WO2019045311A1 - Method for improving life of lithium-sulfur secondary battery - Google Patents

Abstract

Disclosed is a method for improving the life of a lithium-sulfur secondary battery capable of improving the life properties of the secondary battery by inducing uniform reaction of the lithium-sulfur secondary battery. The method for improving the life of the lithium-sulfur secondary battery includes a self-activation step of charging to reach a zone in which a long-chain lithium polysulfide is formed in the process of charging the lithium-sulfur secondary battery, and then allowing ten minutes to four hours of rest time.

Description

How to improve the life of lithium-sulfur secondary battery

This application claims the benefit of priority based on Korean Patent Application No. 10-2017-0111825, filed on Sep. 1, 2017, the entire contents of which are incorporated herein by reference.

The present invention relates to a lithium-sulfur secondary battery, and more particularly, to a method for improving the service life of a lithium-sulfur secondary battery, which can improve the life characteristics of a secondary battery by inducing a uniform reaction of the lithium- .

Lithium-sulfur secondary batteries among secondary batteries whose application fields are expanding to electric vehicles and energy storage devices are attracting attention as a next generation secondary battery technology such that a relatively high energy storage density can be realized. Such a lithium-sulfur secondary battery means a battery using a sulfur-based material having S-S bond (Sulfur-Sulfur Bond) as a cathode active material and using lithium metal as an anode active material. In particular, the sulfur used as the main material of the cathode active material is very rich in resources, is not toxic, and has a low atomic weight.

The lithium-sulfur secondary battery is ionized and oxidized while lithium, which is a negative electrode active material, discharges electrons, and the sulfur-based material, which is a positive electrode active material, receives and reduces the released electrons (lithium- And the reduction reaction of sulfur is a process in which the SS bond is converted into a sulfur anion form by accepting two electrons). At this time, the lithium cation generated by the oxidation reaction of lithium is transferred to the anode through the electrolyte, and forms a salt by binding with the sulfur anion generated by the reduction reaction of sulfur. More specifically, sulfur before the discharge is a cyclic S ₈ structure, which is converted to lithium polysulfide by a reduction reaction, and lithium sulfide (Li ₂ S) is produced when the lithium polysulfide is completely reduced .

As described above, the lithium-sulfur (Li-S) secondary battery has many advantages and has attracted much attention. However, the conventional lithium-sulfur secondary battery has a problem that the capacity retention rate is lowered due to active material loss due to the dissolution of lithium polysulfide (LiPS). In addition, in a lithium-sulfur secondary battery having a high energy density, the concentration of lithium polysulfide (LiPS) in the electrolyte rapidly increases. At this time, the mobility of the electrolyte decreases and the reactivity of sulfur decreases, The battery exhibits a non-uniform reaction pattern and the degradation of the battery is accelerated.

In this connection, attempts have been made to design a sulfur anode structure or to apply a redox mediator as a cathode additive as a means for inducing a uniform reaction of a lithium-sulfur secondary battery. In this case, however, the process of dispersing the sulfur anode additive is complicated and it is difficult to synthesize the redox relay material. As a more effective method for inducing a uniform reaction of the lithium-sulfur secondary battery, attempts have been made to improve the sulfur anion reactivity through controlling the discharge rate or changing the electrolyte composition using an additive, So far it has not shown a great effect.

Accordingly, an object of the present invention is to provide a lithium-sulfur secondary battery which is capable of inducing a uniform reaction of a lithium-sulfur secondary battery by introducing a self-activation step of charging the battery to a predetermined region and then giving a rest time And a method for improving the service life of a lithium-sulfur secondary battery, which can be improved.

In order to accomplish the above object, the present invention provides a method of manufacturing a lithium-sulfur secondary battery, which comprises charging the lithium-sulfur secondary battery to reach a region where a long chain lithium polysulfide is formed, And a self-activation step of imparting a rest time to the lithium-sulfur secondary battery.

According to the method for improving the service life of a lithium-sulfur secondary battery according to the present invention, by introducing a self-activating step of charging the battery to a predetermined region and then giving a rest time, a uniform reaction of the lithium- And the lifetime characteristics of the secondary battery can be improved.

1 is a graph comparing lifetime characteristics of a lithium-sulfur secondary battery according to an embodiment of the present invention and a lithium-sulfur secondary battery according to a comparative example.

2 and 3 are graphs showing a charge profile of a lithium-sulfur secondary battery according to an embodiment of the present invention.

4 is a graph showing a charging profile of a lithium-sulfur secondary battery according to a comparative example of the present invention.

Hereinafter, the present invention will be described in detail.

The method for improving the service life of a lithium-sulfur secondary battery according to the present invention is a method for charging a lithium-sulfur secondary battery so as to reach a region where a long chain lithium polysulfide (LiPS) is formed, And a self-activation step to give a rest time of 10 minutes to 4 hours.

