KR101610446B1 - A separator of lithium sulfur secondary battery - Google Patents

Abstract

The present invention relates to a lithium sulfur battery having a separator membrane capable of sufficiently liquefying an electrolyte solution on the anode side of a sulfur-conductive material of a lithium sulfur battery and having a separator membrane by using an ionomer membrane on the lithium anode side.

Description

[0001] The present invention relates to a separator of lithium sulfur secondary battery,

The present invention relates to a lithium sulfur battery having a separator membrane capable of sufficiently liquefying an electrolyte solution on the anode side of a sulfur-conductive material of a lithium sulfur battery and having a separator membrane by using an ionomer membrane on the lithium anode side.

In order to solve the problem of shuttle phenomenon and reduction of Coulomb efficiency by suppressing polysulfide migration, Li is substituted in the SO ₃ H ^- group of the PFSA membrane used for the fuel cell, There are studies that apply to batteries.

In particular, when applying the H ⁺ cations by substitution with lithium for a lithium sulfur battery, chemically stable and has high proton conductivity and lithium transference number close to 1, since the unique structure of its own as a characteristic to block the movement of polysulfide anions Li ⁺ only It is advantageous to move.

However, unlike the conventional mechanism in which lithium ions are transferred by dissolving lithium polysulfide by using a liquid electrolyte, there is no room for electrolyte retention by using a membrane separation membrane. Therefore, a positive electrode having a low sulfur loading amount There is a limitation that it must be used, and in particular, there is a big problem that the lithium ion conductivity is low (see FIG. 1).

(See FIG. 3), a method of separating Nafion ion from a nonionic surfactant is disclosed in the following article: [Application of lithiated Nafion ionomer film for lithium sulfur cells, Journal of Power Sources 218 (2012) 163-167, Zhaoqing Jin, Kai Xie, Xiaobin Hong, Zongqian Hu, Xiang Liu] The reaction mechanism of the PFSA membrane is as follows.

- (CF ₂ CF ₂ ) _m - (CF ₂ CF (OCF ₂ CF (CF ₃ ) OCF ₂ -CF ₂ SO ₃ H)) _n

⇒ -OCF ₂ CF (CF ₃ ) OCF ₂ -CF ₂ SO ₃ Li (presence of a pendent side chain)

⇒ -SO ₃ ^- + Li ⁺ decomposition (Li ⁺ ion transport, -SO ₃ ^- electric field formation)

When the above mechanism is followed, the PS movement is blocked to suppress side reaction with the Li cathode, thereby improving cell performance and lifetime, preventing loss of active material, and improving cell performance and lifetime.

However, due to the low lithium ion conductivity, there is a critical limitation in increasing the cell energy density.

As a conventional technology for the separation membrane of a secondary battery, in KR 10-2012-0135808,

A hydrophilic polysulfide confinement layer is formed between the anode and the separator. The polysulfide confinement layer (4) has a porous structure having a plurality of through holes to smoothly diffuse and move the migration material in the electrolyte during the charging / discharging reaction. The porous PE membrane is subjected to an oxygen plasma treatment to oxidize the surface, and then the PEG polymer membrane is bonded to the surface of the porous PE membrane by reacting the PEG with the silane attached thereto. Porous hydrophilic membrane.

KR 10-2012-0104358 (WO 2011/084649)

Wherein the redox flow energy storage device comprises a positive electrode active material, a negative electrode active material, and an ion-permeable medium separating the positive electrode active material from the negative electrode active material, flow cell comprising a multistage galvanostatic charge / discharge of a LiCoO ₂ suspension continuously flowing at a rate of ₁ mL / mL / min.

KR 10-2005-0021131,

Disclosed is a method for preventing loss of sulfur by dissolving sulfur in an anode toward a cathode by using a separator coated with gold (Au), which is a material having excellent conductivity, to prevent loss of sulfur electrodes and improve electrical conductivity.

However, this method is insufficient as a structure for liquefying an electrolyte together with an ionomer membrane of a lithium-sulfur secondary battery, especially a lithium ion-substituted separator.

