KR20030092662A - Lithium/sulfur battery with four discharge plateau - Google Patents

Abstract

PURPOSE: Provided is a lithium/sulfur battery having improved discharge property, which decreases an interfacial contact resistance between a polymer electrolyte and electrodes and the crystallinity of the electrolyte and increases ion conductivity. CONSTITUTION: The lithium/sulfur battery representing 4-step discharge reactions(Li2S8, Li2S4, Li2S2, Li2S) is obtained by adding alumina, lead oxide or carbon as a ceramic filler to the polymer electrolyte having the formula of PEOnLix(n=10, 6, and LiX= LiCF3SO3 or LiBF4), ball milling the mixture for 2-12 hours to form a composite polymer electrolyte, and then applying the composite polymer electrolyte to a litium/50 wt% sulfur battery.

Description

Lithium / sulfur battery with four stage discharge flatness {Lithium / sulfur battery with four discharge plateau}

The present invention adds lead oxide (PbO), alumina (Al ₂ O ₃ ), or carbon to the PEO _n LiX (n = 10, 6, LiX = LiCF ₃ SO ₃ or LiBF ₄ ) polymer electrolyte as a ceramic filler The present invention relates to a lithium / sulfur battery having a four-stage flat discharge section obtained by applying a composite polymer electrolyte prepared by a ball milling method to a lithium / sulfur battery.

The excellent properties of electrolytes in batteries are important factors in determining the characteristics of the batteries. There is a ball milling method different from the general magnetic stirring method to prepare such an electrolyte, the method serves to lower the crystallinity of polyethylene oxide (Poly (ethylene oxide) (PEO)).

In the preparation of PEO _n LiX (n = 10, 6, LiX = LiCF ₃ SO ₃ or LiBF ₄ ) polymer electrolyte, the electrolyte prepared by the ball milling method is much lower in crystallinity than the electrolyte prepared by the magnetic stirring method. It was reported that the ion conductivity was higher than that without ball milling, and that the ball milling with the addition of ceramic filler was higher than that with ball milling. [Korean Patent Application No. 2001-015706] Reference].

When the PEO _n LiX (n = 10, 6, LiX = LiCF ₃ SO ₃ or LiBF ₄ ) polymer electrolyte prepared by the ball milling method is applied to a lithium (Li) / sulfur (S) battery, a difference in discharge curve appears. When LiBF ₄ is used, it has been reported to have different discharge flatness intervals than LiCF ₃ SO ₃ [JHShin, YTLim, KWKim, HJAhn, JHAhn, Effect of ball milling on structural and electrochemical properties of (PEO) _n LiX (LiX = LiCF ₃ SO ₃ and LiBF ₄ ) polymer electrolytes,, Journal of Power Source, 107 (2002), p 103-109].

The chemical reaction of a lithium / sulfur battery composed of such an electrolyte reacts with two lithium and one sulfur to form lithium sulfide (Li ₂ S) to generate energy of 1672 (mAh / g-sulfur). It is reported that the reaction between lithium and sulfur leads to the formation of an intermediate product, lithium polysulfide (Li ₂ Sn n> 2) [D. Marmorstein, Electrochemical performance of lithium / sulfur cells with three different polymer electrolytes, Journal of Power Source 89 (2000), p219-226. Discharge flat section is generated by the lithium polysulfide and lithium sulfide (Li ₂ S). The flat section generated from the upper part is generated by the production of lithium polysulfide (Li ₂ S _n , n = 8, 4, 2) as an intermediate product. Some polysulfides are known to have a reduction potential in the 2 V range consistent with the gastric flat area [RD Rauh, RSShuker, JMMarston, SBBrummer, J. Inorg. Nucl. Chem. 39 (1977) 1761.

In addition, the convex shape between the upper and lower planar regions exhibits a changing potential as a shift in equilibrium between other polysulfides, and the lower discharge planar region has been reported to be due to the formation of Li ₂ S as a separate phase [ D. Marmorstein, Electrochemical performance of lithium / sulfur cells with three different polymer electrolytes, Journal of Power Source 89 (2000), p219-226].

When discharged at a discharge depth of 10 to 12% of the total discharge capacity (1672 mAh / g-sulfur), the active sulfur changes to Li ₂ S ₈ , at twice the discharge depth (20 to 25%). Li ₂ S ₄ is produced upon discharge, and these two polysulfides are reported to be able to react with non-equilibrium and partially irreversible products that lead to an increase in capacity reduction electrochemically.

In addition, Maltec Co. says that the production of lithium polysulfide, an intermediate product in the reaction of lithium and sulfur, will proceed to the next step.

