Multilayer separator for lithium secondary battery, method for manufacturing same, and lithium secondary battery including same

Abstract

A multilayer separator for a lithium secondary battery according to an embodiment of the present invention comprises: a porous substrate; a first coating layer disposed on one surface of the porous substrate and including a first binder, a nitric acid compound, and a first lithium salt; and a second coating layer disposed on a surface of the porous substrate opposite to a surface in contact with the first coating layer, and including a second binder and a second lithium salt.

Description

Technical Field

[0001] This invention relates to a multilayer separator for lithium secondary batteries, a method for manufacturing the same, and a lithium secondary battery including the multilayer separator. Background Technology

[0002] Rechargeable batteries are batteries that can be repeatedly charged and discharged. With the development of the information communication and display industries, rechargeable batteries are widely used as power sources for portable electronic communication devices such as portable cameras, mobile phones, and laptops (PCs). In addition, in recent years, battery packs that include rechargeable batteries have been developed for use as power sources for environmentally friendly vehicles such as electric vehicles.

[0003] Among secondary batteries, lithium secondary batteries have high operating voltage and energy density per unit weight, and are advantageous for charging speed and lightweight design, so they are being actively researched and developed.

[0004] In recent years, in order to realize batteries with high energy density and fast charging characteristics, lithium secondary batteries including lithium metal electrodes are being researched and developed.

[0005] During repeated charge-discharge cycles, lithium dendrites may form in lithium secondary batteries, including those with lithium metal electrodes, due to uneven lithium deposition on the electrode surface. These lithium dendrites, protruding from the lithium metal surface, can damage or penetrate the separator and come into contact with the positive electrode, significantly reducing battery life.

[0006] Therefore, it may be necessary to modify, process, or improve the properties of the electrode or separator to improve the lifespan and stability of the lithium metal electrode. Summary of the Invention

[0007] (a) Technical problems to be solved

[0008] One technical problem of the present invention is to provide a multilayer separator for lithium secondary batteries with improved mechanical and physical properties.

[0009] One technical problem of the present invention is to provide a lithium secondary battery comprising a multilayer separator.

[0010] One technical problem of the present invention is to provide a method for manufacturing the multilayer separator for the lithium secondary battery.

[0011] (II) Technical Solution

[0012] The multilayer separator for lithium secondary batteries according to the present invention comprises: a porous substrate; a first coating disposed on one side of the porous substrate and comprising a first adhesive, a nitrate compound and a first lithium salt; and a second coating disposed on the opposite side of the porous substrate to the side in contact with the first coating and comprising a second adhesive and a second lithium salt.

[0013] In an exemplary embodiment, the nitrate compound may include at least one selected from lithium nitrate, cerium nitrate, and silver nitrate.

[0014] In an exemplary embodiment, the content of the nitrate compound may be from 5 to 15 parts by weight relative to 100 parts by weight of the first adhesive.

[0015] In an exemplary embodiment, the second coating may not contain the nitrate compound.

[0016] In an exemplary embodiment, the first lithium salt and the second lithium salt may each independently contain a compound selected from LiF, LiCl, LiBr, LiI, LiNO3, LiN(CN)2, LiBF4, LiClO4, LiPF6, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiCF3CF2SO3, Li(CF3SO2)2N, and Li(FSO2)2N. 、 At least one of LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, Li(SF5)3C, Li(CF3SO2)3C, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2, LiSCN and Li(CF3CF2SO2)2N.

[0017] In an exemplary embodiment, the thickness of the porous substrate can be from 2 μm to 20 μm. The thicknesses of the first coating and the second coating can each be independently from 1 μm to 20 μm.

[0018] In an exemplary embodiment, the first adhesive and the second adhesive may each independently include at least one selected from polyethylene oxide (PEO), polytetrafluoroethylene (PTFE), polyurethane (PU), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP).

[0019] In an exemplary embodiment, the first coating and the second coating may each independently further comprise at least one selected from oxide-based solid electrolytes, sulfide-based solid electrolytes, and polymer electrolytes.

[0020] In an exemplary embodiment, the porous substrate may include: a first region adjacent to the first coating, wherein the pores contain a first adhesive, a nitrate compound, and a first lithium salt; and a second region adjacent to the second coating, wherein the pores contain a second adhesive and a second lithium salt.

[0021] The lithium secondary battery according to the present invention includes: a positive electrode; a negative electrode disposed opposite to the positive electrode and comprising a lithium metal layer; and the aforementioned multilayer separator for lithium secondary batteries disposed between the positive electrode and the negative electrode.

