KR20240145674A - Composite separator and secondary battery comprising the same - Google Patents

Abstract

A composite membrane having excellent long-term cycle stability and rate performance is provided. According to one aspect of the present invention, a composite membrane is provided, comprising: a membrane; and a protective layer disposed on at least one surface of the membrane; wherein the protective layer comprises a first repeating unit derived from a polymer having a functional group, a second repeating unit derived from a crosslinking agent, and a third repeating unit derived from a fluorinated carbon material.

Description

COMPOSITE SEPARATOR AND SECONDARY BATTERY COMPRISING THE SAME

The present invention relates to a composite separator, and more specifically, to a composite separator and a secondary battery including the same.

A typical example of a secondary battery is a lithium-ion battery, which is composed of a cathode, a separator, an anode, and an electrolyte as its basic components. It is a representative energy device with high energy density that can be charged and discharged while reversibly converting chemical energy and electrical energy, and can be widely used in small electronic devices such as mobile phones and laptops. Recently, lithium-ion batteries are rapidly expanding their application to hybrid electric vehicles (HEVs), plug-in electric vehicles (plug-in EVs), electric bicycles (E-bikes), and energy storage systems (ESS) in response to environmental issues, high oil prices, and energy efficiency and storage.

Meanwhile, in the case of lithium metal batteries using lithium metal as an electrode, the energy density of the battery can be improved due to the high theoretical capacity (3860 mAh/g) and low thermodynamic redox potential of lithium metal. However, these lithium metal batteries have the problem of risk due to the high reactivity of lithium metal, and research is continuously being conducted to solve this problem.

Lithium dendrite is a branch-shaped crystal that is formed when using a lithium-ion battery, and refers to the crystal that accumulates on the surface of the negative electrode during the charging process of the battery. When lithium metal is used as the negative electrode, the solid-electrolyte-interphase (SEI) film becomes unstable as the battery continues to charge and discharge, and dead lithium is formed inside the SEI film, and lithium dendrite that consumes electrolyte and lithium metal is formed on the surface of the electrode. As a result, there is a problem that the cycle performance and rate performance of the secondary battery are reduced.

The purpose of the present invention is to provide a composite separator that improves the cycle stability of a secondary battery.

Another object of the present invention is to provide a composite separator that improves the rate performance of a secondary battery.

Another object of the present invention is to provide a composite separator having excellent performance of a secondary battery even when the reversible capacity ratio (N/P ratio) of the positive and negative electrodes is lowered.

Another object of the present invention is to provide a method for manufacturing the composite membrane.

The purposes of the present invention are not limited to the purposes mentioned above, and other purposes and advantages of the present invention which are not mentioned can be understood by the following description, and will be more clearly understood by the embodiments of the present invention. In addition, it will be easily understood that the purposes and advantages of the present invention can be realized by the means and combinations thereof indicated in the claims.

According to a first aspect of the present invention for achieving the above object, a composite separator is provided, comprising: a separator; and a protective layer disposed on at least one surface of the separator, wherein the protective layer comprises a first repeating unit derived from a polymer having a functional group, a second repeating unit derived from a crosslinking agent, and a third repeating unit derived from a fluorinated carbon material.

According to the present invention, the functional group may include a carboxyl group.

According to the present invention, the crosslinking agent may include a compound having two or more crosslinking reaction functional groups at the terminal.

According to the present invention, the crosslinking reaction functional group may include at least one of an epoxy group and an isocyanate group.

According to the present invention, the crosslinking agent may be one or a mixture of two or more selected from the group consisting of PEGDE (poly(ethylene glycol) diglycidyl ether), DGEBA (diglycidyl ether of bisphenol-A), RDGE (resorcinol diglycidyl ether), TGIC (triglycidyl isocyanurate), TMPTGE (trimethylolpropane triglycidyl ether), TPEGE (tetraphenylolethane glycidyl ether), and TPGECMS (tetrakis(propyl glycidyl ether)cyclomethylsiloxane).

According to the present invention, the fluorinated carbon material may be a fluorinated compound, at least one of which is selected from the group consisting of graphene oxide and modified carbon nanotubes.

According to the present invention, the second repeating unit can be connected to the first repeating unit and the third repeating unit.

According to the present invention, a secondary battery can be provided, which includes a composite separator; a cathode disposed on one side of the composite separator; a cathode disposed on the other side of the composite separator; and an electrolyte.

According to the present invention, the negative electrode may include lithium metal (Li metal).

According to the present invention, the negative electrode may include at least one negative electrode active material selected from a carbonaceous material and a silicon-based material.

According to the present invention, the reversible capacity ratio (N/P ratio) of the positive electrode and the negative electrode may be 1.5 or less.

According to the present invention, the reversible capacity ratio (N/P ratio) of the positive electrode and the negative electrode may be 1.1 or less.

The above-mentioned solution to the problem does not enumerate all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail by referring to the specific examples below.

According to one aspect of the present invention, a composite separator can be provided that improves charge transfer between the interface of an electrode and a separator by increasing lithiophilicity as the surface of the separator becomes more hydrophilic.

According to another aspect of the present invention, a composite separator having excellent electrochemical stability and being robust can be provided.

According to another aspect of the present invention, a secondary battery having improved cycle stability and excellent rate performance can be provided by implementing a stable voltage profile for a long period of time.

According to another aspect of the present invention, a secondary battery can be provided that improves the mobility of lithium ions in an electrolyte through the formation of a uniform SEI film.

In addition to the effects described above, the specific effects of the present invention are described below together with the specific contents for carrying out the invention.

Figure 1 is a schematic diagram of a ring-opening reaction of an epoxy structure by a hydroxyl group and a ring-opening reaction of an epoxy structure by a carboxylate group.
Figure 2 is a schematic diagram showing the growth of lithium dendrites in a conventional lithium metal battery.
Figure 3 is a schematic diagram of a secondary battery according to one embodiment of the present invention.
Figure 4a is a digital photograph of graphite powder.
Fig. 4b is a digital photograph of FGO powder.
Figure 5 is a SEM (Scanning Electron Microscope) photograph of FGO according to Manufacturing Preparation Example 1.
Figure 6a is an EDS (Energy Dispersive X-ray Spectrometer) mapping result for carbon elements in FGO of Manufacturing Preparation Example 1.
Figure 6b is an EDS mapping result for fluorine element of FGO of Manufacturing Preparation Example 1.
Figure 6c is an EDS mapping result for the oxygen element of FGO of Manufacturing Preparation Example 1.
Figure 7 is an XRD (X-ray diffraction) pattern of graphite, graphene oxide (GO), and FGO according to Manufacturing Preparation Example 1.
Figure 8 shows the XPS comparative analysis results of GO and FGO.
Figure 9 is the FT-IR (Fourier-transform infrared spectroscopy) of each GO and FGO according to Manufacturing Preparation Example 1.
Figure 10 illustrates the synthesis steps of PAA-PEGDE according to one embodiment of the present invention.
Figure 11a is the FT-IR (Fourier-transform infrared spectroscopy) of each PAA and PAALi.
Figure 11b is the FT-IR (Fourier-transform infrared spectroscopy) of each PAALi, PEGDE, and PAA-PEGDE.
Figure 12a shows the glass transition temperatures of PAA and PAA-PEGDE measured using DSC (Differential Scanning Calorimetry).
Figure 12b shows the results of TGA (Thermogravimetric analysis) of PAA and PAA-PEGDE.
Figure 13 is a digital photograph of a dispersion containing FGO (3 mg) and PEGDE (1 mg) completely dispersed in deionized water before and after cross-linking reaction.
Figure 14 is an elemental mapping photograph of SEM-EDS for the protective layer in the composite membrane of Example 1.
Figure 15a shows the contact angle of deionized water on the surface of the membrane according to Comparative Example 1.
Figure 15b shows the contact angle of deionized water on the surface of the protective layer of the membrane according to Example 1.
Figure 16a shows the results of evaluating the electrochemical performance of a lithium-lithium symmetric cell including the separator of Comparative Example 1 and Example 1.
Figure 16b shows the results of evaluating the electrochemical performance of a lithium-lithium symmetric cell including the separator of Comparative Example 2.
Figure 17 shows the results of a linear sweep voltammetry (LSV) test of a lithium-lithium symmetric cell including the separator of Comparative Example 1 and Example 1.
Figure 18 is a schematic diagram to demonstrate the effect of a protective layer at the interface of a separator and lithium metal (electrode).
Figure 19 shows the results of EIS (Electrochemical impedance spectroscopy) evaluation of lithium-lithium symmetric cells including the separators of Comparative Example 1 and Example 1 after the first and fifth cycles.
Figure 20a is a ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometer) depth profile of LiF ^- after the fifth cycle.
Figure 20b is a ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometer) depth profile of C ₂ H ₃ O ^- after the fifth cycle.
Figure 20c is a ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometer) depth profile of CF ₃ ^- after the fifth cycle.
Figure 21a shows the results of evaluating the electrochemical performance of the LFP-Li complete paper according to Comparative Example 1 and the LFP-Li complete paper according to Example 1 when the loading amount of LFP is 9.1 mg/cm ² .
Figure 21b shows the electrochemical performance evaluation results of the LFP-Li full-cell when the reversible capacity ratio (N/P ratio) of the positive and negative electrodes is 1.0 when the LFP loading amount is 24.5 mg/cm ² .
Figure 21c shows the rate performance of LFP-Li full-sheet when the loading amount of LFP is 9.1 mg/cm ² .
Figure 22a is a SEM cross-sectional image of a lithium metal electrode in the LFP-Li complete cell according to Comparative Example 1 after the 10th lithium stripping.
Figure 22b is a SEM cross-sectional image of a lithium metal electrode in the LFP-Li full-cell according to Example 1 after the 10th lithium stripping.
Figure 23 shows the results of evaluating the coulombic efficiency of a Micro Silicon-Li battery according to Comparative Example 1 and a Micro Silicon-Li battery according to Example 1.