Li ₂ S ₈ → Li ₂ S ₄ → Li ₂ S ₂ → Li ₂ S

Although the lithium / sulfur battery satisfies the above hypothesis, it should have four stages of discharge flatness, but the current discharge curve shows only two stages of discharge flatness. However, in the present invention, after preparing the PEO-LiCF ₃ SO ₃ or LiBF ₄ composite polymer electrolyte, it is confirmed that the discharge flat section of the four stages similar to the hypothesis of the discharge behavior of the lithium / sulfur battery composed of a lithium / sulfur battery When the heating / cooling process was performed on the lithium / sulfur battery, it was confirmed that the battery had four more steps and an increase in capacity was also obtained.

Recently, many researches have been conducted to develop batteries for electric vehicles to prevent environmental pollution caused by vehicle exhaust gas.In addition, as the demand and technology of portable electronic devices such as camcorders, laptops, and mobile phones have developed rapidly, Is a major source of energy as well as product performance.

Therefore, many studies on lithium secondary batteries, which are known to have the best discharge performance, are being conducted. Among them, polymer batteries have been studied the most due to high energy density and discharge voltage, and are currently commercialized in camcorders and mobile phones. It is becoming.

Although lithium polymer batteries have been considerably developed, the lithium polymer batteries of classical PEO-LiX based electrolytes still have some problems. These problems are as follows.

(Iii) cycle characteristics and stability due to the reactivity of the lithium anode,

(Ii) The limitation of the operating temperature due to the thermal dependence of lithium ion migration in the polymer electrolyte and the composite polymer electrolyte for solving the problem have been studied considerably in recent years due to the excellent mechanical stability and high ionic conductivity.

Accordingly, an object of the present invention is to provide lead oxide (PbO), alumina (Al ₂ O ₃ ), or the like in a PEO _n LiX (n = 10, 6, LiX = LiCF ₃ SO ₃ or LiBF ₄ ) polymer electrolyte. The composite polymer electrolyte is prepared by ball milling by adding a ceramic filler such as carbon, and then applied to a lithium / sulfur battery to reduce the contact resistance at the interface between the composite polymer electrolyte and lithium and sulfur electrodes. The lithium / sulfur battery discharged by improving the ion conductivity by reducing the crystallinity of the electrolyte when the lithium / sulfur battery was heated at 80 ° C. for 1 hour and then heated / cooled at room temperature for 1 hour. To improve it.

An object of the present invention is a four-stage discharge flat section by a ceramic filler added to the PEO _n LiX (n = 10, 6, LiX = LiCF ₃ SO ₃ or LiBF ₄ ) solid polymer electrolyte to improve the ionic conductivity and mechanical properties (Li ₂ S ₈ , Li ₂ S ₄ , Li ₂ S ₂ , Li ₂ S) To provide a lithium / sulfur battery showing an increase in the characteristics and capacity.

1 is a graph showing the results of a cyclic voltage / current test on a lithium / sulfur battery according to Example 1 of the present invention.

Figure 2 is a graph showing the discharge curve according to the discharge current density for the lithium / sulfur battery according to Example 1 of the present invention.

3 is a graph showing a discharge curve cycled with a battery voltage of 2.4 (V) for a lithium / sulfur battery according to Example 1 of the present invention.

4 is a graph showing a discharge curve cycled with a battery voltage of 2.1 (V) for a lithium / sulfur battery according to Example 1 of the present invention.

5 is a graph showing a discharge curve cycled with a battery voltage of 1.7 (V) for a lithium / sulfur battery according to Example 1 of the present invention.

6 is a graph showing a discharge cycle discharged at a depth of about 12% (200 mAh / g sulfur) for the lithium / sulfur battery according to Example 1 of the present invention.

7 is a graph showing a discharge cycle discharged at a depth of about 24% (400 mAh / g sulfur) of the lithium / sulfur battery according to Example 1 of the present invention.

8 is a graph showing a discharge curve according to the discharge current density for the lithium / sulfur battery according to Example 2 of the present invention.

The above objects of the present invention may all be achieved by the present invention described below.

The present invention is added to the PEO _n LiX (n = 10, 6, LiX = LiCF ₃ SO ₃ or LiBF ₄ ) polymer electrolyte for 2 to 12 hours by adding 5 to 15% by weight of alumina, lead oxide or carbon as a ceramic filler It is a lithium / sulfur battery that shows a four-step discharge reaction (Li ₂ S ₈ , Li ₂ S ₄ , Li ₂ S ₂ , Li ₂ S) by applying the composite polymer electrolyte prepared by milling to a lithium / 50 wt% sulfur battery. It is characterized by.

