CN106816634B - pseudo high-concentration lithium-sulfur battery electrolyte and lithium-sulfur battery - Google Patents

Abstract

The invention discloses a pseudo-high-concentration lithium-sulfur battery electrolyte and a lithium-sulfur battery. The electrolyte contains a lithium salt, an ether solvent and a non-solvent solution, and the concentration of the lithium salt in the ether solvent is higher than 3.0 mol/ L, the concentration of lithium salt in the pseudo high-concentration electrolyte is not less than 0.5mol/L. The battery electrolyte provided by the invention can improve the problems of high viscosity and low conductivity of lithium-sulfur battery electrolyte with high concentration of lithium salt, has non-flammability, and can significantly improve the electrochemical performance and safety of the lithium-sulfur battery.

Description

A pseudo-high concentration lithium-sulfur battery electrolyte and lithium-sulfur battery

technical field

The invention relates to a pseudo-high-concentration lithium-sulfur battery electrolyte and a corresponding lithium-sulfur battery, in particular to a lithium-sulfur battery electrolyte containing a high-concentration lithium salt of a fluoroether.

Background technique

The theoretical specific capacity of the lithium-sulfur battery is 1675mAh/g, and the theoretical specific energy is 2600Wh/Kg, much higher than the existing lithium-ion batteries. Moreover, sulfur reserves are abundant, the price is low, low toxicity and no pollution. Therefore, lithium-sulfur batteries have become candidates for next-generation high-energy lithium batteries and have attracted worldwide attention. Lithium polysulfide, an intermediate product of lithium-sulfur batteries, will react directly with esters. Lithium-sulfur batteries generally use ethers as electrolyte solvents instead of carbonates and carboxylates used in lithium-ion battery electrolytes. The solubility of lithium polysulfide in the ether electrolyte is high, and the lithium polysulfide dissolved in the electrolysis during charging and discharging will migrate to the negative electrode and corrode with the metal lithium negative electrode, and consume the active materials of the positive and negative electrodes at the same time, resulting in battery failure. Poor cycle performance and low Coulombic efficiency. At the same time, the insulation of sulfur and Li ₂ S and the volume expansion during charging and discharging also seriously affect the utilization rate of active materials and cycle stability of lithium-sulfur batteries, seriously hindering its practical application.

Aiming at the problems of lithium-sulfur batteries, researchers around the world have carried out a lot of research work on the microstructure design of sulfur cathodes, the preparation of functional separators, the modification of electrolytes, and the protection of metal lithium anodes, and have achieved remarkable results. Effect. Among them, a lot of work has been carried out around the modification of lithium-sulfur battery electrolytes, such as using ionic liquids that can dissolve lithium salts but not lithium polysulfides as electrolyte co-solvents to inhibit the dissolution and migration of lithium polysulfides in the electrolyte . LiNO ₃ , KNO ₃ , P ₂ S ₅ , etc. are used as electrolyte additives to assist the formation of a relatively stable SEI film on the lithium metal negative electrode side and inhibit the corrosion reaction between lithium polysulfide dissolved in the electrolyte and lithium metal. In addition, more lithium salts are dissolved in ether solvents to form high-concentration lithium salt electrolytes. Most of the solvent molecules in the high-concentration lithium salt solution participated in the solvation of lithium ions in the lithium salt, and there were few free solvent molecules, so lithium polysulfide could no longer be dissolved. Moreover, the viscosity of the high-concentration lithium salt electrolyte is very high, which significantly reduces the migration of lithium polysulfide in the electrolyte. Therefore, the electrolyte with a high concentration of lithium salt can significantly reduce the impact of the shuttle effect caused by the dissolution and migration of lithium polysulfide in the electrolyte and the corrosion of the metal lithium negative electrode on battery performance, which is helpful to obtain high-performance lithium-sulfur batteries. .

The conductivity in the electrolyte of lithium-ion batteries depends on the mobility of lithium ions in the electrolyte and the viscosity of the electrolyte. However, most solvent molecules in high-concentration lithium-salt electrolytes participate in the solvation of lithium ions and there are few free solvent molecules. The main problems of high-concentration lithium-sulfur battery electrolytes are high viscosity and low conductivity of the electrolyte. High viscosity is not conducive to the migration of lithium ions and the full contact between the electrolyte and the electrode, and the corresponding lithium-sulfur battery has a high impedance value. The lower conductivity indicates that the migration rate of lithium ions in the electrolyte is slower, which is not conducive to the rate performance and low temperature performance of the battery. Therefore, finding new ways to solve the problems of high viscosity and low conductivity of high-concentration lithium-sulfur battery electrolytes is an important way to promote the practical application of high-concentration lithium-sulfur battery electrolytes and accelerate the commercialization of lithium-sulfur batteries.