The method for improving the lifetime of a lithium-sulfur secondary battery according to the present invention is to apply the same or similar materials to existing materials without additional materials such as a conventional anode, cathode, electrolyte and separator. However, it has a feature of further including a self-activation step during the charging process, and it can be confirmed that the lifetime characteristics of the battery are improved.

The lithium polysulfide is a salt formed by combining a lithium cation generated by an oxidation reaction of lithium upon discharge of a lithium-sulfur secondary battery and a sulfone anion generated by a reduction reaction of sulfur, and the long-chain lithium polysulfide long chain lithium polysulfide (LiPS) refers to a long chain salt formed by the combination of a lithium cation and a sulfur anion. At this time, the long-chain lithium polysulfide includes 4 to 8, preferably 4 to 7, more preferably 5 or 6 (in the molecule) sulfur atoms. When the number of sulfur atoms in the long-chain lithium polysulfide molecule is out of the above range, it may be difficult to improve the lifetime characteristics even when the self-activation step is performed. On the other hand, single-chain lithium polysulfide is defined as containing 1 to 3 sulfur atoms in the molecule.

As described above, the reason why only the region where the long-chain lithium polysulfide is formed during filling of the lithium-sulfur secondary battery is to be filled is that when the chain length of the lithium polysulfide is short, the solubility in the electrolyte is low, , And even if a resting period is given thereafter, the redox transit action effect of lithium polysulfide may not be realized.

That is, the method for improving the service life of a lithium-sulfur secondary battery according to the present invention is characterized in that when a lithium-sulfur secondary battery is charged to a region where long-chain lithium polysulfide is formed and then a rest period is provided, Sulfur secondary battery, thereby making the lithium-sulfur secondary battery perform a uniform reaction upon charging, thereby improving or improving the lifetime characteristics of the battery. In other words, the present invention enables a uniform reaction of the lithium-sulfur secondary battery at the time of charging, thereby improving the lifetime characteristics of the battery without adding a redox relay material. In addition, after the idle period ends, the remaining charge can proceed.

The rest time is 10 minutes to 4 hours, preferably 30 minutes to 2 hours, more preferably 30 to 60 minutes. When the resting period is less than 10 minutes, the long-chain lithium polysulfide does not perform the role of redox relay, and it may be difficult to induce a uniform reaction when the lithium-sulfur secondary battery is charged. If the rest period exceeds 4 hours, There is a fear that the lifetime characteristic is not improved due to an excessively long rest period and an increase in self-discharge. However, the rest period may vary depending on the performance of the lithium-sulfur secondary battery, the charging environment, and the like.

In addition, the lifetime characteristics of the lithium-sulfur secondary battery according to the present invention are influenced by the charging voltage when the long-chain lithium polysulfide (LiPS) is mainly formed in addition to the appropriate rest period. The charging voltage just before the resting period can be varied depending on the capacity and the charging environment of the lithium-sulfur secondary battery. For example, the lifetime characteristics of the lithium-sulfur secondary battery under the same resting period are as follows: (LiPS) is formed is 2.34 to 2.40 V (volt), and the case where the charging voltage is 2.37 to 2.39 V is better. Here, the long-chain lithium polysulfide when a charging voltage of 2.37 to 2.39 V is formed corresponds to Li ₂ S ₆ containing six sulfur atoms.

Meanwhile, the positive electrode of the lithium-sulfur secondary battery used in the present invention includes a positive electrode collector and a positive electrode active layer formed on the positive electrode collector. The cathode current collector may be any material selected from among stainless steel, aluminum, nickel, titanium, sintered carbon and aluminum, and may be formed of carbon , Nickel, titanium or silver can be further performed. The shape of the positive electrode collector may be a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric. The thickness is not particularly limited, and the mechanical strength, productivity And the capacity of the battery.

The method for forming the positive electrode active layer on the current collector may be a known coating method such as a bar coating method, a screen coating method, a doctor blade method, a dipping method, a reverse roll method, a direct roll method, a gravure method, And is not particularly limited. The amount of the positive electrode active layer to be coated on the current collector is not particularly limited, and can be adjusted in consideration of the final thickness of the target positive electrode active layer. Before or after the step of forming the positive electrode active layer, a known process required for the production of the positive electrode, for example, a rolling process or a drying process, may be performed.

The negative electrode constituting the lithium-sulfur secondary battery of the present invention comprises a negative electrode collector and a negative electrode active material layer formed on the negative electrode collector, wherein the negative electrode active material layer includes a negative electrode active material, a binder and a conductive material. Examples of the negative electrode active material include a material capable of reversibly intercalating or deintercalating lithium ions (Li +), a material capable of reacting with lithium ions to reversibly form a lithium-containing compound, a lithium metal or a lithium alloy Can be used.