The battery system of the present invention ranges from 2.8 to 2.6V corresponding to Li ₂ S ₈ formation reaction, from 2.6 to 2.4V corresponding to Li ₂ S ₄ formation reaction, and from 2.4 to 2.1V corresponding to Li ₂ S ₂ formation reaction. And charge / discharge cycle performance in the range of 2.1 to 1.7V corresponding to Li ₂ S production reaction.

In the composite polymer electrolyte of the present invention, polyethylene oxide (PEO) is used as the ion conductor and the binder, and acetonitrile is used as the polymer solvent, and 5% by weight of a lithium salt such as LiBF ₄ or LiCF ₃ SO ₃ is added thereto. 5 to 15 wt% of lead oxide (PbO), alumina (Al ₂ O ₃ ) or carbon as a ceramic additive is added and ball milling using a ball mill for 2 to 12 hours.

Then, the composite polymer electrolyte prepared above is coated on a glass plate and dried in air for about 24 hours to prepare a thin electrolyte film, and then dried in a vacuum atmosphere for 24 hours and then assembled in a lithium / sulfur battery.

According to the battery system of the present invention, lithium is used as the negative electrode and 50 wt% sulfur electrode is used as the positive electrode. The sulfur electrode is titrated with carbon, polyethylene oxide, and sulfur at 50 wt% or more of the total weight ratio. , The solvent is put into a mixer using acetonitrile, mixed for 48 hours, dissolved, then poured into a glass plate and dried, and then vacuum dried at 10 ^-3 Torr 60 ℃ for 24 hours to prepare a film.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

The negative electrode was titrated with lithium foil (350 μm in thickness), 0.16 g of carbon, 0.301 g of polyethylene oxide, and 0.5 g of sulfur (50% by weight of the total weight) as the positive electrode. The solvent was placed in a mixer using 44 ml of acetonitrile. After mixing and dissolving for a period of time, it was poured into a glass plate and dried, and then a composite sulfur electrode prepared in a film by vacuum drying at 10 ^-3 Torr for 60 hours for 24 hours was used.

Next, 0.852 g of polyethylene oxide (PEO), 0.0426 g of LiBF _{4 and} 42 ml of acetonitrile were mixed, and 0.0852 g of alumina (Al ₂ O ₃ ) purchased from Aldrich was added as a ceramic additive, followed by 2 to 12 hours. After ball milling using a miller, an amount of the electrolyte slurry was applied on a glass plate and dried in air for about 24 hours to prepare a thin electrolyte film.

After drying the electrolyte prepared in the above manner in a vacuum atmosphere for 24 hours to assemble a lithium / sulfur battery.

The following experiment was performed. The discharge characteristics of the battery were tested after heating the lithium / sulfur battery at 80 ° C. for 1 hour and then performing pretreatment of heating / cooling at room temperature for 1 hour.

(1) Cyclic current / voltage test to check the four stages of the discharge curve

The experiment was performed at a scanning speed of 0.01 mV / sec and was performed at 80 ° C. in the voltage range from 3.0 (V) to 1.6 (V). The results are shown in FIG. 1, and it can be seen from FIG. 1 that peaks appear at voltages consistent with the four steps of the discharge curve.

(2) Discharge test according to discharge current density

This experiment was conducted with discharge current densities of C / 32 (0.063 mA / cm 2) and C / 16 (0.125 mA / cm 2). The results are shown in FIG. 2, and according to FIG. 2, it could be seen that four discharge flat sections existed, and more stable discharge flat sections could be seen when performed at a low current density.

(3) Li ₂ S ₄ Cycle experiment with generation voltage 2.4 (V)

This experiment was conducted by charging and discharging at current densities of C / 32 (0.063 mA / cm 2) and C / 16 (0.125 mA / cm 2). As shown in FIG. 3, the capacity is consistent with 400 (mAh / g sulfur), which is the capacity at which theoretical Li ₂ S ₄ production is completed at the initial discharge. Lithium polysulfide (Li ₂ S _n n> 2) is a non-equilibrium property that melts well into the electrolyte, so the cycle does not work well. However, the lithium polysulfide in the present embodiment shows that although the capacity decrease occurs, the cycle occurs. I could confirm it.

(4) Li ₂ S ₂ Cycle experiment with generation voltage of 2.1 (V)

The same charging and discharging conditions as above (3) were carried out. According to Figure 4, it can be confirmed that the three discharge flat intervals, lithium polysulfide in the present embodiment was confirmed that the cycle is performed, although the capacity decrease occurs.

