Lithium-sulfur battery electrolyte, preparation method and application thereof, and lithium-sulfur battery

Abstract

The invention relates to the technical field of lithium-sulfur batteries, and discloses a lithium-sulfur battery electrolyte, a preparation method and application thereof, and a lithium-sulfur battery, the electrolyte comprises an inner-layer organic solvent, an organic lithium salt, an inorganic lithium additive and an outer-layer protective agent; and the outer layer protective agent is biphenyl. The electrolyte adopts biphenyl as an outer layer protective agent to form a double-layer solvation structure of lithium polysulfide, so that the lithium polysulfide has good reaction kinetics on one side of a positive electrode, irreversible chemical reaction between the lithium polysulfide and a lithium metal negative electrode is inhibited, the cycling stability of the lithium metal negative electrode is improved, and the service life of the lithium metal negative electrode is prolonged. And the long cycle stability of the lithium-sulfur battery is ensured.

Description

Technical Field

[0001] This invention relates to the field of lithium-sulfur battery technology, specifically to a lithium-sulfur battery electrolyte, its preparation method and application, and lithium-sulfur batteries. Background Technology

[0002] Commercially available lithium-ion batteries have been widely used in consumer electronics, smart grids, and electric vehicles. However, due to their intercalation mechanism, the energy density of lithium-ion batteries is nearing its limit. Therefore, there is a need to develop novel high-energy-density rechargeable battery systems to meet future application demands for high energy density. Among these novel battery systems, lithium-sulfur batteries, with a high theoretical energy density of 2600 Wh / kg, are considered the most likely rechargeable battery technology to break through the energy density limit of lithium-ion batteries.

[0003] The uneven deposition and removal of lithium metal anodes in lithium-sulfur batteries leads to severe volume deformation during cycling. Furthermore, lithium polysulfides dissolved in the electrolyte are generated during cycling. These lithium polysulfides diffuse to the lithium metal anode side and react with it, increasing the inhomogeneity of the solid electrolyte interphase (SEI) film on the lithium metal surface. This further exacerbates the uneven deposition and removal of lithium metal, significantly increasing the irreversible consumption rate of lithium metal and promoting the accumulation of large amounts of inactive lithium. These processes drastically reduce the cycling stability of lithium metal, ultimately causing rapid failure of both the lithium metal anode and the lithium-sulfur battery.

[0004] As a crucial component of lithium-sulfur batteries, the electrolyte is used to regulate the solvation structure of lithium polysulfides by adding an outer solvent with low reactivity to lithium metal. This regulation of lithium polysulfide reactivity effectively suppresses side reactions between lithium polysulfides and the lithium metal anode. For example, some researchers have designed electrolytes encapsulating lithium polysulfides within two solvation shells to suppress side reactions between lithium polysulfides and lithium metal. The inner solvent, such as ethylene glycol dimethyl ether, possesses strong solvation capabilities, ensuring that lithium polysulfides can undergo solid-liquid-solid conversion on the positive electrode side. The outer solvent, such as isopropyl ether, has low solvation capabilities and strong reduction stability, suppressing the reactivity between lithium polysulfides and lithium metal, thereby weakening the shuttle effect of lithium polysulfides and improving the cycle stability of the lithium metal anode and the cycle life of the lithium-sulfur battery. While these strategies can extend the cycle life of lithium-sulfur batteries to some extent, they still cannot meet the practical application requirements of lithium-sulfur batteries.

[0005] Therefore, how to effectively extend the cycle life of lithium-sulfur batteries while ensuring their cycle stability is a problem that needs to be solved. Summary of the Invention

[0006] The purpose of this invention is to overcome the problem of poor cycle stability of the negative electrode in existing lithium-sulfur batteries, and to provide a lithium-sulfur battery electrolyte, its preparation method and application, and a lithium-sulfur battery. The electrolyte uses biphenyl as an outer protective agent to form a bilayer solvation structure of lithium polysulfides. While ensuring that lithium polysulfides have good reaction kinetics on the positive electrode side, it inhibits irreversible chemical reactions between lithium polysulfides and the lithium metal negative electrode, improves the cycle stability of the lithium metal negative electrode, and ensures the long-term cycle stability of the lithium-sulfur battery.

