KR102024889B1 - Semi-Interpenetrating Polymer Networks Polymer Electrolyte and All-Solid-State Battery comprising The Same - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polymer electrolyte applied to an all-solid-state battery using lithium as a negative electrode, and more particularly, includes a polyethylene oxide (PEO) -based polymer and a lithium salt, and is crosslinked by a crosslinkable monomer. To a semi-penetrating polymer network (semi-IPN) structure. When the high-strength polymer electrolyte of the present invention is applied to an all-solid-state battery, the ionic conductivity can be maintained while maintaining the current level, and the micro-short phenomenon caused by lithium dendrite growth can be alleviated.

Description

Semi-Interpenetrating Polymer Networks Polymer Electrolyte and All-Solid-State Battery comprising The Same

The present invention relates to a solid polymer electrolyte of an all-solid-state battery in which a lithium-based material is applied as a negative electrode, and more particularly, includes a polyethylene oxide (PEO) -based polymer and a lithium salt. The present invention relates to a solid polymer electrolyte which is cross-linked to form a semi interpenetrating polymer network (semi-IPN) structure.

In recent years, miniaturization, light weight, thinness, and portability of portable cordless products such as portable audio devices, multimedia players, mobile phones, smart phones, notebook personal computers, tablet devices, video cameras, and the like are required. In addition, from the viewpoint of environmental problems such as air pollution and increased carbon dioxide, the development and practical use of hybrid vehicles and electric vehicles are in progress. Accordingly, these electronic devices and electric vehicles require excellent secondary batteries having characteristics such as high efficiency, high output, high energy density, and light weight, and various secondary batteries have been developed and researched as secondary batteries having such characteristics. It is done.

A rechargeable battery that is capable of being charged and discharged has a structure in which electrical direct contact between the positive electrode and the negative electrode is usually prevented by separating the positive electrode (cathode) and the negative electrode (negative electrode, anode) from the porous polymer film containing an organic electrolyte solution. It is. To date, V ₂ O ₅ , Cr ₂ O ₅ , MnO ₂ , TiS _2, and the like are known as positive electrode active materials of this nonaqueous electrolyte secondary battery, and LiCoO is a 4V class positive electrode active material in lithium ion batteries that are currently commercialized. ₂ , LiMn ₂ O ₄ , LiNiO ₂ and the like are used. On the other hand, as a negative electrode, much examination has been made about alkali metals including metal lithium. In particular, lithium is considered to be an ideal cathode material because it has a very high theoretical energy density (weight capacity density 3861 mAh / g) and a low charge and discharge potential (-3.045 V vs. SHE).

As the electrolyte, for example, a lithium salt dissolved in a non-aqueous organic solvent is used, which has good ion conductivity. During charging, lithium ions move from the positive electrode to the negative electrode, and during discharge, lithium ions move in the reverse direction and return from the negative electrode to the positive electrode.

However, in order to use lithium metal as a negative electrode, there are the following problems. During charging, dendritic lithium (lithium dendrite) precipitates on the lithium surface of the negative electrode. When charge and discharge are repeated, the dendrite lithium grows, peeling from lithium metal occurs, and the cycle characteristics are lowered. In the worst case, it grows enough to break through the separator, causing a short circuit of the battery and causing battery ignition. Therefore, in order to use lithium metal as a negative electrode, it is necessary to solve the problem of lithium dendrites.

Accordingly, studies have been made on various carbon materials capable of occluding and releasing lithium, metals such as aluminum, alloys, and oxides. However, using these negative electrode materials is effective in suppressing the growth of lithium dendrites, but on the other hand, there is a problem of lowering the capacity as a battery.

For example, when inorganic fillers were applied in an attempt to suppress the growth of dendrites, dendrite growth could be delayed (J. Mater. Chem. A, 2013, 1, 4949-4955), and polymers made of polystyrene blocks ( Block copolymers, including polystyrene blocks, also increased strength significantly and were effective in inhibiting dendrites (Nano Lett., Vol. 9, No. 3, 2009).

However, in the case of the inorganic filler (Filler), it is necessary to add a large amount to suppress the dendrites, in which case the energy density is lowered and the ion conductivity is also lowered. Block copolymers also have very low ionic conductivity, and are brittle in nature, making it difficult to apply to batteries.

