SEPARATOR FOR RECHARGEABLE LITHIUM BATTERY AND RECHARGEABLE LITHIUM BATTERY INCLUDING THE SAME

Examples of the present disclosure include a separator for a rechargeable lithium battery, and a rechargeable lithium battery including the separator, and the separator for a rechargeable lithium battery includes a porous substrate and a coating layer located on at least one surface of the porous substrate. The coating layer includes a cross-linked product of a binder and a cross-linking agent, a filler, and an adhesive binder. The binder includes a (meth)acryl-based binder including one or more of a first structural unit derived from (meth)acrylamide, a second structural unit derived from (meth)acrylic acid or (meth)acrylate, and a third structural unit derived from (meth)acrylamido sulfonic acid or a salt thereof, the cross-linking agent includes an aziridine-based cross-linking agent. The filler includes a filler having a particle diameter D100 of about 0.5 μm or less, and the adhesive binder includes a cross-linked (meth)acryl-based polymer or copolymer.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of priority to Korean Patent Application No. 10-2024-0058156, filed on Apr. 30, 2024 in the Korean Intellectual Property Office, the entire disclosure of which being incorporated herein by reference.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates to a separator for a rechargeable lithium battery, and a rechargeable lithium battery including the separator.

2. Discussion of Related Art

With increasing presence of electronic devices using batteries, such as, e.g., mobile phones, notebook computers, electric vehicles, and the like, the demand for secondary batteries having high energy density and high capacity is rapidly increasing. Therefore, improving the performance of rechargeable lithium batteries may be advantageous.

A rechargeable lithium battery typically includes a positive electrode and a negative electrode that include an active material capable of the intercalation and deintercalation of lithium ions, and produces electrical energy by oxidation and reduction reactions when the lithium ions are intercalated into and deintercalated from the positive electrode and the negative electrode.

The rechargeable lithium battery may further include a separator between the positive electrode and the negative electrode. The separator may have a low membrane resistance and a high heat resistance, resulting in low heat shrinkage.

SUMMARY

One example embodiment includes a separator for a rechargeable lithium battery, which increases the stability of the battery by having a low dry shrinkage rate and a low shrinkage rate in an electrolyte.

Another example embodiment includes a rechargeable lithium battery including the separator.

Another example embodiment includes a separator for a rechargeable lithium battery.

The separator for a rechargeable lithium battery includes a porous substrate and a coating layer located on at least one surface of the porous substrate. The coating layer includes a cross-linked product of a binder and a cross-linking agent, a filler, and an adhesive binder. The binder includes a (meth)acryl-based binder including one or more of a first structural unit derived from (meth)acrylamide; and one or more of a second structural unit derived from (meth)acrylic acid or (meth)acrylate and a third structural unit derived from (meth)acrylamido sulfonic acid or a salt thereof. The cross-linking agent includes an aziridine-based cross-linking agent. The filler has a particle diameter D100 of about 0.5 μm or less, and the adhesive binder includes a cross-linked (meth)acryl-based polymer or copolymer.

According to another example embodiment, a rechargeable lithium battery includes a positive electrode, a negative electrode, and the separator located between the positive electrode and the negative electrode.

DETAILED DESCRIPTION

Hereinafter, example embodiments of the present disclosure are described in detail. However, the embodiments are presented as examples, and the present disclosure is not limited thereto, and the present disclosure is only defined by the scope of the appended claims.

Unless otherwise stated herein, when a part such as a layer, a membrane, an area, a plate, and the like, is described as being disposed “on” another part, it includes not only a case where the part is “directly on” another part, but also a case where there are other parts therebetween.

Unless otherwise stated herein, the singular may also include the plural. In addition, unless otherwise stated, the term “A or B” may indicate “including A, including B, or including A and B.”

In the present specification, “a combination thereof” may indicate a mixture, stack, composite, copolymer, alloy, blend, or reaction product of constituents.

Unless otherwise defined herein, “particle diameter D100” refers to a diameter of a particle with a cumulative volume of 100% by volume in a particle diameter distribution. The particle diameter D100 may be measured by methods known to those skilled in the art and for example, may be measured using a particle size analyzer, a transmission electron microscope photograph, or a scanning electron microscope photograph. As another method, the particle diameter D100 may be obtained by measuring the particle diameter using a measuring device using dynamic light scattering, performing data analysis to count the number of particles for each particle size range, and then calculating the particle diameter D100 therefrom. Alternatively, the particle diameter D100 may be measured using a laser diffraction method. When measuring the particle diameter by the laser diffraction method, for example, the particle diameter D100 based on 100% of a particle diameter distribution in the measuring device may be calculated by dispersing particles to be measured in a dispersion medium, then introducing the dispersion medium into a commercially available laser diffraction particle diameter measuring device (e.g., Microtrac's MT 3000), and radiating ultrasonic waves of about 28 kHz with an output of 60 W.

A particle diameter may be a particle size.

Unless otherwise defined herein, “particle diameter D50” may be an average particle diameter D50, which refers to a diameter of a particle with a cumulative volume of 50% by volume in a particle diameter distribution. The particle diameter distribution may be obtained from the above method in the particle diameter D100.

In the present specification, “(meth)acryl” refers to acryl and/or methacryl. Hereinafter, unless otherwise defined, “substitution” indicates that hydrogen in a compound is substituted with a substituent such as or including at least one of a C1 to C30 alkyl group, a C2 to C30 alkenyl group, a C2 to C30 alkynyl group, a C6 to C30 aryl group, a C7 to C30 alkylaryl group, a C1 to C30 alkoxy group, a C1 to C30 heteroalkyl group, a C3 to C30 heteroalkylaryl group, a C3 to C30 cycloalkyl group, a C3 to C15 cycloalkenyl group, a C6 to C30 cycloalkynyl group, a C2 to C30 heterocycloalkyl group, a halogen (F, Cl, Br, or I), a hydroxy group (—OH), a nitro group (—NO2), a cyano group (—CN), an amino group (—NRR′) (here, R and R′ are each independently hydrogen or a C1 to C6 alkyl group), a sulfobetaine group (—RR′N+(CH2)nSO3—, n is a natural number from 1 to 10), a carboxybetaine group (—RR′N+(CH2)nCOO—, n is a natural number from 1 to 10) (here, R and R′ are each independently a C1 to C20 alkyl group), an azido group (—N3), an amidino group (—C(═NH)NH2), a hydrazino group (—NHNH2), a hydrazono group (═N(NH2)), a carbamoyl group (—C(O)NH2), a thiol group (—SH), an acyl group (—C(═O)R, here, R denotes hydrogen, a C1 to C6 alkyl group, a C1 to C6 alkoxy group, or a C6 to C12 aryl group), a carboxyl group (—COOH) or a salt thereof (—C(═O)OM, here, M denotes an organic or inorganic cation), a sulfonic acid group (—SO3H) or a salt thereof (—SO3M, here, M denotes an organic or inorganic cation), a phosphate group (—PO3H2) or a salt thereof (—PO3MH or —PO3M2, here, M denotes an organic or inorganic cation), and a combination thereof.