[0007] To achieve the above objectives, the first aspect of the present invention provides a lithium-sulfur battery electrolyte, wherein the electrolyte comprises an inner organic solvent, an organic lithium salt, an inorganic lithium additive, and an outer protective agent.

[0008] The outer protective agent is biphenyl.

[0009] Preferably, the molar ratio of the outer protective agent to the inner organic solvent is 1:2-5, and more preferably 1:3-4.

[0010] A second aspect of the present invention provides a method for preparing the lithium-sulfur battery electrolyte described in the first aspect, wherein the method includes the following steps:

[0011] Under a protective atmosphere, an organic lithium salt, an inorganic lithium additive, an inner organic solvent, and an outer protective agent are mixed to obtain the lithium-sulfur battery electrolyte.

[0012] The third aspect of this invention provides an application of the lithium-sulfur battery electrolyte described in the first aspect in a lithium-sulfur battery.

[0013] A fourth aspect of the present invention provides a lithium-sulfur battery, the lithium-sulfur battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte;

[0014] The electrolyte is the lithium-sulfur battery electrolyte described in the first aspect.

[0015] The beneficial effects achieved through the above technical solution are as follows:

[0016] The lithium-sulfur battery electrolyte provided by this invention uses biphenyl as an outer protective agent to form a bilayer solvation structure of lithium polysulfides. While ensuring that lithium polysulfides have good reaction kinetics on the positive electrode side, it inhibits irreversible chemical reactions between lithium polysulfides and lithium metal negative electrodes, improves the cycle stability of lithium metal negative electrodes, and ensures the long-term cycle stability of lithium-sulfur batteries. Detailed Implementation

[0017] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0018] The first aspect of the present invention provides a lithium-sulfur battery electrolyte, wherein the electrolyte comprises an inner organic solvent, an organic lithium salt, an inorganic lithium additive, and an outer protective agent;

[0019] The outer protective agent is biphenyl.

[0020] In this invention, the electrolyte contains an inner organic solvent and an outer protective agent. The outer protective agent, biphenyl, has low solvation ability and high reduction stability for lithium polysulfides, which can effectively construct a double-layer solvation structure for lithium polysulfides, reduce the side reaction activity between lithium polysulfides and lithium metal, reduce the irreversible corrosion reaction between lithium polysulfides and lithium metal anode, effectively protect the lithium metal anode and improve the long-cycle stability of lithium-sulfur battery devices.

[0021] According to the present invention, preferably, the molar ratio of the outer protective agent to the inner organic solvent is 1:2-5, for example 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, or any range between the two, preferably 1:3-4.

[0022] In this invention, the organic lithium salt is insoluble in the outer protective agent. Adding the outer protective agent effectively constructs a lithium polysulfide bilayer solvation structure. The inner solvent fully solvates the lithium polysulfide and lithium salt, ensuring rapid positive electrode reaction kinetics and high ionic conductivity. The outer protective agent cannot solvate the lithium salt and lithium polysulfide, thus forming a protective sheath in the solvated outer layer. This suppresses side reactions between lithium polysulfide and metallic lithium, as well as the irreversible decomposition of the lithium salt on the negative electrode surface, thereby improving the stability of the negative electrode and the cycle life of the lithium-sulfur battery. If too little outer protective agent is added, a sufficiently effective bilayer solvation structure cannot be formed; if too much is added, the bilayer solvation structure will be destroyed, preventing sufficient solvation of the lithium polysulfide and lithium salt, resulting in a decrease in the energy density of the lithium-sulfur battery and limited improvement in its lifespan.

[0023] According to the present invention, the inner organic solvent can be selected from a wide range of conventional organic solvents in the art. Preferably, the inner organic solvent is ethylene glycol dimethyl ether and / or 1,3-dioxolane, more preferably ethylene glycol dimethyl ether and 1,3-dioxolane.

[0024] According to the present invention, preferably, the volume ratio of ethylene glycol dimethyl ether to 1,3-dioxolane in the inner organic solvent is 1:0.1-5, for example 1:0.1, 1:0.5, 1:0.8, 1:1, 1:1.2, 1:1.5, 1:2, or any range between the two, preferably 1:0.5-2, more preferably 1:0.8-1.2. In the present invention, the organic solvent prepared according to the above volume ratio can effectively dissolve the organic lithium salt, improve the solubility of the organic lithium salt in the electrolyte, and thus improve the conductivity of the electrolyte. The above-mentioned inner organic solvent and outer protective agent are fully mixed to form a double-layer solvation structure, improving the cycle stability of the lithium metal anode and ensuring the long-term cycle stability of the lithium-sulfur battery.