The numerical range indicated by the term "to" in this specification refers to a numerical range that includes the values described before and after the term as the lower limit and the upper limit, respectively. When multiple numerical values are disclosed as the upper limit and the lower limit of an arbitrary numerical range, the numerical range disclosed in this specification can be understood as an arbitrary numerical range that uses any one of the multiple lower limit values and any one of the multiple upper limit values as the lower limit and the upper limit, respectively.

According to one aspect of the present invention, a composite separator is provided, comprising: a separator; and a protective layer disposed on at least one surface of the separator; wherein the protective layer comprises a first repeating unit derived from a polymer having a functional group, a second repeating unit derived from a crosslinking agent, and a third repeating unit derived from a fluorinated carbon material.

In the past, when implementing a secondary battery with a general separator without a protective layer, a highly porous SEI film was formed along with the growth of lithium dendrites during the lithium stripping/plating process, and dead lithium was also generated inside the SEI film. Due to this unstable SEI film, there was a problem in that the electrolyte was continuously decomposed, consumed, and the performance of the battery deteriorated.

According to one aspect of the present invention, a composite separator can be provided in which a protective layer is disposed on at least one surface of a separator, thereby making the surface of the separator more hydrophilic, thereby increasing lithiophilicity and improving charge transfer between the interface of the electrode and the separator. According to another aspect of the present invention, a composite separator having excellent electrochemical stability and a strong protective layer can be provided through a cross-linking agent capable of inducing cross-linking between a fluorinated carbon material and a polymer. According to another aspect of the present invention, a secondary battery can be provided in which cycle stability is improved and rate performance is excellent by implementing a stable voltage profile for a long period of time. According to another aspect of the present invention, a secondary battery can be provided in which the mobility of lithium ions in an electrolyte is improved by forming a uniform SEI (solid-electrolyte-interphase) film between the interface of the electrode and the protective layer.

Below, the composition of the present invention is described in more detail.

1. Composite membrane and its manufacturing method

Membrane

The separation membrane according to the present invention may include a porous substrate.

The porous substrate according to the present invention can be a porous structure with fine pore diameters that can electrically insulate the negative electrode and the positive electrode to prevent short circuits while providing a path for lithium ions to move.

As a constituent material of the porous substrate, any organic or inorganic material having electrical insulation can be used without particular limitation. The porous substrate may include, for example, at least one selected from the group consisting of polyolefin, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyvinyl terephthalate, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalene, and specifically may include polyolefin. Polyolefin not only has excellent coatability, but also can increase the ratio of the electrode active material layer in the battery by making the separator thin, thereby increasing the capacity per volume. Specifically, the weight average molecular weight (M _w ) of the polyolefin can be 100,000 to 500,000 g/mol. If the weight average molecular weight of the polyolefin is less than the above numerical range, it may be difficult to secure sufficient mechanical properties, and if it exceeds the above numerical range, the shutdown function may not be implemented or molding may become difficult. The shutdown function refers to the function of blocking the movement of ions and preventing thermal runaway of the battery by melting the thermoplastic resin and closing the pores of the porous substrate when the temperature of the secondary battery increases.

The thickness of the porous substrate may be, for example, 3 to 50 μm or 4 to 15 μm. If the thickness of the porous substrate is less than the numerical range, the function of the conductive barrier may not be sufficient, and if it exceeds the numerical range, the resistance of the separator may excessively increase.

The average diameter of the pores included in the porous substrate may be, for example, 10 to 100 nm. The pores included in the porous substrate have a structure in which they are interconnected with each other, so that gas or liquid can pass from one side of the porous substrate to the other side.

The separation membrane according to the present invention may further include a coating layer disposed on at least one surface of the porous substrate.

The coating layer according to the present invention can improve the mechanical strength and heat resistance of a separator for a secondary battery, and increase the ion conductivity within the secondary battery.

The coating layer according to the present invention may include a binder polymer and inorganic particles.

The binder polymer according to the present invention can connect inorganic particles and stably fix them. The above binder polymers include, for example, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, poly(ethylene-co-vinyl acetate), polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, One or more selected from the group consisting of cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, acrylonitrile-styrene butadiene copolymer, polyimide, and styrene-butadiene rubber may be used in combination.

The inorganic particles according to the present invention can contribute to improving the mechanical strength and heat resistance of a separator for a secondary battery. Specifically, the inorganic particles are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles that can be used in the present invention are not particularly limited as long as they do not undergo oxidation and/or reduction reactions in the operating voltage range of the applied secondary battery (e.g., 0 to 5 V based on Li/Li ⁺ ).

For example, when using inorganic particles having a high dielectric constant as inorganic particles, it can contribute to increasing the degree of dissociation of an electrolyte salt, such as a lithium salt, in a liquid electrolyte, thereby improving the ionic conductivity of the electrolyte. For the reasons described above, the inorganic particles may be inorganic particles having a dielectric constant of 5 or higher, inorganic particles having a lithium ion transfer capability, or a mixture thereof.

The inorganic particles having a dielectric constant of 5 or more are one selected from the group consisting of Al ₂ O ₃ , SiO ₂ , ZrO ₂ , AlO(OH), Al(OH) ₃ , Mg(OH) ₂ , BaSO ₄ , TiO ₂ , BaTiO ₃ , Pb(Zr _x Ti _1-x )O ₃ (PZT, where 0<x<1), Pb _1-x La _x Zr _1-y Ti _y O ₃ (PLZT, where 0<x<1 and 0<y<1), (1-x)Pb(Mg _1/3 Nb _2/3 )O _3-x PbTiO ₃ (PMN-PT, where 0<x<1), HfO ₂ , SrTiO ₃ , SnO ₂ , CeO ₂ , MgO, NiO, CaO, ZnO, and SiC. Or it may be a mixture of two or more.

