TECHNICAL FIELD

The present invention relates to a separator including a functional binder, and more particularly to a separator configured such that one surface of a polyolefin substrate is coated with inorganic particles and a functional binder having improved properties, and to an electrochemical device including the separator.

BACKGROUND ART

Thorough research is ongoing with the goal of increasing the stability of a battery separator, especially a separator for a lithium ion secondary battery. A safety-reinforced separator (SRS), which is currently mass-produced, is configured such that the surface of a polyolefin-based substrate is coated with inorganic particles and a binder. A volume in which a liquid electrolyte is placed is increased by virtue of the porous structure due to the inorganic particles and the binder to thus increase lithium ion conductivity and an electrolyte impregnation ratio, whereby both the performance and the stability of an electrochemical device using the separator may be improved. In this regard, Patent Document 1 discloses an organic-inorganic hybrid porous separator and an electrochemical device using the same.

In order to maintain the stability of the SRS, the adhesion characteristics of a binder-inorganic material and a substrate-binder for preventing the inorganic material from being stripped from the surface of a polyolefin substrate are regarded as very important. A binder that is currently available is of a PVDF type and has low adhesion characteristics. Hence, there is a need to develop a novel binder in order to improve stability.

A PVDF-based binder may exhibit high adhesion to a cathode but low adhesion to an anode using an SBR (Styrene-Butadiene Rubber)-based binder. Furthermore, a PVDF-based binder is hydrophobic and thus exhibits poor wetting properties of a separator in an electrolyte, undesirably deteriorating the power output of a battery.

A currently available SRS is problematic in that it cannot withstand the dissolution of a transition metal of a cathode material under conditions of high temperature and high voltage, which may frequently occur during the driving of an electric vehicle. A conventional binder is incapable of adsorbing the dissolved transition metal, undesirably shortening the life cycle of the battery.

Since damage to the separator due to heat and pressure is irreversible, problems of low power output and low safety of the battery attributed to the separator once damaged cannot be overcome.

Many attempts have been made to introduce a compound containing a hydrophilic group in order to improve the properties of a separator. Non-Patent Document 1, which is aimed at improving the wetting properties of a commercially available PP (Polypropylene) separator of a lithium ion battery, discloses a surface-coating process for immersing a PP substrate in tannic acid, which is a natural vegetable polyphenyl. However, the surface of PP is hydrophobic, and thus tannic acid, having a large number of hydroxyl groups, is not uniformly distributed on the surface of PP, and the coating layer does not exhibit sufficient durability during the manufacture of an electrochemical device using the same and upon the charge/discharge thereof.

Non-Patent Document 2, which is aimed at improving the wetting properties of a PP substrate serving as a separator of a lithium ion battery, discloses a coating process using pyrogallic acid. Like Non-Patent Document 1, Non-Patent Document 2 enables realization of a uniform coating layer in a short time because pyrogallic acid has low physicochemical affinity to PP, which is hydrophobic, but the coating layer does not have sufficient durability and uniformity during the manufacture of an electrochemical device using the same and during charge/discharge thereof.

Non-Patent Documents 3 to 6 disclose a binder having self-healing performance, such as a capsule-shaped binder for improving the properties of an anode containing silicon. Such a binder is developed in order to solve the problem that occurs when using a silicon anode having high capacity, that is, low bondability of a conventional PVDF to silicon, but is difficult to directly apply to a currently useful SRS binder.

With the goal of improving the wetting properties of a separator, the technique for disposing a binder film on the outermost layer of an SRS is disclosed in Patent Document 2. Although hydrophilicity is improved due to the film of the outermost layer, it is difficult to access the pores formed by the inorganic particles and the binder, making it impossible to improve the performance and stability of an electrochemical device.

Patent Document 3 discloses a battery separator and a non-aqueous electrolyte battery using the same. In the battery separator including a resin porous film composed mainly of a thermoplastic resin and a multilayered porous film comprising a heat-resistant porous layer composed mainly of heat-resistant particles, the heat-resistant porous layer has a thickness of 1 to 15 μm, and the peel strength of the resin porous film and the heat-resistant porous layer at 180° is 0.6 N/cm or more.

