WO2019059440A1 - Lithium secondary battery separator employing inorganic liquid electrolyte - Google Patents

Abstract

The present invention provides a lithium secondary battery separator characterized in that: (a) the lithium secondary battery comprises a lithium salt-containing inorganic liquid electrolyte in which sulfur dioxide is injected; and (b) coating layers made of a polymer material are formed on both surfaces of the separator. The present invention has greatly improved wettability of a separator in an electrolyte, ultimately and remarkably improves the charge/discharge characteristics and cycle lifespan of a lithium secondary battery and, particularly, can be stably operated even at room temperature, thereby being applicable to an energy storage device such as a portable electronic apparatus or an electric vehicle.

Description

Separator for Lithium Secondary Battery Employing Inorganic Liquid Electrolyte

The present invention relates to a separator for a lithium secondary battery employing an inorganic liquid electrolyte and a method for producing the same, and more particularly, to a separator for a lithium secondary battery employing an inorganic liquid electrolyte based on sulfur dioxide, To a separation membrane for a lithium secondary battery.

In recent years, the development of the technology has widened the use of batteries not only for portable electronic devices but also for power sources for electric vehicles and power storage devices for medium and large-sized energy storage systems. Accordingly, researches on lithium secondary batteries capable of satisfying various demands have been carried out. In particular, safety of batteries is an important issue. Specifically, the separator, which is one of the key components of the lithium secondary battery according to the necessity of a high output and a large capacity battery in an electric vehicle or a medium and large-sized energy storage system including a light weight and miniaturization of a portable electronic device, A material that can be improved is required.

Generally, the lithium secondary battery includes an electrode assembly composed of an anode, a cathode, and a separator disposed between the anode and the cathode, and an electrolyte containing a lithium salt. In such a lithium secondary battery, charging and discharging proceed while repeating the process of inserting and separating lithium ions from each other between the positive electrode and the negative electrode. At this time, since the separation membrane includes micro pores, it provides a path through which lithium ions move through the pores, and physically separates the positive and negative electrodes to perform a function of being electrically insulated. On the other hand, as the thickness of the separator is reduced, the distance between the anode and the cathode is reduced, which is advantageous in terms of high output and high energy density of the battery. However, since the pore size must be very small in order to maintain the insulating property, . On the other hand, when a thick separator is used, the volume of the electrode assembly itself may become large and the volume-capacity may be deteriorated.

In addition, even if the physical properties are satisfied, if the wettability of the electrolytic solution can not be secured, the electrochemical characteristics may be deteriorated. If the electrode is partially depleted due to the local depletion of the electrolyte or if the reaction is locally concentrated, lithium metal may precipitate and there is a risk of resin dendrite growth of lithium. Therefore, the electrolyte must be rapidly permeated into the separation membrane, and the state must be maintained uniformly even after wet. In addition, the interface between the electrode and the separation membrane and the electrolyte must be maintained firmly to achieve excellent charge / discharge characteristics and cycle life.

However, the polyolefin separator commercialized for lithium secondary batteries is excellent in chemical stability and mechanical strength, but has a disadvantage in that it has poor affinity with electrolytic solution, especially inorganic liquid electrolyte, and has poor thermal stability. Therefore, a system requiring high output and safety It is necessary to improve it. For this purpose, an example using a composite membrane in which a polyolefin separator is coated with a polymer material or ceramic has been known. However, most of them have been developed for a lithium secondary battery employing a non-aqueous electrolyte in which a lithium salt is added to a mixed solvent of ethylene carbonate or propylene carbonate which is a high-dielectric solvent and dimethyl carbonate or diethyl carbonate which is a low-viscosity solvent, And a cycle life is limited, and a lithium secondary battery employing an inorganic liquid electrolyte having excellent wettability of a separator has not been reported.

Therefore, the present inventors have found that when the surface of the separator is coated with a polymer material, the wettability of the separator to the electrolyte is greatly improved, and the sulfur dioxide-based The charging / discharging characteristics and the cycle life of the lithium secondary battery including the inorganic liquid electrolyte and the separator can be remarkably improved. The present invention has been accomplished based on this finding.