The inorganic particles having the lithium ion transport capability are lithium phosphate (Li ₃ PO ₄ ), lithium titanium phosphate (Li _x Ti _y (PO ₄ ) ₃ , 0<x<2, 0<y<3), lithium aluminum titanium phosphate (Li _x Al _y Ti _z (PO ₄ ) ₃ , 0<x<2, 0<y<1, 0<z<3), (LiAlTiP) _x O _y series glass (0<x<4, 0<y<13), lithium lanthanum titanate (Li _x La _y TiO ₃ , 0<x<2, 0<y<3), lithium germanium thiophosphate (Li _x Ge _y P _z S _w , 0<x<4, 0<y<1, 0<z<1, 0<w<5), lithium nitride (Li _x N _y , 0<x<4, The glass may be one or a mixture of two or more selected from the group consisting of SiS ₂ series glass (Li _x Si _y S _z , 0<x<3, 0<y<2, 0<z<4) and P ₂ S ₅ series glass (Li _x P _y S _z , 0<x<3, 0<y<3, 0<z<7).

For example, the average particle diameter (D ₅₀ ) of the above-mentioned inorganic particles may be 1 nm to 10 μm, specifically 10 nm to 2 μm, and more specifically 50 nm to 1 μm, for forming a coating layer of uniform thickness and an appropriate porosity. The "average particle diameter (D ₅₀ )" refers to the particle diameter at the 50% point of the cumulative distribution of the number of particles according to particle diameter. The average particle diameter can be measured using a laser diffraction method. Specifically, after the target powder for measurement is dispersed in a dispersion medium, it is introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac S3500) and the difference in diffraction pattern according to particle size is measured when the particles pass through a laser beam, thereby calculating the particle size distribution.

According to another embodiment of the present invention, the weight ratio of the inorganic particles and the binder polymer (inorganic particles: binder polymer) may be 50:50 to 99:1, and specifically, 70:30 to 95:5. When the content ratio of the inorganic particles to the binder polymer is less than the above numerical range, the content of the binder polymer increases, which may deteriorate the thermal safety improvement performance of the separator, and the pore size and porosity may decrease due to a decrease in the empty space formed between the inorganic particles, which may cause a deterioration in the performance of the final battery, and when the numerical range is exceeded, the content of the binder polymer is too small, which may weaken the peeling resistance of the coating layer.

For example, the thickness of the coating layer may be 0.1 to 10 μm, specifically 1 to 3 μm, and more specifically 1.4 to 1.6 μm. When the thickness of the coating layer satisfies the numerical range, the insulation and thermal stability of the separator can be increased, and the energy density of the battery can be improved.

For example, the packing density of the coating layer may be 0.1 to 20 g/cm ³ , specifically 0.5 to 12 g/cm ³ , and more specifically 1 to 3 g/cm ³ . When the packing density of the coating layer satisfies the numerical range, the permeability of lithium ions may be advantageous, and the heat resistance of the separator may be maintained at an appropriate level. Meanwhile, the packing density is the density of the coating layer loaded at a height of 1 μm per unit area (m ² ) of the porous substrate.

Protective layer

The composite separator according to the present invention comprises a protective layer disposed on at least one side of the separator. Specifically, the protective layer may be disposed directly on one side of the separator or directly on both sides of the separator.

A protective layer according to one aspect of the present invention can improve the mobility of lithium ions between the interface of the electrode and the separator by increasing the lithium affinity (lithiophilicity) of the surface of the separator. A protective layer according to another aspect of the present invention can effectively prevent the formation of lithium dendrites by forming a uniform SEI film at the interface of the negative electrode and the protective layer through interaction with the negative electrode.

A protective layer according to the present invention comprises a first repeating unit derived from a polymer having a functional group, a second repeating unit derived from a crosslinking agent, and a third repeating unit derived from a fluorinated carbon material.

A polymer having a functional group according to the present invention can increase lithium affinity on the surface of a separator and form a cross-linking bond with a fluorinated carbon material through a cross-linking agent.

The polymer having a functional group according to the present invention may be a polymer compound capable of forming a lithium salt by an acid-base reaction with OH ^- , specifically, a polymer compound including at least one of a carboxyl group, a hydroxyl group, and a thiol group, and more specifically, a polymer compound including a carboxyl group, and more specifically, an acrylic acid copolymer in which acrylic acid and one or more monomers selected from the group consisting of methacrylate, methyl methacrylate, ethylene, acrylonitrile, ethylene glycol, ethylene oxide, propylene oxide, propylene carbonate, and phenylene sulfide are copolymerized; alginic acid; carboxymethyl cellulose, and combinations thereof, and more specifically, polyacrylic acid. According to one embodiment of the present invention, when the polymer compound including the carboxyl group is used as the polymer having the functional group, a polymer in the form of a lithium salt can be easily synthesized by an acid-base reaction, so that the synthesis process of the crosslinking reaction can be easily performed.

The weight average molecular weight (Mw) of the polymer having a functional group according to the present invention may vary depending on the type of the polymer compound. For example, when the polymer having the functional group is polyacrylic acid, the weight average molecular weight of the polymer having the functional group may be 300,000 to 600,000 g/mol, 400,000 to 500,000 g/mol, 430,000 to 470,000 g/mol, or 450,000 to 470,000 g/mol.

When the weight average molecular weight of the polymer having the functional group has a weight average molecular weight (Mw) in the above-described range, one polymer chain can widely network the membrane to strengthen the structure of the protective layer. However, when the weight average molecular weight of the polymer having the functional group exceeds the above-described range, the viscosity of the composition for forming the protective layer increases and the solubility of the polymer having the functional group in a solvent decreases, so that the coating on the membrane may become uneven. When the weight average molecular weight of the polymer having the functional group is smaller than the above-described range, the network between the polymers may not be formed densely, so that the durability of the protective film may decrease.

The first repeating unit according to the present invention is derived from a polymer having a functional group and can be analyzed using various analytical methods commonly used in the relevant technical field. Specifically, the first repeating unit can be analyzed using ¹ H-NMR, FT-IR, or a combination thereof.

The crosslinking agent according to the present invention can induce a chemical bond (e.g., a covalent bond) between the polymer having the functional group and the fluorinated carbon material. Specifically, the second repeating unit derived from the crosslinking agent can link the first repeating unit derived from the polymer having the functional group and the third repeating unit derived from the fluorinated carbon material to each other. By crosslinking the polymer having the functional group and the fluorinated carbon material through the crosslinking agent according to the present invention, a composite separator having excellent electrochemical stability and a strong protective layer can be implemented.

According to one embodiment of the present invention, the crosslinking agent may include a compound having two or more crosslinking reaction functional groups at the terminal. Since the crosslinking agent has two or more crosslinking reaction functional groups at the terminal, it may form a crosslinking matrix. For example, the crosslinking reaction functional group may include at least one of an epoxy group and an isocyanate group, and specifically, may include an epoxy group. According to one embodiment of the present invention, when an epoxy group is used as the crosslinking reaction functional group, a polymer compound having a functional group in the form of a lithium salt acts as a nucleophile (Nu ^- ) to provide electrons to a less substituted carbon atom in the epoxy structure of the crosslinking agent. Accordingly, the ring-opening reaction of the epoxy structure easily proceeds, so that the partial crosslinking reaction easily proceeds. Fig. 1 is a schematic diagram of a ring-opening reaction of an epoxy structure by a hydroxyl group and a ring-opening reaction of an epoxy structure by a carboxylate group. Referring to Figure 1, it can be confirmed that the ring opening reaction of the epoxy structure easily proceeds by a hydroxyl group or a carboxyl group.

In one example, the cross-linking agent may be one or a mixture of two or more selected from the group consisting of poly(ethylene glycol) diglycidyl ether (PEGDE), diglycidyl ether of bisphenol-A (DGEBA), resorcinol diglycidyl ether (RDGE), triglycidyl isocyanurate (TGIC), trimethylolpropane triglycidyl ether (TMPTGE), tetraphenylolethane glycidyl ether (TPEGE), and tetrakis(propyl glycidyl ether)cyclomethylsiloxane (TPGEMS).

The second repeating unit according to the present invention can be analyzed using various analytical methods commonly used in the relevant technical field. Specifically, the second repeating unit can be analyzed using ¹ H-NMR, FT-IR, or a combination thereof.