Both the present invention and Patent Document 3 are related to a separator for a lithium secondary battery having improved thermal safety. However, the binder of the present invention may exhibit not only thermal stability but also an enhancement in adhesion, adsorption of a transition metal of a cathode material, self-healing, and the like, which are never noted in Patent Document 3. Hence, there is a significant difference in the molecular structure of a binder therebetween. Specifically, the binder of the present invention contains large numbers of —O and hydroxyl (OH) functional groups in the molecules thereof, whereas the binder of Patent Document 3 includes an N-vinylacetamide polymer or a water-soluble cellulose derivative, along with a crosslinked acrylic resin.

Meanwhile, Patent Document 3 uses xanthan gum, which is used as the binder of the present invention, as a thickener. The binder and the thickener are remarkably different in terms of the amounts thereof. In Patent Document 3, the binder is used in an amount of 1.1 parts by mass or more based on 100 parts by mass of heat-resistant particles, whereas the thickener is used in an amount of 0.1 parts by mass or more based on 100 parts by mass of heat-resistant particles. Here, the binder and the thickener differ by about 10 times in terms of the amounts thereof, and thus it can be found that the binder and the thickener are incompatible with each other. Although the present invention and Patent Document 3 are similar in that they both disclose the use of xanthan gum, the specific end uses thereof are different, and Patent Document 3 does not mention the technical features of the binder that is used in the present invention.

Non-Patent Document 7, which is a review paper on natural materials used in electrochemical energy storage devices, discloses the use of natural materials such as amylopectin, xanthan gum and the like as the binder. However, the binder of Non-Patent Document 7 is used for a cathode or an anode and is thus different in terms of the end use and functionality from those of the binder of the separator used in the present invention. In particular, the binder for a separator of the present invention has properties quite different from those of the material used for a cathode or an anode because the separator includes an inorganic material, and the detailed technical field therebetween is different.

In order to maintain the thermal stability of the SRS including inorganic particles as described above, techniques for increasing the binder-inorganic material adhesion and the substrate-binder adhesion while preventing internal shorting from occurring by performing self-healing of damage to the separator, enhancing the adhesion of the separator to a cathode and an anode, and withstanding the dissolution of a transition metal of a cathode material have not yet been introduced to date.

Korean Patent No. 10-0775310

Korean Patent No. 10-1535198

Korean Patent Application Publication No. 2011-0031998

Lei Pan et al., “Tannic-Acid-Coated Polypropylene Membrane as a Separator for Lithium ion Batteries”, ACS Appl. Mater. Interfaces, 7(29), 16003-16010(2015).

Haibin Wang et al., “Pyrogallic acid coated polypropylene membranes as separators for lithium-ion batteries”, J. Mater. Chem. A, 3, 20535-20540 (2015).

S. R. White et al., “Autonomic healing of polymer composites”, Nature, 409, 794-797 (2001).

Benjamin C-K Tee et al., “Pressure and flexionsensitive electronic skin with repeatable ambient self-healing capability”, Nature nanotechnology, 7, 825 (2012).

Chao Want et al., “Self-healing chemistry enables the stable operation of silicon microparticle anodes for high-energy lithium-ion batteries”, Nature Chemistry, 5, 1042-1048 (2013).

You Kyeong Jeong et al., “Millipede-inspired Structural Design Principles for High Performance Polysaccharide Binders in Silicon Anodes”, Energy & Environmental Science, 8 1224 (2015).

Hua Wang et al., “Nature-Inspired Electrochemical Energy-Storage Materials and Devices”, Adv. Energy Materials 1601709 (2016).