Accordingly, the present invention is an invention (see FIG. 2) in which the capacity of the lithium sulfur battery can be increased by increasing the sulfur loading amount of the lithium sulfur battery by further applying a reflux structure to the PFSA membrane.

The present invention provides a lithium sulfur secondary battery containing a sulfur anode, a lithium anode, and an ionomer membrane, wherein the lithium secondary battery further comprises a lube separation membrane.

The present invention is applied to a lithium sulphate battery. By applying an ionomer membrane having the property of suppressing the movement of lithium polysulfide and moving only lithium ions, the present invention has an effect similar to that of a solid electrolyte, , Which is a main issue of lithium-sulfur battery, which is the main issue of lithium polysulfide shuttle phenomenon, capacity and life-shortening problem due to side reactions at the cathode, etc., cell energy density due to application of low loading sulfur anode The increase limit problem can be remarkably improved.

In detail, the present invention has the following advantages over the conventional structure.

1) Since sufficient performance can be exhibited even with a high sulfur loading amount per unit area (5 ~ 10 mg_ sulfur / cm ² ) containing sufficient electrolyte, when the sulfur loading amount per unit area is increased, the total weight energy density of the cell increases,

2) Through the nonwoven fabric separator applying the coating layer, it can contribute to the safety improvement by closing action in the thermal runaway.

1 shows a lithium sulfur battery structure using only ionomer membrane.
FIG. 2 is a view showing a lithium sulfur battery to which the luminescent separation membrane of the present invention is applied.
FIG. 3 is a view showing the internal structure of a lithium-sulfur battery of a lithium-sulfur battery as a separator for lithium ion secondary battery.
4 is a comparative diagram of the lithium sulfur battery of the present invention and the prior art.
FIG. 5 illustrates a process for producing an ionomer membrane of the present invention.
6 is a microstructure photograph of a glass fiber nonwoven fabric which can be used as a lap separating membrane.
FIG. 7 is a schematic representation of a chemical reaction in a lithium sulfur battery employing a lyse separator membrane and an ionomer membrane.
8 is a schematic view of a specific example using the lyophilic separation membrane of the present invention.

According to the present invention,

A lithium secondary battery comprising a sulfur anode, a lithium anode, and a ionomer membrane, wherein the lithium secondary battery further comprises a lyophobic separation membrane.

The ionomer membrane may be a PFSA (perfluorosulfonic acid) polymer membrane represented by the following formula (1), and the H ⁺ ion of the -SO ₃ H group may be substituted with Li ⁺ .

[Chemical Formula 1]

In the formula 1, m is 0, 1, n is 0 to 5, x is 0 to 15, y is 0 to 2, and the equivalent is 400 to 2000.

The lube separation membrane is preferably located on the anode side with respect to the ionomer membrane, and it is preferable that the porosity is 30 to 80% and the thickness is 30 to 300 占 퐉. The lube separation membrane may be a nonwoven fabric, a cellulose-based natural fiber, or one or more synthetic fibers selected from the group consisting of PE, PP, PTFE and PVDF. On the other hand, it is preferable that the lube separation membrane has a heat insulating coating layer on both sides or a cross section, and the heat insulating coating layer may be made of a polyolefin type.

In addition, a heat insulating coating layer may be present inside the lumped liquid separation membrane, or may be formed of a polyolefin type.

The lithium sulfur secondary battery to which the lap-liquid separating membrane of the present invention is applied can produce a sulfur loading amount of sulfur anode up to 7 mg / cm ² .

More specifically, the lithiated ionomer membrane is prepared by replacing the H ⁺ cations of the PFSA membrane with Li ⁺ ions, and applied as a separator to form a lithium sulfur cell. A cell is prepared by inserting a lithium-substituted membrane between a positive electrode containing sulfur and a conductive material and a lithium negative electrode, and an electrolyte is added to the positive electrode. In this case, the types and composition ratios of the sulfur, the conductive material and the binder are not limited thereto. The types of electrolytes include all of carbonate, ether, ester, and sulfone.

As the discharge reaction progresses, the anions of the polysulfide can not move toward the cathode due to the electric field formation, and only the lithium ions migrate to the hopping mode, which can suppress the side reaction with the lithium anode of the polysulfide, the active material loss, and the shuttle phenomenon.