The material capable of reversibly intercalating or deintercalating lithium ions (Li +) is, for example, crystalline carbon, amorphous carbon, or a mixture thereof, and can react reversibly with the lithium ion (Li +) to form a lithium- The lithium alloy is, for example, sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium Is an alloy of lithium and a metal such as Be, Mg, Ca, Sr, Ba, Ra, Al or Sn.

The binder may include, for example, an acrylic polymer, and the like. Any conventional binder that can be used in the art can be used without limitation. In addition, the negative electrode active material, conductive material, Without limitation.

The electrolyte solution constituting the lithium-sulfur secondary battery of the present invention includes solvents (Solvents) and lithium salts (lithium salts), and may further include additives, if necessary. As the solvent, a conventional non-aqueous solvent serving as a medium through which ions involved in an electrochemical reaction of a battery can move can be used without any particular limitation. Examples of the non-aqueous solvent include a carbonate-based solvent, an ester-based solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent and an amphoteric solvent.

More specifically, examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate ), Ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). Examples of the ester solvent include methyl acetate, ethyl acetate, n-propyl acetate, Propanolate, propionate, ethyl propionate,? -Butyrolactone, decanolide, valerolactone, mevalonolactone, and carprolactone. The ether solvents include di Ethyl ether, dipropyl ether, dibutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, diglyme, triglyme, tetraglyme, tetra 2-methyltetrahydrofuran, and polyethylene glycol dimethyl ether. Examples of the ketone-based solvent include cyclohexanone, and the alcohol-based solvent includes ethyl alcohol and isopropyl alcohol. Examples of the non-protonic solvent include nitriles such as acetonitrile, amides such as dimethylformamide Dioxolanes such as Dole, 1,3-dioxolane (DOL), and sulfolane. The nonaqueous solvent may be used singly or in combination of two or more. When two or more of them are mixed, the mixing ratio may be appropriately adjusted according to the performance of the objective battery, and 1,3-dioxolane and dimethoxyethane In a volume ratio of 1: 1.

The separation membrane constituting the lithium-sulfur secondary battery of the present invention has a function of physically separating the electrodes and can be used without any particular limitation. However, it is also possible to separate or insulate the positive electrode from the negative electrode, It is preferable that it is made of a porous, non-conductive or insulating material which enables transport of ions and has a porosity of 30 to 50%.

Examples of the separation membrane include a porous polymer film made of a polyolefin-based polymer such as an ethylene polymer, a propylene polymer, an ethylene / butene copolymer, an ethylene / hexene copolymer and an ethylene / methacrylate copolymer, And the like, and the use of the porous polymer film is more preferable. In addition, the general configuration of a lithium-sulfur secondary battery, which is not described, can be generally applied to the configuration of a lithium-sulfur secondary battery.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention as set forth in the appended claims. Such changes and modifications are intended to be within the scope of the appended claims.

[Example 1] Charging of lithium-sulfur secondary battery using rest period

According to the following Table 1, lithium-sulfur secondary batteries using an electrolyte including a lithium salt (1.0 M LiTFSI (lithium bis (trifluoromethanesulfonyl) imide) + 1 wt% LiNO ₃ ) and a solvent (DOL / DME in a volume ratio of 1: 1) The battery was charged to a current of 2.40 V at a current rate of 0.2 and then had a rest time of stopping charging for 1 hour. After the rest period was completed, the recharging was continued to complete the charging of the lithium-sulfur secondary battery.

[Example 2] Charging of lithium-sulfur secondary battery using resting period

The charging of the lithium-sulfur secondary battery was completed in the same manner as in Example 1, except that the lithium-sulfur secondary battery was charged to 2.38 V and then a rest period was set according to Table 1 below.

[Example 3] Charging of lithium-sulfur secondary battery using rest period

The charging of the lithium-sulfur secondary battery was completed in the same manner as in Example 1, except that the lithium-sulfur secondary battery was filled up to 2.36 V according to the following Table 1, and then the battery had a rest period.

[Example 4] Charging of lithium-sulfur secondary battery using resting period

The charging of the lithium-sulfur secondary battery was completed in the same manner as in Example 1, except that the lithium-sulfur secondary battery was charged to 2.34 V, and then the lithium-sulfur secondary battery was allowed to have a rest period.

[Example 5] Charging of lithium-sulfur secondary battery using resting period

The procedure of Example 1 was repeated, except that the lithium-sulfur secondary battery was charged to 2.38 V and then allowed to have a rest period and the rest period was changed from 1 hour to 30 minutes in accordance with the following Table 1 to obtain lithium-sulfur The charging of the secondary battery was completed.