[0022] In an exemplary embodiment, the multilayer separator for the lithium secondary battery can be configured such that the first coating is adjacent to the lithium metal layer of the negative electrode, and the second coating is adjacent to the positive electrode.

[0023] In an exemplary embodiment, the lithium secondary battery may further include a nitrogen-based solid electrolyte interface layer disposed between the first coating and the lithium metal layer.

[0024] According to the method for manufacturing a multilayer separator for lithium secondary batteries of the present invention, a first coating composition comprising a first binder, a nitrate compound, a first lithium salt, and a solvent is coated onto one side of a release substrate. A porous substrate is then laminated on the coated first coating composition. A second coating composition comprising a second binder, a second lithium salt, and a solvent is then coated onto the porous substrate. Drying can be performed after coating the first and / or second coating compositions.

[0025] In an exemplary embodiment, the content of the first lithium salt in the first coating composition can be from 0.5M to 2M. The content of the second lithium salt in the second coating composition can be from 0.5M to 2M.

[0026] (III) Beneficial Effects

[0027] By inducing uniform lithium deposition through a multilayer separator in a lithium secondary battery according to an exemplary embodiment of the present invention, the formation of lithium dendrites can be suppressed.

[0028] The multilayer separator for lithium secondary batteries according to an exemplary embodiment of the present invention has excellent mechanical and physical properties, and can suppress damage even if lithium dendrites are formed.

[0029] The lithium secondary battery according to an exemplary embodiment of the present invention can have improved lifespan characteristics by including the multilayer separator for the lithium secondary battery.

[0030] The lithium secondary battery of this invention can be widely used in green technology fields such as electric vehicles, battery charging stations, and other battery-based solar and wind power generation. Furthermore, the lithium secondary battery of this invention can be used in eco-friendly electric vehicles and hybrid vehicles to prevent climate change by suppressing air pollution and greenhouse gas emissions. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the cross-section of a multilayer separator for a lithium secondary battery according to an exemplary embodiment.

[0032] Figure 2 This is a schematic diagram of the cross-section of a lithium secondary battery according to an exemplary embodiment.

[0033] Figure 3 This is a graph showing the capacity retention rates according to Examples 1 to 6.

[0034] Figure 4 This is a graph showing the current and voltage according to Comparative Example 1.

[0035] Figure 5 This is a graph showing the current and voltage according to Comparative Example 2. Detailed Implementation

[0036] According to an exemplary embodiment of the present invention, a multilayer separator for a lithium secondary battery (hereinafter, may be simply referred to as a multilayer separator) includes a first coating and a second coating having mutually different compositions on both sides of a porous substrate. Furthermore, a method for manufacturing the multilayer separator and a lithium secondary battery including the multilayer separator are provided.

[0037] The present invention will now be described in detail with reference to the accompanying drawings. However, this is merely exemplary, and the present invention is not limited to the specific embodiments described herein.

[0038] Figure 1 This is a schematic diagram of the cross-section of a multilayer separator for a lithium secondary battery according to an exemplary embodiment.

[0039] Reference Figure 1 The multilayer membrane 100 includes a porous substrate 10, a first coating 20 disposed on one side of the porous substrate 10, and a second coating 30 disposed on the other side of the porous substrate 10.

[0040] The porous substrate 10 may have a structure including three-dimensional pores and may be a porous polymer membrane commonly used as a diaphragm.

[0041] The porous substrate 10 may include a porous polymer membrane made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer. The porous substrate 10 may also include a nonwoven fabric formed from high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

[0042] The thickness of the porous substrate 10 can be from 2 μm to 20 μm. In some embodiments, the thickness of the porous substrate 10 can be from 2 μm to 10 μm. Within the above range, the electrodes can be electrically separated without significantly increasing the volume of the battery.

[0043] The first coating 20 may comprise a first binder, a nitrate compound, and a first lithium salt. For example, the first coating 20 may be relative to the lithium metal electrode described later. In this case, during repeated charge and discharge cycles of the battery, a nitrogen-containing solid electrolyte interphase (SEI) layer derived from the nitrate compound of the first coating 20 can be formed on the surface of the lithium metal electrode. Therefore, lithium can be uniformly electrodeposited on the surface of the lithium metal electrode, thereby suppressing the formation of lithium dendrites.

[0044] Furthermore, even if the nitrogen-based solid electrolyte interface layer is damaged or consumed due to continuous repeated battery charging and discharging, nitrogen can be continuously replenished from the first coating 20, thereby firmly maintaining the nitrogen-based solid electrolyte interface layer. Therefore, the battery's lifespan characteristics can be further improved.