Hereinafter, the C1 to C3 alkyl group may be or include at least one of a methyl group, an ethyl group, or a propyl group. The C1 to C10 alkylene group may be or include, for example, at least one of a C1 to C6 alkylene group, a C1 to C5 alkylene group, or a C1 to C3 alkylene group and may be or include, for example, at least one of a methylene group, an ethylene group, or a propylene group. The C3 to C20 cycloalkylene group may be or include, for example, a C3 to C10 cycloalkylene group, or a C5 to C10 cycloalkylene group, for example, a cyclohexylene group. The C6 to C20 arylene group may be or include, for example, a C6 to C10 arylene group, for example, a phenylene group. The C3 to C20 heterocyclic group may be or include, for example, a C3 to C10 heterocyclic group, for example, a pyridine group.

Hereinafter, “hetero” indicates including one or more heteroatoms such as or including at least one of N, O, S, Si, and P.

In addition, in the chemical formulas, the symbol * refers to a part that is connected to the same or different atom, group, or structural unit.

Hereinafter, “alkali metal” refers to an element belonging to Group 1 of the periodic table, such as lithium, sodium, potassium, rubidium, cesium, or francium and may be present in a cationic or neutral state.

In the present specification, when describing a numerical range, “X to Y” indicates “X or more and Y or less (X≤ and ≤Y).”

A separator for a rechargeable lithium battery according to one example embodiment includes a porous substrate and a coating layer located on at least one surface of the porous substrate. The coating layer includes a cross-linked product of a binder and a cross-linking agent, a filler, and an adhesive binder, the binder includes a (meth)acryl-based binder including a first structural unit derived from (meth)acrylamide; one or more of a second structural unit derived from (meth)acrylic acid or (meth)acrylate, and a third structural unit derived from (meth)acrylamido sulfonic acid or a salt thereof. The cross-linking agent includes an aziridine-based cross-linking agent. The filler has a particle diameter D100 of about 0.5 μm or less. The adhesive binder includes a cross-linked (meth)acryl-based polymer or copolymer.

According to one example embodiment, the coating layer may include a heat-resistant layer formed of or including a composition including the (meth)acryl-based binder, the aziridine-based cross-linking agent, and the filler having a particle diameter D100 of about 0.5 μm or less, and an adhesive layer located on the heat-resistant layer and including the adhesive binder.

According to one example embodiment, the cross-linked product may be or include a heat cross-linked product.

Because the coating layer includes the cross-linked product, the filler, and the adhesive binder, the separator for a rechargeable lithium battery may have both a significantly low dry shrinkage rate and shrinkage rate in an electrolyte.

According to one example embodiment, the dry shrinkage rate of the separator for a rechargeable lithium battery may be in a range of about 5% or less, for example, 3% or less, and the shrinkage rate in the electrolyte may be 5% or less, for example, 3% or less or 2% or less.

According to one example embodiment, the separator for a rechargeable lithium battery exhibits a significantly low shrinkage rate in the electrolyte. The shrinkage rate in the electrolyte is obtained in consideration of an application location of the separator in the rechargeable lithium battery. The separator may be saturated with the electrolyte. A separator with a low shrinkage rate in an electrolyte can increase the stability of the battery by maintaining heat resistance properties substantially without weakening the mechanical properties of the (meth)acryl-based binder when the separator is saturated with the electrolyte.

A separator formed of or including a composition including the (meth)acryl-based binder but not including an aziridine-based cross-linking agent as a cross-linking agent, or including a cross-linking agent other than the aziridine-based cross-linking agent, may not satisfy the above shrinkage rate range in the electrolyte. For example, when an epoxy-based cross-linking agent is included instead of the aziridine-based cross-linking agent, the viscosity stability may not be sufficient in a slurry state for a coating layer, and the above-described shrinkage rate in the electrolyte may not be satisfied. According to one example embodiment, the aziridine-based cross-linking agent may be included in an amount of about 95 wt % or more, for example, ranging from 98 wt % to 100 wt %, for example, 100 wt % of the total cross-linking agent in the composition.

A separator formed of or including a composition including the (meth)acryl-based binder but not including a filler having a particle diameter D100 of about 0.5 μm or less, or including a filler having a particle diameter D100 of more than about 0.5 μm, may not satisfy the above shrinkage rate range in the electrolyte. According to one example embodiment, the filler having a particle diameter D100 of about 0.5 μm or less may be included in an amount in a range of about 95 wt % or more, for example, ranging 95 wt % to 100 wt %, from 99 wt % to 100 wt %, or 100 wt % of the fillers in the coating layer.

A separator formed of or including a composition including the aziridine-based cross-linking agent and the filler but not including the (meth)acryl-based binder, or including a binder other than the (meth)acryl-based binder, may not satisfy the above dry shrinkage rate and shrinkage rate ranges in electrolyte. According to one example embodiment, the (meth)acryl-based binder may be included in an amount in a range of about 95 wt % or more, for example, ranging from about 98 wt % to 100 wt %, or for example, 100 wt % of the total binder in the composition.

Coating Layer

The binder includes a (meth)acryl-based binder including a first structural unit derived from (meth)acrylamide; and one or more of a second structural unit derived from (meth)acrylic acid or (meth)acrylate, and a third structural unit derived from (meth)acrylamido sulfonic acid or a salt thereof.

According to one example embodiment, the binder may include the (meth)acryl-based binder in an amount in a range of about 95 wt % or more, for example, ranging from 95 wt % to 100 wt % or 100 wt %.

The (meth)acryl-based binder is or includes a water-based heat-resistant binder, and may fix the filler to a porous substrate, provide bonding strength so that the coating layer is bonded to the porous substrate and an electrode, and contribute to increasing the heat resistance, air permeability, and oxidation resistance of the separator.

The first structural unit derived from (meth)acrylamide has an amide functional group (—(C═O)—NH2) in the structural unit. The —(C═O)—NH2 functional group can increase the bonding characteristics with the porous substrate and the electrode, and more firmly fix inorganic particles in the coating layer by forming a hydrogen bond with the —OH functional group of the filler, thereby reinforcing the heat resistance of the separator.

The second structural unit derived from (meth)acrylic acid or (meth)acrylate may be configured to fix the filler to the porous substrate, and provide bonding strength so that the coating layer is bonded to the porous substrate and the electrode, and contribute to increasing the heat resistance and air permeability of the separator. In addition, the structural unit derived from (meth)acrylic acid or (meth)acrylate may include a carboxyl functional group (—C(═O)O—) in the structural unit, thereby contributing to improving the dispersibility of a coating slurry.

The third structural unit derived from (meth)acrylamido sulfonic acid or a salt thereof may include a bulky functional group, thereby reducing the mobility of a copolymer including the bulky functional group, and reinforcing the heat resistance of the separator.

