprises an ether solvent, and the ether solvent comprises DOL and DME.

[0018] In a preferred technical solution of the above electrolyte, the electrolyte further comprises a lithium salt, and the lithium salt is at least one of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(difluorosulfonylimide) and lithium bis(trifluoromethylsulfonylimide).

[0019] In a second aspect, the present invention further provides an electrolyte for a lithium-sulfur battery, characterized in that the electrolyte comprises a lithium salt, an ether solvent and an additive, wherein the lithium salt is at least one of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(difluorosulfonylimide) and lithium bis(trifluoromethylsulfonylimide); the additive is a chain compound or a cyclic compound containing a carbon-sulfur bond, and its structural formula is shown in Formula 1, Formula 2 or Formula 3:

[0020]

[0021]

[0022]

[0023] Wherein, R1, R2, R3 and R4 in Formula 1 and Formula 2 are independently selected from H, C1-C12 alkyl or benzyl, and n in Formula 1 satisfies 1≤n≤10.

[0024] In the case of adopting the above technical solution, by adding a chain compound or a cyclic compound containing a carbon-sulfur bond to the electrolyte, the reaction kinetics of polysulfides are effectively improved under the action of carbon-sulfur double bonds and carbon-sulfur single bonds, and the conversion of polysulfides is promoted, so as to reduce the content of polysulfides in the electrolyte. At the same time, the lithium salt is set to at least one of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(difluorosulfonylimide) and lithium bis(trifluoromethylsulfonylimide), so as to improve the electrochemical stability and conductivity of the electrolyte. In this way, by optimizing the composition of the electrolyte, the stability and life of the lithium-sulfur battery with the electrolyte can be better ensured.

[0025] In a third aspect, the present invention further provides a lithium-sulfur battery, comprising a positive electrode, a separator, a negative electrode and an electrolyte, wherein the electrolyte is the electrolyte described above.

[0026] In the preferred technical solution of the above lithium-sulfur battery, the liquid-sulfur ratio of the lithium-sulfur battery is less than 10 μL / mg.

[0027] It should be noted that the lithium-sulfur battery has all the technical effects of the aforementioned electrolyte, which will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings, in which:

[0029] Figure 1 is a trend diagram of the discharge specific capacity of the lithium-sulfur battery obtained in Examples 1 to 4 of the present invention at 0.5C rate performance as the number of cycles changes;

[0030] Figure 2 is a trend diagram of the discharge specific capacity of the lithium-sulfur battery obtained in Example 3 of the present invention and Comparative Example 1 at 0.5C rate performance as the number of cycles changes;

[0031] Figure 3 This is a trend diagram of the discharge specific capacity of the lithium-sulfur battery obtained in Example 5 of the present invention at 0.1C rate performance as the number of cycles changes. DETAILED DESCRIPTION

[0032] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood by those skilled in the art that these embodiments are only used to explain the technical principles of the present invention and are not intended to limit the protection scope of the present invention.

[0033] The present invention provides an electrolyte for a lithium-sulfur battery, the electrolyte comprising an additive, the additive comprising a chain compound or a cyclic compound containing a carbon-sulfur bond, the structural formula of which is shown in Formula 1, Formula 2 or Formula 3 below:

[0034]

[0035]

[0036]

[0037] Wherein, R1, R2, R3 and R4 in Formula 1 and Formula 2 are independently selected from H, C1-C12 alkyl or benzyl, and n in Formula 1 satisfies 1≤n≤10.

[0038] During the charge and discharge process of the lithium-sulfur battery, the polysulfide (Li2Sx) intermediate product generated by the positive electrode will dissolve into the electrolyte, pass through the diaphragm, diffuse to the negative electrode, and directly react with the metallic lithium of the negative electrode, which ultimately causes the irreversible loss of effective substances in the lithium-sulfur battery, thereby causing the rapid decay of the battery capacity of the lithium-sulfur battery. The electrolyte of the present invention is added with an additive comprising a chain compound or a cyclic compound containing a carbon-sulfur bond, wherein the carbon-sulfur bond comprises a carbon-sulfur double bond and a carbon-sulfur single bond, the sulfur-carbon double bond in the additive has good catalytic activation ability, and the presence of the sulfur-carbon single bond is conducive to the catalytic activation ability of the sulfur-carbon double bond, so that the intermediate polysulfide in the electrolyte can be efficiently captured, the reaction kinetics of the polysulfide is improved, and the conversion of the polysulfide during the charge and discharge process is promoted. Furthermore, since chain compounds or cyclic compounds containing carbon-sulfur bonds are usually well miscible with the electrolyte, only a small amount of additives is needed to fully contact the polysulfides in the electrolyte and promote the conversion of the polysulfides, thereby slowing down the aggregation and diffusion of the polysulfides, reducing the content of polysulfides in the electrolyte, and improving the utilization rate of the active sulfur in the positive electrode of the lithium-sulfur battery, thereby improving the stable cycle of the lithium-sulfur battery and increasing the life of the lithium-sulfur battery.