However, if there is no liquor separation membrane, it is necessary to fabricate the cell with a low loading sulfur (loading ~ 1 mg / cm ² ) as an anode. In order to increase cell energy density, it is necessary to increase the amount of sulfur loading. There is a problem that it is inappropriate to use only the membrane. In addition, since the ion conduction method moves only lithium ions, there is a disadvantage that the ion conductivity is lower than that of the conventional one.

Accordingly, the present invention is an invention (see FIG. 2) in which the capacity of the lithium sulfur battery can be increased by increasing the sulfur loading amount of the lithium sulfur battery by further applying a reflux structure to the PFSA membrane. ionomer membranes were prepared by substituting Li ⁺ ions for H ⁺ ions in the SO ₃ H groups of the PFSA polymer membrane, which has a - (CF ₂ CF ₂ ) _x - (CF ₂ CF) _y backbone and a side chain of SO ₃ ^- At this time, the thickness is limited to a range of 10 to 100 mu m, preferably 20 to 50 mu m.

The polymer structure is preferably a polymer film having m = 0, 1, n = 0 to 5, x = 0 to 15, y = 0 to 2 and an equivalent weight of 400 to 2000 (see Table 1 below).

The mass ratio of the SO ₃ H functional groups of the PFSA polymer membrane satisfying the condition H ⁺ ions in the process for replacing the Li ⁺ PFSA membranes with LiOH solution of 1: in the range of 1000: 1 to 3 days.

 When the double membrane is used, the lube separator located on the anode side is filled with the electrolytic solution. Therefore, it is expected that the effect of increasing the amount of lithium ion by making polysulfide by dissolving the sulfur sufficiently in the high loading sulfur anode can be expected, Can block the anion of the polysulfide and move only the lithium ions sufficiently dissolved in the anode side to the cathode side, thereby improving problems such as side reactions and depletion of the electrolyte caused by the contact of the polysulfide and the lithium anode.

In order to overcome the above-mentioned problems, a separation membrane capable of sufficiently retaining the electrolyte solution on the anode side of the sulfur-conductive material of the lithium-sulfur battery was further applied, and a lithium-sulfur battery having the dual separation membrane was formed by using an ionomer membrane on the lithium- (See Fig. 4).

 As mentioned above, the lube separation membrane preferably has a porosity of 30 to 80% and a thickness of 30 to 300 μm, and is chemically stable to the organic solvent (electrolyte), and is located on the side of the sulfur anode. The non-woven fabric may be a glass fiber (see FIG. 6), a natural fiber (cellulose-based material), synthetic fibers (PE, PP, PTFE, PVDF) or the like. Also, in order to close the nonwoven fabric itself during thermal runaway, a coating layer may be provided on both sides of the nonwoven fabric separator membrane to provide a closure function at a temperature rise.

Hereinafter, the secondary battery of the present invention will be described in further detail with reference to the following examples, which should not be construed as limiting the scope of the claimed invention.

<Commercial PFSA Polymer Membrane  H + ions Li + Substitution>

 Using a Nafion 212 from DuPont, LiOH aqueous solution and ethanol were mixed in a 1: 1 mass ratio to prepare a solution in a beaker. The mixture was heated with stirring in a heating mantle at 80 ° C for 12 hours or more (see FIG. 5).

The higher the concentration of Li ⁺ ions in the solution, the easier the Li substitution in the membrane. In this embodiment, the Li ion replacement process was performed at a mass ratio of 1: 100 between the membrane and the solution. After the substitution reaction, the membrane was washed with distilled water and dried in a vacuum oven at 120 ° C. for one day to prepare a Li ion-exchanged ionomer membrane and vacuum-stored in a glove box.

< Ionomer With membrane Liquor  By applying a membrane Lithium sulfur  Manufacture of batteries>

A separation membrane for electrolyte solution is formed on the sulfur anode side, and a lithium ion exchange membrane and a lithium negative electrode are sequentially arranged to constitute a cell.

