An electrolyte for a lithium-sulfur battery and a preparation method thereof

Abstract

The present invention discloses an electrolyte for a lithium-sulfur battery. The preparation raw materials include, by weight percentage: 5-20% of a lithium salt, 1-10% of a phosphazene derivative composite material, and the balance is made up of an organic solvent. In the electrolyte of the present invention, a cyclotriphosphazene derivative-graphene oxide composite material is introduced, which can inhibit the "shuttle" problem of polysulfides at the positive electrode, and at the same time inhibit the corrosion and dendrite growth problems at the negative electrode. The cyclotriphosphazene derivative and graphene oxide are in a weight ratio of (0.05-5):(0.01-4), which can reduce the capacity attenuation of the lithium-sulfur battery and improve the cycle life of the lithium-sulfur battery. The preparation method of the present invention is simple, can be compatible with existing lithium battery production equipment, does not require additional equipment modification, has a low production cost, and is suitable for large-scale industrial production.

Description

Technical Field

[0001] The present invention relates to an electrolyte for a lithium-sulfur battery and a preparation method thereof, relates to H01M, and specifically relates to the field of devices that directly convert chemical energy into electrical energy. Background Art

[0002] In order to promote the high-quality development of renewable energy and develop a new generation of secondary batteries with high specific energy, after exploration, the lithium-sulfur battery, as a new type of secondary battery, has a theoretical specific energy of up to 2600 Wh / kg -1 , which is much higher than that of existing lithium batteries and has high commercial value. It is an electrochemical energy storage technology with great development potential. However, there are still some technical problems in the practical application of lithium-sulfur batteries. The "shuttle" problem of soluble polysulfides will lead to the loss of positive active materials, the surface of the negative electrode of the lithium-sulfur battery will be corroded, and the uneven deposition of lithium ions will trigger the formation of lithium dendrites on the surface of the negative electrode, resulting in a rapid decline in the capacity of the lithium-sulfur battery. Currently, the methods to solve such technical problems include: designing a sulfur carrier to inhibit the shuttle problem of polysulfides, modifying the negative electrode, introducing a protective layer, preparing a film-forming additive, etc. to prevent the corrosion of the negative electrode surface. However, the improved process is complex and the conditions of the manufacturing environment are harsh, which is not suitable for large-scale production.

[0003] Chinese Patent CN201610881442.0 discloses a graphene-based protective layer on the surface of a lithium metal anode and a corresponding lithium-sulfur battery. By attaching a graphene-based composite protective layer on the surface of the lithium anode, the stacked structure between the graphene sheets can play a role in inhibiting the growth of lithium dendrites. The inorganic material between the graphene sheets forms a lithium ion channel through an in-situ electrochemical reaction with the lithium anode, thereby isolating the direct contact between the electrolyte and the lithium metal and inhibiting the occurrence of side reactions. However, the process conditions for attaching the graphene protective layer on the surface of the lithium anode are complex and the production difficulty is high. Chinese Patent CN201410848143.8 discloses a sulfur-carbon composite positive electrode material for a lithium-sulfur battery and a preparation method thereof. Using sodium polysulfide as a raw material, nanoscale sulfur particles are generated by high-speed ball milling to promote chemical reactions and melt into the carbon pores of conductive carbon black. The composite material has a high discharge specific capacity and a long cycle life. However, the battery efficiency of the formed sulfur-carbon battery is inferior to that of the lithium-sulfur battery, and the actual application effect of the battery is not good. Summary of the Invention

[0004] In order to reduce the attenuation problem of the new lithium-sulfur battery and improve the cycle life of the lithium-sulfur battery, the first aspect of the present invention provides an electrolyte for a lithium-sulfur battery. The preparation raw materials include, by weight percentage: 5-20% of a lithium salt, 1-10% of a phosphazene derivative composite material, and 70-94% of an organic solvent.

[0005] As a preferred embodiment, the phosphazene derivative composite material includes a phosphazene derivative and graphene oxide, and the phosphazene derivative is cyclotriphosphazene.

[0006] As a preferred embodiment, the cyclotriphosphazene is selected from one or a combination of hydroxy(pentafluoro) cyclotriphosphazene, amino(pentafluoro) cyclotriphosphazene, epoxy(pentafluoro) cyclotriphosphazene, alkoxy(pentafluoro) cyclotriphosphazene, aryloxy(pentafluoro) cyclotriphosphazene, and hexachlorocyclotriphosphazene.

[0007] As a preferred embodiment, the cyclotriphosphazene is hexachlorocyclotriphosphazene.