[0039] In a possible embodiment, R1, R2, R3 and R4 are independently selected from H, methyl or benzyl, and n in Formula 1 satisfies n=1. That is, the additive can be dimethyl trithiocarbonate, S,S-dibenzyl trithiocarbonate, 1,3-disulfuric acid-2-thione, etc. These compounds all include carbon-sulfur single bonds and carbon-sulfur double bonds. The carbon-sulfur double bond has good catalytic activation ability, and the carbon-sulfur single bond helps the catalytic activation of the carbon-sulfur double bond, thereby effectively improving the kinetics of the conversion reaction.

[0040] In one possible embodiment, the additive is ethylene trithiocarbonate, vinylene trithiocarbonate, S,S-dibenzyl trithiocarbonate or dimethyl trithiocarbonate. These compounds all include carbon-sulfur single bonds and carbon-sulfur double bonds. The carbon-sulfur single bond helps the catalytic activation of the carbon-sulfur double bond and can effectively improve the kinetics of the conversion reaction.

[0041] It should be noted that the additive can also be a carbon-sulfur bond-containing chain compound such as bis(dodecylthiocarbonyl) disulfide, bis(carboxymethyl) trithiocarbonate, or a carbon-sulfur bond-containing cyclic compound such as 4,5-ethylenedithio-1,3-dithiol-2-thione, 1,3-dithiocyclopentadiene-2-thione-4,5-dimethyl carboxylate, 5-propyl-1,3-dithiol and [4,5-d][1,3]dithiocyclopentadiene-2-thione. Without departing from the basic principles of the present application, those skilled in the art can flexibly select a specific carbon-sulfur bond-containing chain compound or cyclic compound according to the specific application scenario, as long as it can capture polysulfides and promote polysulfide conversion.

[0042] In one possible embodiment, the weight average molecular weight of the additive is less than 900. The additive with a weight average molecular weight within this range can be miscible with other components of the electrolyte, capture polysulfides and promote the conversion of polysulfides, and will not affect the proportion of active substances in the electrolyte due to excessive molecular weight. That is, while ensuring the conversion rate of polysulfides, the content of effective active substances in the electrolyte is ensured, thereby ensuring the energy density of the lithium-sulfur battery.

[0043] In a possible embodiment, the electrolyte also includes an auxiliary agent, which is an inorganic lithium salt. The main function of the electrolyte is to transfer ions in the battery. Taking metallic lithium as the negative electrode as an example, during the charging process of the battery, the lithium ions detach from the negative electrode and attach to the surface of the negative electrode. During the discharge process, the lithium ions detach from the negative electrode, transfer to the positive electrode through the electrolyte and release energy. However, in the early stage of the first charge, the lithium salt and ether solvent in the electrolyte will react with the lithium ions on the surface of the negative electrode to generate solid products such as Li2CO3, LiF, LiOH and organic lithium compounds, which will be deposited on the surface of the negative electrode. These solid products will not affect the conduction of lithium ions, that is, lithium ions can freely pass through these solid products to ensure the transmission of lithium ions. At the same time, these solid products can prevent the electrolyte from passing through, thereby preventing the electrolyte from continuing to react at the negative electrode, thereby greatly reducing the irreversible reaction of the lithium-sulfur battery and ensuring the stability of the lithium-sulfur battery cycle capacity.

[0044] Preferably, the inorganic lithium salt selected in the present invention is LiNO3, which can protect the metallic lithium negative electrode, and is beneficial to improving the coulombic efficiency and the cycle stability and capacity retention rate of the lithium-sulfur battery.

[0045] It should be noted that, in practical applications, those skilled in the art may choose inorganic lithium salts such as lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluorophosphate (LiPF6) and lithium nitrate (LiNO3), etc. In practical applications, those skilled in the art may also choose not to add inorganic lithium salts to the electrolyte. Such adjustments and changes to the inorganic lithium salts do not deviate from the principles and scope of the present invention and should be included in the protection scope of the present invention.

[0046] In a possible embodiment, the content of the additive in the electrolyte is 0.01 to 0.2 mol / L. Taking ethylene trithiocarbonate as an example, the content of ethylene trithiocarbonate in the electrolyte can be 0.01, or 0.05, or 0.1, or 0.15, or 0.2, and so on. By controlling the molar concentration of the additive, the amount of ethylene trithiocarbonate added to the electrolyte can be accurately controlled, so that the conversion of polysulfides can be promoted by ethylene trithiocarbonate, the conversion efficiency of polysulfides can be improved, and the stability of the lithium-sulfur battery can be ensured, and the effective ingredients in the electrolyte can be avoided from being affected by excessive addition of additives, thereby affecting the energy density of the lithium-sulfur battery and reducing the battery performance of the lithium-sulfur battery. Preferably, the content of ethylene trithiocarbonate in the electrolyte is 0.05 to 0.15 mol / L.

[0047] In practical applications, the content of ethylene trithiocarbonate in the electrolyte can also be selected in other proportions according to different design requirements or usage scenarios of the lithium-sulfur battery. Such adjustment and change of the content of ethylene trithiocarbonate in the electrolyte does not deviate from the principle and scope of the present invention and should be included in the protection scope of the present invention.