In one example embodiment, the (meth)acryl-based binder may be or include a terpolymer including a first structural unit derived from (meth)acrylamide, a second structural unit derived from (meth)acrylic acid or (meth)acrylate, and a third structural unit derived from (meth)acrylamido sulfonic acid or a salt thereof.

In one example embodiment, the (meth)acryl-based binder may be or include a binary polymer including a first structural unit derived from (meth)acrylamide and a third structural unit derived from (meth)acrylamido sulfonic acid or a salt thereof.

For example, with respect to 100 mol % of the (meth)acryl-based binder, the first structural unit may be included in an amount ranging from about 80 mol % to 85 mol %, the second structural unit may be included in an amount ranging from about 10 mol % to 15 mol %, and the third structural unit may be included in an amount ranging from about 5 mol % to 10 mol %.

When the content of each structural unit is within the above range, the heat resistance and bonding strength of the separator can be further increased.

The first structural unit may be represented by Chemical Formula 1 below:

In Chemical Formula 1, R1 and R2 are or include hydrogen or a methyl group. The second structural unit may be, for example, represented by any one or more of Chemical Formulas 2, 3, 4 below, and a combination thereof:

In Chemical Formulas 2 to 4, R3, R4, R6, R7, R8, and R9 may each independently be or include hydrogen or a methyl group,

R5 is or includes a substituted or unsubstituted C1 to C20 alkyl group, and M is or includes an alkali metal.

The alkali metal may be or include, for example, at least one of lithium, sodium, potassium, rubidium, or cesium.

The structural unit derived from (meth)acrylate may be derived from (meth)acrylic acid alkyl esters, (meth)acrylic acid perfluoroalkyl esters, and (meth)acrylate having a functional group in a side chain, such as a (meth)acrylic acid alkyl ester. In addition, the carbon number of an alkyl group or perfluoroalkyl group bonded to the non-carbonyl oxygen atom of the (meth)acrylic acid alkyl ester or the (meth)acrylic acid perfluoroalkyl ester may range from about 1 to about 20, for example, from 1 to 10, for example, from 1 to 5.

Examples of (meth)acrylic acid alkyl ester in which the alkyl group or perfluoroalkyl group bound to the non-carbonyl oxygen atom has 1 to 5 carbon atoms may include at least one of acrylic acid alkyl esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, or t-butyl acrylate, acrylic acid-2-(perfluorobutyl)ethyls such as acrylic acid-2-(perfluorobutyl)ethyl, or acrylic acid-2-(perfluoropentyl)ethyl, alkyl methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, and t-butyl methacrylate, and methacrylic acid-2-(perfluoroalkyl)ethyls such as methacrylic acid-2-(perfluorobutyl)ethyl, methacrylic acid-2-(perfluoropentyl)ethyl, or methacrylic acid-2-(perfluoroalkyl)ethyl.

Examples of other (meth)acrylic acid alkyl esters may include acrylic acid alkyl esters having 6 to 18 carbon atoms in the alkyl group bonded to the non-carbonyl oxygen atom such as at least one of acrylic acid n-hexyl, acrylic acid-2-ethylhexyl, acrylic acid nonyl, acrylic acid lauryl, acrylic acid stearyl, acrylic acid cyclohexyl, or acrylic acid isobornyl; methacrylic acid alkyl esters having 6 to 18 carbon atoms in the alkyl group bonded to the non-carbonyl oxygen atom such as at least one of methacrylic acid n-hexyl, methacrylic acid-2-ethylhexyl, methacrylic acid octyl, methacrylic acid isodecyl, methacrylic acid lauryl, methacrylic acid tridecyl, methacrylic acid stearyl, or methacrylic acid cyclohexyl; acrylic acid-2-(perfluoroalkyl)ethyls having 6 to 18 carbon atoms in the perfluoroalkyl group bonded to the non-carbonyl oxygen atom such as at least one of acrylic acid-2-(perfluorohexyl)ethyl, acrylic acid-2-(perfluorooctyl) ethyl, acrylic acid-2-(perfluoronitrile) ethyl, acrylic acid-2-(perfluorodecyl)ethyl, acrylic acid-2-(perfluorodecyl)ethyl, acrylic acid-2-(perfluorotetradecyl)ethyl, or acrylic acid-2-(perfluorokexadecyl)ethyl; and methacrylic acid-2-(perfluoroalkyl)ethyls having 6 to 18 carbon atoms in the perfluoroalkyl group bonded to the non-carbonyl oxygen atom such as at least one of methacrylic acid-2-(perfluorohexyl)ethyl, methacrylic acid-2-(perfluorooctyl)ethyl, methacrylic acid-2-(perfluoronyl)ethyl, methacrylic acid-2-(perfluorodecyl)ethyl, methacrylic acid-2-(perfluorododecyl)ethyl, methacrylic acid-2-(perfluorotetradecyl)ethyl, or methacrylic acid-2-(perfluorohexadecyl)ethyl.

The structural unit derived from (meth)acrylic acid or (meth)acrylate may include one or both of the structural unit represented by Chemical Formula 2 and the structural unit represented by Chemical Formula 3, and when both are included, the structural unit represented by Chemical Formula 2 and the structural unit represented by Chemical Formula 3 may be included in a molar ratio of about 10:1 to about 1:1, for example, 6:1 to 1:1, and for example, 3:1 to 1:1.

The third structural unit may be or include a structural unit derived from (meth)acrylamido sulfonic acid or (meth)acrylamido sulfonate, and the (meth)acrylamido sulfonate may be or include a conjugate base of (meth)acrylamido sulfonic acid, (meth)acrylamido sulfonate, or a derivative thereof. The third structural unit may be, for example, represented by any one or more of Chemical Formulas 5, 6, and 7 below, and a combination thereof:

In Chemical Formulas 5 to 7, R10, R11, R12, R13, R14, and R15 may each independently be or include a hydrogen or methyl group,

As an example, in Chemical Formulas 5 to 7, L1, L2, and L3 may each independently be or include a substituted or unsubstituted C1 to C10 alkylene group, and a, b, and c may each be equal to 1.

The structural unit derived from (meth)acrylamido sulfonic acid or a salt thereof may include one or two or more of the structural unit represented by Chemical Formula 5, the structural unit represented by Chemical Formula 6, and the structural unit represented by Chemical Formula 7. As an example, the above structural unit may include the structural unit represented by Chemical Formula 6, and as another example, include the structural unit represented by Chemical Formula 6 and the structural unit represented by Chemical Formula 7 together.

When the structural unit represented by Chemical Formula 6 and the structural unit represented by Chemical Formula 7 are included together, the structural unit represented by Chemical Formula 6 and the structural unit represented by Chemical Formula 7 may be included in a molar ratio of about 10:1 to about 1:2, for example, 5:1 to 1:1, or for example, 3:1 to 1:1.