(Prior art document)

(Patent Literature)

Patent Document 1. Korean Patent Publication No. 10-2016-0109227

Patent Document 2. Korean Patent No. 10-1470696

Patent Document 3: JP-A-2016-081606

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and it is an object of the present invention to provide a lithium secondary battery, which is capable of significantly improving the wettability of a separator for an inorganic liquid electrolyte and significantly improving charge / discharge characteristics and cycle life, The present invention provides a separator for a lithium secondary battery employing a sulfur dioxide-based inorganic liquid electrolyte capable of being stably driven even when the separator is used.

According to an aspect of the present invention, there is provided a separation membrane for a lithium secondary battery, comprising: (a) the lithium secondary battery includes a lithium salt-containing inorganic liquid electrolyte into which sulfur dioxide is injected; (b) a separation layer for a lithium secondary battery, wherein a coating layer of a polymer material is formed on both sides of the separation layer.

The lithium salt is characterized by at least one selected from the group consisting of LiAlCl ₄ , LiGaCl ₄ , Li ₂ CoCl ₄ , Li ₂ NiCl ₄ , Li ₂ CuCl ₄ , Li ₂ MnCl ₄ , Li ₂ ZnCl ₄ and LiAlBr ₄ .

The sulfuric acid-impregnated lithium salt-containing inorganic liquid electrolyte is characterized by being LiAlCl ₄ -3 SO ₂ .

(b) the separator is a separator selected from the group consisting of polyethylene, polypropylene, polyester, polyamide, polyimide, polycarbonate, polyacetal, polyether sulfone, polyetheretherketone, polyphenylene oxide, polyphenylene sulfide Or more porous substrate.

The polymer material may be selected from the group consisting of polyethylene oxide, polyvinyl pyrrolidone, polyacrylonitrile, polyvinylidene fluoride, polymethylmethacrylate, polyvinyl acetate, ethylene vinyl acetate copolymer, cellulose acetate, carboxymethylcellulose and polyimide And at least one selected from the group consisting of

The coating layer has a thickness of 1 to 10 mu m.

The present invention also provides a lithium secondary battery including an anode, an anode, an electrode assembly composed of a porous separator disposed between the anode and the cathode, and an electrolyte, wherein the porous separator has a coating layer of a polymer material on both sides, Wherein the electrolyte is a lithium salt-containing inorganic liquid electrolyte into which sulfur dioxide is injected.

The present invention also provides an energy storage element comprising the lithium secondary battery.

According to the present invention, the polymer material is coated on the porous separator for a lithium secondary battery, and the wettability of the separator to the electrolyte is greatly improved by employing an inorganic liquid electrolyte based on sulfur dioxide, and ultimately the charge / Life can be remarkably improved. In particular, it can be stably driven at a low temperature, so that it can be applied to energy storage devices such as portable electronic devices or electric vehicles.

1 is a scanning electron microscope (SEM) image of a membrane according to Example 1 (PEO coated PE separator), PE separator and glass fiber filter of the present invention.

2 is a photograph of the wettability of the separator according to Example 1 (PEO coated PE), Comparative Example 1 (PE) and Comparative Example 2 (Glass fiber filter) of the present invention.

3 is a graph showing characteristics of an initial cell of a lithium secondary battery manufactured from Example 1 and Comparative Examples 1 to 3 of the present invention.

4 is a graph showing the charge-discharge cycle capacity and lifetime characteristics of a lithium secondary battery manufactured from Example 1 and Comparative Examples 1 to 3 of the present invention.

5 is a graph showing the coulomb efficiency in the charge and discharge cycles of the lithium secondary battery manufactured from Example 1 and Comparative Examples 1 to 3 of the present invention.

6 is a graph showing the relationship between the initial discharge capacity of the lithium secondary battery fabricated in Example 1 of the present invention and the initial discharge capacity of the lithium secondary battery according to Example 1 (75 wt%, 50 wt%, 25 wt%, 10 Weight%) A graph comparing initial discharge capacities of the produced lithium secondary batteries.