[0048] In a possible embodiment, the lithium salt is lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), and the addition of LiTFSI can improve the electrochemical stability and conductivity of the electrolyte. In addition, LiTFSI does not react with water, so there is no problem of gas generation due to the reaction of lithium salt with water, which causes the bloating of the lithium-sulfur battery.

[0049] It should be noted that, in practical applications, the lithium salts that can be selected by technicians in this field may also be lithium bis(oxalatoborate) (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium bis(difluorosulfonyl)imide (LiFSI), etc. Such adjustments and changes to the lithium salts do not deviate from the principle and scope of the present invention and should be included in the protection scope of the present invention.

[0050] Since adding too much auxiliary agent will inhibit the conversion of polysulfide, thus causing the capacity of lithium-sulfur battery to decay, but adding too little auxiliary agent will not effectively protect the positive electrode material, and adding too much lithium salt will corrode the current collector, but adding too little will make it difficult for the lithium-sulfur battery to have high electrochemical stability and conductivity. For this reason, the molar ratio of lithium salt to auxiliary agent in the electrolyte of the present invention is (2-6):1, for example, 2:1, 5:1, 6:1, etc. The lithium-sulfur battery including the electrolyte composed of lithium salt and auxiliary agent within this ratio has high electrochemical stability and conductivity, and the lithium salt under this addition amount will not corrode the current collector, making the current collector relatively stable, thereby improving the charge and discharge efficiency and service life of the lithium-sulfur battery.

[0051] In practical applications, those skilled in the art can adjust the molar ratio of lithium salt and additive according to the actual design data of lithium-sulfur batteries. Such adjustment and change of the molar ratio of lithium salt and additive does not deviate from the principle and scope of the present invention and should be included in the scope of the present invention.

[0052] In a possible embodiment, the electrolyte also includes an ether solvent, which is a mixed solution of DOL (1,3-dioxolane) and DME (ethylene glycol dimethyl ether). The ether solvent can better dissolve additives and other components in the electrolyte, ensure that the electrolyte has good fluidity, and improve the ionic conductivity of the electrolyte, thereby improving the charge and discharge capacity of the lithium-sulfur battery.

[0053] In a possible embodiment, the ether solvent is a mixed solution composed of DOL and DME in a volume ratio of 1:1. 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME) have similar physicochemical properties. The melting point of 1,3-dioxolane (DOL) is -95°C and the boiling point is 75°C. The melting point of ethylene glycol dimethyl ether (DME) is -58°C and the boiling point is 82-83°C. Therefore, the mixed solution composed of DOL and DME in a volume ratio of 1:1 will not crystallize in a wide range (for example, 0°C to 70°C). Then, the electrolyte using the mixed solution as a solvent will not undergo obvious crystallization in a wide temperature range (for example, 0°C to 70°C). The electrolyte has high ionic conductivity, thermodynamic stability, and a wide electrochemical window in the liquid state, and has the potential for low-temperature applications. Therefore, by using a mixed solvent composed of DOL and DME in a volume ratio of 1:1 as the solvent of the electrolyte, it can be ensured that no obvious crystallization will occur in the electrolyte at low temperatures, thereby effectively improving the ionic conductivity of the electrolyte and thus improving the charge and discharge capacity of the lithium-sulfur battery.

[0054] It should be noted that, in practical applications, those skilled in the art may choose other volume ratios of DOL and DME to form the mixed solution, for example, a larger volume ratio such as 1.5:1, 2:1, or a smaller volume ratio such as 0.9:1, 0.8:1, etc. Such adjustments and changes to the volume ratio of DOL and DME do not deviate from the principle and scope of the present invention, and should be included in the scope of the present invention.

[0055] It should also be noted that, in practical applications, the ether solvent may also be a mixed solution of one or more of tetrahydrofuran, 2-methyltetrahydrofuran, petroleum ether, etc., or a mixed solution formed by mixing one or more of the aforementioned ether solvents with DOL and / or DME, etc. Such adjustments and changes to the ether solvent do not deviate from the principle and scope of the present invention and should be included in the protection scope of the present invention.

[0056] The present invention also provides a lithium-sulfur battery, which comprises a positive electrode, a separator, a negative electrode and an electrolyte, wherein the electrolyte is the electrolyte for the lithium-sulfur battery mentioned above.

[0057] Specifically, the mass ratio of the amount of electrolyte to sulfur in the lithium-sulfur battery, that is, the liquid-sulfur ratio (E / S) is less than 10 μL / mg. In other words, each milligram of sulfur has a more suitable amount of electrolyte. For example, each milligram of sulfur corresponds to 1 μL, 2 μL, 4 μL, or 6 μL, or 8 μL, or 10 μL of electrolyte. The present invention controls the liquid-sulfur ratio within a suitable range, thereby obtaining a lithium-sulfur battery with good service life, electrochemical stability, and conductivity, and by adjusting the liquid-sulfur ratio of the electrolyte, it is beneficial to prepare lithium-sulfur batteries of various capacities, so that the lithium-sulfur battery is suitable for different use environments. Preferably, the liquid-sulfur ratio (E / S) is 10 μL / mg.