(5) Li ₂ Cycle experiment with 1.7V of S generation section

The same charging and discharging conditions as above (3) were carried out. According to FIG. 5, the flat section of the Li ₂ S reaction has almost no capacity reduction, while the discharge section due to the generation of lithium polysulfide (Li ₂ S _n n> 2) shows a decrease in capacity but a cycle. .

(6) Discharge cycle experiment discharged at 200 (mAh / g sulfur) of 12% depth of discharge

Discharge was performed at 200 (mAh / g sulfur), which is 12% of the discharge depth of the total discharge capacity (1672 mAh / g sulfur), and the cycle was performed until the battery voltage became 1.7 (V) or less. 6 is shown. This experiment was carried out under the same charging and discharging conditions as in (3) above. During discharge, the active sulfur reacts with lithium to change to Li ₂ S ₈ . I could confirm that I had. The first step confirmed that the capacity was 200 (mAh / g of sulfur) and did not reappear after the first cycle. Therefore, lithium polysulfide of Li ₂ S ₈ is considered to be an irreversible intermediate product.

(7) Discharge cycle experiment discharged at 400 (mAh / g sulfur) of 24% discharge depth

Discharge was performed at 400 (mAh / g sulfur), which is 24% of the discharge depth of the total discharge capacity (1672 mAh / g sulfur), and the cycle was performed until the battery voltage became 1.7 (V) or less. 7 is shown. This experiment was conducted under the same charging and discharging conditions as above (3). When only 400 (mAh / g of sulfur) was discharged at a discharge depth of 24%, the active sulfur reacted with lithium to pass Li ₂ S ₈ to Li. ₂ is changed to S _4, the following cycle of Li2S8 reaction will not appear, as the cycle is continued it was confirmed that with the discharge voltage of the flat section of the step 4.

Example 2

A lithium / sulfur battery was assembled in the same manner as in Example 1 except that LiCF ₃ SO ₃ was used as the polymer electrolyte and carbon was added as a ceramic additive.

The discharge experiment according to the discharge storage density of the battery was carried out at a discharge storage density of C / 16 (0.125 mA / cm 3) and the results are shown in FIG. 8.

According to FIG. 8, although the discharge flat period was formed differently from the case of FIG. 2, it was confirmed that four discharge flat periods existed.

As a result of confirming through the above examples, the discharge test results of lithium / sulfur battery using a composite polymer electrolyte in which a ceramic filler is added to PEO ₆ (LiBF ₄ or LiCF ₃ SO ₃ ) solid polymer electrolyte prepared by ball milling according to the present invention In addition, it has four stages of discharge flatness, which is different from the two-stage discharge flatness shown in previous studies, which is lithium polysulfide as an intermediate product during the reaction of finally forming lithium sulfide by reaction of lithium and sulfur. It is related to the generation of. The stoichiometric capacity of this process of converting Li ₂ S ₈ → Li ₂ S ₄ → Li ₂ S ₂ to lithium polysulfide is about 800 (mAh / g-sulfur), which is consistent with the capacity up to step 3 of the present invention.

In the present invention, it has been confirmed that the discharge flat section has three and four stages according to the type of ceramic filler, and heating / cooling pretreatment is performed in order to improve the adhesion between the interface between lithium and the electrolyte and the electrolyte and the sulfur electrode, and to lower the crystallinity. In this case, there is an effect of forming a certain four-stage flat section with increasing capacity.

Claims (5)

PEO _n LiX (n = 10, 6, LiX = LiCF ₃ SO ₃ or LiBF ₄ ) prepared by ball milling for 2 to 12 hours by adding 5 to 15% by weight of alumina, lead oxide or carbon as a ceramic filler Lithium / Sulfur battery showing a four-stage discharge reaction (Li ₂ S ₈ , Li ₂ S ₄ , Li ₂ S ₂ , Li ₂ S) characterized by applying the composite polymer electrolyte to a lithium / 50 wt% sulfur battery . The lithium / sulfur battery of claim 1, wherein the battery has charge / discharge cycle performance in a range of 2.8 to 2.6V corresponding to a Li ₂ S ₈ formation reaction. The lithium / sulfur battery of claim 1, wherein the battery has charge / discharge cycle performance in a range of 2.6 to 2.4V corresponding to a Li ₂ S ₄ formation reaction. The lithium / sulfur battery of claim 1, wherein the battery has charge / discharge cycle performance in a range of 2.4 to 2.1 V corresponding to a Li ₂ S ₂ formation reaction. The lithium / sulfur battery of claim 1, wherein the battery has charge / discharge cycle performance in a range of 2.1 to 1.7 V corresponding to a Li ₂ S production reaction.