FIG. 7 is a graph comparing the initial discharge capacities of the lithium secondary batteries manufactured in Example 2 and Comparative Examples 4 to 7, according to Examples and Comparative Examples 1 and 2 of the present invention.

8 is a graph showing characteristics of an initial cell by driving the lithium secondary battery fabricated from Example 1 and Comparative Example 3 of the present invention at a low temperature.

Hereinafter, a separator for a lithium secondary battery employing a sulfur dioxide-based inorganic liquid electrolyte according to the present invention and a lithium secondary battery including the same will be described in detail with reference to the accompanying drawings.

The present invention relates to a separator for a lithium secondary battery, comprising: a) the lithium secondary battery comprises a lithium salt-containing inorganic liquid electrolyte into which sulfur dioxide is injected; (b) a separation layer for a lithium secondary battery, wherein a coating layer of a polymer material is formed on both sides of the separation layer.

Conventionally, the electrolyte for a lithium secondary battery is mainly composed of a non-aqueous electrolyte containing lithium salt and lithium, and non-aqueous electrolyte, organic solid electrolyte and inorganic solid electrolyte are used as the non-aqueous electrolyte. Examples of the nonaqueous electrolytic solution include nonaqueous electrolytes such as N-methyl-2-pyrrolidinone, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, dimethylformamide, dimethylacetamide, dimethylsulfoxide or tetrahydrofuran A non-magnetic organic solvent is used.

As the organic solid electrolyte, a polymer including an ionic dissociation group such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphoric acid ester polymer, and a polyvinyl alcohol is used.

As the inorganic solid electrolyte, a nitride, a halide or a sulfate of lithium is used.

In particular, a lithium salt such as LiCl or LiPF ₆ is added to a mixed solvent of ethylene carbonate or propylene carbonate which is a high-dielectric solvent and dimethyl carbonate or diethyl carbonate which is a low-viscosity solvent as a non-aqueous electrolyte, However, since the charge and discharge characteristics and the cycle life are limited, the need for development of new electrolytes has been steadily raised.

Accordingly, in the present invention, the wettability of the separator, which is a core component of the lithium secondary battery, is further improved by employing the inorganic salt electrolyte containing lithium salt into which sulfur dioxide is injected as a novel type of electrolyte.

At this time, at least one selected from the group consisting of LiAlCl ₄ , LiGaCl ₄ , Li ₂ CoCl ₄ , Li ₂ NiCl ₄ , Li ₂ CuCl ₄ , Li ₂ MnCl ₄ , Li ₂ ZnCl ₄ and LiAlBr ₄ is used as the lithium salt , And LiAlCl ₄ -3 SO ₂ in the form of sulfur dioxide is more preferably used as an inorganic liquid electrolyte.

Conventionally, there are many examples in which a commercialized glass fiber filter is used as a separator for a lithium secondary battery. However, since the lithium secondary battery employing the glass fiber filter has a disadvantage that the capacity retention rate drops sharply after several decades, ) The separation membrane may be formed of a polyolefin (polyethylene or polypropylene), a polyester, a polyamide, a polyimide, a polycarbonate, a polyacetal, a polyether sulfone, a polyether ether ketone, a polyphenylene oxide, May be used as the porous substrate.

However, since the separation membrane (porous substrate) has a disadvantage in that the wettability of the inorganic salt liquid electrolyte containing lithium salt injected with sulfur dioxide is somewhat low, the present invention is not limited thereto. In the present invention, both sides of the separation membrane are coated with a polymer material, Thereby greatly improving the wettability of the lithium salt-containing inorganic liquid electrolyte.

Examples of the polymer material include polyethylene oxide, polyvinyl pyrrolidone, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, polyvinyl acetate, ethylene vinyl acetate copolymer, cellulose acetate, carboxymethyl cellulose and polyimide At least one selected from the group consisting of polyethylene oxide is preferably used, and polyethylene oxide is more preferably used.

The coating layer of the polymer material preferably has a thickness of 1 to 10 mu m. If the thickness of the coating layer is less than 1 mu m, the affinity with the inorganic liquid electrolyte is poor and the wettability may deteriorate. If the thickness of the coating layer is less than 10 mu m , The initial discharge capacity may be lowered.