[0058] It should be noted that, in practical applications, those skilled in the art may also select other liquid-sulfur ratios according to different use environments of lithium-sulfur batteries. Such adjustments and changes to the liquid-sulfur ratio do not deviate from the principle and scope of the present invention and should be included in the protection scope of the present invention.

[0059] In a possible embodiment, the positive electrode is prepared by coating a current collector with sulfur element loaded on multi-walled carbon nanotubes. Specifically, multi-walled carbon nanotubes are used as the positive electrode substrate, the positive electrode substrate is mixed with sulfur solid powder in proportion, and heated for a period of time at a preset temperature in an inert atmosphere to obtain a positive electrode intermediate, the positive electrode intermediate is mixed with a binder and a dispersant to form a slurry, the slurry is coated on the current collector, and dried to obtain the positive electrode.

[0060] It should be noted that, in practical applications, those skilled in the art can use graphite as the positive electrode substrate, load sulfur on the graphite and then coat it on the current collector to obtain the positive electrode. Of course, other materials can also be used as the positive electrode substrate to load sulfur to prepare the positive electrode. Such adjustments and changes to the positive electrode substrate do not deviate from the principle and scope of the present invention and should be included in the protection scope of the present invention.

[0061] It should be noted that, in actual applications, the positive electrode substrate is mixed with sulfur solid powder in proportion, and the positive electrode intermediate can be prepared by heating at 155°C for 12 hours in an inert atmosphere of argon, or by heating at 155°C for 12 hours in an inert atmosphere of nitrogen, etc. Such adjustments and changes to the inert atmosphere used in the preparation of the positive electrode intermediate do not deviate from the principle and scope of the present invention, and should be included in the protection scope of the present invention.

[0062] It should also be noted that when the positive electrode intermediate is prepared, other preset temperatures and reaction times may be used, for example, heating at 145°C for 14 hours, or heating at 165°C for 11 hours, etc. Such adjustments and changes to the reaction temperature and reaction time do not deviate from the principles and scope of the present invention, and should be included in the scope of protection of the present invention.

[0063] Specifically, the binders that can be selected include polyvinylidene fluoride (PVDF), polyacrylic acid (PAA) and its salt binders, polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), etc. Such adjustments and changes to the binders do not deviate from the principles and scope of the present invention and should be included in the protection scope of the present invention.

[0064] Specifically, the dispersants that can be selected include paraffins, fatty acids, aliphatic amides and esters, N-methylpyrrolidone (NMP), etc. Such adjustments and changes to the dispersants do not deviate from the principles and scope of the present invention and should be included in the protection scope of the present invention.

[0065] Specifically, the sulfur loading on the positive electrode is 2 mg / cm 2 ~5mg / cm 2 That is to say, 2 to 5 mg of sulfur is loaded on each square centimeter of the positive electrode. For example, 2 mg, 3 mg, 4 mg, or 5 mg of sulfur is loaded on each square centimeter of the positive electrode.

[0066] It should be noted that, in practical applications, those skilled in the art can design other sulfur loading amounts according to the usage scenarios of the lithium-sulfur battery. Such adjustments and changes to the sulfur loading amount of the lithium-sulfur battery do not deviate from the principle and scope of the present invention and should be included in the protection scope of the present invention.

[0067] The electrolyte for lithium-sulfur battery and the lithium-sulfur battery of the present invention are described in detail below in conjunction with Examples 1 to 7 and Comparative Example 1.

[0068] It should be noted that in each embodiment and comparative example, the main parameters of the negative electrode and the separator are as follows:

[0069] The negative electrode is a metallic lithium negative electrode with a thickness of 450 μm and a diameter of 15.6 mm;

[0070] The separator is a microporous polypropylene membrane (Celgard 2400) with a diameter of 19 mm.

[0071] Various raw materials, base materials, etc. used in the preparation process are all commercially available products.

[0072] Embodiment 1

[0073] The lithium salt, ether solvent and auxiliary agent are mixed to obtain a lithium-sulfur battery electrolyte, wherein the lithium salt is LiTFSI, the ether solvent is a mixed solution of DOL and DME in a volume ratio of 1:1, and the auxiliary agent is LiNO3, wherein the concentration of LiTFSI is 1 mol / L, and the concentration of LiNO3 is 0.20 mol / L. Ethylene trithiocarbonate is added to the lithium-sulfur battery electrolyte, and the content of ethylene trithiocarbonate in the electrolyte after the addition is 0.005 mol / L, and the electrolyte is stirred at 25°C until dissolved to obtain a lithium-sulfur battery electrolyte containing additives.

[0074] The sulfur solid powder and multi-walled carbon nanotubes were mixed in a mass ratio of 8:2, and then heated at 155°C for 12 hours under an inert atmosphere to prepare a carbon nanotube positive electrode loaded with sulfur.

[0075] The prepared carbon nanotube positive electrode loaded with sulfur and the binder polyvinylidene fluoride (PVDF) were weighed at a mass ratio of 9:1, and an appropriate amount of N-methylpyrrolidone (NMP) was added to mix and adjusted into a slurry. The slurry was evenly coated on the aluminum foil current collector using a coating machine, and the positive electrode was obtained after drying. The positive electrode was cut into a positive electrode sheet with a diameter of 14 mm and weighed, wherein the sulfur loading of the positive electrode sheet was 2 mg / cm 2 .