[0008] As a preferred embodiment, the weight ratio of the phosphazene derivative to graphene oxide is (0.05 - 5):(0.01 - 4).

[0009] The applicant found during the experiment that introducing the cyclotriphosphazene derivative-graphene oxide composite material into the electrolyte can inhibit the "shuttle" problem of the positive polysulfide, and at the same time inhibit the corrosion and dendrite growth problems of the negative electrode, reduce the battery capacity attenuation, and improve the cycle life of the lithium-sulfur battery. The possible reason is speculated as follows: Using the cyclotriphosphazene derivative can adsorb polysulfides and avoid the shuttle effect. However, the reaction activity of the cyclotriphosphazene derivative is relatively active and it is easy to react with the electrolyte. Loading the cyclotriphosphazene derivative with graphene can pre-consume the active groups in the cyclotriphosphazene derivative, reduce the initial activity of the cyclotriphosphazene derivative, and thus achieve a stable effect of inhibiting the "shuttle", enabling it to stably exist in the electrolyte, thereby improving the cycle specific capacity and average Coulomb efficiency of the lithium-sulfur battery and optimizing the battery performance.

[0010] The applicant further found during the experiment that a weight ratio of (0.05 - 5):(0.01 - 4) of the cyclotriphosphazene derivative to graphene oxide can achieve the best cycle performance. The reason may be that under the preferred weight ratio, the ionic conductivity of the electrolyte is the best, enabling a long-life cycle charge and discharge effect.

[0011] As a preferred embodiment, the raw materials for preparing the phosphazene derivative composite material include: 0.05 - 5 parts by mass of the phosphazene derivative, 0.01 - 4 parts by mass of graphene oxide, 1 - 15 parts by mass of an acid-binding agent. The raw materials also include 20 - 50 parts by mass of a solvent and 30 - 60 parts by mass of a detergent.

[0012] As a preferred embodiment, the solvent is selected from one or a combination of dimethylformamide, toluene, dichloromethane, acetonitrile, n-heptane, cyclohexane, and chloromethane.

[0013] As a preferred embodiment, the detergent is a combination of water and absolute ethanol. Preferably, the volume ratio of water to absolute ethanol is 1:1.

[0014] As a preferred embodiment, the acid-binding agent is selected from one or a combination of several of pyridine, triethylamine, ethylenediamine, sodium carbonate, potassium carbonate, and sodium acetate.

[0015] As a preferred embodiment, the method for preparing the phosphazene derivative composite material comprises the following steps:

[0016] S1 Add graphene oxide to a solvent for dispersion, then add a phosphazene derivative and perform ultrasonic treatment to make it uniformly dispersed;

[0017] S2 Then carry out a low-temperature water bath, add an acid-binding agent, and stir slowly;

[0018] S3 Subsequently, heat. After the reaction is complete, centrifuge, remove the supernatant, wash the precipitate with a detergent to obtain the phosphazene derivative composite material.

[0019] As a preferred embodiment, the ultrasonic treatment time in step S1 is 10 - 120 min.

[0020] As a preferred embodiment, the low-temperature water bath temperature in step S2 is 0 - 20 °C, the stirring time is 10 - 120 min, and the stirring speed is 200 - 1000 rpm.

[0021] As a preferred embodiment, the heating rate of heating in step S3 is 1 - 10 °C / min, the final heating temperature is 60 - 100 °C, the centrifugation speed is 5000 - 10000 rpm, and the centrifugation time is 1 - 10 min.

[0022] As a preferred embodiment, the number of times of washing with the detergent in step S3 is 1 - 10 times.

[0023] As a preferred embodiment, the lithium salt is selected from one or a combination of several of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium trifluoromethanesulfonate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

[0024] As a preferred embodiment, the lithium salt is lithium bis(trifluoromethylsulfonyl)imide.

[0025] As a preferred embodiment, the organic solvent is selected from one or a combination of several of ethylene glycol dimethyl ether, tetrahydrofuran, 1,3-dioxolane, triethylene glycol dimethyl ether, tris(ethylene glycol) dimethyl ether, diethylene glycol dimethyl ether, and polyethylene glycol dimethyl ether.

[0026] As a preferred embodiment, the organic solvent is a combination of ethylene glycol dimethyl ether and 1,3-dioxolane, and the volume ratio of ethylene glycol dimethyl ether to 1,3-dioxolane is (1-2):(1-2). Further preferably, the volume ratio of ethylene glycol dimethyl ether to 1,3-dioxolane is 1:1.