The conventional lithium secondary battery includes an anode, a cathode, an electrode assembly composed of a porous separator disposed between the anode and the cathode, and an electrolyte. In the present invention, an anode, a cathode, And the electrolyte is a lithium salt-containing inorganic liquid electrolyte into which sulfur dioxide is injected. The lithium secondary battery according to any one of claims 1 to 3, wherein the porous separator has a porous coating layer on both surfaces thereof. To provide lithium secondary batteries.

Here, the porous separator (base material), the polymer material coated on both sides of the porous separator, and the lithium salt-containing inorganic liquid electrolyte into which the sulfur dioxide is injected are as described above, and thus the further explanation is omitted.

Also, the lithium secondary battery according to the present invention ultimately significantly improves the charge / discharge characteristics and cycle life of the lithium secondary battery, and can be stably driven even at a low temperature, Therefore, the present invention can provide an energy storage device including the lithium secondary battery.

Hereinafter, specific examples and comparative examples will be described in detail.

(Example 1)

A polymer solution (90 wt%) in which polyethylene oxide (PEO) powder was dissolved in acetonitrile solvent was coated on both sides of a commercially available porous polyethylene (PE) separator (thickness 16 탆) by casting method, PEO coated PE separator was prepared by drying for more than 2 hours. The thickness of the coating layer was adjusted to 6 ㎛. As a lithium salt-containing inorganic liquid electrolyte in which sulfur dioxide was injected, a battery (2032 coin type) was produced by using LiAlCl ₄ -3 SO _2. Graphite was used as a working electrode and lithium metal was used as a counter electrode.

(Example 2)

A battery (2032 coin type) was fabricated in the same manner as in Example 1 except that the thickness of the coating layer was adjusted to 2 탆.

(Comparative Example 1)

A cell (2032 coin type) was fabricated in the same manner as in Example 1, except that a PE separator without a PEO coating was used.

(Comparative Example 2)

A cell (2032 coin type) was fabricated in the same manner as in Example 1 except that a glass fiber filter not coated with PEO was used as a separator.

(Comparative Example 3)

Example 1 was repeated except that 1 M LiPF ₆ was added to a mixed organic solvent (1: 1, volume ratio) of ethylene carbonate (EC) / dimethyl carbonate (DMC) instead of the inorganic liquid electrolyte of the above- A battery (2032 coin type) was produced in the same manner.

(Comparative Examples 4 to 7)

Except for the difference in thickness of the coating layer (12 占 퐉 (Comparative Example 4), 20 占 퐉 (Comparative Example 5), 28 占 퐉 (Comparative Example 6), and 34 占 퐉 (Comparative Example 7) A battery (2032 coin type) was fabricated.

1 shows a scanning electron microscope (SEM) image of a separation membrane according to Example 1 (PEO coated PE separator), Comparative Example 1 (PE separator) and Comparative Example 2 (Glass fiber filter) It can be confirmed that polyethylene oxide (PEO) was uniformly coated on the porous PE separator according to Example 1 of the present invention.

FIG. 2 is a photograph showing the wettability of the membrane according to Example 1 (PEO coated PE), Comparative Example 1 (PE) and Comparative Example 2 (Glass fiber filter) of the present invention. As shown in FIG. 2, in Example 1 and Comparative Examples 1 and 2, opaque white faces are shown in a state where they are not in contact with the electrolytic solution. However, it can be seen that the electrolyte solution is impregnated into the electrolyte solution at the instant of contact with the electrolytic solution, while the electrolyte solution is floated on the electrolyte solution layer in the case of Comparative Example 1 without color change. In addition, when the time elapsed to 1 day, 7 days, and 21 days, Example 1 and Comparative Example 2 did not melt or shrink without reacting with the electrolytic solution, while in the case of Comparative Example 1, It can be seen that it is floating without being impregnated.