Contents of the invention

The purpose of the present invention is to provide a pseudo-high-concentration lithium-sulfur battery electrolyte, which can solve the problems of high viscosity and low conductivity of the high-concentration lithium-sulfur battery electrolyte.

The present invention provides concrete technical scheme as follows:

A pseudo-high-concentration lithium-sulfur battery electrolyte, characterized in that:

The electrolyte contains lithium salt, ether solvent and non-solvent liquid;

The concentration of the lithium salt in the ether solvent is not less than 3mol/L;

The solubility of the non-solvent liquid to lithium salt is lower than 0.1mol/L.

Preferably, the molar concentration of the lithium salt in the ether solvent is higher than 3.0 mol/L, and the overall concentration of the lithium salt in the pseudo high-concentration electrolyte is higher than 0.5 mol/L.

Preferably, the non-solvent liquid is preferably selected from at least one of the fluoroethers represented by the following structural formula (I):

Wherein: Rf ₁ and Rf ₂ are independently selected from C1-C10 alkyl groups or C1-C10 fluoroalkyl groups, and at least one is selected from C1-C10 fluoroalkyl groups. .

More preferably, the Rf ₁ and Rf ₂ are independently preferably selected from C1-C6 fluoroalkyl groups or C1-C6 alkyl groups, and at least one is selected from C1-C6 fluoroalkyl groups.

Preferably, in the electrolyte, the mass fraction of the fluoroether is 5-90%, and the mass fraction of the ether solvent is 20-98%.

More preferably, in the electrolyte, the mass fraction of fluoroether is 30-60%, and the mass fraction of ether solvent is 40-70%.

Preferably, the lithium salt is selected from LiPF ₆ , LiBF ₄ , LiBOB, LiDFOB, LiPO ₂ F ₂ , LiSO ₃ CF ₃ , lithium bistrifluoromethylsulfonylimide (LiTFSI) and lithium bisfluorosulfonylimide (LiFSI) at least one.

Preferably, the ether solvent is selected from tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, ethylene glycol dimethyl ether, dimethoxymethane, 1,2-dimethoxyethane and At least one of diglyme;

A lithium-sulfur battery adopts the above electrolyte.

The invention adds a proper amount of non-solvent liquid, especially fluorinated ether, to the high-concentration lithium-sulfur battery electrolyte. First of all, the fluorine in fluoroethers has strong electronegativity and weak polarity, so the solubility of ether solvents is greatly reduced after being fluorinated, and many of them cannot dissolve lithium salts and lithium polysulfides. Therefore, the addition of fluoroether will not change the solvation state of lithium ions and solvent molecules in the high-concentration lithium-salt electrolyte, and the performance of the new electrolyte is similar to that of the high-concentration lithium-salt electrolyte. The concentration of liquid lithium salt is reduced, and this new electrolyte is named pseudo-high-concentration lithium-salt lithium-sulfur battery electrolyte.

Compared with the high-concentration lithium-salt electrolyte, an appropriate amount of fluoroether is added to the pseudo-high-concentration lithium-salt electrolyte. Fluoroethers have very low viscosity and have good wettability to electrodes and separators. Therefore, the viscosity of the pseudo-high-concentration lithium salt electrolyte decreased significantly, causing an increase in the conductivity of the electrolyte. In addition, the fluoroether itself is non-flammable, so the addition of fluoroether can also reduce the flammability of the electrolyte to a certain extent, and even obtain a non-flammable electrolyte.

Detailed ways

The present invention will be further described below in conjunction with specific examples, but the present invention is not limited to these specific implementations. Those skilled in the art will realize that the present invention covers all alternatives, modifications and equivalents as may be included within the scope of the claims.