[0025] According to the present invention, the type of organolithium salt is not particularly limited, and is a conventional organolithium salt in the art. Those skilled in the art can select a suitable organolithium salt. Preferably, the organolithium salt is lithium bis(trifluoromethanesulfonyl)imide and / or lithium bis(fluorosulfonyl)imide.

[0026] According to the present invention, preferably, the concentration of the organic lithium salt in the electrolyte is 300-1800 mmol / L, for example, 500 mmol / L, 600 mmol / L, 700 mmol / L, 800 mmol / L, 900 mmol / L, 1000 mmol / L, or any range between the two, preferably 500-1000 mmol / L. In the present invention, using an appropriate amount of the above-mentioned organic lithium salt can provide more active lithium ions in the electrolyte, thereby improving the cycle performance of the lithium-sulfur battery.

[0027] According to the present invention, preferably, the mass fraction of the inorganic lithium additive in the electrolyte is 1-5 wt%, for example, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt%, or any range between the two, preferably 2-4 wt%.

[0028] According to the present invention, preferably, the inorganic lithium additive is lithium nitrate. Adding an appropriate amount of inorganic lithium additive can form a protective SEI layer on the surface of the lithium metal anode, which can further improve the cycle performance of the lithium-sulfur battery.

[0029] A second aspect of the present invention provides a method for preparing the lithium-sulfur battery electrolyte described in the first aspect, wherein the method includes the following steps:

[0030] Under a protective atmosphere, an organic lithium salt, an inorganic lithium additive, an inner organic solvent, and an outer protective agent are mixed to obtain the lithium-sulfur battery electrolyte.

[0031] In this invention, preferably, the preparation method of the lithium-sulfur battery electrolyte is simple, which involves mixing organic lithium salt and inorganic lithium additives with an organic solvent and an outer protective agent under a protective atmosphere to obtain the lithium-sulfur battery electrolyte.

[0032] In this invention, the protective atmosphere has the conventional meaning in the art, referring to an oxygen-free atmosphere, preferably selected from at least one of argon, nitrogen, and helium.

[0033] In this invention, the method and equipment for mixing are not particularly limited, as long as the organolithium salt is fully dissolved in the inner organic solvent. Preferably, the mixing is performed in a glove box. Preferably, the mixing is carried out under stirring conditions. The stirring rate is not particularly limited, as long as the electrolyte is mixed uniformly.

[0034] According to a preferred embodiment of the present invention, the preparation method includes the following steps:

[0035] 1) Under a protective atmosphere, organic lithium salts and inorganic lithium additives are mixed with an inner organic solvent;

[0036] 2) Mix the product from step 1) with the outer protective agent to obtain the lithium-sulfur battery electrolyte.

[0037] In this invention, organic lithium salts and inorganic lithium additives are dissolved in an inner organic solvent and uniformly dispersed, and then an outer protective agent is added. The organic lithium salts and inorganic lithium additives are insoluble in the outer protective agent, which has low solvation ability and high reduction stability, effectively constructing a bilayer solvation structure of lithium polysulfides.

[0038] The third aspect of this invention provides an application of the lithium-sulfur battery electrolyte described in the first aspect in a lithium-sulfur battery.

[0039] In this invention, the lithium-sulfur battery electrolyte is applied to lithium-sulfur batteries. The double-layer solvation structure of the electrolyte can reduce the side reaction activity between lithium polysulfide and lithium metal, reduce the irreversible corrosion reaction between lithium polysulfide and lithium metal anode, effectively protect the lithium metal anode and improve the long-cycle stability of lithium-sulfur battery devices.

[0040] A fourth aspect of the present invention provides a lithium-sulfur battery, the lithium-sulfur battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte;

[0041] The electrolyte is the lithium-sulfur battery electrolyte described in the first aspect.

[0042] The lithium-sulfur battery preparation method described in this invention is simple and produces excellent battery performance, providing a technical solution for constructing practical, long-cycle, high-energy-density lithium-sulfur batteries.