A sulfonate group in the structural unit derived from (meth)acrylamido sulfonic acid or a salt thereof may be or include, for example, at least one of vinyl sulfonic acid, allyl sulfonic acid, styrene sulfonic acid, anethole sulfonic acid, acrylamidoalkane sulfonic acid, sulfoalkyl (meth)acrylate, or a functional group derived from a salt thereof.

Here, the alkane may be or include at least one of a C1 to C20 alkane, a C1 to C10 alkane, or a C1 to C6 alkane, and the alkyl may be or include at least one of a C1 to C20 alkyl, a C1 to C10 alkyl, or a C1 to C6 alkyl. The salt refers to a salt composed of the above-described sulfonic acid and a desired ion. The ion may be or include, for example, an alkali metal ion, and in this case, the salt may be or include a sulfonic acid alkali metal salt.

The (meth)acrylamidoalkane sulfonic acid may be or include, for example, 2-(meth)acrylamido-2-methylpropane sulfonic acid, and the sulfoalkyl (meth)acrylate may be or include, for example, at least one of 2-sulfoethyl (meth)acrylate, 3-sulfopropyl (meth)acrylate, and the like.

The (meth)acryl-based binder may be, for example, represented by Chemical Formula 8 below:

In Chemical Formula 8, R1, R2, R12, R13, R16, and R17 each independently is or includes hydrogen or a methyl group,

The alkali metal may be or include, for example, at least one of lithium, sodium, potassium, rubidium, or cesium.

As an example, in Chemical Formula 8, l+m+n=1. In addition, as an example,

As an example, in Chemical Formula 8, L2 may be or include a substituted or unsubstituted C1 to C10 alkylene group, and b may be equal to 1.

In the (meth)acryl-based binder, the structural unit substituted with an alkali metal (M+) may be present in an amount ranging from about 50 mol % to about 100 mol %, for example, from 60 mol % to 90 mol % or from 70 mol % to 90 mol % with respect to 100 mol % of the total amount of (meth)acrylamido sulfonic acid structural unit. When the above range is satisfied, the (meth)acryl-based binder, and the separator including the (meth)acryl-based binder, can exhibit desired or improved bonding strength, heat resistance, and oxidation resistance.

The (meth)acryl-based binder may further include other units in addition to the above-described units. For example, the (meth)acryl-based binder may include a unit derived from an alkyl (meth)acrylate, a unit derived from a diene-based binder, a unit derived from a styrene-based binder, an ester group-containing unit, a carbonate group-containing unit, or combinations thereof.

The (meth)acryl-based binder may be in various forms, such as an alternating polymer in which the units are alternately distributed, a random polymer in which the units are randomly distributed, or a graft polymer in which some structural units are grafted.

A weight average molecular weight of the (meth)acryl-based binder may range from about 350,000 to about 970,000, for example, from 450,000 to 970,000 or from 450,000 to 700,000. When the weight average molecular weight of the (meth)acryl-based binder satisfies the above range, the (meth)acryl-based binder, and the separator including the (meth)acryl-based binder, can exhibit desired or improved bonding strength, heat resistance, and air permeability. The weight average molecular weight may be or include a polystyrene-converted average molecular weight measured using gel permeation chromatography.

The (meth)acryl-based binder may be prepared by various known methods such as, e.g., emulsion polymerization, suspension polymerization, bulk polymerization, or solution polymerization.

The (meth)acryl-based binder may be prepared by, e.g., a solution polymerization method.

According to one example embodiment, the (meth)acryl-based binder may be included in the coating layer of the separator in the form of a film.

The cross-linking agent includes an aziridine-based cross-linking agent. The aziridine-based cross-linking agent may crosslink the (meth)acryl-based binder, and allow the separator to readily satisfy the above dry shrinkage rate and shrinkage rate ranges in the electrolyte.

The aziridine-based cross-linking agent may be or include a bi-functional or higher aziridine-based cross-linking agent. Here, “bi-functional or higher” indicates that two or more aziridine groups are present in a molecule. According to one example embodiment, the aziridine-based cross-linking agent may be or include at least one of a bi- or tri-functional aziridine-based cross-linking agent.

The filler has a particle diameter D100 in a range of about 0.5 μm or less. Within the above range, the separator may readily satisfy the dry shrinkage rate and the shrinkage rate in the electrolyte when the (meth)acryl-based binder is combined with the aziridine-based cross-linking agent. For example, the filler may have a particle diameter D100 ranging from 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5 μm, 0.1 μm to 0.5 μm.

According to one example embodiment, the filler may have a particle diameter D50 of about 0.4 μm or less, for example, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, μm 0.35 μm or less, 0.25 μm or less, or ranging from 0.1 μm to 0.25 μm. Within the above range, the shrinkage in the electrolyte may be reduced.

The filler may be or include, for example, at least one of an inorganic filler, an organic filler, an organic-inorganic composite filler, or a combination thereof. The inorganic filler may be or include a ceramic material that can increase heat resistance. The inorganic filler may include, for example, at least one of a metal oxide, a metalloid oxide, a metal fluoride, a metal hydroxide, or a combination thereof. The inorganic filler may include, for example, at least one of Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, or a combination thereof, but is not limited thereto. The organic filler may include at least one of an acrylic compound, an imide compound, an amide compound, or a combination thereof, but is not limited thereto. The organic filler may have a core-shell structure, but is not limited thereto.

The filler may be substantially spherical, substantially plate-shaped, substantially cubic, or amorphous. For example, the filler may be plate-shaped.

For example, the filler may be or include plate-shaped boehmite.

The filler may be included in a desired content with respect to the binder, for example, the (meth)acryl-based binder. According to one example embodiment, the (meth)acryl-based binder and the filler may be included in a mass ratio of 1:10 to 1:50, for example, 1:10, 1:15, 1:20, 1:25, 1:30, 1:35, 1:40, 1:45, 1:50, 1:15 to 1:40 or 1:20 to 1:30. Within the above range, the dry shrinkage rate and the shrinkage rate in the electrolyte can be reduced.

The filler may be included in an amount ranging from about 50 wt % to about 99 wt % of the total amount of the coating layer, for example, from 70 wt % to 99 wt %, for example, from 75 wt % to 99 wt %, for example, from 80 wt % to 99 wt %, for example, from 85 wt % to 99 wt %, for example, from 90 wt % to 99 wt %, or for example, from 95 wt % to 99 wt %. When the filler is included within the above range, the separator can exhibit desired or improved heat resistance, durability, oxidation resistance, and stability.

The adhesive binder may be configured to secure bonding strength with the electrode of the separator. Because heat resistance and bonding strength are physical properties that have a trade-off relationship, in one example embodiment, the coating layer may further include the adhesive binder together with the (meth)acryl-based binder so that the (meth)acryl-based binder and the adhesive binder are each independently present in the coating layer, thereby forming a separator with desired or improved heat resistance and bonding strength.

In addition, by the adhesive binder, the separator can maintain the heat resistance and the bonding strength, increase the stability and lifetime of a battery, and also increase the resistance of the battery when the separator is included in the battery.