The abbreviations of fluoroethers described in the following examples are as follows:

HFMOP is (CF ₃ ) ₂ CHOCH ₃ , HFEOP is (CF ₃ ) ₂ CHOCH ₂ CH ₃ , HFTFPOP is (CF ₃ ) ₂ CHOCH ₂ CF ₂ CF ₂ H, TFEOTFP is HCF ₂ CF ₂ OCH ₂ CF ₂ CF ₂ H , TFEOPFP is HCF ₂ CF ₂ OCH ₂ CF ₂ CF ₃ , HFPEE is CF ₃ CF ₂ CHFOCH ₂ CH ₃ .

Example 1

A lithium-ion battery electrolyte, including 1,3 dioxolane (DOL) and ethylene glycol dimethyl ether (DME) two ether solvents, with LiTFSI as lithium salt. The preparation method is as follows: mix DOL and DME at a volume ratio of 1:1, and then add LiTFSI to make the concentration reach 5.0 mol/L. Afterwards, HFMOP is added to the electrolyte, so that the mass fraction of HFMOP in the electrolyte is 40%, so as to obtain the lithium-sulfur battery electrolyte.

Example 2

A lithium-ion battery electrolyte, including 1,3 dioxolane (DOL) and ethylene glycol dimethyl ether (DME) two ether solvents, with LiTFSI as lithium salt. The preparation method is as follows: mix DOL and DME at a volume ratio of 1:1, and then add LiTFSI to make the concentration reach 5.0 mol/L. Then add HFEOP to the electrolyte, so that the mass fraction of HFEOP in the electrolyte is 50%, to obtain the lithium-sulfur battery electrolyte.

Example 3

A lithium-ion battery electrolyte, including 1,3 dioxolane (DOL) and ethylene glycol dimethyl ether (DME) two ether solvents, with LiTFSI as lithium salt. The preparation method is as follows: mix DOL and DME at a volume ratio of 1:1, and then add LiTFSI to make the concentration reach 5.0 mol/L. Then add HFTFPOP to the electrolyte, so that the mass fraction of HFTFPOP in the electrolyte is 60%, to obtain the lithium-sulfur battery electrolyte.

Example 4

A lithium-ion battery electrolyte, including 1,3 dioxolane (DOL) and ethylene glycol dimethyl ether (DME) two ether solvents, with LiTFSI as lithium salt. The preparation method is as follows: mix DOL and DME at a volume ratio of 1:1, and then add LiTFSI to make the concentration reach 5.0 mol/L. Afterwards, TFEOTFP is added to the electrolyte so that the mass fraction of TFEOTFP in the electrolyte is 60%, so as to obtain an electrolyte for a lithium-sulfur battery.

Example 5

A lithium-ion battery electrolyte, including 1,3 dioxolane (DOL) and ethylene glycol dimethyl ether (DME) two ether solvents, with LiTFSI as lithium salt. The preparation method is as follows: mix DOL and DME at a volume ratio of 1:1, and then add LiTFSI to make the concentration reach 7.0 mol/L. Afterwards, TFEOPFP is added to the electrolyte, so that the mass fraction of TFEOPFP in the electrolyte is 60%, so as to obtain the lithium-sulfur battery electrolyte.

Example 6

A lithium-ion battery electrolyte, including 1,3 dioxolane (DOL) and ethylene glycol dimethyl ether (DME) two ether solvents, with LiTFSI as lithium salt. The preparation method is as follows: mix DOL and DME at a volume ratio of 1:1, and then add LiTFSI to make the concentration reach 7.0 mol/L. Then add HFPEE to the electrolyte, so that the mass fraction of HFPEE in the electrolyte is 60%, to obtain the lithium-sulfur battery electrolyte.

Comparative example 1

A lithium-ion battery electrolyte, including 1,3 dioxolane (DOL) and ethylene glycol dimethyl ether (DME) two ether solvents, with LiTFSI as lithium salt. The preparation method is as follows: mix DOL and DME at a volume ratio of 1:1, and then add LiTFSI to make the concentration reach 1.0 mol/L.

Comparative example 2

A lithium-ion battery electrolyte, including 1,3 dioxolane (DOL) and ethylene glycol dimethyl ether (DME) two ether solvents, with LiTFSI as lithium salt. The preparation method is as follows: mix DOL and DME at a volume ratio of 1:1, and then add LiTFSI to make the concentration reach 5.0 mol/L.

Comparative example 3

A lithium-ion battery electrolyte, including 1,3 dioxolane (DOL) and ethylene glycol dimethyl ether (DME) two ether solvents, with LiTFSI as lithium salt. The preparation method is as follows: mix DOL and DME at a volume ratio of 1:1, and then add LiTFSI to make the concentration reach 7.0 mol/L.