[0027] The second aspect of the present invention provides a method for preparing an electrolyte for a lithium-sulfur battery, comprising the following steps:

[0028] M1 Add a lithium salt to the organic solvent and disperse it evenly by magnetic stirring;

[0029] M2 Prepare a phosphazene derivative composite material;

[0030] M3 Add the prepared phosphazene derivative composite material to the solution in step M1, and ultrasonically disperse it evenly to obtain an electrolyte for a lithium-sulfur battery.

[0031] As a preferred embodiment, the magnetic stirring time in step M1 is 5-60 min; the ultrasonic time in step M3 is 5-60 min.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (1) For the electrolyte for a lithium-sulfur battery of the present invention, introducing a cyclotriphosphazene derivative-graphene oxide composite material into the electrolyte can inhibit the "shuttle" problem of polysulfides at the positive electrode, and at the same time inhibit the corrosion and dendrite growth problems at the negative electrode.

[0034] (2) For the electrolyte for a lithium-sulfur battery of the present invention, the weight ratio of the cyclotriphosphazene derivative to graphene oxide is (0.05-5):(0.01-4), which can reduce the capacity decay of the lithium-sulfur battery and improve the cycle life of the lithium-sulfur battery.

[0035] (3) For the electrolyte for a lithium-sulfur battery of the present invention, the preparation method is simple, it can be compatible with existing lithium battery production equipment, no additional equipment modification is required, the production cost is low, and it is suitable for large-scale industrial production. Description of the Drawings

[0036] Figure 1 It is a test picture of the capacity retention rate of a lithium-sulfur battery prepared with the electrolyte prepared in Example 1.

[0037] Figure 2 It is a test picture of the capacity retention rate of a lithium-sulfur battery prepared with the electrolyte prepared in Comparative Example 1. Detailed Embodiments

[0038] Example 1

[0039] An electrolyte for a lithium-sulfur battery, the preparation raw materials including, by weight percentage: 10% of a lithium salt, 5% of a phosphazene derivative composite material, and 85% of an organic solvent.

[0040] The lithium salt is lithium bis(trifluoromethanesulfonyl)imide; the organic solvent is a combination of ethylene glycol dimethyl ether and 1,3-dioxolane, with a volume ratio of 1:1.

[0041] The preparation raw materials of the phosphazene derivative composite material include: 0.1 part by mass of a phosphazene derivative, 0.7 part by mass of graphene oxide, 3 parts by mass of an acid-binding agent, and the preparation raw materials also include 30 parts by mass of a solvent and 40 parts by mass of a detergent.

[0042] The phosphazene derivative is hexachlorocyclotriphosphazene, and the acid-binding agent is pyridine. The solvent is acetonitrile, and the detergent is a combination of water and absolute ethanol, with a volume ratio of 1:1.

[0043] The preparation method of the phosphazene derivative composite material includes the following steps:

[0044] S1 Add graphene oxide to the solvent for dispersion, then add the phosphazene derivative and perform ultrasonic treatment to make it evenly dispersed;

[0045] S2 Then carry out a low-temperature water bath, add the acid-binding agent, and stir slowly;

[0046] S3 Subsequently, heat. After the reaction is complete, centrifuge, remove the supernatant, wash the precipitate with the detergent to obtain the phosphazene derivative composite material.

[0047] The ultrasonic treatment time in step S1 is 45 min.

[0048] The low-temperature water bath temperature in step S2 is 5°C, the stirring time is 60 min, and the stirring speed is 600 rpm.

[0049] The heating rate of heating in step S3 is 5°C / min, the final heating temperature is 80°C, the centrifugation speed is 7000 rpm, and the centrifugation time is 3 min.

[0050] The number of cleaning times of the detergent in step S3 is 6 times.

[0051] A preparation method of an electrolyte for a lithium-sulfur battery includes the following steps:

[0052] M1 Add the lithium salt to the organic solvent and disperse it evenly by magnetic stirring;

[0053] M2 Prepare the phosphazene derivative composite material;

[0054] M3 Add the prepared phosphazene derivative composite material to the solution in step M1, and after ultrasonic dispersion, obtain the electrolyte for the lithium-sulfur battery.

[0055] In the step M1, the magnetic stirring time is 20 min; in the step M3, the ultrasonic time is 15 min.

[0056] Example 2

[0057] An electrolyte for a lithium-sulfur battery, the specific implementation manner is the same as that of Example 1, the difference is that the preparation raw materials include, by weight percentage: 10% of lithium salt, 1% of phosphazene derivative composite material, and 89% of organic solvent.