The adhesive binder may be cross-linked. According to one example embodiment, the adhesive binder is or includes a (meth)acryl-based adhesive binder and may be or include a cross-linked (meth)acrylate-based polymer or copolymer. For example, the adhesive binder may include a cross-linked polymethyl (meth)acrylate-based polymer.

To prepare the cross-linked (meth)acryl-based polymer, a cross-linking agent may be further added during a polymerization process.

When the (meth)acryl-based adhesive binder has a glass transition temperature ranging from about 50° C. or higher and about 110° C. or lower. Within the above range, not only the bonding strength of the electrode is desired or improved, but also ionic conductivity is sufficient.

The adhesive binder may have a predetermined or desired swelling degree with respect to an electrolyte. For example, a mass increase rate (swelling degree) due to the electrolyte when the adhesive binder is left at about 60° C. for about 72 hours may range from about 50% to 500%. Within the above electrolyte swelling degree range, a bonding area of the coating layer in the electrolyte can be increased, and there can be no or less issues of reduced bonding strength due to swelling and increased battery resistance due to an Li ion movement path being blocked. Here, as an electrolyte included to measure the electrolyte swelling degree, LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) (volume mixing ratio: EC/DEC/DMC=2/4/4, 1 mol/L of LiPF6 as a supporting electrolyte) is included. In addition, the electrolyte swelling degree may be measured as follows.

First, a polymer is prepared. Then, a film is manufactured using the prepared polymer. For example, when the polymer is a solid, a film with a thickness of 0.5 mm is manufactured by drying the polymer at a temperature of 85° C. for 48 hours and then molding the polymer into a film. In addition, for example, when the polymer is a solution or a dispersion, such as latex, a film with a thickness of 0.5 mm is manufactured by placing the solution or dispersion in a polytetrafluoroethylene dish and drying the solution or dispersion at a temperature of 85° C. for 48 hours.

Next, a test sample is obtained by cutting the film manufactured as above into 1 cm squares. A weight of the test sample is measured and set as W0. In addition, the test sample is immersed in the electrolyte at a temperature of 60° C. for 72 hours, and the test sample is taken out of the electrolyte. The electrolyte on the surface of the test sample, which has been taken out of the electrolyte, is wiped off, and a weight W1 of the test sample after immersion is measured.

In addition, using the weights W0 and W1, the swelling degree S (times) is calculated as S═W1/W0.

In addition, a method of adjusting the electrolyte swelling degree of the polymer may include, for example, appropriately selecting the type and amount of monomers for preparing the polymer in consideration of a solubility parameter (SP) value of the electrolyte.

The adhesive binder may be included in an amount ranging from about 1 wt % to about 20 wt %, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 wt %, 5 to 20 wt %, for example, 5 to 15 wt % with respect to the total amount of the coating layer. Within the above range, bonding strength to the electrode can be exhibited, battery resistance does not increase, and thus there may be no limitation in capacity implementation.

The coating layer may have a thickness ranging from about 0.01 μm to about 20 μm, and within the above range, may have a thickness ranging from 1 μm to 10 μm, 1μ m to 5 μm, or 1 μm to 3μ m.

According to one example embodiment, each heat-resistant layer may have a thickness ranging from about 0.8 μm to about 2.5 μm, for example, from 0.9 μm to 2.2 μm, or from 1.0 μm to 2.0 μm.

According to one example embodiment, the adhesive layers may have a total thickness ranging from 0.5 μm to 2.0 μm, for example, from 0.5 μm to 1.5 μm.

A ratio of the thickness of the coating layer to the thickness of the porous substrate may range from about 0.05 to about 0.5, for example, from 0.05 to 0.4, from 0.05 to 0.3, or from 0.1 to 0.2. Within the above range, the separator can exhibit desired or improved air permeability, heat resistance, bonding strength, and the like. Here, “thickness of the coating layer” indicates a thickness of one coating layer when the coating layer is formed on only one surface of the porous substrate, and indicates a thickness of two coating layers when the coating layer is formed on both surfaces of the porous substrate.

Porous Substrate

The porous substrate may be or include a substrate having multiple pores and commonly included in electrochemical devices. The porous substrate may be or include a polymer membrane formed of or including any one polymer such as or including at least one of a polyolefin such as polyethylene or polypropylene, polyester such as polyethylene terephthalate or polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyether ether ketone, polyaryl ether ketone, polyetherimide, polyamideimide, polybenzimidazole, polyethersulfone, polyphenylene oxide, a cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon, and polytetrafluoroethylene, or a copolymer or mixture of two or more types thereof.

The porous substrate may be or include, for example, a polyolefin-based substrate including a polyolefin, and the polyolefin-based substrate may have a desired or improved shutdown function, thereby contributing to increasing the safety of the battery. The polyolefin-based substrate may be of include, for example, at least one of a polyethylene single film, a polypropylene single film, a polyethylene/polypropylene double film, a polypropylene/polyethylene/polypropylene triple film, and a polyethylene/polypropylene/polyethylene triple film. In addition, the polyolefin-based resin may include a non-olefin resin in addition to an olefin resin or include a copolymer of olefin and non-olefin monomers.

The porous substrate may have a thickness ranging from about 1 μm to about 40 μm, for example, from 1 μm to 30 μm, from 1 μm to 20 μm, or from 5 μm to 15 μm.

The separator for a rechargeable lithium battery according to one example embodiment may have desired or improved bonding strength. For example, the separator for a rechargeable lithium battery may have a bonding strength of about 0.05 gf/mm or more, for example, ranging from 0.05 gf/mm to 0.1 gf/mm, for example, from 0.05 gf/mm to 0.2 gf/mm. The bonding strength may be measured by the following method.

A separator for a rechargeable lithium battery is located between a positive electrode and a negative electrode, and the separator is bonded to the positive electrode and the negative electrode by passing between rolls with a pressure of 250 kgf at a speed of 150 mm/sec in an 80° C. chamber. A sample is produced by cutting the separator bonded to the positive electrode and the negative electrode to a width of 25 mm and a length of 50 mm. In the above sample, the separator is separated from a negative electrode plate by about 10 mm to 20 mm, then the separator is fixed to an upper grip and the negative electrode plate is fixed to a lower grip so that a gap between the grips is 20 mm, and then peeled by being pulled in a 180° direction. After the peeling is started at a peeling speed of 20 mm/min, an average value was obtained by measuring a force required to peel 40 mm three times. The average value is calculated as the average value of the measured values.

The separator for a rechargeable lithium battery according to one example embodiment may exhibit desired or improved air permeability and have an air permeability value of, for example, less than about 200 sec/100 cc, for example, 190 sec/100 cc or less, or 180 sec/100 cc or less. That is, the separator may have an air permeability value of less than about 40 sec/100 cc 1 μm per unit thickness, for example, 30 sec/100 cc· 1 μm or less, or 25 sec/100 cc 1 μm or less. Herein, the air permeability refers to the time (seconds) it takes for 100 cc of air to pass through the unit thickness of the separator. The air permeability per unit thickness may be obtained by measuring the air permeability for the total thickness of the separator and dividing the air permeability by the thickness. The air permeability may be obtained by measuring the time it takes for 100 cc of air to pass through the separator using an air permeability measurement device (EG01-55-1MR, Asahi Seiko Co., Ltd.).