DISCLOSURE

Technical Problem

Accordingly, the present invention is intended to provide a functional binder and a battery separator including the functional binder, in which the separator includes a porous polyolefin substrate and an organic-inorganic hybrid porous coating layer formed on at least one surface of the substrate and configured to include inorganic particles and a binder compound, thus increasing binder-inorganic material adhesion and substrate-binder adhesion while preventing internal shorting from occurring by performing self-healing of damage to the separator, enhancing the adhesion of the separator to a cathode and an anode, and withstanding the dissolution of a transition metal of a cathode material.

Technical Solution

Therefore, a first aspect of the present invention provides a battery separator, comprising (a) a polyolefin-based substrate and (b) an active layer comprising a mixture of inorganic particles and a binder, in which the inorganic particles in the active layer are connected and fixed to each other by the binder, and a porous structure is formed due to the interstitial volume between the inorganic particles, wherein the active layer is present as a coating on at least one region selected from the group consisting of (a) a surface of the substrate, and (b) a portion of pores in the substrate, and the binder contains 10 wt % or more of a hydroxyl group per molecule thereof.

Specifically, the binder may include at least one selected from among tannic acid, pyrogallic acid, amylose, amylopectin, and xanthan gum, or the binder may be a mixture comprising 1) at least one selected from among tannic acid, pyrogallic acid, amylose, amylopectin, and xanthan gum and 2) at least one selected from among polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, an acrylonitrilestyrene-butadiene copolymer, and polyimide.

The inorganic particles may be at least one selected from the group consisting of inorganic particles having a dielectric constant of 5 or more, inorganic particles having piezoelectricity, and inorganic particles having lithium ion transfer capacity.

The inorganic particles having a dielectric constant of 5 or more may be SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, Y2O3, Al2O3, TiO2or SiC; the inorganic particles having piezoelectricity may be BaTiO3, Pb(Zr,Ti)O3(PZT), Pb1-xLaxZr1-yTiyO3(PLZT), Pb(Mg3Nb2/3)O3-PbTiO3(PMN-PT) or hafnia (HfO2); and the inorganic particles having lithium ion transfer capacity may be lithium phosphate (Li3PO4), lithium titanium phosphate (LixTiy(PO4)3, 0<x<2, 0<y<3), lithium aluminum titanium phosphate (LixAlyTiz(PO4)3, 0<x<2, 0<y<1, 0<z<3), (LiAlTiP)xOy-based glass (0<x<4, 0<y<13), lithium lanthanum titanate (LixLayTiO3, 0<x<2, 0<y<3), lithium germanium thiophosphate (LixGeyPzSw, 0<x<4, 0<y<1, 0<z<1, 0<w<5), lithium nitride (LixNy, 0<x<4, 0<y<2), SiS2(LixSiySz, 0<x<3, 0<y<2, 0<z<4)-based glass or P2S5(LixPySz, 0<x<3, 0<y<3, 0<z<7)-based glass.

Specifically, the inorganic particles may include at least one selected from among Al2O3, AlOOH, and Mg(OH)2.

The polyolefin-based substrate comprise at least one selected from the group consisting of high-density polyethylene, low-density polyethylene, linear low-density polyethylene, ultrahigh-molecular-weight polyethylene, and polypropylene.

A second aspect of the present invention provides a method of manufacturing the battery separator, comprising the steps of 1) preparing a binder solution by dissolving a binder in a solvent, 2) adding and mixing the binder solution obtained in the step 1) with inorganic particles, and 3) coating at least one region selected from the group consisting of (a) a surface of a polyolefin-based substrate and (b) a portion of pores in the substrate with the solution obtained in the step 2) and performing drying, the hinder containing 10 wt % or more of a hydroxyl group per molecule thereof.

The solvent may include water and acetone mixed at a weight ratio of 100:0 to 0:100, preferably 95:5 to 0:100, more preferably 75:25 to 0:100, much more preferably 60:40 to 0:100, even more preferably 50:50 to 0:100, and most preferably 40:60 to 0:100. In an embodiment of the present invention, the optimal ratio thereof is 75:25.

A third aspect of the present invention provides an electrochemical device comprising the battery separator of the present invention. Here, the electrochemical device may be a lithium secondary battery.