[0076] The lithium-sulfur battery electrolyte containing the additive prepared above is assembled with a positive electrode plate, a separator, and a metal lithium negative electrode into a lithium-sulfur battery, and the liquid-sulfur ratio (E / S) in the lithium-sulfur battery is 10 μL / mg.

[0077] Embodiment 2

[0078] The lithium salt, ether solvent and auxiliary agent are mixed to obtain a lithium-sulfur battery electrolyte, wherein the lithium salt is LiTFSI, the ether solvent is a mixed solution of DOL and DME in a volume ratio of 1:1, and the auxiliary agent is LiNO3, wherein the concentration of LiTFSI is 1 mol / L, and the concentration of LiNO3 is 0.20 mol / L. Ethylene trithiocarbonate is added to the lithium-sulfur battery electrolyte, and the content of ethylene trithiocarbonate in the electrolyte after the addition is 0.01 mol / L, and the electrolyte is stirred at 25°C until dissolved to obtain a lithium-sulfur battery electrolyte containing additives.

[0079] The sulfur solid powder and multi-walled carbon nanotubes were mixed in a mass ratio of 8:2, and then heated at 155°C for 12 hours under an inert atmosphere to prepare a carbon nanotube positive electrode loaded with sulfur.

[0080] The prepared carbon nanotube positive electrode loaded with sulfur and the binder polyvinylidene fluoride (PVDF) were weighed at a mass ratio of 9:1, and an appropriate amount of N-methylpyrrolidone (NMP) was added to mix and adjusted into a slurry. The slurry was evenly coated on the aluminum foil current collector using a coating machine, and the positive electrode was obtained after drying. The positive electrode was cut into a positive electrode sheet with a diameter of 14 mm and weighed, wherein the sulfur loading of the positive electrode sheet was 2 mg / cm 2 .

[0081] The lithium-sulfur battery electrolyte containing the additive prepared above is assembled with a positive electrode plate, a separator, and a metal lithium negative electrode into a lithium-sulfur battery, and the liquid-sulfur ratio (E / S) in the lithium-sulfur battery is 10 μL / mg.

[0082] Embodiment 3

[0083] The lithium salt, ether solvent and auxiliary agent are mixed to prepare a lithium-sulfur battery electrolyte, wherein the lithium salt is LiTFSI, the ether solvent is a mixed solution of DOL and DME in a volume ratio of 1:1, and the auxiliary agent is LiNO3, wherein the concentration of LiTFSI is 1 mol / L, and the concentration of LiNO3 is 0.20 mol / L. Ethylene trithiocarbonate is added to the lithium-sulfur battery electrolyte, and the content of ethylene trithiocarbonate in the electrolyte after the addition is 0.05 mol / L, and the electrolyte is stirred at 25°C until dissolved to obtain a lithium-sulfur battery electrolyte containing additives.

[0084] The sulfur solid powder and multi-walled carbon nanotubes were mixed in a mass ratio of 8:2, and then heated at 155°C for 12 hours under an inert atmosphere to prepare a carbon nanotube positive electrode loaded with sulfur.

[0085] The prepared carbon nanotube positive electrode loaded with sulfur and the binder polyvinylidene fluoride (PVDF) were weighed at a mass ratio of 9:1, and an appropriate amount of N-methylpyrrolidone (NMP) was added to mix and adjusted into a slurry. The slurry was evenly coated on the aluminum foil current collector using a coating machine, and the positive electrode was obtained after drying. The positive electrode was cut into a positive electrode sheet with a diameter of 14 mm and weighed, wherein the sulfur loading of the positive electrode sheet was 2 mg / cm 2 .

[0086] The lithium-sulfur battery electrolyte containing the additive prepared above is assembled with a positive electrode plate, a separator, and a metal lithium negative electrode into a lithium-sulfur battery, and the liquid-sulfur ratio (E / S) in the lithium-sulfur battery is 10 μL / mg.

[0087] Embodiment 4

[0088] The lithium salt, ether solvent and auxiliary agent are mixed to obtain a lithium-sulfur battery electrolyte, wherein the lithium salt is LiTFSI, the ether solvent is a mixed solution of DOL and DME in a volume ratio of 1:1, and the auxiliary agent is LiNO3, wherein the concentration of LiTFSI is 1 mol / L, and the concentration of LiNO3 is 0.20 mol / L. Ethylene trithiocarbonate is added to the lithium-sulfur battery electrolyte, and the content of ethylene trithiocarbonate in the electrolyte is 0.1 mol / L after the addition is completed, and the electrolyte is stirred at 25°C until dissolved to obtain a lithium-sulfur battery electrolyte containing additives.

[0089] The sulfur solid powder and multi-walled carbon nanotubes were mixed in a mass ratio of 8:2, and then heated at 155°C for 12 hours under an inert atmosphere to prepare a carbon nanotube positive electrode loaded with sulfur.