The fluorinated carbon material according to the present invention imparts conductive properties to a protective layer, and forms a cross-linking bond with a polymer having the functional group through a cross-linking agent to realize a composite separator having excellent electrochemical stability and a strong protective layer.

According to the present invention, any carbon material capable of forming a partial cross-linking with the cross-linking agent can be used without limitation as the fluorinated carbon material. For example, the fluorinated carbon material may be a compound in which at least one or more is fluorinated and is selected from the group consisting of graphene oxide and modified carbon nanotubes, and specifically, may be a compound in which graphene oxide is fluorinated. Here, the modified carbon nanotube is a compound in which at least one functional group among a carboxyl group and a hydroxyl group is introduced to a carbon nanotube by a method known in the relevant technical field, and a partial cross-linking reaction with the cross-linking agent can proceed through the functional group.

For example, the fluorine content of the fluorinated graphene oxide may be 1 to 10 wt%, 1 to 7 wt%, or 1 to 5 wt%. The fluorine content in the fluorinated graphene oxide can be analyzed by an XPS analysis method. When the fluorine content in the fluorinated graphene oxide satisfies the numerical range, the cross-linking reaction with the cross-linking agent can effectively proceed.

The third repeating unit according to the present invention is derived from a fluorinated carbon material and can be analyzed in the same manner as the first and second repeating units.

The second repeating unit according to the present invention can be connected to the first repeating unit and the third repeating unit. Specifically, the second repeating unit can be connected to the first repeating unit through a partial first cross-linking reaction and can be connected to the third repeating unit through a partial second cross-linking reaction.

The thickness of the protective layer according to the present invention may be 3 to 30 μm or 5 to 20 μm. When the thickness of the protective layer satisfies the above numerical range, the mechanical properties and ionic conductivity of the composite membrane can be balancedly controlled.

Manufacturing method

A method for manufacturing a composite separator according to the present invention may include a step of (S1) coating a composition for forming a protective layer on at least one surface of a separator; and a step of (S2) drying the separator coated with the composition for forming a protective layer to manufacture a composite separator.

The composition for forming a protective layer according to the present invention may include a fluorinated carbon material, a polymer having a functional group, a polymer-crosslinking agent that is partially crosslinked with a crosslinking agent, and a solvent. For example, the content of the fluorinated carbon material based on the total weight of the composition for forming a protective layer may be 10 to 90 wt%, specifically 25 to 85 wt%. In addition, the content of the polymer-crosslinking agent based on the total weight of the composition for forming a protective layer may be 10 to 90 wt%, specifically 15 to 75 wt%. Here, the polymer-crosslinking agent may be, for example, PAA-PEGDE in which polyacrylic acid and a crosslinking agent (PEGDE) are chemically bonded (e.g., covalently bonded).

In the step (S1), a general coating method commercially available in the relevant technical field can be used as a means for coating the composition for forming the protective layer. For example, as the coating method, at least one method selected from the group consisting of a coating method using a doctor blade, a slot-die coating method, a dip coating method, and a gravure coating method can be used.

According to one embodiment of the present invention, the step (S2) may include a first drying step and a second drying step that is different from the first drying step. Here, the first drying step may be a step for inducing a crosslinking reaction between compounds included in the composition for forming the protective layer. The second drying step may be a step for evaporating a solvent included in the composition for forming the protective layer. For example, the first drying step may be a step performed at 100 to 140° C. for 2 to 4 hours, and the second drying step may be a step performed in a vacuum oven at 60 to 80° C. for 20 to 24 hours.

2. Secondary battery

According to another embodiment of the present invention, a secondary battery can be provided, including the composite separator; a cathode disposed on one side of the composite separator; a cathode disposed on the other side of the composite separator; and an electrolyte. The above-described parts and repeated descriptions are briefly described or omitted.

cathode

The negative electrode according to the present invention may be a negative electrode generally used in the relevant technical field, and may specifically include lithium metal. According to one embodiment of the present invention, when lithium metal is used as the negative electrode, a uniform SEI film can be formed at the interface between the negative electrode and the composite separator.

Figure 2 is a schematic diagram showing the growth of lithium dendrites in a conventional lithium metal battery.

Referring to Fig. 2, when lithium metal is generally used as an anode, a highly porous SEI film is formed along with the growth of lithium dendrites during the lithium stripping/plating process, and dead lithium is also generated inside the SEI film. Due to this unstable SEI film, there was a problem in that the electrolyte was continuously decomposed, resulting in consumption of the electrolyte and deterioration of the battery performance.

According to an embodiment of the present invention, since the protective layer of the composite separator is adjacent to the negative electrode, a stable voltage profile is implemented for a long period of time, thereby improving cycle stability and providing a secondary battery with excellent rate performance. According to another aspect of the present invention, since the protective layer of the composite separator is adjacent to the negative electrode, a secondary battery can be implemented that improves the mobility of lithium ions in the electrolyte.

Figure 3 is a schematic diagram of a secondary battery according to one embodiment of the present invention.

The SEI film refers to a solid film formed when lithium ions in a battery move to the negative electrode during the process of charging the battery, and the electrolyte is reduced/decomposed on the surface of the negative electrode during the process. According to one embodiment of the present invention, since a uniform SEI film is formed between the negative electrode and the composite separator, long-term cycle stability and rate performance can be improved.

Specifically, the SEI film may include LiF. Here, LiF may be derived from an electrolyte component containing a fluorine atom. According to one embodiment of the present invention, the mobility of lithium ions may be further improved by forming a uniform and durable SEI film.

Meanwhile, according to another embodiment of the present invention, the negative electrode may include, in addition to lithium metal, at least one negative electrode active material selected from carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, and silicon-based materials such as Si, SiO _β (0<β<2).

anode

The positive electrode according to the present invention may include a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer may include a positive electrode active material, a conductive material, and a positive electrode binder.

For example, the positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Specifically, the positive electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, an aluminum-cadmium alloy, or the like. The positive electrode current collector may typically have a thickness of 6 to 20 μm.

The cathode active material is a compound capable of reversible absorption and release of lithium, and specifically, may include a lithium composite metal oxide including lithium and at least one metal selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), and aluminum (Al).

More specifically, the lithium composite metal oxide is a lithium-manganese oxide (e.g., LiMnO ₂ , LiMn ₂ O ₄ , etc.), a lithium-cobalt oxide (e.g., LiCoO ₂ , etc.), a lithium-nickel oxide (e.g., LiNiO ₂ , etc.), a lithium-nickel-manganese oxide (e.g., LiNi _1-y1 Mn _y1 O ₂ (wherein, 0＜y1＜1), LiMn _2-z1 Ni _z O ₄ (wherein, 0＜Z1＜2)), etc.), a lithium-nickel-cobalt oxide (e.g., LiNi _1-y2 Co _y2 O ₂ (wherein, 0＜y2＜1)), etc.), a lithium-manganese-cobalt oxide (e.g., LiCo _1-y3 Mn _y3 O ₂ (wherein, 0＜y3＜1), LiMn _2-z2 Co _z2 O ₄ (wherein, 0＜Z2＜2) etc.), lithium-nickel-manganese-cobalt oxides (e.g., Li(Ni _p1 Co _q1 Mn _r1 )O ₂ (wherein, 0＜p1＜1, 0＜q1＜1, 0＜r1＜1, p1+q1+r1=1), lithium-nickel-manganese-aluminum oxides Li(Ni _p2 Co _q2 Al _r2 )O ₂ (wherein, 0＜p2＜2, 0＜q2＜2, 0＜r2＜2, p2+q2+r2=1) etc.), or lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li(Ni _p3 Co _q3 Mn _r3 M _S3 )O ₂ (wherein, M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p3, q3, r3 and s3 is an atomic fraction of an element, and 0＜p3＜1, 0＜q3＜1, 0＜r3＜1, 0＜s3＜1, p3+q3+r3+s3=1)) can be included, and these can be included alone or can include two or more of them.