< Example  1 to 3>

Sulfur: Conductive material (VGCF): Mixed with binder (PVdF) = 70 wt%: 20 wt%: 10 wt%, slurry is cast on aluminum foil and dried at 80 degrees for 24 hours to produce a 14 pt positive electrode . The cathode is prepared in 16 pie size using lithium foil (100 micrometer thick). The ion-exchange membrane and the ionomer membrane were used simultaneously. The ionomer membrane was placed on the lithium foil and the electrolyte membrane was inserted thereinto. Then, 1M LiTFSI in TEGDME: DIOX (1: 1) (Refer to the use of the liner separator of Fig. 8).

< Comparative Example  1 to 2>

Sulfur: Conductive material (VGCF): Mixed with binder (PVdF) = 70 wt%: 20 wt%: 10 wt%, slurry is cast on aluminum foil and dried at 80 degrees for 24 hours to produce a 14 pt positive electrode . The cathode is prepared in 16 pie size using lithium foil (100 micrometer thick). The ionomer membrane was used as the separator, and an ionomer mebrane was placed on the lithium foil as the anode. Then, an anode electrode was placed on the lithium foil, and a coin cell was formed after the electrolyte 1M LiTFSI in TEGDME: DIOX (1: .

Capacitance characteristics of the examples according to the use of the lumped liquid separation membrane of the high loading sulfur (loading amount 5 mg / cm ² ) electrode are shown in the following graph and table 1.

Primary discharge capacity (mAh / g) Discharge voltage (V) Comparative Example 1 207 - Example 1 1086 2.09 Example 2 1015 2.05 Example 3 1075 2.07

The evaluation of the lifetime characteristics of the battery with and without the lube liquid separator is shown in the following graph and Table 2.

% Discharge capacity 1 cycle 30 cycles Comparative Example 2 11 7.7 Example 1 92 50

Loading amount 2 mg / cm ² In the case of the high loading sulfur electrode, the performance such as capacity and lifetime can not be exhibited in the absence of the liquefied separator, and it is confirmed that when the membrane and the lube separator are simultaneously applied, the lifetime performance is improved.

That is, the sulfur loading amount of the anode can be used from the low load to the high loading (~ 5 mg / cm ² ) by the laminar separation membrane, and the lysate separator can dissolve the lithium polysulfide from the sulfur anode, Since the lithium polysulfide does not move further toward the cathode by the ionomer membrane and only the lithium ions migrate to the cathode (see FIG. 7). By applying a high loading anode, it is possible to prevent side reactions with the lithium anode by shutting off the movement of the polysulfide and improving the energy density, and the shuttle phenomenon and the clone efficiency increase effect can be expected.

The pervaporation separation membrane of the present invention can be utilized as the four concrete examples of Fig.

Claims (10)

A lithium secondary battery comprising a sulfur anode, a lithium negative electrode and an ionomer membrane, wherein the positive electrode further comprises a lyophobic separation membrane made of a glass fiber nonwoven fabric. The secondary battery according to claim 1, wherein the ionomer membrane is a PFSA (perfluorosulfonic acid) polymer membrane represented by the following formula (1), and the H ⁺ ion of the -SO ₃ H group is substituted with Li ⁺ .
[Chemical Formula 1]

In the formula 1, m = 0, 1, n = 0 to 5, x = 0 to 15, y = 0 to 2, The secondary battery according to claim 1, wherein the lyophobic separation membrane is located on the anode side with respect to the ionomer membrane. The secondary battery according to claim 1, wherein the pore-isolation membrane has a porosity of 30 to 80% and a thickness of 30 to 300 탆. delete The secondary battery according to claim 1, wherein a heat insulating coating layer is present on both sides or a cross section of the luminal liquid separating film. The secondary battery according to claim 1, wherein the sulfur loading amount of the sulfur anode is 7 mg / cm ² or less. The secondary battery according to claim 6, wherein the heat insulating coating layer is made of polyolefin. The secondary battery according to claim 1, wherein a heat insulating coating layer is present in the luminal liquid separating film. The secondary battery according to claim 9, wherein the heat insulating coating layer is made of a polyolefin type.

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