[0090] The prepared carbon nanotube positive electrode loaded with sulfur and the binder polyvinylidene fluoride (PVDF) were weighed in a mass ratio of 9:1, and an appropriate amount of N-methylpyrrolidone (NMP) was added to mix and adjusted into a slurry. The slurry was evenly coated on the aluminum foil current collector using a coating machine, and after drying, it was cut into a positive electrode sheet with a diameter of 14 mm and weighed, wherein the sulfur loading of the positive electrode sheet was 2 mg / cm 2 .

[0091] The lithium-sulfur battery electrolyte containing the additive prepared above is assembled with a positive electrode plate, a separator, and a metal lithium negative electrode into a lithium-sulfur battery, and the liquid-sulfur ratio (E / S) in the lithium-sulfur battery is 10 μL / mg.

[0092] Embodiment 5

[0093] The lithium salt, ether solvent and auxiliary agent are mixed to prepare a lithium-sulfur battery electrolyte, wherein the lithium salt is LiTFSI, the ether solvent is a mixed solution of DOL and DME in a volume ratio of 1:1, and the auxiliary agent is LiNO3, wherein the concentration of LiTFSI is 1 mol / L, and the concentration of LiNO3 is 0.20 mol / L. Ethylene trithiocarbonate is added to the lithium-sulfur battery electrolyte, and the content of ethylene trithiocarbonate in the electrolyte after the addition is 0.05 mol / L, and the electrolyte is stirred at 25°C until dissolved to obtain a lithium-sulfur battery electrolyte containing additives.

[0094] The sulfur solid powder and multi-walled carbon nanotubes were mixed in a mass ratio of 8:2, and then heated at 155°C for 12 hours under an inert atmosphere to prepare a carbon nanotube positive electrode loaded with sulfur.

[0095] The prepared carbon nanotube positive electrode loaded with sulfur and the binder polyvinylidene fluoride (PVDF) were weighed at a mass ratio of 9:1, and an appropriate amount of N-methylpyrrolidone (NMP) was added to mix and adjust into a slurry. The slurry was evenly coated on the aluminum foil current collector using a coating machine, and after drying, it was cut into positive electrode pieces with a diameter of 14 mm and weighed, wherein the sulfur loading of the positive electrode piece was 5 mg / cm 2 .

[0096] The lithium-sulfur battery electrolyte containing the additive prepared above is assembled with a positive electrode plate, a separator, and a metallic lithium negative electrode into a lithium-sulfur battery, and the liquid-sulfur ratio (E / S) in the lithium-sulfur battery is 4 μL / mg.

[0097] Embodiment 6

[0098] The lithium salt, ether solvent and auxiliary agent are mixed to obtain a lithium-sulfur battery electrolyte, wherein the lithium salt is LiTFSI, the ether solvent is a mixed solution of DOL and DME in a volume ratio of 1:1, and the auxiliary agent is LiNO3, wherein the concentration of LiTFSI is 1 mol / L, and the concentration of LiNO3 is 0.20 mol / L. Ethylene trithiocarbonate is added to the lithium-sulfur battery electrolyte, and the content of ethylene trithiocarbonate in the electrolyte is 0.15 mol / L after the addition is completed, and the electrolyte is stirred at 25°C until dissolved to obtain a lithium-sulfur battery electrolyte containing additives.

[0099] The sulfur solid powder and multi-walled carbon nanotubes were mixed in a mass ratio of 8:2, and then heated at 155°C for 12 hours under an inert atmosphere to prepare a carbon nanotube positive electrode loaded with sulfur.

[0100] The prepared carbon nanotube positive electrode loaded with sulfur and the binder polyvinylidene fluoride (PVDF) were weighed in a mass ratio of 9:1, and an appropriate amount of N-methylpyrrolidone (NMP) was added to mix and adjusted into a slurry. The slurry was evenly coated on the aluminum foil current collector using a coating machine, and after drying, it was cut into a positive electrode sheet with a diameter of 14 mm and weighed, wherein the sulfur loading of the positive electrode sheet was 2 mg / cm 2 .

[0101] The lithium-sulfur battery electrolyte containing the additive prepared above is assembled with a positive electrode plate, a separator, and a metal lithium negative electrode into a lithium-sulfur battery, and the liquid-sulfur ratio (E / S) in the lithium-sulfur battery is 10 μL / mg.

[0102] Embodiment 7

[0103] The lithium salt, ether solvent and auxiliary agent are mixed to obtain a lithium-sulfur battery electrolyte, wherein the lithium salt is LiTFSI, the ether solvent is a mixed solution of DOL and DME in a volume ratio of 1:1, and the auxiliary agent is LiNO3, wherein the concentration of LiTFSI is 1 mol / L, and the concentration of LiNO3 is 0.20 mol / L. Dimethyl trithiocarbonate is added to the lithium-sulfur battery electrolyte, and the content of dimethyl trithiocarbonate in the electrolyte after the addition is 0.05 mol / L, and the electrolyte is stirred at 25°C until dissolved to obtain a lithium-sulfur battery electrolyte containing additives.