The conductive material used in the above positive electrode can improve conductivity between active material particles in the electrode or with a metal current collector and prevent the binder from acting as an insulator. The conductive material may be, for example, a mixture of one or two or more conductive materials selected from the group consisting of graphite, carbon black, carbon fibers, metal fibers, metal powders, conductive whiskers, conductive metal oxides, activated carbon, and polyphenylene derivatives, and more specifically, may be a mixture of one or more conductive materials selected from the group consisting of natural graphite, artificial graphite, Super-p, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, Denka black, aluminum powder, nickel powder, zinc oxide, potassium titanate, and titanium oxide.

The binder used in the above positive electrode can suppress separation between positive electrode active material particles, or between the positive electrode active material layer and the current collector. A polymer commonly used in electrodes in the relevant technical field can be used as the positive electrode binder. These positive binders include, but are not limited to, polyvinylidene fluoride, poly(vinylidene fluoride co -hexafluoropropylene), poly(vinylidene fluoride- co -trichloroethylene), poly(methylmethacrylate), poly(ethylhexylacrylate), poly(butylacrylate), poly(acrylonitrile), poly(vinylpyrrolidone), poly(vinyl acetate), poly(ethylene-co-vinyl acetate), poly(ethylene oxide), polyacrylate, cellulose acetate, cellulose It may include at least one selected from the group consisting of cellulose acetate butyrate, cellulose acetate propionate, cyano ethyl pullulan, cyano ethyl poly(vinylalcohol), cyanoethylcellulose, cyano ethylsucrose, pullulan, and carboxyl methyl cellulose.

The reversible capacity ratio (N/P ratio) of the positive electrode and the negative electrode according to the present invention may be 1.5 or less, 1.1 or less, or 1.0 or less, and specifically, may be 0.9 to 1.1. Since a general lithium metal battery uses lithium in excess, the N/P ratio is very high (e.g., N/P ratio≥50). In a lithium metal battery, an attempt to lower the reversible capacity ratio (N/P ratio) of the positive electrode and the negative electrode in the electrode manufacturing process (e.g., a method of increasing the loading of the positive electrode or a method of lowering the loading of the lithium metal by making the lithium foil thin) is desirable from the viewpoint of energy density, but there is difficulty in the battery manufacturing process.

According to one aspect of the present invention, by using the composite separator, even if the reversible capacity ratio (N/P ratio) of the positive electrode and the negative electrode is lowered, a secondary battery in which lithium ions are easily moved and which has excellent long-term cycle stability and rate performance can be implemented.

Electrolyte

The electrolyte according to the present invention may include a solvent and a lithium salt.

The solvent according to the present invention may be, for example, one or a mixture of two or more selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma butyrolactone (GBL), fluoroethylene carbonate (FEC), methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, pentyl acetate, methyl propionate, ethyl propionate, ethyl propionate and butyl propionate.

The lithium salt according to the present invention may include anions such as, for example, NO ^3- , F ^- , Cl ^- , Br ^- , I ^- , PF ₆ ^- , FSI ^- , TFSI ^- . Here, FSI is bis(fluorosulfonyl)imide and TFSI is bis(trifluoromethylsulfonyl)imide.

If necessary, the electrolyte according to the present invention may further include an additive that contributes to protecting the surface of the electrode so that charge and discharge can be performed more stably when lithium ions move. For example, the additive may be one or a mixture of two or more selected from the group consisting of fluoroethylene carbonate (FEC), cyclohexylbenzene, biphenyl, vinylene carbonate, succinic anhydride, ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, butane sultone, methyl methane sulfonate, methyl toluene sulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, thioanisole, diphenyl disulfide, and dipyridinium disulfide.

Application

According to another embodiment of the present invention, any one of a battery module including the secondary battery as a unit battery, a battery pack including the battery module, and a device including the battery pack as a power source may be provided. Specific examples of the device include, but are not limited to, a power tool that is powered by an electric motor and moves; an electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), etc.; an electric two-wheeled vehicle including an electric bicycle (E-bike) and an electric scooter (E-scooter); an electric golf cart; and a power storage system.

Hereinafter, embodiments of the present invention will be described in detail so that a person having ordinary skill in the art to which the present invention pertains can easily implement the present invention, but this is only an example, and the scope of the rights of the present invention is not limited by the following contents.

[Manufacturing Preparation Example 1: Synthesis of Fluorinated Graphene Oxide (FGO)]

Preparation of graphite dispersion

A graphite dispersion was prepared by stirring 1 g of graphite in a solution containing H ₃ PO ₄ (10 mL), HF (10 mL), and H ₂ SO ₄ (90 mL) at 50 °C for 2 hours.

Synthesis of FGO powder

After KMnO ₄ (4.5 g) was added to the graphite dispersion, the mixture was stirred at 50°C for another 12 hours. The temperature of the stirred mixture was lowered to 25°C using ice (100 mL), and H ₂ O ₂ (2 mL) was slowly added until the color of the resultant changed to brown, thereby preparing a fluorinated graphene oxide (FGO) dispersion. The FGO dispersion was sequentially centrifuged using HCl, deionized water (DI water), ethanol, and diethyl ether using a centrifuge, and then vacuum-dried at 25°C to remove the residual solvent, and finally FGO in the form of a brown powder was obtained.

[Experimental Example 1: Digital photographs of graphite and FGO]

Fig. 4a is a digital photograph of graphite powder. Fig. 4b is a digital photograph of FGO powder.

Referring to FIGS. 4a and 4b, it can be confirmed that the color of the powder changes to brown as the dark-colored graphite powder is converted into FGO synthesized by the method according to Manufacturing Preparation Example 1.

[Experimental Example 2: SEM image of FGO]

Fig. 5 is a SEM (Scanning Electron Microscope) image of FGO according to Manufacturing Preparation Example 1. Referring to Fig. 5, it can be confirmed that FGO was synthesized through the method according to Manufacturing Preparation Example 1.

[Experimental Example 3: EDS Mapping of FGO]

Fig. 6a is an EDS (Energy Dispersive X-ray Spectrometer) mapping result for the carbon element of FGO of Manufacturing Preparation Example 1. Fig. 6b is an EDS mapping result for the fluorine element of FGO of Manufacturing Preparation Example 1. Fig. 6c is an EDS mapping result for the oxygen element of FGO of Manufacturing Preparation Example 1.

Referring to FIGS. 6a to 6c, it can be confirmed that carbon, oxygen, and fluorine elements are distributed in the FGO of Manufacturing Preparation Example 1, and through this, it can be confirmed that the fluorine element is introduced into graphene oxide (hereinafter referred to as 'GO').

[Experimental Example 4: XRD Patterns of Graphite, GO, and FGO]

Figure 7 is an XRD (X-ray diffraction) pattern of graphite, graphene oxide (GO), and FGO according to Preparation Example 1. The XRD pattern was measured in an angle range of 5° to 65° using XRD (Smart lab, Rigaku, Japan).

Referring to Fig. 7, it can be confirmed that although FGO did not exhibit the characteristic (002) peak of graphite at 26.5°, a peak corresponding to the characteristic peak of GO (2θ = 10.74°) appears at 10.40°. This suggests that the exfoliation and oxidation of graphite were successfully achieved.

[Experimental Example 5: XPS Analysis of FGO]

Figure 8 shows the XPS analysis results of GO and FGO.

Referring to Figure 8, unlike GO, it was confirmed that FGO showed a peak corresponding to the F1s region, which confirmed that a fluorine atom was introduced into the composite according to Manufacturing Preparation Example 1.

[Experimental Example 6: FT-IR Analysis of GO and FGO]

Figure 9 shows the results of FT-IR (Fourier-transform infrared spectroscopy) analysis of GO and FGO according to Manufacturing Preparation Example 1.

Referring to Fig. 9, unlike GO, FGO exhibited a peak in the 1080 cm ^-1 wavelength region corresponding to the CF bond. This confirms that fluorine atoms were introduced into the composite according to Preparation Example 1.

[Preparation Example 2: Synthesis of PAA-PEGDE]

Figure 10 illustrates the synthesis steps of PAA-PEGDE according to one embodiment of the present invention.

Referring to FIG. 10, a method for synthesizing PAA-PEGDE according to one embodiment of the present invention may include the following steps (a) and (b).