The separator for a rechargeable lithium battery according to one example embodiment may be formed by applying a composition for forming a coating layer on one surface or both surfaces of the porous substrate, drying, and then curing the composition. The curing may be performed using conventional methods known to those skilled in the art.

FIG. 1 is a cross-sectional view illustrating a separator for a rechargeable lithium battery, according to one example embodiment. Referring to FIG. 1, the separator for a rechargeable lithium battery includes a porous substrate 1 and a coating layer 2 located on both surfaces of the porous substrate 1. The coating layer 2 may include a heat-resistant layer 5 including a filler 3 and a cross-linked product 4 of a (meth)acryl-based binder and a cross-linking agent, and an adhesive layer 7 located on the heat-resistant layer 5 and including an adhesive binder 6.

Rechargeable Lithium Battery

According to one example embodiment, the rechargeable lithium battery includes the separator for a rechargeable lithium battery, a positive electrode, and a negative electrode.

The separator for rechargeable lithium battery refers to the description described above. The separator for rechargeable lithium battery may be positioned between the positive electrode and the negative electrode.

Positive Electrode

A positive electrode for a rechargeable lithium battery may include a current collector and a positive electrode active material layer on the current collector. The positive electrode active material layer may include a positive electrode active material, and may further include a binder and/or a conductive material.

For example, the positive electrode may further include an additive that can be configured as a sacrificial positive electrode.

Positive Electrode Active Material

The positive electrode active material may include a compound (lithiated intercalation compound) that is capable of intercalating and deintercalating lithium. For example, at least one of a composite oxide of lithium and a metal such as or including at least one of cobalt, manganese, nickel, and combinations thereof may be included.

The composite oxide may be or include a lithium transition metal composite oxide. Examples of the composite oxide may include at least one of lithium nickel-based oxide, lithium cobalt-based oxide, lithium manganese-based oxide, lithium iron phosphate-based compound, cobalt-free nickel-manganese-based oxide, or a combination thereof.

In the above Chemical Formulas, A is or includes at least one of Ni, Co, Mn, or a combination thereof; X is or includes at least one of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element or a combination thereof; D is or includes at least one of O, F, S, P, or a combination thereof; G is or includes at least one of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; and L1 is or includes at least one of Mn, Al, or a combination thereof.

The positive electrode active material may be or include, for example, a high nickel-based positive electrode active material having a nickel content that is greater than or equal to about 80 mol %, greater than or equal to about 85 mol %, greater than or equal to about 90 mol %, greater than or equal to about 91 mol %, or greater than or equal to about 94 mol % and less than or equal to about 99 mol % based on 100 mol % of the metal excluding lithium in the lithium transition metal composite oxide. The high-nickel-based positive electrode active material may be capable of achieving high capacity and can be applied to a high-capacity, high-density rechargeable lithium battery.

An amount of the positive electrode active material may be in a range of about 90 wt % to about 99.5 wt % based on 100 wt % of the positive electrode active material layer. Amounts of the binder and the conductive material may be in a range of about 0.5 wt % to about 5 wt %, respectively, based on 100 wt % of the positive electrode active material layer.

The binder is configured to attach the positive electrode active material particles to each other, and to attach the positive electrode active material to the current collector. Examples of the binder may include at least one of polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, a polymer including ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, a styrene-butadiene rubber, a (meth)acrylated styrene-butadiene rubber, an epoxy resin, a (meth)acrylic resin, a polyester resin, nylon, and the like, as non-limiting examples.

The conductive material may be included to impart conductivity (e.g., electrical conductivity) to the electrode. Any material that does not cause chemical change (e.g., that does not cause an undesirable chemical change in the rechargeable lithium battery), and that conducts electrons, can be included in the battery. Examples of the conductive material may include a carbon-based material such as at least one of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, a carbon fiber, a carbon nanofiber, and carbon nanotube; a metal-based material including at least one of copper, nickel, aluminum, silver, and the like, in a form of a metal powder or a metal fiber; a conductive polymer such as a polyphenylene derivative; or a mixture thereof.

Al may be included as the current collector, but is not limited thereto.

Negative Electrode

The negative electrode for a rechargeable lithium battery may include a current collector and a negative electrode active material layer on the current collector. The negative electrode active material layer may include a negative electrode active material, and may further include a binder and/or a conductive material (e.g., an electrically conductive material).

For example, the negative electrode active material layer may include about 90 wt % to about 99 wt % of the negative electrode active material, about 0.5 wt % to about 5 wt % of the binder, and about 0 wt % to about 5 wt % of the conductive material.

Negative Electrode Active Material

The negative electrode active material may include at least one of a material that reversibly intercalates/deintercalates lithium ions, a lithium metal, a lithium metal alloy, a material capable of doping/dedoping lithium, or a transition metal oxide.

The material that reversibly intercalates/deintercalates lithium ions may include a carbon-based negative electrode active material, such as, for example, crystalline carbon, amorphous carbon or a combination thereof. The crystalline carbon may be or include graphite such as non-shaped, sheet-shaped, flake-shaped, sphere-shaped, or fiber-shaped, natural graphite or artificial graphite. The amorphous carbon may be or include at least one of a soft carbon, a hard carbon, a mesophase pitch carbonization product, calcined coke, and the like.

The lithium metal alloy includes an alloy of lithium and a metal such as or including at least one of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn.

The material capable of doping/dedoping lithium may be or include a Si-based negative electrode active material or a Sn-based negative electrode active material. The Si-based negative electrode active material may include at least one of silicon, a silicon-carbon composite, SiOx (0<x<2), a Si-Q alloy (where Q is or includes at least one of an alkali metal, an alkaline-earth metal, a Group 13 element, a Group 14 element (excluding Si), a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and a combination thereof). The Sn-based negative electrode active material may include at least one of Sn, SnO2, a Sn-based alloy, or a combination thereof.

The silicon-carbon composite may be or include a composite of silicon and amorphous carbon. According to an example embodiment, the silicon-carbon composite may be in the form of silicon particles and amorphous carbon coated on the surface of the silicon particles. For example, the silicon-carbon composite may include a secondary particle (core) in which primary silicon particles are assembled, and an amorphous carbon coating layer (shell) on the surface of the secondary particle. The amorphous carbon may also be between the primary silicon particles, and, for example, the primary silicon particles may be coated with the amorphous carbon. The secondary particle may be dispersed in an amorphous carbon matrix.

The silicon-carbon composite may further include crystalline carbon. For example, the silicon-carbon composite may include a core including crystalline carbon and silicon particles and an amorphous carbon coating layer on a surface of the core.