[0104] The sulfur solid powder and multi-walled carbon nanotubes were mixed in a mass ratio of 8:2, and then heated at 155°C for 12 hours under an inert atmosphere to prepare a carbon nanotube positive electrode loaded with sulfur.

[0105] The prepared carbon nanotube positive electrode loaded with sulfur and the binder polyvinylidene fluoride (PVDF) were weighed at a mass ratio of 9:1, and an appropriate amount of N-methylpyrrolidone (NMP) was added to mix and adjusted into a slurry. The slurry was evenly coated on the aluminum foil current collector using a coating machine, and the positive electrode was obtained after drying. The positive electrode was cut into a positive electrode sheet with a diameter of 14 mm and weighed, wherein the sulfur loading of the positive electrode sheet was 2 mg / cm 2 .

[0106] The lithium-sulfur battery electrolyte containing the additive prepared above is assembled with a positive electrode plate, a separator, and a metal lithium negative electrode into a lithium-sulfur battery, and the liquid-sulfur ratio (E / S) in the lithium-sulfur battery is 10 μL / mg.

[0107] Comparative Example 1

[0108] A lithium salt, an ether solvent and an auxiliary agent are mixed to prepare a lithium-sulfur battery electrolyte, wherein the lithium salt is LiTFSI, the ether solvent is a mixed solution of DOL and DME in a volume ratio of 1:1, and the auxiliary agent is LiNO3, wherein the concentration of LiTFSI is 1 mol / L, and the concentration of LiNO3 is 0.20 mol / L.

[0109] The sulfur solid powder and multi-walled carbon nanotubes were mixed in a mass ratio of 8:2, and then heated at 155°C for 12 hours under an inert atmosphere to prepare a carbon nanotube positive electrode loaded with sulfur.

[0110] The prepared carbon nanotube positive electrode loaded with sulfur and the binder polyvinylidene fluoride (PVDF) were weighed in a mass ratio of 9:1, and an appropriate amount of N-methylpyrrolidone (NMP) was added to mix and adjust into a slurry. The slurry was evenly coated on the aluminum foil current collector with a coating machine, and after drying, it was cut into positive electrode sheets with a diameter of 14 mm and weighed, wherein the sulfur loading of the positive electrode sheet was 2 mg / cm 2 .

[0111] The prepared lithium-sulfur battery electrolyte was assembled with a positive electrode sheet, a separator, and a metallic lithium negative electrode into a lithium-sulfur battery, wherein the liquid-sulfur ratio (E / S) of the lithium-sulfur battery was 10 μL / mg.

[0112] The lithium-sulfur batteries prepared in the above-mentioned Examples 1 to 5 and Comparative Example 1 were subjected to electrochemical tests, wherein the standard specific capacity of the lithium-sulfur battery was 1672 mAh / g. The specific results obtained from the tests are as follows: Figures 1 to 3 shown.

[0113] Among them, the experiment detected that the lithium-sulfur batteries prepared in Examples 1 to 5 have improved coulombic efficiency compared with traditional lithium-sulfur batteries, and the change in coulombic efficiency is small as the number of cycles increases, that is, the coulombic efficiency is greater than 99.9%.

[0114] Specifically, in this embodiment, the lithium-sulfur batteries obtained in the above embodiments and comparative examples are electrochemically tested using the blue electric tester system produced by Shenzhen Kejing Zhida Technology Co., Ltd. at 25°C, and the charge-discharge cycle test is as follows: the test process is first discharged and then charged, the charge-discharge cut-off voltage is 1.7-2.8V, and the current density is 0.5C or 0.1C (1C=1675mAh), and then the same conditions are repeated for multiple cycles to examine the change in the discharge specific capacity of the lithium-sulfur battery prepared under the conditions in each embodiment with the number of cycles. Among them, one cycle is to complete one discharge and one charge.

[0115] in, Figure 1 and Figure 2 This is the experimental result obtained by testing at 25℃ and current density of 0.5C. Figure 3 These are the experimental results obtained at 25°C and a current density of 0.1C.

[0116] in, Figure 1 In the figure, “-■-” represents the test curve of the first embodiment, “-·-” represents the test curve of the second embodiment, “-□-” represents the test curve of the third embodiment, and “-○-” represents the test curve of the fourth embodiment. Figure 2 In the figure, “-□-” represents the test curve of Example 3, and “-★-” represents the test curve of Comparative Example 1. Figure 3 middle It represents the test curve of Example 5.

[0117] like Figure 1 As shown, according to the discharge specific capacity test data of the lithium-sulfur battery of Examples 1 to 4, it can be seen that under the same sulfur loading amount and liquid sulfur ratio of the lithium-sulfur battery, as the amount of ethylene trithiocarbonate added to the electrolyte increases, the discharge specific capacity of the lithium-sulfur battery first increases and then decreases. In Example 3, when the addition amount of ethylene trithiocarbonate is 0.050 mol / L, the discharge specific capacity of the lithium-sulfur battery is the highest. This also means that the conversion efficiency of ethylene trithiocarbonate to polysulfide is the highest at this time. At this addition amount of ethylene trithiocarbonate, the shuttle effect of polysulfide in the electrolyte can be significantly reduced, avoiding the loss of active substances in the positive electrode of the lithium-sulfur battery and avoiding the capacity decay of the lithium-sulfur battery.