Korean Unexamined Patent Publication No. 2013-0142224 "Solid Polymer Electrolyte, Manufacturing Method Thereof, and Lithium Battery Including It"

J. Mater. Chem. A, 2013, 1, 4949-4955 Nano Lett. Vol. 9, No. 3, 2009

In order to solve the above problems, the inventors of the present invention have studied various means for more firmly suppressing the growth of dendrites, without degrading the performance of the battery. As a result, the weight average molecular weight (Mw) is relatively high. (ethylene oxide): Cross-linking the PEO-based polymer was confirmed to physically suppress the generation of lithium dendrites and completed the present invention.

Accordingly, an object of the present invention is to provide an all-solid-state battery having an effect of inhibiting dendrites by improving mechanical strength while maintaining current levels of ion conductivity.

The present invention is devised to achieve the above object, the weight average molecular weight (Mw) of 1,000,000 to 8,000,000 polyethylene oxide-based polymer and lithium salt, the polyethylene oxide-based polymer is-(CH ₂ -CH ₂ -O ) —Crosslinked by a crosslinkable monomer comprising repeating units and alkylene unsaturated bonds within the range of 2 to 8 polymerizable at both ends to form semi-IPN (semi-Interpenetrating Polymer Networks) Provided is a polymer electrolyte for an all-solid-state battery.

When the solid polymer electrolyte of the present invention is applied to an all-solid-state battery, the ionic conductivity can be maintained while maintaining the current level, and the micro-short phenomenon caused by lithium dendrite growth can be alleviated. In addition, the resistance of the battery as a whole may be reduced by reducing the thickness of the solid polymer electrolyte, thereby improving electrical characteristics.

1 is data showing discharge capacity and coulombic efficiency for each charge / discharge cycle of an all-solid-state battery including the polymer electrolyte membrane of Comparative Example 2. FIG.
FIG. 2 is data showing discharge capacity and coulombic efficiency according to charge / discharge cycles of an all-solid-state battery including the polymer electrolyte membrane of Example 1. FIG.

The solid polymer electrolyte applied to the all-solid-state battery of the present invention includes a polyethylene oxide (PEO) -based polymer and a lithium salt, and is cross-linked by a crosslinkable monomer to form a semi-interpenetrating polymer. Networks: form a semi-IPN) structure.

The semi-interpenetrating polymer network structure of the present invention can increase the strength of the solid polymer electrolyte, and the higher the strength, the more physically it can suppress the generation of lithium dendrites on the electrode surface. However, if the semi-interpenetrating polymer network structure is too excessively formed, the ion conductivity of lithium ions is lowered. In the present invention, while maintaining the ion conductivity of lithium ions, but in order to increase the mechanical strength, the ion conductive polymer is formed of a high-density semi-interpenetrating polymer network structure using a relatively large weight average molecular weight (Mw), lithium It is characterized by selecting and using a specific polymer so that ions can move smoothly. Hereinafter, each component of the polymer electrolyte of the present invention will be described in detail.

Ion conductive polymer

The present invention applies a polyethylene oxide (Poly (ethylene oxide): PEO) -based polymer (derivative) as the ion conductive polymer.

At this time, the weight average molecular weight (Mw) of the polyethylene oxide polymer is preferably 1,000,000 to 8,000,000. In the related art, in consideration of a decrease in the ion conductivity of a solid electrolyte, a relatively low molecular weight has been selected and used as the weight average molecular weight (Mw) of the ion conductive polymer is 3,000 to 9,000. However, the present invention is characterized by applying a large polymer as compared to the weight average molecular weight (Mw) used in the prior art, which contributes to the formation of a semi-penetrating polymer network structure to a higher density. Therefore, the mechanical strength of the solid polymer electrolyte can be further improved.

In addition, since the polyethylene oxide polymer according to the present invention has ethylene glycol (Ethylene glycol) chain having a high donor number, it is possible to suppress ionic conductivity decrease due to crosslinking as much as possible.

Crosslinkable  Monomer

The crosslinkable monomer according to the present invention crosslinks the polyethylene oxide polymer to form a semi-interpenetrating polymer network structure.

For such crosslinking, the crosslinkable monomer may be a bifunctional or more than one polyfunctional monomer, and includes an alkylene within a range of 2 to 8 including — (CH ₂ —CH ₂ —O) — repeating units and polymerizable at both ends. It is preferable to include an unsaturated bond. The alkylenically unsaturated bond is a hydrocarbon group including at least one carbon-carbon double bond or triple bond, an ethenyl group, 1-propenyl group, 2-propenyl group, 2-methyl-1-propenyl group, 1-butenyl group, 2-butenyl group, ethynyl group, 1-propynyl group, 1-butynyl group, 2-butynyl group, and the like, but are not limited thereto. Such alkylenically unsaturated bonds act as a crosslinking point to crosslink the polyethylene oxide polymer through a polymerization process to form a crosslinking network structure.