The Si-based negative electrode active material or the Sn-based negative electrode active material may be combined with a carbon-based negative electrode active material.

The binder may be configured to attach the negative electrode active material particles to each other, and to attach the negative electrode active material to the current collector. The binder may include a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof.

The aqueous binder may be or include at least one of a styrene-butadiene rubber, a (meth)acrylated styrene-butadiene rubber, a (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, a butyl rubber, a fluoro rubber, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrine, polyphosphazene, poly(meth)acrylonitrile, an ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, a polyester resin, a (meth)acrylic resin, a phenol resin, an epoxy resins, polyvinyl alcohol, and a combination thereof.

When an aqueous binder is included as the negative electrode binder, a cellulose-based compound capable of imparting viscosity may be further included. The cellulose-based compound may include at least one of carboxymethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, or an alkali metal salt thereof. The alkali metal may include at least one of Na, K, or Li.

The dry binder may be or include a polymer material that is capable of being fibrous. For example, the dry binder may be or include at least one of polytetrafluoroethylene, polyvinylidene fluoride, a polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

The conductive material may be included to impart conductivity (e.g., electrical conductivity) to the electrode. Any material that does not cause chemical change (e.g., that does not cause an undesirable chemical change in the rechargeable lithium battery), and that conducts electrons, can be included in the battery. Non-limiting examples thereof may include a carbon-based material such as at least one of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, a carbon fiber, a carbon nanofiber, and a carbon nanotube; a metal-based material including at least one of copper, nickel, aluminum, silver, and the like. in a form of a metal powder or a metal fiber; a conductive polymer such as a polyphenylene derivative; or a mixture thereof.

The negative current collector may include at least one of a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, or a combination thereof.

The rechargeable lithium battery may further include an electrolyte solution.

Electrolyte Solution

The electrolyte solution for a rechargeable lithium battery may include a non-aqueous organic solvent and a lithium salt.

The non-aqueous organic solvent may be configured as a medium for transmitting ions taking part in the electrochemical reaction of a battery.

The non-aqueous organic solvent may be or include at least one of a carbonate-based, ester-based, ether-based, ketone-based, or alcohol-based solvent, an aprotic solvent, or a combination thereof.

The ether-based solvent may include at least one of dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydrofuran, and the like. In addition, the ketone-based solvent may include cyclohexanone, and the like. The alcohol-based solvent may include at least one of ethanol, isopropyl alcohol, and the like and the aprotic solvent may include at least one of nitriles such as R—CN (wherein R is a C2 to C20 linear, branched, or cyclic hydrocarbon group, a double bond, an aromatic ring, or an ether bond, and the like); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane, 1,4-dioxolane, and the like; sulfolanes, and the like.

The non-aqueous organic solvents may be included alone or in combination of two or more solvents.

In addition, when using a carbonate-based solvent, a cyclic carbonate and a chain carbonate may be mixed, and the cyclic carbonate and the chain carbonate may be mixed in a volume ratio of about 1:1 to about 1:9.

The lithium salt dissolved in the organic solvent is configured to supply lithium ions in a battery, to enable a basic operation of a rechargeable lithium battery, and to improve transportation of the lithium ions between positive and negative electrodes. Examples of the lithium salt include at least one of LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LIN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide, LiFSI), LiC4F9SO3, LiN(CxF2x+1SO2) (CyF2y+1SO2) (wherein x and y are integers of 1 to 20), lithium trifluoromethane sulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalato)phosphate (LiDFOB), and lithium bis(oxalato) borate (LiBOB).

The rechargeable lithium battery may be classified into cylindrical, prismatic, pouch, or coin-type batteries, and the like depending on their shape.

FIGS. 2 to 5 are schematic views illustrating a rechargeable lithium battery according to an example embodiment. FIG. 2 illustrates a cylindrical battery, FIG. 3 illustrates a prismatic battery, and FIGS. 4 and 5 illustrate pouch-type batteries. Referring to FIGS. 2 to 5, the rechargeable lithium battery 100 may include an electrode assembly 40 including a separator 30 between a positive electrode 10 and a negative electrode 20, and a case 50 in which the electrode assembly 40 is included. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte solution (not shown). The rechargeable lithium battery 100 may include a sealing member 60 sealing the case 50, as shown in FIG. 2. In FIG. 3, the rechargeable lithium battery 100 may include a positive lead tab 11, a positive terminal 12, a negative lead tab 21, and a negative terminal 22. As shown in FIGS. 4 and 5, the rechargeable lithium battery 100 may include an electrode tab 70 illustrated in FIG. 5, or, for example, a positive electrode tab 71 and a negative electrode tab 72 illustrated in FIG. 4, the electrode tabs 70/71/72 forming an electrical path for inducing the current formed in the electrode assembly 40 to the outside of the battery 100.

The rechargeable lithium battery according to an example embodiment may be applicable to, e.g., automobiles, mobile phones, and/or various types of electric devices, as non-limiting examples.

Hereinafter, examples and comparative examples of the present disclosure are described. However, the following examples are merely example embodiments of the present disclosure, and the present disclosure is not limited to the following examples.

Preparation Example 1

In a 10 L four-necked flask provided with a stirrer, a thermometer, and a cooling tube, a process of adding distilled water (6361 g), acrylic acid (AA) (604.1 g, 8.5 mol), acrylamide (AM) (72.06 g, 1.0 mol), potassium persulfate (2.7 g, 0.01 mol), 2-acrylamido-2-methylpropane sulfonic acid (AMPS) (103.6 g, 0.5 mol), and a 5N aqueous lithium hydroxide solution (1.05 equivalents with respect to a total amount of 2-acrylamido-2-methylpropanesulfonic acid), then reducing an internal pressure to 10 mmHg using a diaphragm pump, and returning the internal pressure to a normal pressure using nitrogen was repeated three times.

The reaction was carried out for 12 hours while controlling the temperature of the reaction solution to be stable between 65° C. and 70° C. After cooling to room temperature, the pH of the reaction solution was adjusted to 7 to 8 using a 25% aqueous ammonia solution.

Accordingly, poly(acrylic acid-co-acrylamide-co-2-acrylamido-2-methylpropanesulfonic acid) lithium salt was prepared. A molar ratio of acrylic acid, acrylamide, and 2-acrylamido-2-methylpropane sulfonic acid was 10:85:5. A non-volatile component in about 10 mL of the reaction solution (reaction product) was measured and the measurement result was 9.5 wt % (theoretical value: 10%).

Preparation Example 2

An acryl-based copolymer was prepared in the same manner as in Preparation Example 1, with a difference that acrylic acid (50 g, 0.69 mol) and 2-acrylamido-2-methylpropane sulfonic acid (50 g, 0.24 mol) were used and acrylonitrile was not used. A molar ratio of acrylic acid and 2-acrylamido-2-methylpropane sulfonic acid was 74:26. The non-volatile component of the reaction solution was 9.0 wt % (theoretical value: 10%).