In addition, Figure 3 in Example 1 and Comparative Examples 1 to 3 was it exhibited an initial battery characteristics of the lithium secondary battery manufactured in a graph bar, amperage 20 mA g from the present ^invention result of driving the each cell to ^1, the arms the initial capacity of the lithium secondary battery employing the liquid electrolyte is not as big a Comparative example 1, Comparative example 2 and carrying respective capacitance differences in ^{356.7 mAh g -1, 354.2 mAh g} -1, 356.9 mAh g -1 in example 1 . As the graphite inserts and removes lithium, the flat voltage appears, and the deviation between the three cells is not so large.

However, in the lithium secondary battery of Comparative Example 3 employing the organic liquid electrolyte, the initial capacity is 340.9 mAh g ^-1 , which is much smaller than that of the inorganic liquid electrolyte. This is because unlike the case of the inorganic liquid electrolyte, the flat voltage is not clearly distinguished, and when a conventional organic liquid electrolyte is employed, the coating layer of the polymer material acts as a resistance of lithium movement, and the resistance of the battery increases.

FIG. 4 is a graph showing the charge-discharge cycle capacity and lifetime characteristics of the lithium secondary battery manufactured from Example 1 and Comparative Examples 1 to 3 of the present invention. In Example 1, since the separation membrane having a thinner thickness and improved electrolyte wettability is used, an inorganic liquid electrolyte based on sulfur dioxide is employed, so the conductivity of lithium ion is increased and the internal resistance is lowered, thereby improving the life characteristic. It can be seen that after 20 cycles, the capacity retention rate rapidly decreases in the case of the comparative example 1, and the capacity retention rate is the lowest in comparison with the comparative example 2 and the example 1. [ The capacity was 221.1 mAh g- ^{1 on} a 47-cycle basis with a 61.9% capacity reduction over the first cycle. In the case of Comparative Example 2, the capacity retention rate rapidly dropped after 30 cycles. The capacity retention rate of Comparative Example 2 based on 30 cycles was maintained at a capacity of 350.5 mAh g ^{-1 and} a capacity of 98.9% based on the first cycle However, the capacity of 115.4 mAh g ^{-1 on} a 100-cycle basis shows a significant reduction in capacity to 32.5% of the initial cycle. On the other hand, from Example 1, the sulfuric acid-based inorganic liquid electrolyte was used to maintain a capacity of 99.8% as compared with the first cycle at a capacity of 356.5 mAh g ^-1 at 100 cycles, and at the same time, a polymer material The performance of the battery can be remarkably improved. In addition, in the case of the lithium secondary battery manufactured from Comparative Example 3, the capacity developed over time seems to be increased, but it can be said that the capacity that has not been developed due to a high initial resistance factor is recovered.

In addition, FIG. 5 is a graph showing the coulombic efficiency in the charge / discharge cycle of the lithium secondary battery manufactured in Example 1 of the present invention and Comparative Examples 1 to 3. In Example 1, the efficiency was 97 to 98.8% While Comparative Example 1 is maintained at a low efficiency of 94.5 to 96%, while Comparative Example 2 shows a gradual decrease to 95%. Therefore, it can be seen that the lithium secondary battery manufactured from Example 1 has remarkably improved battery performance as compared with the lithium secondary battery manufactured from Comparative Example 1 and Comparative Example 2. [ Also, in the case of the lithium secondary battery manufactured from Comparative Example 3, as shown in FIGS. 3 and 4, the capacity is gradually increased due to the initial low capacity expression, and the efficiency is maintained to be high.

6 shows the initial discharge capacity of the lithium secondary battery fabricated in Example 1 of the present invention and the initial discharge capacity of the lithium secondary battery according to Example 1 (75 wt%, 50 wt%, 25 wt% , 10% by weight). The graph of the initial discharge capacity of the produced lithium secondary battery is shown in FIG. As shown in FIG. 6, the lithium secondary battery (having a concentration of the polymer solution for coating of 90 wt%) manufactured from Example 1 of the present invention and the remaining lithium produced by varying the concentration of the polymer solution for coating It was found that the concentration of the polymer solution for the coating was not a large variable affecting the initial discharge capacity. However, in the lithium secondary battery manufactured in Example 1, It can be seen that the initial discharge capacity of the battery is relatively high.