[Example 6] Charging of lithium-sulfur secondary battery using resting period

The procedure of Example 1 was repeated, except that the lithium-sulfur secondary battery was charged to 2.38 V, followed by a rest period, and the rest period was changed from 1 hour to 10 minutes. The charging of the secondary battery was completed.

[Example 7] Charging of lithium-sulfur secondary battery using resting period

The procedure of Example 1 was repeated, except that the lithium-sulfur secondary battery was charged to 2.38 V and then allowed to have a rest period and the rest period was changed from 1 hour to 2 hours in accordance with the following Table 1 to obtain lithium-sulfur The charging of the secondary battery was completed.

[Example 8] Charging of lithium-sulfur secondary battery using resting period

The procedure of Example 1 was repeated, except that the lithium-sulfur secondary battery was charged to 2.38 V, followed by a rest period, and the rest period was changed from 1 hour to 4 hours. The charging of the secondary battery was completed.

[Comparative Example 1] Charging of a conventional lithium-sulfur secondary battery

According to the following Table 1, a lithium-sulfur secondary battery using an electrolyte containing a lithium salt (1.0 M LiTFSI + 1 wt% LiNO ₃ ) and a solvent (DOL / DME in a volume ratio of 1: 1) The charging was completed at once.

	Hold voltage (charge, unit: V)	Rest time
Example 1	2.40	1 h
Example 2	2.38	1 h
Example 3	2.36	1 h
Example 4	2.34	1 h
Example 5	2.38	30 min
Example 6	2.38	10 min
Example 7	2.38	2 h
Example 8	2.38	4 h
Comparative Example 1	-	-

[Examples 1 to 8, Comparative Example 1] Evaluation of life characteristics of lithium-sulfur secondary batteries

The remaining capacities of the lithium-sulfur secondary batteries packed in Examples 1 to 8 and Comparative Example 1 were checked after repeated charging and discharging, and the life characteristics of each lithium-sulfur secondary battery were evaluated. FIG. 1 is a graph comparing life characteristics of a lithium-sulfur secondary battery according to an embodiment of the present invention and the life characteristics of a lithium-sulfur secondary battery according to a comparative example. FIGS. 2 and 3 are cross- - A graph showing a charging profile of a sulfur secondary battery, wherein FIGS. 2A to D correspond to the first to fourth embodiments, and FIGS. 3A to D correspond to the fifth to eighth embodiments, respectively. 4 is a graph showing a charge profile of a lithium-sulfur secondary battery according to a comparative example of the present invention.

First, the lithium-sulfur secondary battery of Comparative Example 1 which was buffered (i.e., continuously charged) in a single cycle without a rest period had a battery deterioration at about 15 cycles (cycles) as shown in Figs. 1A and 1B It was confirmed that the phenomenon occurred. On the other hand, it was found that the lithium-sulfur secondary batteries of Examples 1 to 8 having a rest period during charging exhibited excellent life characteristics as compared with Comparative Example 1.

On the other hand, as shown in FIGS. 1A and 1B, among the Examples 1 to 8 having a rest period during charging, different life characteristics results were shown depending on the voltage or the resting time. In other words, through the embodiments 1 to 4, it is possible to confirm the optimum voltage immediately before the rest period (2.38 V corresponding to the embodiment 2) (see FIG. 1 A), and in the embodiments 2 and 5-8, It can be seen that not only the confirmation of the resting time (30 minutes corresponding to the example 5) but also the effect of the life characteristic is reduced as the resting period becomes longer (refer to FIG. 1B).

Claims (8)

1. In the charging process of the lithium-sulfur secondary battery,

   And a self-activation step of charging to reach a region where a long chain lithium polysulfide is formed, followed by a rest time of 10 minutes to 4 hours, A method for improving the lifetime of a sulfur secondary battery.
2. The method for improving the service life of a lithium-sulfur secondary battery according to claim 1, wherein the dormant period is from 30 minutes to 2 hours.
3. The method for improving the service life of a lithium-sulfur secondary battery according to claim 1, wherein the rest period is 30 to 60 minutes.
4. The method of claim 1, wherein the charge voltage before the dormant period is 2.34 to 2.40 V.
5. The method of claim 1, wherein the charge voltage before the dormant period is 2.37 to 2.39 V.
6. The method for improving the service life of a lithium-sulfur secondary battery according to claim 1, wherein the long-chain lithium polysulfide comprises 4 to 8 sulfur atoms.
7. The lithium-sulfur secondary battery according to claim 1, wherein the long-chain lithium polysulfide serves as a redox relay, and the lifetime characteristics of the lithium-sulfur secondary battery are improved by uniformly reacting the lithium- How to improve life.
8. The method for improving the service life of a lithium-sulfur secondary battery according to claim 1, wherein the remaining charge is continued after the idle period ends.

Images