For example, the crosslinkable monomer may include polyethylene glycol diacrylate (PEGDA), polyethylene glycol dimethacrylate (PEGDMA), polypropylene glycol diacrylate (Poly ( propylene glycol diacrylate: PPGDA), poly (propylene glycol) dimethacrylate (PPGDMA) and combinations thereof, and preferably polyethylene glycol diacrylate (PEGDA). .

In addition, it is preferable to form a cross-penetrating polymer network structure suitable for the purpose of the present invention that the crosslinkable monomer is contained in an amount of 5 to 50 wt% based on the weight of the polyethylene oxide-based polymer.

There is no restriction | limiting in particular in the method of the said crosslinkable monomer crosslinking between the said polyethylene oxide type polymers, Preferably, after adding a thermal initiator, it can crosslinking, maintaining suitable temperature conditions. In this case, benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN), and the like may be applied as thermal initiators.

Lithium salt

In the polymer electrolyte for solid-state batteries of the present invention, lithium ions can penetrate into the semi-interpenetrating polymer network structure and move freely. At this time, it is possible to operate a basic lithium battery as a source of lithium ions, and any lithium salt can be used as long as it is commonly used in lithium batteries, but preferably LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, LiSCN, LiC (CF ₃ SO ₂ ) ₃ , (CF ₃ SO ₂ ) ₂ NLi, (FSO ₂ ) ₂ NLi, chloroborane lithium, lower aliphatic lithium carbonate, lithium 4-phenyl borate, imide and combinations thereof, more preferably (CF Lithium bis (trifluoromethane sulfonyl) imide (LiTFSI), represented by ₃ SO ₂ ) ₂ NLi, is possible.

In order to secure the practical performance of the lithium salt, the lithium salt, the EO: Li molar ratio of the polyethylene oxide-based polymer and the lithium salt may be appropriately selected and applied within the range of 30: 1 to 3: 1 for the purpose of the present invention. . When the concentration of the lithium salt is included in the above range, since the electrolyte has an appropriate conductivity and viscosity, excellent electrolyte performance can be exhibited, and lithium ions can be effectively moved.

The method for producing a solid polymer electrolyte having a semi-interpenetrating polymer network structure of the present invention is not limited in the present invention, and a mixing and molding process may be used by a wet or dry method as is known. In addition, the thickness of the solid polymer electrolyte of the present invention may be selectively prepared within the range of 5 ~ 50 ㎛.

Hereinafter, an all-solid-state battery including the solid polymer electrolyte of the present invention interposed between a positive electrode and a negative electrode, and an all-solid-state battery forming a semi-interpenetrating polymer network structure by the crosslinkable monomer will be described in detail.

All solid  battery

The electrode active material may be a positive electrode active material when the electrode proposed in the present invention is a positive electrode, a negative electrode active material when the negative electrode. At this time, each electrode active material can be any active material applied to a conventional electrode, and is not particularly limited in the present invention.

The positive electrode active material may vary depending on the use of the lithium secondary battery, and the specific composition uses a known material. For example, any one selected from the group consisting of lithium-phosphate-iron compound, lithium cobalt-based oxide, lithium manganese-based oxide, lithium copper oxide, lithium nickel-based oxide and lithium manganese composite oxide, lithium-nickel-manganese-cobalt-based oxide And lithium transition metal oxides. More specifically, in the lithium metal phosphate represented by Li _{1 + a} M (PO _4-b ) X _b , M is at least one selected from metals of Groups 2 to 12, and X is F, S and As at least 1 type selected from N, it is preferable that they are -0.5 <= <= 0.5, and 0 <= b <= 0.1.

In this case, the negative electrode active material may be one selected from the group consisting of lithium metal, lithium alloy, lithium metal composite oxide, lithium-containing titanium composite oxide (LTO), and combinations thereof. At this time, the lithium alloy may be an alloy consisting of lithium and at least one metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al and Sn. In addition, the lithium metal composite oxide is any one metal (Me) oxide (MeO _x ) selected from the group consisting of lithium and Si, Sn, Zn, Mg, Cd, Ce, Ni, and Fe, for example, Li _x Fe ₂ O ₃ (0 <x ≦ 1) or Li _x WO ₂ (0 <x ≦ 1).