[0043] In this invention, the types of positive and negative electrodes for the lithium-sulfur battery are relatively wide, and can be conventional lithium-sulfur battery positive and negative electrode materials in the art. According to a preferred embodiment of the invention, the positive electrode is a carbon-sulfur composite positive electrode. The negative electrode is metallic lithium. These can be commercially available or prepared using existing methods.

[0044] In this invention, the type of diaphragm can be selected from a wide range, and is a conventional diaphragm in the art, preferably selected from at least one of polyethylene, polypropylene, and polyvinylidene fluoride. The diaphragm can be a single-layer film or a multi-layer composite film, without particular limitation.

[0045] In this invention, the lithium-sulfur battery electrolyte is assembled with a positive electrode, a negative electrode, and a separator to obtain a lithium-sulfur battery. Charge-discharge performance and cycle performance are tested at 25°C. In a 3Ah lithium-sulfur pouch battery, an initial energy density of over 370Wh / kg can be achieved, and the cycle life exceeds 60 cycles when the capacity reaches 80% of the initial capacity, thus improving the cycle life and coulombic efficiency of the lithium-sulfur battery.

[0046] In this invention, there are no particular limitations on the assembly method of the positive electrode, negative electrode, separator and the lithium-sulfur battery electrolyte. The lithium-sulfur battery can be obtained by assembling it in a conventional manner in the art.

[0047] According to a particularly preferred embodiment of the present invention, a lithium-sulfur battery electrolyte comprises an inner organic solvent, an organic lithium salt, a lithium nitrate additive, and an outer protective agent.

[0048] The outer protective agent is biphenyl;

[0049] The molar ratio of the outer protective agent to the inner organic solvent is 1:3-4;

[0050] The volume ratio of ethylene glycol dimethyl ether to 1,3-dioxolane in the inner organic solvent is 1:0.5-2.

[0051] The present invention will be described in detail below through examples and comparative examples. Unless otherwise specified, all reagents used in the following examples and comparative examples are commercially available.

[0052] Example 1

[0053] 1) In an argon atmosphere glove box, weigh out the organic lithium salt (lithium bis(trifluoromethanesulfonyl)imide) and lithium nitrate additive according to the stoichiometric ratio and add them to the reagent bottle. Then add the inner organic solvent ethylene glycol dimethyl ether and 1,3-dioxolane. The volume ratio of ethylene glycol dimethyl ether and 1,3-dioxolane is 1:1. Stir to completely dissolve the organic lithium salt and lithium nitrate additive.

[0054] 2) Add biphenyl to the solution obtained in step 1) and stir until completely dissolved. The molar ratio of biphenyl to the inner organic solvent is 1:3. Stir until homogeneous.

[0055] The concentration of organic lithium salt in the lithium-sulfur battery electrolyte is 1000 mmol / L, and the mass fraction of lithium nitrate additive is 2 wt%.

[0056] Example 2

[0057] The electrolyte was prepared according to the method of Example 1, except that the amount of biphenyl added in step 2) was changed, and the molar ratio of the outer protective agent to the inner organic solvent was 1:4, to obtain the lithium-sulfur battery electrolyte. Other components and preparation process were the same as in Example 1.

[0058] Example 3

[0059] The electrolyte was prepared according to the method of Example 1, except that the amount of biphenyl added in step 2) was changed and the molar ratio of the outer protective agent to the inner organic solvent was 1:2 to obtain the lithium-sulfur battery electrolyte. Other components and preparation process were the same as in Example 1.

[0060] Example 4

[0061] 1) In an argon atmosphere glove box, weigh out the organic lithium salt (lithium bis(fluorosulfonyl)imide) and lithium nitrate additive according to the stoichiometric ratio and add them to the reagent bottle. Then add the inner organic solvent ethylene glycol dimethyl ether and 1,3-dioxolane dropwise. The volume ratio of ethylene glycol dimethyl ether to 1,3-dioxolane is 1:0.5. Stir until the organic lithium salt and lithium nitrate additive are completely dissolved.

[0062] 2) Add biphenyl to the solution obtained in step 1) and stir until completely dissolved. The molar ratio of biphenyl to the inner organic solvent is 1:3.5. Stir until homogeneous.

[0063] The concentration of organic lithium salt in the lithium-sulfur battery electrolyte is 700 mmol / L, and the mass fraction of lithium nitrate additive is 3 wt%.