[0045] In exemplary embodiments, the nitrate compound may include a metal nitrate. In some embodiments, the nitrate compound may include at least one selected from lithium nitrate, cerium nitrate, and silver nitrate. For example, the nitrate compound may include lithium nitrate.

[0046] The nitrate compound can decompose during repeated charge and discharge of the battery, thereby forming a nitrogen-based solid electrolyte interface layer on the surface of the lithium metal electrode.

[0047] In an exemplary embodiment, the content of the nitrate compound relative to 100 parts by weight of the first adhesive can be from 5 parts by weight to 15 parts by weight. In some embodiments, the content of the nitrate compound relative to 100 parts by weight of the first adhesive can specifically be from 5 parts by weight to 12 parts by weight, or more specifically from 8 parts by weight to 12 parts by weight. Within the above ranges, a robust nitrogen-based solid electrolyte interface layer can be formed on the surface of the lithium metal electrode without reducing the migration rate of lithium ions.

[0048] The first adhesive is a polymer-based compound that can form the framework of the first coating 20. The first coating 20 may contain a nitrate compound and a first lithium salt contained in the matrix formed by the first adhesive.

[0049] The first adhesive can be polyethylene oxide (PEO), polytetrafluoroethylene (PTFE), polyurethane (PU), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), etc. These can be used alone or in combination of two or more.

[0050] The first lithium salt can be, for example, LiF, LiCl, LiBr, LiI, LiNO3, LiN(CN)2, LiBF4, LiClO4, LiPF6, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiCF3CF2SO3, Li(CF3SO2)2N, or Li(FSO2)2N. 、 LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, Li(SF5)3C, Li(CF3SO2)3C, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2, LiSCN, Li(CF3CF2SO2)2N, etc. These can be used alone or in combination of two or more.

[0051] The thickness of the first coating 20 can be from 1 μm to 20 μm. According to some embodiments, the thickness of the first coating 20 can specifically be from 2 μm to 18 μm, or more specifically from 2 μm to 15 μm, or even more specifically from 5 μm to 15 μm. Within the above range, a nitrogen-based solid electrolyte interface can be formed and maintained on the surface of the lithium metal electrode, and a thinner battery can be provided.

[0052] The second coating 30 may contain a second adhesive and a second lithium salt.

[0053] In an exemplary embodiment, the second adhesive is a polymer-based compound that can form the backbone of the second coating 30. The second coating 30 may contain a second lithium salt contained in the matrix formed by the second adhesive.

[0054] The second adhesive can be polyethylene oxide (PEO), polytetrafluoroethylene (PTFE), polyurethane (PU), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), etc. These can be used alone or in combination of two or more.

[0055] The second lithium salt can be, for example, LiF, LiCl, LiBr, LiI, LiNO3, LiN(CN)2, LiBF4, LiClO4, LiPF6, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiCF3CF2SO3, Li(CF3SO2)2N, or Li(FSO2)2N. 、 LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, Li(SF5)3C, Li(CF3SO2)3C, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2, LiSCN, Li(CF3CF2SO2)2N, etc. These can be used alone or in combination of two or more.

[0056] The second adhesive and the second lithium salt may be the same as or different from the first adhesive and the first lithium salt described above.

[0057] The second coating 30 may not contain nitrate compounds. For example, the second coating 30 may be adjacent to the positive electrode, and the nitrate compounds may decompose on the surface of the positive electrode to form byproducts. Therefore, battery performance may be reduced due to byproducts formed inside the battery.

[0058] The thickness of the second coating 30 can be from 1 μm to 20 μm. According to some embodiments, the thickness of the second coating 30 can specifically be from 1 μm to 14 μm, or more specifically from 1 μm to 10 μm. Within the above range, a nitrogen-based solid electrolyte interface can be formed and maintained on the surface of the lithium metal electrode, and a thinner battery can be provided.

[0059] In an exemplary embodiment, the first coating 20 and the second coating 30 may each independently further include a solid electrolyte, which may include an all-solid electrolyte or a semi-solid electrolyte. For example, the first coating 20 and the second coating 30 may contain at least one selected from oxide-based solid electrolytes, sulfide-based solid electrolytes, polymer electrolytes, etc.