(a) a step of inducing lithium substitution of PAA;

A solution (PAA ^- Li) was prepared by completely dissolving 0.5 g of poly(acrylic acid) (hereinafter referred to as 'PAA') having a weight average molecular weight of 450,000 g/mol and LiOH (0.289 g) in 50 mL of deionized water.

(b) Synthesis step of PAA-PEGDE;

40 mg of PEGDE (Poly(ethylene glycol)diglycidyl ether), which acts as a cross-linking agent, was added to the solution (PAA ^- Li) and stirred at 60°C for 1 hour. Next, the stirred resultant was dialyzed for 2 days using a semipermeable tube (Dialysis tube) to remove impurities and unreacted compounds, and then freeze-dried for 3 days. As a result, PAA-PEGDE in the form of a white powder was finally synthesized.

[Manufacturing Example 1: Manufacturing of a membrane]

<Example 1: Preparation of composite membrane including protective layer (PAA-PEG-FGO)>

A protective layer-forming composition, in which FGO (3 mg) and PAA-PEGDE (1 mg) synthesized according to the method of Manufacturing Preparation Example 2 were completely dispersed in deionized water (10 mL), was coated on one side of a polypropylene separator (thickness: 25 μm; porosity: 55%). The polypropylene separator coated with the protective layer-forming composition was vacuum-dried at 110°C for 3 hours to induce a cross-linking reaction, and then additionally dried in a vacuum oven at 70°C for one day. As a result, a composite separator was manufactured in which a protective layer (PAA-PEG-FGO) having a thickness of 10 μm was formed on one side of the polypropylene separator.

<Comparative Example 1: Preparation of a commercial PP membrane without a protective layer>

A commercial polypropylene membrane (thickness: 25 μm; porosity: 55%) was prepared.

<Comparative Example 2: Preparation of composite membrane including comparative protective layer (PAA+FGO)>

A comparative protective layer-forming composition, in which FGO (3 mg) and PAA (1 mg) were completely dispersed in deionized water (10 mL), was coated on one side of a polypropylene separator (thickness: 25 μm; porosity: 55%). The polypropylene separator coated with the comparative protective layer-forming composition was vacuum-dried at 110° C. for 3 hours and then additionally dried in a vacuum oven at 70° C. for one day. As a result, a composite separator was manufactured in which a comparative protective layer was finally formed on one side of the polypropylene separator.

[Experimental Example 7: FT-IR of PAA and PAALi]

Figure 11a is the FT-IR (Fourier-transform infrared spectroscopy) of each PAA and PAALi.

Referring to Fig. 11a, the FT-IR spectrum of PAALi exhibited characteristic peaks of symmetric and asymmetric carboxylate groups at 1409 cm ^-1 and 1552 cm ^-1 , respectively, which can be confirmed to be distinct from the carboxylic acid peak of PAA, which exhibits a peak at 1701 cm ^-1 .

Figure 11b is the FT-IR (Fourier-transform infrared spectroscopy) of each PAALi, PEGDE, and PAA-PEGDE.

Referring to Fig. 11b, the FT-IR spectrum of PAA-PEGDE showed peaks at 946 cm ^-1 and 1,096 cm ^-1 , which are related to the bending vibration of the epoxy group, along with the peaks of PAALi described above. In addition, it can be inferred that PEGDE not only formed a branched structure by bonding to PAALi, but also acted as a cross-linking agent to connect PAALi chains.

[Experimental Example 8: Thermal property evaluation to confirm the crosslinked structure of PAA-PEGDE]

Figure 12a shows the glass transition temperatures of PAA and PAA-PEGDE measured using DSC (Differential Scanning Calorimetry). The glass transition temperatures (T _g ) of the polymers were measured using DSC (DSC 400, PerkinElmer, USA) at a ramping rate of 10°C/min.

Referring to Figure 12a, the glass transition temperature of PAA-PEGDE was 136°C, which was higher than that of PAA (128°C). This can be presumed to be due to the weakened chain mobility caused by cross-linking between the epoxy group of one end of the PEGDE chain and the PAALi polymer.

Figure 12b shows the results of TGA (Thermogravimetric analysis) of PAA and PAA-PEGDE. TGA was measured in a nitrogen (N ₂ ) atmosphere in the range of 100 to 1000°C at a ramping rate of 10°C/min.

Referring to Figure 12b, it can be confirmed that PAA-PEGDE, unlike PAA, forms a cross-linked network structure, thereby restricting chain rotation and mobility, thereby further improving thermal stability. As a result of TGA analysis, it can be confirmed that PAA-PEGDE has a lower weight loss rate than PAA under the same temperature conditions.

[Experimental Example 9: Digital photos before and after cross-linking reaction]

Figure 13 is a digital photograph of a dispersion containing FGO (3 mg) and PEGDE (1 mg) completely dispersed in deionized water before and after cross-linking reaction.

Referring to Figure 13, the crosslinking reaction between FGO and PEGDE can be visually confirmed. Specifically, before crosslinking, the physical mixture of FGO and PEGDE (left photo) showed fluidity when the vial was inverted, whereas when heat treatment was applied under vacuum, a new phase such as a gel (right photo) was formed.

[Experimental Example 10: SEM-EDS Mapping of the Composite Membrane of Example 1]

Figure 14 is an elemental mapping photograph of SEM-EDS for the protective layer in the composite membrane of Example 1.

Referring to Fig. 14, it can be confirmed that the FGO particles are uniformly distributed throughout the protective layer. Due to the uniform distribution of the FGO particles, when the battery is cycled, a SEI film (LiF-rich SEI) in which LiF is uniformly formed can be effectively formed.

[Experimental Example 11: Contact angle measurement on each surface of the PP separator and protective layer]

Deionized water was dropped onto the surface of each membrane according to Comparative Example 1 and Example 1, and the contact angle of the deionized water on each surface was measured using a contact angle measuring device (DSA100, Kruss, Germany).

Figure 15a shows the contact angle of deionized water on the surface of the separator according to Comparative Example 1. Figure 15b shows the contact angle of deionized water on the surface of the protective layer of the separator according to Example 1.

Referring to FIGS. 15a and 16b, it can be confirmed that the contact angle of deionized water on the surface of the separator according to Comparative Example 1 is 115°, and the contact angle of deionized water on the surface of the protective layer of the separator according to Example 1 is 74°. Through this, it can be inferred that by introducing the protective layer (PAA-PEG-FGO), the surface of the separator becomes more hydrophilic, thereby increasing the lithiophilicity and improving the charge transfer dynamics between the interface of the lithium metal and the separator.

[Manufacturing Example 2: Manufacturing of Li-Li symmetric cell]

A symmetrical electrode assembly was manufactured by sequentially stacking lithium metal used as a negative electrode (thickness of negative electrode: 300 μm); a separator (thickness: 35 μm) manufactured by the method according to Comparative Examples 1 and 2 and Example 1; and lithium metal used as a positive electrode (thickness: 300 μm). The electrode assembly was loaded into a battery case (a battery case for a CR2032 type coin cell), and then an electrolyte including 0.2 M LiNO ₃ dissolved in a mixed solvent of 1 M LiTFSI and DOL/DME mixed in a volume ratio of 1:1 was injected into the battery case, thereby finally manufacturing a lithium-lithium symmetrical cell.

[Experimental Example 12: Electrochemical Performance Evaluation of Lithium-Lithium Symmetric Cell]

Fig. 16a shows the results of the electrochemical performance evaluation of a lithium-lithium symmetric cell including the separator of Comparative Example 1 and Example 1. Fig. 16b shows the results of the electrochemical performance evaluation of a lithium-lithium symmetric cell including the separator of Comparative Example 2. Specifically, the electrochemical performance of the lithium-lithium symmetric cell was evaluated under the conditions of a current density of 1 mA/cm ² and a battery capacity of 1 mAh/cm ² .