[0064] Example 5

[0065] The electrolyte was prepared according to the method of Example 1, except that the amount of biphenyl added in step 2) was changed, and the molar ratio of the outer protective agent to the inner organic solvent was 1:5, to obtain the lithium-sulfur battery electrolyte. Other components and preparation process were the same as in Example 1.

[0066] Comparative Example 1

[0067] The electrolyte was prepared according to the method of Example 1, except that biphenyl was not added to the solution prepared in step 1), while the other components of the electrolyte remained the same.

[0068] Comparative Example 2

[0069] The electrolyte was prepared according to the method of Example 1, except that in step 2), 1,4-difluorobenzene with the same molar amount as biphenyl was added to the solution prepared in step 1), the molar ratio of 1,4-difluorobenzene to the inner organic solvent was 1:3, and the other components of the electrolyte remained the same.

[0070] Test case

[0071] The lithium-sulfur battery electrolytes prepared in the examples and comparative examples were tested in 3Ah lithium-sulfur pouch cells. The results of the first-cycle energy density and cycle life when the capacity reached 80% of the initial capacity are shown in Table 1.

[0072] Lithium-sulfur pouch batteries:

[0073] The cathode is a carbon-sulfur composite cathode with a sulfur content of 63 wt% and a sulfur surface loading of 7 mg / cm³. 2 ;

[0074] The negative electrode is metallic lithium, with a thickness of 85 micrometers;

[0075] The diaphragm is a polypropylene diaphragm with a thickness of 25 micrometers;

[0076] Test conditions: 3Ah lithium-sulfur pouch battery, after assembly and resting for 12 hours, constant current charge and discharge at 0.05C, voltage range 1.8V-2.6V. The energy density of the first cycle is the battery discharge energy divided by the battery mass.

[0077] Table 1

[0078]

[0079] As can be seen from the results in Table 1, the lithium-sulfur battery electrolyte prepared using the embodiments of the present invention, with the addition of an outer protective agent, can effectively improve the cycle life and coulombic efficiency of lithium-sulfur pouch batteries with a capacity of 3Ah.

[0080] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium-sulfur battery electrolyte, characterized by, The electrolyte comprises an inner layer organic solvent, an organic lithium salt, an inorganic lithium additive and an outer layer protective agent. The outer layer protective agent is biphenyl.

2. The electrolyte of claim 1, wherein, The molar ratio of the outer layer protective agent to the inner layer organic solvent is 1:2-5, preferably 1:3-4.

3. The electrolyte of claim 1 or 2, wherein, The inner layer organic solvent is ethylene glycol dimethyl ether and / or 1,3-dioxolane, preferably ethylene glycol dimethyl ether and 1,3-dioxolane.

4. The electrolyte of claim 3, wherein, The volume ratio of ethylene glycol dimethyl ether to 1,3-dioxolane in the inner layer organic solvent is 1:0.1-5, preferably 1:0.5-2.

5. The electrolyte according to any one of claims 1 to 4, wherein, The organic lithium salt is lithium bis(trifluoromethylsulfonyl)imide and / or lithium bis(fluorosulfonyl)imide.

6. The electrolyte according to any one of claims 1 to 5, wherein, The concentration of the organic lithium salt in the electrolyte is 300-1800 mmol / L, preferably 500-1000 mmol / L.

7. The electrolyte according to any one of claims 1 to 6, wherein, The mass fraction of the inorganic lithium additive in the electrolyte is 1-5 wt%, preferably 2-4 wt%. Preferably, the inorganic lithium additive is lithium nitrate.

8. The method for preparing a lithium-sulfur battery electrolyte according to any one of claims 1 to 7, characterized in that, The method comprises the following steps: Mixing the organic lithium salt, the inorganic lithium additive, the inner layer organic solvent and the outer layer protective agent under a protective atmosphere to obtain the lithium-sulfur battery electrolyte.

9. The lithium-sulfur battery electrolyte according to any one of claims 1-7 for use in a lithium-sulfur battery.

10. A lithium-sulfur battery, characterized by, The lithium-sulfur battery comprises a positive electrode, a negative electrode, a separator and an electrolyte; The electrolyte is the lithium-sulfur battery electrolyte according to any one of claims 1-7.

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