[0060] The oxide-based solid electrolyte may include, for example, lithium lanthanum titanium oxide (LLTO)-based oxides, lithium lanthanum zirconium oxide (LLZO)-based oxides, and Li... 6.4 La3Zr 1.4 Ta 0.6 O 12 Lithium lanthanum zirconium tantalum oxide (LLZTO) based oxide, Li6La2CaTa2O 12 Li6La2ANb2O 12 (A is Ca or Sr), Li2Nd3TeSbO 12 Li3BO 2.5 N 0.5 Li9SiAlO8, lithium aluminum germanium phosphate (LAGP) based oxides, lithium aluminum titanium phosphate (LATP) based oxides, Li 1+x Ti 2-x Al x Si y (PO4)3(0≤x≤1,0≤y≤1), LiAl x Zr 2-x (PO4)3 (0≤x≤1), LiTi x Zr 2-x (PO4)3 (0≤x≤1), lithium superionic conductor (LISICON) based oxides, lithium phosphorus oxynitride (LIPON) based oxides, perovskite based oxides, sodium superionic conductor (NASICON) based oxides, Al2O3, ZnO2, Ce2O2, TiO2, ZrO2, HfO2, MnO2, MgO, WO2, V2O5, and other metal oxides, etc. These can be used alone or in combination of two or more.

[0061] The sulfide-based solid electrolyte may include, for example, Li2S-P2S5, Li2S-P2S5-LiCl, Li2S-P2S5-LiBr, Li2S-P2S5-LiCl-LiBr, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, and Li2S-P2S5-Z. m S n (m and n are positive numbers, Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (p and q are positive numbers, M is P, Si, Ge, B, Al, Ga, or In), Li 7-x PS 6-x Cl x (0≤x≤2), Li 7-x PS 6-x Br x (0≤x≤2), Li 7-x PS 6-x I x (0≤x≤2), etc. These can be used individually or in combination of two or more.

[0062] The polymer electrolyte can refer not only to an all-solid electrolyte, but also to a gel polymer electrolyte containing an electrolyte within a polymer-based electrolyte. For example, the gel polymer electrolyte can be prepared by swelling the electrolyte after preparing the polymer matrix (physical gel) or by curing a composition containing an electrolyte and monomers (chemical gel), depending on the preparation method.

[0063] The gel polymer electrolyte can be prepared from a precursor for forming a gel polymer electrolyte. This precursor can be a precursor that can form a polymer after the multilayer separator 100 is assembled into a battery and exposed to energy such as light irradiation. For example, the precursor can contain a monomer or oligomer for forming a gel polymer electrolyte.

[0064] In an exemplary embodiment, the porous substrate 10 may include: a first region adjacent to the first coating 20, and the pores therein containing a first adhesive, a nitrate compound, and a first lithium salt; and a second region adjacent to the second coating 30, and the pores therein containing a second adhesive and a second lithium salt.

[0065] The first region and the second region can be formed by the manufacturing process of the multilayer membrane 100 described later. For example, the first region can be formed by coating a composition for forming the first coating 20 (first coating composition) and then stacking the porous substrate 10, and the second region can be formed by coating the porous substrate 10 with a composition for forming the second coating 30 (second coating composition).

[0066] In an exemplary embodiment, the porosity of the multilayer membrane 100 can be from 20% to 90%. The porosity can be expressed as a percentage representing the volume of pores in each layer of the total volume of the porous substrate 10, the first coating 20, and the second coating 30.

[0067] According to an exemplary embodiment, the elongation of the multilayer diaphragm 100 can be from 20% to 300%. The elongation can be a percentage value of the difference between the length of the multilayer diaphragm 100 before extension and the length of the multilayer diaphragm 100 increased due to extension in the length direction, relative to the length of the multilayer diaphragm 100 before extension.

[0068] Within the aforementioned range, the mechanical and physical properties of the multilayer separator 100 can be improved, so that even if lithium dendrites are formed, the multilayer separator 100 will not be easily damaged.

[0069] The pore diameter of the pores contained in the porous substrate 10, the first coating 20 and the second coating 30 can be from 10 nm to 5 μm.

[0070] According to an exemplary embodiment of the present invention, a method for manufacturing the multilayer separator for lithium secondary batteries is provided.

[0071] First, a first coating composition can be applied to one side of a release substrate. The first coating composition may contain a first adhesive, a nitrate compound, a first lithium salt, and a solvent.

[0072] The release substrate can be peeled off after the manufacturing of the multilayer diaphragm is completed.

[0073] The first adhesive, nitrate compound, and first lithium salt can be the same as described above.

[0074] The solvent can be an organic solvent. For example, the solvent can be propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran, etc. These can be used alone or in combination of two or more.

[0075] The first coating composition can be prepared by adding a first binder and a nitrate compound to a solution containing a first lithium salt dissolved in a solvent.

[0076] The content of the first lithium salt in the solution can be from 0.5M to 2M. In some embodiments, the content of the first lithium salt in the solution can be from 0.8M to 1.2M. Within the above range, the migration rate of lithium ions in the first coating can be relatively fast.