7 is a graph showing the initial discharge capacities of the lithium secondary batteries fabricated in Example 1 and Comparative Example 1 and in Example 2 and Comparative Examples 4 to 7 in different coating thicknesses, As can be seen, in the case of the discharge capacity to be developed, although the difference in the average capacity is not large, it is confirmed that the capacity gradually decreases from the case where the thickness of the coating layer exceeds 10 탆, so that the thickness of the coating layer is preferably 1 탆 to 10 탆 Do.

8 is a graph showing the characteristics of an initial cell by driving the lithium rechargeable battery fabricated from Example 1 and Comparative Example 3 at a low temperature. As the driving temperature is gradually lowered, a comparative example 3 shows that the capacity of the lithium secondary battery fabricated from Example 3 is lowered. The results at 10 ℃ to -10 ℃ shown in Figure 8, in capacity of 362.7 mAh g ^-1 which is the lithium produced from example 1 was used for the inorganic liquid electrolyte battery 357.6 mAh g ^{- 1} of about 98.6% while the capacitance is maintained, in the lithium secondary battery produced from the comparative example 3 employing organic liquid electrolyte 334.4 mAh g in capacity of 350.3 mAh g ^{-1 -} it can be seen that significantly less to about 95.4% by ^one. Therefore, it can be confirmed that the lithium secondary battery employing the inorganic liquid electrolyte maintains a higher ionic conductivity than the lithium secondary battery employing the organic liquid electrolyte even at a low temperature, thereby maintaining the capacity for expressing the lithium secondary battery. It can be seen that the separation membrane having the polymer coating layer formed therein as in Example 1 is used and the lithium secondary battery employing the inorganic liquid electrolyte can be stably driven at a low temperature.

Therefore, according to the present invention, since the polymer material is coated on the porous separator for a lithium secondary battery and the inorganic liquid electrolyte based on sulfur dioxide is employed, the wettability of the separator to the electrolyte is greatly improved, and ultimately, The cycle life is remarkably improved, and it can be stably driven even at a low temperature, so that it can be applied to an energy storage device such as a portable electronic device or an electric automobile.

Claims (8)

1. In the separator for a lithium secondary battery,

   (a) the lithium secondary battery comprises a lithium salt-containing inorganic liquid electrolyte into which sulfur dioxide is injected;

   (b) a separator for a lithium secondary battery, wherein a coating layer of a polymer material is formed on both sides of the separator.
2. The lithium secondary battery according to claim 1, wherein the lithium salt is at least one selected from the group consisting of LiAlCl ₄ , LiGaCl ₄ , Li ₂ CoCl ₄ , Li ₂ NiCl ₄ , Li ₂ CuCl ₄ , Li ₂ MnCl ₄ , Li ₂ ZnCl _4, and LiAlBr ₄ Wherein the separator is a lithium secondary battery.
3. The separator for a lithium secondary battery according to claim 1, wherein the sulfuric acid-impregnated lithium salt-containing inorganic liquid electrolyte is LiAlCl ₄ -3 SO ₂ .
4. The method of claim 1, wherein (b) the separation membrane is formed of a material selected from the group consisting of polyethylene, polypropylene, polyester, polyamide, polyimide, polycarbonate, polyacetal, polyethersulfone, polyetheretherketone, polyphenylene oxide, Wherein the porous substrate is at least one porous substrate selected from the group consisting of a porous substrate and a porous substrate.
5. The method of claim 1, wherein the polymeric material is selected from the group consisting of polyethylene oxide, polyvinyl pyrrolidone, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, polyvinylacetate, ethylene vinyl acetate copolymer, cellulose acetate, carboxy Methyl cellulose, and polyimide. The separator for a lithium secondary battery according to claim 1,
6. The separator for a lithium secondary battery according to claim 1, wherein the coating layer has a thickness of 1 to 10 mu m.
7. An electrode assembly comprising a positive electrode, a negative electrode, a porous separator disposed between the positive and negative electrodes, and a lithium secondary battery comprising an electrolyte,

   Wherein the porous separator is coated on both sides with a polymer material, and the electrolyte is an inorganic liquid electrolyte containing a lithium salt into which sulfur dioxide is injected.
8. An energy storage element comprising the lithium secondary battery according to claim 7.

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