In this case, if necessary, a conductive material or a polymer electrolyte may be further added to the active material, and examples of the conductive material may include nickel powder, cobalt oxide, titanium oxide, and carbon. Examples of the carbon include any one selected from the group consisting of Ketjen black, acetylene black, furnace black, graphite, carbon fiber and fullerene, or one or more thereof.

In the manufacture of an all-solid-state battery, an electrode and a solid electrolyte are prepared in a powder state, and then put into a predetermined mold, followed by a dry compression process, or a slurry composition including an active material, a solvent, and a binder, and then coated and dried. It is manufactured through a slurry coating process. The production of the all-solid-state battery having the above configuration is not particularly limited in the present invention, and a known method can be used.

In one example, a solid electrolyte is disposed between the positive electrode and the negative electrode and then compression molded to assemble the cell. The assembled cell is installed in an outer packaging material and then sealed by heat compression. As the exterior material, laminate packs such as aluminum and stainless steel and cylindrical or rectangular metal containers are very suitable.

The method of coating the electrode slurry on the current collector is a method of distributing the electrode slurry on the current collector and then uniformly dispersing it using a doctor blade or the like, die casting, comma coating. And screen printing. In addition, the electrode slurry may be bonded to the current collector by pressing or lamination after molding on a separate substrate. At this time, by adjusting the concentration of the slurry solution, or the number of coating, it is possible to control the coating thickness to be finally coated.

The drying process is a process of removing the solvent and water in the slurry to dry the slurry coated on the metal current collector, and may vary depending on the solvent used. In one example, it is carried out in a vacuum oven at 50 ~ 200 ℃. As a drying method, the drying method by irradiation with warm air, hot air, low humidity wind, vacuum drying, (far) infrared rays, an electron beam, etc. are mentioned, for example. Although it does not specifically limit about drying time, Usually, it carries out in 30 second-24 hours.

After the drying process, a cooling process may be further included, and the cooling process may be slow cooling to room temperature so that the recrystallized structure of the binder is well formed.

In addition, if necessary, in order to increase the capacity density of the electrode after the drying process and increase the adhesion between the current collector and the active materials, a rolling process may be performed in which the electrode is passed between two rolls heated at high temperatures and compressed to a desired thickness. The rolling step is not particularly limited in the present invention, a known rolling step (Pressing) is possible. In one example, it is passed between the rotating rolls or performed using a flat plate press.

Hereinafter, with reference to the preferred embodiments of the present invention and the accompanying drawings will be described in detail the present invention. However, the present invention is not limited by the following examples, and it can be apparent to those skilled in the art that various modifications or changes can be made within the technical idea of the present invention.

Polymer electrolyte membrane manufacturing

<Example 1>

1) Acetonitrile (AN) was mixed with polyethylene oxide (PEO) having a weight average molecular weight (Mw) of 4,000,000 and LiTFSI so that the molar ratio of EO: Li was 9: 1.

2) Poly (ethylene glycol) diacrylate (PEGDA) and initiator benzoyl peroxide (BPO) were mixed in the solution to 10 wt% of the weight sum of polyethylene oxide (PEO) and LiTFSI. After that, stirring is performed. At this time, benzoyl peroxide (BPO) was 1 wt% of polyethylene glycol diacrylate (PEGDA).

3) The obtained mixed solution was cast on a substrate to form a coating film.

4) The coating film was vacuum dried to completely remove acetonitrile (AN), and then cured at 100 ° C. for 24 hours so as not to leave unreacted acrylate (Acrylate). The electrolyte of semi-IPN structure containing PEO-based polymer and lithium salt The membrane was prepared.

Comparative Example 1

1) Acetonitrile (AN) was mixed with polyethylene oxide (PEO) having a weight average molecular weight (Mw) of 4,000,000 and LiTFSI so that the molar ratio of EO: Li was 9: 1.

2) The obtained liquid mixture was cast on the board | substrate, and the coating film was formed.

3) After the coating film was vacuum dried to completely remove acetonitrile (AN), the reaction film was cured at 100 ° C. for 24 hours so that unreacted acrylate remained, thereby preparing an electrolyte membrane including PEO-based polymer and lithium salt.