Referring to FIG. 16a, the lithium-lithium symmetric cell including the separator of Example 1 exhibited a stable voltage profile for 1,200 hours compared to the lithium-lithium symmetric cell including the separator of Comparative Example 1. Specifically, it can be confirmed that the lithium-lithium symmetric cell including the separator of Comparative Example 1 exhibits a high overpotential over time.

Referring to Fig. 16b, it can be confirmed that the lithium-lithium symmetric cell including the separator of Comparative Example 2 exhibits a high overvoltage after 500 hours due to the absence of a cross-linking agent (PEGDE). That is, it can be inferred that an electrochemically stable and robust protective layer can be formed through a cross-linking agent (PEGDE) that can induce cross-linking between the fluorinated carbon material and the polymer.

[Experimental Example 13: LSV Test of Lithium-Lithium Symmetric Cell]

Figure 17 shows the results of a linear sweep voltammetry (LSV) test of a lithium-lithium symmetric cell including the separator of Comparative Example 1 and Example 1. The LSV test was performed in the range of -0.15 to 0.15 V at a scan rate of 1 mV/s.

Meanwhile, the Tafel equation, as shown in Equation 1 below, can be used to quantify the exchange current in the relationship between current and voltage.

[Formula 1]

In the above equation 1, i is the current density, i ₀ is the exchange current density, F is the Faraday constant, η is the overvoltage, R is the ideal gas constant (8.314 J/mol K), and T is the absolute temperature (K).

Referring to the above equation 1 and FIG. 17, the protective layer including PAA-PEG-FGO is interposed between the lithium metal (electrode) and the separator, thereby exhibiting a significantly higher exchange current density (145.4 μAcm ^-2 ) compared to the lithium metal electrode (96.4 μAcm ^-2 ) on which the protective layer is not formed. That is, it can be inferred that the lithium-lithium symmetric cell including the separator of Example 1 implements faster charge transfer kinetics compared to Comparative Example 1.

Figure 18 is a schematic diagram to demonstrate the effect of a protective layer at the interface of a separator and lithium metal (electrode).

Referring to Fig. 18, the cross-linked matrix (PAA-PEG-FGO) structure included in the protective layer can promote desolvation through polar oxygen atoms having lone electron pairs in hydroxyl groups, carboxyl groups, and ethylene glycol groups, ultimately improving interfacial dynamics. In addition, since a LiF-rich and uniform SEI film is formed between the protective layer including PAA-PEG-FGO and the lithium metal (electrode), the mobility of lithium ions can be improved.

[Experimental Example 14: EIS Test of Lithium-Lithium Symmetric Cell]

Figure 19 shows the results of EIS (Electrochemical impedance spectroscopy) evaluation of lithium-lithium symmetric cells including the separators of Comparative Example 1 and Example 1 after the first and fifth cycles.

Referring to Fig. 19, after the first cycle, the lithium metal electrode in the lithium-lithium symmetric cell including the separator of Comparative Example 1 exhibited a combined interfacial resistance (R _total ) of about 175Ω. This can be presumed to be due to the resistance of the SEI film and the resistance to desolvation at the interface. The lithium-lithium symmetric cell including the separator of Comparative Example 1 showed little decrease in interfacial resistance even after the fifth cycle. It can be confirmed that the structural instable and poor conductive properties of the SEI film exhibited a limit stabilization of the SEI film, which caused continuous electrolyte decomposition.

On the other hand, in the lithium-lithium symmetric cell including the separator of Example 1, the R _total of the lithium metal protected with PAA-PEG-FGO was ~93Ω after the first cycle, and finally reached ~15Ω after the 50th cycle. This R _total was significantly less than that of the unprotected lithium electrode in each cycle and continuously decreased as the cycling progressed, confirming that a very stable SEI film was rapidly formed within the fifth cycle.

[Experimental Example 15: ToF-SIMS Results of Lithium-Lithium Symmetric Cell]

Figure 20a is a ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometer) depth profile of LiF ^- after the fifth cycle.

Referring to FIG. 20a, in the lithium-lithium symmetric cell of Example 1 including the separator with the PAA-PEG-FGO protective layer, the LiF ^- signal of the PAA-PEG-FGO sample becomes stronger as sputtering progresses toward the SEI film compared to the lithium-lithium symmetric cell including the PP separator of Comparative Example 1, confirming that LiF is abundantly distributed inside the SEI film. This can be inferred that a significant amount of LiF can be generated through the chemical reaction between the lithium metal (electrode) and the protective layer (PAA-PEG-FGO).

Figure 20b is a ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometer) depth profile of C ₂ H ₃ O ^- after the fifth cycle.

Referring to Fig. 20b, the ToF-SIMS signal of the organic compound C ₂ H ₃ O ^- , a typical SEI component mainly generated from the solvent, can be additionally investigated to confirm the effect of electrolyte decomposition on SEI layer formation.

Figure 20c is a ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometer) depth profile of CF ₃ ^- after the fifth cycle.

Referring to FIG. 20c, it can be confirmed that the intensity of CF ₃ ^- in the SEI film of the lithium-lithium symmetric cell according to Example 1 is significantly lower than that of the SEI film of the lithium-lithium symmetric cell according to Comparative Example 1, which allows us to infer that the PAA-PEG-FGO protective layer effectively prevents the decomposition of the electrolyte. Specifically, although CF ₃ of the PP separator was detected more inside the SEI film, the -CF ₃ component generated by the decomposition of TFSI (Bis(trifluoromethane)sulfonimide) anion cannot sufficiently form a LiF-rich inner SEI film during the initial cycle, and therefore, it can be inferred that it is not advantageous for improving the performance of the secondary battery, unlike LiF derived from FGO.

[Manufacturing Example 3: Manufacturing of LFP-Li Full-Sided Paper]

Manufacturing of the anode

A positive electrode slurry was prepared by dispersing a mixture of LiFePO ₄ (LFP), PVDF (Mw=534,000 g/mol), and Super P in a weight ratio of 90:5:5 in a solvent (NMP). The prepared positive electrode slurry was coated on one surface of an aluminum current collector using a doctor blade, and then vacuum-dried at 60°C to prepare a positive electrode.

Manufacturing of electrode assemblies

An electrode assembly was manufactured using a cathode comprising lithium metal (cathode thickness: 40 or 20 μm); a separator (thickness: 35 μm) manufactured by the method according to each of Comparative Example 1 and Example 1; and a cathode having LiFePO ₄ (LFP) loaded at 9.1 or 24.5 mg/cm ² on one surface of an aluminum current collector (thickness: 20 μm). The electrode assembly was loaded into a battery case (a battery case for a CR2032 type coin cell), and then an electrolyte including 0.2 M LiNO ₃ dissolved in a mixed solvent of 1 M LiTFSI and DOL/DME in a volume ratio of 1:1 was injected into the battery case to finally manufacture an LFP-Li complete battery. Here, when the thickness of the cathode was 40 μm, the loading amount of LFP was 9.1 mg/cm ² , and when the thickness of the cathode was 20 μm, the loading amount of LFP was 24.5 mg/cm ² .

[Experimental Example 16: Evaluation of electrochemical performance and rate performance of LFP-Li full-cell]

Figure 21a shows the results of electrochemical performance evaluation of LFP-Li full paper according to Comparative Example 1 and LFP-Li full paper according to Example 1 when the LFP loading amount is 9.1 mg/cm ² . The electrochemical performance evaluation of the LFP-Li full paper was performed in a potential range of 2.5 to 4.0 V under constant current mode, with one pre-cycle under 0.1 C condition and a subsequent cycle under 0.5 C (82.5 mA/g _cathode ) condition.

Referring to Fig. 21a, the LFP-Li full-cell according to Example 1 exhibited high reversible capacity and excellent cycling performance. In addition, in the case of Example 1, a capacity retention rate of 98.08% was observed even after the 300th cycle, and the Coulombic efficiency (CE) was maintained up to 99.77%.

In contrast, the LFP-Li full-cell according to Comparative Example 1 showed that the coulombic efficiency (CE) was maintained at 99.57% for 240 cycles and then decreased rapidly after the 240th cycle. These results confirm that the SEI film formed on the interface of the negative electrode (Li metal) is not destroyed even after long-term cycling by providing the PAA-PEG-FGO protective layer on the separator of Example 1.