[0058] Example 3

[0059] An electrolyte for a lithium-sulfur battery, the specific implementation manner is the same as that of Example 1, the difference is that the preparation raw materials include, by weight percentage: 10% of lithium salt, 10% of phosphazene derivative composite material, and 80% of organic solvent.

[0060] Example 4

[0061] An electrolyte for a lithium-sulfur battery, the specific implementation manner is the same as that of Example 1, the difference is that the preparation raw materials of the phosphazene derivative composite material include: 0.05 parts by mass of phosphazene derivative, 0.7 parts by mass of graphene oxide, 3 parts by mass of acid-binding agent, and the preparation raw materials further include 30 parts by mass of solvent and 40 parts by mass of detergent.

[0062] Example 5

[0063] An electrolyte for a lithium-sulfur battery, the specific implementation manner is the same as that of Example 1, the difference is that the preparation raw materials of the phosphazene derivative composite material include: 5 parts by mass of phosphazene derivative, 0.7 parts by mass of graphene oxide, 3 parts by mass of acid-binding agent, and the preparation raw materials further include 30 parts by mass of solvent and 40 parts by mass of detergent.

[0064] Example 6

[0065] An electrolyte for a lithium-sulfur battery, the specific implementation manner is the same as that of Example 1, the difference is that the preparation raw materials of the phosphazene derivative composite material include: 0.1 parts by mass of phosphazene derivative, 0.01 parts by mass of graphene oxide, 3 parts by mass of acid-binding agent, and the preparation raw materials further include 30 parts by mass of solvent and 40 parts by mass of detergent.

[0066] Example 7

[0067] An electrolyte for a lithium-sulfur battery, the specific implementation manner is the same as that of Example 1, the difference is that the preparation raw materials of the phosphazene derivative composite material include: 0.1 parts by mass of phosphazene derivative, 4 parts by mass of graphene oxide, 3 parts by mass of acid-binding agent, and the preparation raw materials further include 30 parts by mass of solvent and 40 parts by mass of detergent.

[0068] Example 8

[0069] An electrolyte for a lithium-sulfur battery, the specific implementation manner is the same as that of Example 1, the difference is that the organic solvent is a combination of ethylene glycol dimethyl ether and tetrahydrofuran, and the volume ratio is 1:1.

[0070] Example 9

[0071] An electrolyte for a lithium-sulfur battery, the specific implementation manner is the same as that of Example 1, the difference is that the lithium salt is lithium tetrafluoroborate.

[0072] Example 10

[0073] An electrolyte for a lithium-sulfur battery, the specific implementation manner is the same as that of Example 1, the difference is that the phosphazene derivative is hydroxy(pentafluoro)cyclotriphosphazene.

[0074] Comparative Example 1

[0075] An electrolyte for a lithium-sulfur battery, the preparation raw materials include, by weight percentage: 10% of lithium salt and 90% of organic solvent.

[0076] The lithium salt is lithium bis(trifluoromethanesulfonyl)imide; the organic solvent is a combination of ethylene glycol dimethyl ether and 1,3-dioxolane, and the volume ratio is 1:1.

[0077] A preparation method of an electrolyte for a lithium-sulfur battery includes the following steps:

[0078] Add the lithium salt to the organic solvent, stir magnetically for 20 min, and uniformly disperse to obtain the electrolyte for the lithium-sulfur battery.

[0079] Performance test

[0080] Match the electrolytes prepared in the examples and comparative examples with a sulfur positive electrode, a lithium metal negative electrode, and a polypropylene separator to assemble a lithium-sulfur battery. The preparation steps are as follows:

[0081] (1) Preparation of the sulfur positive electrode sheet: Mix and grind carbon nanotubes and elemental sulfur according to a mass ratio of 3:7. Transfer the mixed powder to a 50 mL polytetrafluoroethylene reaction kettle and heat at 155 °C for 12 h to obtain the positive electrode active material. Mix the positive electrode active material, acetylene black, and polyvinylidene fluoride according to a mass ratio of 7:2:1, add N-methyl-2-pyrrolidone, and grind for 30 min to make a slurry, which is coated on an aluminum current collector. Dry at 80 °C for 12 h and slice to obtain a sulfur positive electrode sheet with a diameter of Ф12 mm and a surface loading of 2.5 mg / cm².