[0118] like Figure 2 As shown, the discharge specific capacity of the lithium-sulfur battery in Comparative Example 1 is significantly lower than the discharge specific capacity of the lithium-sulfur battery in Example 3. Therefore, it can be seen that after adding ethylene trithiocarbonate to the electrolyte, the discharge specific capacity of the lithium-sulfur battery can be significantly improved. This also shows that the addition of ethylene trithiocarbonate can significantly improve the reaction kinetics of polysulfides in the lithium-sulfur battery, slow down the aggregation and diffusion of polysulfides, and can achieve stable circulation of the lithium-sulfur battery, thereby improving the service life of the lithium-sulfur battery.

[0119] like Figure 3 As shown, in Example 5, the content of ethylene trithiocarbonate in the electrolyte is 0.05 mol / L, and the sulfur loading is 5 mg / cm 2 , liquid sulfur ratio (E / S) is 4μL / mg, according to Figure 3 Discharge capacity of lithium-sulfur battery (Example 5). In Example 5, the lithium-sulfur battery still has a relatively stable discharge capacity when ethylene trithiocarbonate is used as an additive and the sulfur loading is high and the liquid-sulfur ratio is low. The conventional lithium-sulfur battery has a relatively stable discharge capacity when the sulfur loading is 5 mg / cm 2When the liquid sulfur ratio (E / S) is 4 μL / mg, the discharge capacity of the conventional lithium-sulfur battery is difficult to measure, and the performance of the conventional lithium-sulfur battery is unstable. Therefore, in this embodiment, the use of ethylene trithiocarbonate as an additive in the electrolyte can make the lithium-sulfur battery have a relatively stable charge and discharge capacity, and can design a lithium-sulfur battery with a high sulfur loading according to the specific use environment of the lithium-sulfur battery.

[0120] The additive of the present invention is ethylene trithiocarbonate or dimethyl trithiocarbonate. The additive can efficiently "capture" liquid phase intermediate species polysulfide by using a small amount. The additive can catalyze the conversion of polysulfide when applied to lithium-sulfur batteries, significantly improve the reaction kinetics of polysulfide, slow down the aggregation and diffusion of polysulfide, and realize stable circulation of lithium-sulfur batteries under the conditions of high sulfur loading and low electrolyte dosage, thereby improving the service life of lithium-sulfur batteries.

[0121] So far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present invention.

Claims

1. An electrolyte for a lithium-sulfur battery, characterized in that: The electrolyte includes an additive, and the additive is a chain compound or a cyclic compound containing a carbon-sulfur bond, and its structural formula is shown in Formula 1, Formula 2 or Formula 3: Wherein, R1, R2, R3 and R4 in Formula 1 and Formula 2 are independently selected from H, C1-C12 alkyl or benzyl, and n in Formula 1 satisfies 1≤n≤10.

2. The electrolyte according to claim 1, wherein R1, R2, R3 and R4 are independently selected from H, methyl or benzyl, and n in Formula 1 satisfies n=1.

3. The electrolyte according to claim 1, wherein the additive is ethylene trithiocarbonate, vinylene trithiocarbonate, S,S-dibenzyl trithiocarbonate or dimethyl trithiocarbonate. The electrolyte according to claim 1 , wherein the weight average molecular weight of the additive is less than 900. 5 . The electrolyte according to claim 1 , wherein the content of the additive in the electrolyte is 0.005 mol / L to 0.2 mol / L.

6. The electrolyte according to claim 5, characterized in that The content of the additive in the electrolyte is 0.05 mol / L to 0.15 mol / L.

7. The electrolyte according to claim 6, characterized in that The electrolyte further includes an ether solvent, and the ether solvent includes 1,3-dioxolane and ethylene glycol dimethyl ether.

8. The electrolyte according to claim 1, characterized in that The electrolyte further includes a lithium salt, and the lithium salt is at least one of lithium bis(oxalatoborate), lithium difluoro(oxalatoborate), lithium bis(difluorosulfonylimide) and lithium bis(trifluoromethylsulfonylimide).

9. An electrolyte for a lithium-sulfur battery, characterized in that: The electrolyte includes a lithium salt, an ether solvent and an additive, wherein the lithium salt is at least one of lithium bis(oxalatoborate), lithium difluoro(oxalatoborate), lithium bis(difluorosulfonyl)imide and lithium bis(trifluoromethylsulfonyl)imide; and the additive is a chain compound or a cyclic compound containing a carbon-sulfur bond, and its structural formula is shown in Formula 1, Formula 2 or Formula 3: Wherein, R1, R2, R3 and R4 in Formula 1 and Formula 2 are independently selected from H, C1-C12 alkyl or benzyl, and n in Formula 1 satisfies 1≤n≤10.

10. A lithium-sulfur battery, characterized in that: The lithium-sulfur battery comprises a positive electrode, a separator, a negative electrode and an electrolyte, and the electrolyte is the electrolyte according to any one of claims 1 to 9.

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