Figure 21b shows the electrochemical performance evaluation results of the LFP-Li full-cell when the reversible capacity ratio (N/P ratio) of the positive and negative electrodes is 1.0 when the LFP loading amount is 24.5 mg/cm ² .

Referring to FIG. 21b, the LFP-Li full cell according to Example 1 exhibited a capacity retention rate of 93.8% even after the 100th cycle, whereas the LFP-Li full cell according to Comparative Example 1 exhibited a rapid decrease in capacity after the 54th cycle. The Coulombic efficiency of the LFP-Li full cell according to Example 1 was 99.87% for 100 cycles, which is much higher than the Coulombic efficiency (99.55%) of the full cell according to Comparative Example 1 that underwent 54 cycles.

In general, a battery having a limited lithium supply source with a capacity ratio (N/P ratio) of close to 1 relative to the opposing areas of the positive and negative electrodes may have a problem in that it is difficult to maintain long-term cycle performance because the cycle performance of the battery may deteriorate when the lithium metal is depleted. The LFP-Li full-cell according to Example 1 can realize long-term cycle stability even if the reversible capacity ratio (N/P ratio) of the positive and negative electrodes is low through a protective layer.

Figure 21c shows the rate performance of LFP-Li full-sheet when the loading amount of LFP is 9.1 mg/cm ² .

Referring to Fig. 21c, the LFP-Li full cell according to Example 1 exhibited specific capacities of 145.8 and 136.3 mAh/g _cathode at 1C and 2C, respectively. On the other hand, the LFP-Li full cell according to Comparative Example 1 exhibited specific capacities of 131.6 and 109.0 mAh/g _cathode , respectively, under the same rate conditions.

[Experimental Example 17: SEM cross-sectional image of lithium metal electrode in LFP-Li full-cell]

Figure 22a is a SEM cross-sectional photograph of a lithium metal electrode in the LFP-Li complete electrode according to Comparative Example 1 after the 10th lithium stripping.

Figure 22b is a SEM cross-sectional image of a lithium metal electrode in the LFP-Li full-cell according to Example 1 after the 10th lithium stripping.

Referring to FIGS. 22a and 23b, in the case of Comparative Example 1, it can be confirmed that a highly porous SEI film (thickness = 13 μm) is observed along with the indiscriminate growth of lithium dendrites on the exposed Li electrode after the 10th lithium stripping. This may suggest poor reversibility of the Li plating/stripping process due to uncontrolled side reactions. In addition, the lithium metal observed in the SEI film may mean dead lithium.

In contrast, in the case of Example 1, it can be confirmed that lithium dendrites were not observed at all due to the presence of the protective layer. This absence of lithium dendrites is due to the effect of the protective layer (PAA-PEG-FGO), which can separate Li metal from the electrolyte and improve charge transfer at the interface.

In comprehensive consideration of the above experimental results, according to one aspect of the present invention, a composite separator can be provided in which a protective layer is disposed on at least one surface of a separator, thereby making the surface of the separator more hydrophilic and thereby increasing lithium affinity, thereby improving charge transfer between the interface of the electrode and the separator. According to another aspect of the present invention, a composite separator having excellent electrochemical stability and a strong protective layer can be provided through a crosslinking agent (PEGDE) capable of inducing crosslinking between a fluorinated carbon material and a polymer. According to another aspect of the present invention, unlike a secondary battery manufactured with a general separator which exhibits an unstable voltage profile due to a higher overpotential over time, a secondary battery which implements a stable voltage profile for a long period of time can be provided. According to another aspect of the present invention, a secondary battery which improves the mobility of lithium ions in an electrolyte can be provided by forming an SEI film rich in LiF and having excellent uniformity between the protective layer and the electrode.

[Manufacturing Example 4: Manufacturing of Micro Silicon-Li Battery]

Manufacturing of electrodes

A slurry was prepared by dispersing a mixture of micro Silicon (Si), SBR (styrene-butadiene rubber) - CMC (carboxymethyl cellulose), and CNT in a weight ratio of 80:19:1 in distilled water. The prepared slurry was coated on one surface of a copper current collector using a doctor blade, and then vacuum dried at 60°C to prepare an electrode.

Manufacturing of electrode assemblies

An electrode assembly was manufactured using an electrode having Micro Silicon (Si) loaded on one surface of lithium metal (cathode thickness: 300 μm); each of the separators (thickness: 35 μm) manufactured by the methods according to Comparative Example 1 and Example 1; and a copper current collector (thickness: 20 μm). The electrode assembly was loaded into a battery case (a battery case for a CR2032 type coin cell), and then an electrolyte including 1.3 M LiPF ₆ and 10 wt% FEC dissolved in a mixed solvent of EC/EMC/DEC in a volume ratio of 3:5:2 was injected into the battery case to finally manufacture a Micro Silicon-Li battery.

[Experimental Example 18: Evaluation of Electrochemical Performance and Rate Performance of Micro Silicon-Li Battery]

Figure 23 shows the results of evaluating the coulombic efficiency of a Micro Silicon-Li battery according to Comparative Example 1 and a Micro Silicon-Li battery according to Example 1. The electrochemical performance evaluation of the Micro Silicon-Li battery was performed under 0.5C conditions in a potential range of 0.01 to 1.5 V in constant current-constant voltage mode during charging and in constant current mode during discharge.

Referring to FIG. 23, it was confirmed that the Micro Silicon-Li battery according to Example 1 exhibited higher Coulombic efficiency than the Micro Silicon-Li battery according to Comparative Example 1.

Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements made by those skilled in the art using the basic concept of the present invention defined in the following claims also fall within the scope of the present invention.

Claims (13)

Membrane; and
A protective layer disposed on at least one surface of the above membrane;
The above protective layer
A first repeating unit derived from a polymer having a functional group,
The second repeating unit derived from the crosslinker and
Comprising a third repeating unit derived from a fluorinated carbon material,
Composite membrane. In the first paragraph,
The above functional group comprises a carboxyl group,
Composite membrane. In the first paragraph,
The weight average molecular weight (Mw) of the polymer having the above functional group is 300,000 to 600,000 g/mol.
Composite membrane. In the first paragraph,
The above cross-linking agent is,
Comprising a compound having two or more cross-linking reaction functional groups at the terminal,
Composite membrane. In paragraph 4,
The above cross-linking reaction functional group is,
Containing at least one of an epoxy group and an isocyanate group,
Composite membrane. In the first paragraph,
The above cross-linking agent is,
One or a mixture of two or more selected from the group consisting of PEGDE (Poly(ethylene glycol) diglycidyl ether), DGEBA (diglycidyl ether of bisphenol-A), RDGE (resorcinol diglycidyl ether), TGIC (triglycidyl isocyanurate), TMPTGE (trimethylolpropane triglycidyl ether), TPEGE (tetraphenylolethane glycidyl ether), and TPGECMS (tetrakis(propyl glycidyl ether)cyclomethylsiloxane).
Composite membrane. In the first paragraph,
The above fluorinated carbon material is,
A compound comprising at least one fluorinated compound selected from the group consisting of graphene oxide and modified carbon nanotubes.
Composite membrane. In the first paragraph,
The second repeating unit is connected to the first repeating unit and the third repeating unit.
Composite membrane. A composite membrane according to any one of claims 1 to 8;
A cathode disposed on one side of the composite membrane on which the protective layer is disposed;
an anode disposed on the other side of the composite membrane; and
containing electrolyte;
Secondary battery. In Article 9,
The above negative electrode contains lithium metal (Li metal).
Secondary battery. In Article 9,
The above negative electrode comprises at least one negative electrode active material selected from carbonaceous materials and silicon-based materials.
Secondary battery. In Article 9,
The reversible capacity ratio (N/P ratio) of the positive and negative electrodes is 1.5 or less,
Secondary battery. In Article 9,
The reversible capacity ratio (N/P ratio) of the positive and negative electrodes is 1.1 or less.
Secondary battery.