Comparative Example 2

1) Acetonitrile (AN) was mixed with polyethylene oxide (PEO) having a weight average molecular weight (Mw) of 4,000,000 and LiTFSI so that the molar ratio of EO: Li was 9: 1.

2) Al ₂ O ₃ as an inorganic filler was mixed in the mixed solution so that 30 wt% of the weight sum of polyethylene oxide (PEO) and LiTFSI was added.

3) The obtained mixed solution was cast on a substrate to form a coating film.

4) The coating film was vacuum dried to completely remove acetonitrile (AN), and then cured at 100 ° C. for 24 hours so as not to leave unreacted acrylate (Acrylate), thereby preparing an electrolyte membrane including PEO-based polymer, lithium salt and filler. .

TABLE 1

Ionic Conductivity Measurement

The electrolyte membranes of Example 1 and Comparative Examples 1 and 2 were placed between electrodes (stainless steel and SUS), respectively, and the ions at 60 ° C. were obtained from the response obtained by applying alternating current through two blocking electrodes using an impedance analyzer (Zahner, IM6). Conductivity was measured. The electrolyte membrane of Comparative Example 1 had the highest ionic conductivity, but in Example 1 and Comparative Example 2, the ionic conductivity was measured to be somewhat low.

Interface resistance measurement

The electrolyte membrane of Example 1 of the present invention had the thinnest thickness, and therefore the interfacial resistance was also measured lowest. Therefore, it was confirmed that the influence of the somewhat lowered ionic conductivity of Example 1 can be compensated for by lowering the interfacial resistance.

All solid  Battery manufacturing

The polymer electrolyte membranes of Example 1 and Comparative Example 2 prepared above were stacked between a lithium negative electrode and a lithium-phosphate-iron (LFP, LiFePO ₄ ) positive electrode to produce an all-solid-state battery.

1 and 2 are data showing discharge capacity and coulombic efficiency for each charge / discharge cycle of an all-solid-state battery including the polymer electrolyte membranes of Comparative Examples 2 and 1 above. According to the data shown in FIG. 1, when the 80 μm polymer electrolyte membrane including the inorganic filler is applied, a micro-short phenomenon occurs, and thus the coulombic efficiency value decreases from 28 cycle points.

In comparison, according to the data shown in FIG. 2 according to the present invention, an all-solid-state battery to which a polymer electrolyte membrane having a semi-interpenetrating polymer network structure is applied has a stable discharge capacity and coulombic efficiency up to 50 cycles despite a thickness of 20 μm. And no micro-short phenomenon occurred.

Claims (8)

In an all-solid-state battery comprising a positive electrode, a negative electrode and a solid polymer electrolyte interposed therebetween,
The solid polymer electrolyte may include a polyethylene oxide (PEO) -based polymer having a weight average molecular weight (Mw) of 1,000,000 to 8,000,000; And lithium salts;
The polyethylene oxide-based polymer is cross-linked by a crosslinkable monomer including a — (CH ₂ —CH ₂ —O) — repeating unit to form a semi-interpenetrating polymer network (semi-IPN),
The solid polymer electrolyte has a thickness of 5 ~ 20㎛ all-solid-state battery, characterized in that.
The method of claim 1,
The crosslinkable monomer is an all-solid-state battery, characterized in that it comprises from 2 to 8 alkylene unsaturated bonds.
The method of claim 1,
The crosslinkable monomer is polyethylene glycol diacrylate (PEGDA), polyethylene glycol dimethacrylate (PEGDMA), polypropylene glycol diacrylate (Poly (propylene glycol)). An all-solid-state battery comprising one kind selected from diacrylate: PPGDA), poly (propylene glycol) dimethacrylate (PPGDMA), and a combination thereof.
The method of claim 1,
The cross-linkable monomer is an all-solid-state battery, characterized in that 5 to 50 wt% based on the weight of the polyethylene oxide-based polymer.
The method of claim 1,
The lithium salt is LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, LiSCN, LiC (CF ₃ SO ₂ ) ₃ , (CF ₃ SO ₂ ) ₂ NLi, (FSO ₂ ) ₂ NLi, chloroborane lithium, lower aliphatic carbonate, lithium 4-phenylborate, imide And one selected from a combination thereof.
The method of claim 1,
An all-solid-state battery according to claim 1, wherein the polyethylene oxide polymer and the lithium salt have an EO: Li molar ratio of 30: 1 to 3: 1. delete delete

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