[0082] (2) Assembly of lithium-sulfur button battery: Inside an argon atmosphere glove box (oxygen content < 0.01 ppm, water content < 0.01 ppm), assemble the positive electrode sheet, polypropylene separator, electrolyte, lithium sheet, stainless steel gasket, stainless steel spring piece, and negative electrode case into a button battery. After standing for 12 h, perform charge-discharge cycle tests at a rate of 0.5C, and the results are shown in Table 1. The charge-discharge cycle test diagrams of Example 1 and Comparative Example 1 are shown in Figure 1-2 .

[0083] Table 1

[0084]

[0085] From Table 1, it can be seen that for Example 1 and Comparative Example 1, the lithium-sulfur battery electrolyte provided by the present invention can significantly improve the capacity retention rate and average Coulombic efficiency of the lithium-sulfur battery. According to Examples 1-3, it can be found that electrolytes composed of different ratios of organic solvents, lithium salts, and hexachlorocyclotriphosphazene-graphene oxide composites all improve the capacity retention rate and average Coulombic efficiency of the lithium-sulfur battery to varying degrees. According to Examples 1, 4-7 and Comparative Example 1, it is found that different contents of hexachlorocyclotriphosphazene or graphene oxide can all improve the capacity retention rate and average Coulombic efficiency of the lithium-sulfur battery to varying degrees. According to Examples 8-10 and Comparative Example 1, it is found that in the electrolyte system of this patent invention, changing the organic solvent, lithium salt, and cyclotriphosphazene derivative-graphene oxide composite can all improve the capacity retention rate and average Coulombic efficiency of the lithium-sulfur battery to varying degrees.

Claims

1. An electrolyte for a lithium-sulfur battery, characterized in that: The raw materials for preparation include, by weight percentage: 5-20% lithium salt, 1-10% phosphazene derivative composite material, and 70-94% organic solvent; The phosphazene derivative composite material comprises a phosphazene derivative and graphene oxide, wherein the phosphazene derivative is cyclotriphosphazene; The cyclotriphosphazene is selected from one or a combination of hydroxy (pentafluoro) cyclotriphosphazene, amino (pentafluoro) cyclotriphosphazene, epoxy (pentafluoro) cyclotriphosphazene, alkoxy (pentafluoro) cyclotriphosphazene, aryloxy (pentafluoro) cyclotriphosphazene and hexachloro cyclotriphosphazene; The weight ratio of the phosphazene derivative and graphene oxide is (0.05-5): (0.01-4); The raw materials for preparing the phosphazene derivative composite material include: 0.05-5 parts by weight of phosphazene derivatives, 0.01-4 parts by weight of graphene oxide, 1-15 parts by weight of an acid binding agent, and the raw materials for preparing the composite material also include 20-50 parts by weight of a solvent and 30-60 parts by weight of a detergent; The acid binding agent is selected from one or a combination of pyridine, triethylamine, ethylenediamine, sodium carbonate, potassium carbonate, and sodium acetate; The method for preparing the phosphazene derivative composite material comprises the following steps: S1: adding graphene oxide into a solvent for dispersion, then adding a phosphazene derivative and performing ultrasonic treatment to make it evenly dispersed; S2 is then placed in a low temperature water bath, and an acid binding agent is added and stirred slowly; S3 is then heated, and after the reaction is complete, centrifuged, the supernatant is removed, and the precipitate is washed with a detergent to obtain a phosphazene derivative composite material; In step S2, the low-temperature water bath temperature is 0-20° C., the stirring time is 10-120 min, and the stirring speed is 200-1000 rpm.

2. The electrolyte for lithium-sulfur batteries according to claim 1, characterized in that: The lithium salt is selected from the group consisting of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium trifluoromethanesulfonate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(fluorosulfonyl imide), and lithium bis(trifluoromethylsulfonyl imide), or a combination thereof.

3. The electrolyte for lithium-sulfur battery according to claim 1, characterized in that: The organic solvent is selected from one or a combination of ethylene glycol dimethyl ether, tetrahydrofuran, 1,3-dioxolane, triethylene glycol dimethyl ether, tri(ethylene glycol) dimethyl ether, diethylene glycol dimethyl ether, and polyethylene glycol dimethyl ether.

4. A method for preparing an electrolyte for a lithium-sulfur battery according to any one of claims 1 to 3, characterized in that: The following steps are involved: M1 adds lithium salt to the organic solvent and disperses it evenly by magnetic stirring; M2 preparation of phosphazene derivative composite materials; M3: adding the prepared phosphazene derivative composite material into the solution of step M1, and obtaining an electrolyte for lithium-sulfur battery after ultrasonic dispersion.

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