Table 1 below illustrates the molar ratio of each monomer in the copolymers prepared in Preparation Examples 1 and 2.

AM
AA
AMPS

A dispersion was prepared by mixing the acryl-based binder (10 wt % in distilled water) prepared in Preparation Example 1 and boehmite (particle diameter D100: 0.5 μm, particle diameter D50: 0.2 μm, plate-shaped, Anhui Estone) as a filler in a mass ratio of 1:20 for the acryl-based copolymer and the filler based on solid content, adding the mixture to a water solvent, and then milling and dispersing the mixture at 25° C. for 30 minutes using a bead mill.

A composition for forming a heat-resistant layer was prepared by adding trimethylolpropane tris(2-methyl-1-aziridine propionate) as an aziridine-based cross-linking agent in an amount of 0.1 part by weight (content of 10 wt % of the acryl-based copolymer) based on solid content to the dispersion and adding water so that the total solid content became 20 wt %.

Heat-resistant layers were formed by coating both surfaces of a polyethylene film (thickness: 9 μm, SK Company, air permeability: 120 sec/100 cc, and puncture strength: 480 kgf) as a porous substrate with the composition for forming a heat-resistant layer using a die coating method and then drying and aging the heat-resistant layer in an oven at 80° C. for 16 hours.

A separator for a rechargeable lithium battery was manufactured by diluting a cross-linked polymethyl (meth)acrylate polymer (cross-linked PMMA, BM-900B, particle diameter: 0.5 μm) as an adhesive binder to a concentration of 2 wt % solid content, then coating one surface of each heat-resistant layer with the above polymer to 0.5 μm in a loading amount of 1.5 g/m2 with respect to a negative electrode plate surface, and then drying the heat-resistant layer at 50° C. for 10 minutes to form adhesive layers with a total thickness of 1.0 μm.

Examples 2 and 3

Separators for a rechargeable lithium battery were manufactured in the same manner as in Example 1, with a difference that in Example 1, as shown in Table 1 below, boehmite was used as the filler but D50 and D100 were changed, and the content of the aziridine-based crosslinking agent and the loading amount of the adhesive binder were changed.

Comparative Examples 1 to 4

Separators for a rechargeable lithium battery were manufactured in the same manner as in Example 1, with a difference that in Example 1, as shown in Table 1 below, the D50 and D100 of the filler (boehmite), the type of (meth)acryl-based binder, the type of cross-linking agent, the content of the cross-linking agent, the loading amount of the adhesive binder, and the like. were changed. An epoxy-based cross-linking agent is ethylene glycol diglycidyl ether.

Samples were manufactured by cutting the separators for a rechargeable lithium battery of Examples and Comparative Examples to a size of 8 cm×8 cm. A shrinkage rate in each of a machine direction (MD) and a transverse direction (TD) was calculated by drawing a square with a size of 5 cm×5 cm on surfaces of the samples, then putting the samples between pieces of paper or alumina powder, leaving the samples in an oven at 150° C. for 1 hour, taking the sample out, and then measuring the side dimensions of the drawn square. The shrinkage rate was calculated according to Equation 1 below.

L0 denotes an initial length of the separator, and L1 denotes a length of the separator after being left at 150° C. for 1 hour.

Shrinkage Rate in Electrolyte (Units: %)

Samples were manufactured by cutting the separators for a rechargeable lithium battery of Examples and Comparative Examples to a size of 8 cm×8 cm. A square with a size of 5 cm×5 cm was drawn on surfaces of the samples.

A positive electrode slurry was prepared by mixing 97 wt % LiCoNiAl as a positive electrode active material, 1.5 wt % carbon nanotubes, and 1.5 wt % polyvinyl fluoride as a conductive material and adding water thereto. A positive electrode was manufactured by applying the prepared positive electrode slurry on aluminum foil and drying and rolling the prepared positive electrode slurry.

A negative electrode active material slurry was prepared by mixing 97.4 wt % negative electrode active material, 1.0 wt % carboxymethyl cellulose, 1.5 wt % styrene-butadiene-based rubber, and 0.1 wt % carbon nanotubes as a conductive material. A silicon-based negative electrode active material was used as the negative electrode active material. A negative electrode was manufactured by applying the prepared negative electrode slurry on copper foil and drying and rolling the prepared negative electrode slurry.

One sample was located between the positive electrode and the negative electrode to form three sets of positive electrode-sample-negative electrode laminates, which were then put in a pouch. 2 g of an electrolyte (ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (a volume ratio of 30:50:20) in which 1.5M LiPF6 was dissolved) was injected to completely saturate the laminate with the electrolyte, which was sealed and left at 25° C. for 12 hours. Then, a shrinkage rate in each of the machine direction (MD) and the transverse direction (TD) was calculated by leaving the laminate in the oven at 150° C. for 1 hour, then taking the sample out, and measuring the sides dimensions of the drawn square. The shrinkage rate was calculated according to Equation 1.

Presence or Absence of Cross-Linking

Samples were manufactured by cutting the separators for a rechargeable lithium battery of Examples and Comparative Examples to a size of 8 cm×8 cm. When the sample was fully immersed in deionized water at 25° C. and left for 25 hours, whether the filler was detached from the coating layer was visually checked. When the filler is not detached, it indicates that the coating layer composition is cross-linked, and when the filler is detached, it indicates that the coating layer composition is not cross-linked. Although not shown in Table 2 below, the coating layers in all of the Examples and Comparative Examples were crosslinked.

Increase in Air Permeability (Units: Sec/100 c)

The air permeability was measured by a method of measuring the time (units: seconds) it took for 100 cc of air to pass through the separator using a measurement device (EG01-55-1MR, Asahi Seiko).

The air permeability of the separators manufactured in the Examples and Comparative Examples was measured. The air permeability was measured in the same manner after putting the prepared separators in a pouch cell including an electrolyte (same as the electrolyte used to measure the shrinkage rate), compressing the pouch cell, and then taking the separator out, and the difference was calculated. An increase in air permeability is for example 80 sec/100 c or less.

linking

Cross-
agent
Adhesive binder
Dry shrinkage
Shrinkage rate
Increase

Filler

linking
Content

Loading
rate
in electrolyte
in air

Example 1
based

linked

Example 1
based

linked

Example 1
based

linked

Example 1
based

linked

Example 1
based

linked

Example 2
based

linked

Example 1
based

linked

As shown in Table 2, the separators for a rechargeable lithium battery of the Examples can exhibit a significantly low dry shrinkage rate and shrinkage rate in the electrolyte, thereby increasing the stability of the battery.

A separator for a rechargeable lithium battery according to one example embodiment can exhibit a significantly low dry shrinkage rate and shrinkage rate in an electrolyte, thereby increasing the stability of the battery.

Although example embodiments of the present disclosure have been described above, the present disclosure is not limited thereto and may be modified in any form within the scope of the claims, the detailed description of the present disclosure, and the accompanying drawings, and the modifications also fall within the scope of the present disclosure.