The electrolyte solutions prepared in Examples 1 to 6 and Comparative Examples 1 to 3 were tested.

Main test methods:

(1) Use a 2cm*10cm glass fiber cloth to soak in the electrolyte for 1 minute, and test the flammability and self-extinguishing time of the cloth soaked in the electrolyte.

(2) The conductivity, viscosity and contact angle of the electrolyte with the diaphragm at 20°C;

(3) After mixing sulfur and Ketjen black with a mass ratio of 2:1, vacuum treatment at 155 °C for 12 h to obtain a sulfur-carbon composite material. Sulfur-carbon composite material: acetylene black: hydroxymethyl cellulose + styrene-butadiene rubber = 8:1:1 was dispersed in an appropriate amount of water, and the electrode slurry was obtained after ball milling for 6 hours. The obtained slurry was coated on an aluminum foil, dried under an infrared lamp, then dried under vacuum at 60° C. for 12 hours, and cut into electrode sheets with a diameter of 14 mm for use. Afterwards, a lithium-sulfur battery was assembled using the above-mentioned electrolyte, metal lithium as the negative electrode, and Cegrald2400 as the diaphragm, and the cycle performance of the lithium-sulfur battery after 50 cycles was tested at a rate of 0.2C. The test results are as follows:

Table 1

As can be seen from Table 1, compared with the high-concentration lithium salt electrolyte, the pseudo high-concentration lithium salt electrolyte provided by the present invention overcomes its original shortcomings, and has low viscosity, high electrical conductivity, and small contact angle with the separator. advantage. Moreover, the pseudo-high-concentration Li-salt electrolyte retains the advantages of the high-concentration Li-salt electrolyte, such as lower average Coulombic efficiency and higher capacity retention due to the suppressed shuttling effect. At the same time, a certain amount of non-flammable fluoroether is added to the pseudo-high-concentration electrolyte, and the electrolyte as a whole is non-flammable, which further improves the safety of the electrolyte.

Claims (7)

1. A pseudo high concentration lithium sulfur battery electrolyte, characterized in that:

the electrolyte contains lithium salt, ether solvent and non-solvent liquid;

The concentration of the lithium salt in the ether solvent is higher than 3 mol/L;

The solubility of the non-solvent liquid to lithium salt is lower than 0.1 mol/L;

the ether solvent is a mixed solution of 1, 3-dioxolane and glycol dimethyl ether in a volume ratio of 1: 1;

The lithium-sulfur battery is a lithium-sulfur battery with S8 as an active material;

the non-solvent liquid is at least one of fluoroethers shown in the following structural formula (I)

wherein: rf1 and Rf2 are independently selected from C1-C10 alkyl or C1-C10 fluoroalkyl, and at least one is selected from C1-C10 fluoroalkyl.

2. A pseudo high concentration lithium sulfur battery electrolyte according to claim 1 wherein: the molar concentration of the lithium salt in the ether solvent is higher than 3.0mol/L, and the overall concentration of the lithium salt in the pseudo high-concentration electrolyte is higher than 0.5 mol/L.

3. A pseudo high concentration lithium sulfur battery electrolyte according to claim 1 wherein: the Rf1 and the Rf2 are independently selected from C1-C6 fluoroalkyl or C1-C6 alkyl, and at least one is selected from C1-C6 fluoroalkyl.

4. A pseudo high concentration lithium sulfur battery electrolyte according to claim 1 wherein: in the electrolyte, the mass fraction of the fluoroether is 5-90%, and the mass fraction of the ether solvent is 20-98%.

5. The pseudo high concentration lithium sulfur battery electrolyte of claim 4 wherein: in the electrolyte, the mass fraction of the fluoroether is 30-60%, and the mass fraction of the ether solvent is 40-70%.

6. A pseudo high concentration lithium sulfur battery electrolyte according to claim 1 wherein: the lithium salt is at least one selected from LiPF6, LiBF4, LiBOB, LiDFOB, LiPO2F2, LiSO3CF3, lithium bis (trifluoromethyl) sulfonyl imide (LiTFSI) and lithium bis (fluoro) sulfonyl imide (LiFSI).

7. A lithium sulfur battery characterized by: use of an electrolyte as claimed in any of claims 1 to 6.