[0077] The first binder may be added in an amount of 50 to 200 parts by weight relative to 100 parts by weight of the solution. The nitrate compound may be added in an amount of 5 to 15 parts by weight relative to 100 parts by weight of the first binder.

[0078] Next, a porous substrate can be laminated onto the coated first coating composition. When the porous substrate is laminated without drying the first coating composition, the first coating composition can penetrate from the surface of the porous substrate adjacent to the coated first coating composition into the pores of the porous substrate. Therefore, the aforementioned first region can be formed.

[0079] Next, a second coating composition can be applied to the porous substrate. The second coating composition comprises a second binder, a second lithium salt, and a solvent. When the second coating composition is applied to the porous substrate, it can penetrate into the pores of the porous substrate along the direction of gravity. Therefore, the second region can be formed.

[0080] The second adhesive and the second lithium salt can be the same as described above, and the solvent can be the same as that described in the first coating composition.

[0081] The second coating composition can be prepared by adding a second binder to a solution containing a second lithium salt dissolved in a solvent.

[0082] The content of the second lithium salt in the solution can be from 0.5M to 2M. In some embodiments, the content of the second lithium salt in the solution can be from 0.8M to 1.2M. Within the above range, the migration rate of lithium ions in the second coating can be relatively fast.

[0083] The second adhesive may be added in an amount of 50 to 200 parts by weight relative to 100 parts by weight of the solution.

[0084] Subsequently, drying is performed to remove the solvent contained in the first coating composition and the second coating composition, and the first coating and the second coating can be formed. The drying can be carried out at a temperature of, for example, 25°C to 100°C for about 1 hour to 12 hours.

[0085] According to an exemplary embodiment of the present invention, a lithium secondary battery including the multilayer separator is provided.

[0086] Figure 2 This is a schematic diagram of the cross-section of a lithium secondary battery according to an exemplary embodiment.

[0087] Reference Figure 2 The lithium secondary battery includes a positive electrode 300, a negative electrode 200 disposed opposite to the positive electrode 300 and including a lithium metal layer, and a multilayer separator 100 disposed between the positive electrode 300 and the negative electrode 200. The lithium secondary battery may be a lithium metal battery.

[0088] The positive electrode 300 may include a positive electrode current collector 310 and a positive electrode active material layer 320 disposed on at least one side of the positive electrode current collector 310. Figure 2 The diagram shows a case where the positive electrode active material layer 320 is provided on only one side of the positive electrode current collector 310, but the positive electrode active material layer 320 can also be formed on both sides of the positive electrode current collector 310.

[0089] The positive current collector 310 may include metals such as stainless steel, nickel, aluminum, titanium, copper, magnesium, iron, and zinc, or alloys thereof. In one embodiment, the positive current collector 310 may be omitted. In this case, the positive electrode 300 may be composed of a positive electrode active material layer 320.

[0090] The positive electrode active material layer 320 may contain a positive electrode active material. The positive electrode active material may contain compounds capable of reversibly inserting and deintercalating lithium ions.

[0091] According to an exemplary embodiment, the positive electrode active material may comprise a lithium-nickel metal oxide. The lithium-nickel metal oxide may further comprise at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0092] In some embodiments, the positive electrode active material or the lithium-nickel metal oxide may comprise a layered structure or a crystal structure represented by the following chemical formula 1.

[0093] [Chemical Formula 1]

[0094] Li x Ni a M b O 2+z

[0095] In chemical formula 1, the values ​​can be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, and -0.5≤z≤0.1. As mentioned above, M can contain Co, Mn, and / or Al.

[0096] The chemical structure represented by Formula 1 indicates the bonding relationships contained in the layered or crystalline structure of the positive electrode active material, and does not exclude other additional elements. For example, M may contain Co and / or Mn, and Co and / or Mn may be provided together with Ni as the main active element of the positive electrode active material. Formula 1 is provided to represent the bonding relationships of the main active elements, and it should be understood that Formula 1 includes the introduction and substitution of additional elements.

[0097] In one embodiment, in addition to the primary active element, auxiliary elements may be further included to enhance the chemical stability of the positive electrode active material or the layered / crystal structure. These auxiliary elements may be incorporated into the layered / crystal structure to form a bond, and this should be understood to also include the chemical structures represented by Formula 1.

[0098] The auxiliary element may include at least one of, for example, Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, or Zr. The auxiliary element may serve as an auxiliary active element, along with Co or Mn, to contribute to the capacity / power activity of the positive electrode active material; for example, Al.

[0099] For example, the positive electrode active material or the lithium-nickel metal oxide may contain a layered structure or a crystal structure represented by the following chemical formula 1-1.

[0100] [Chemical Formula 1-1]

[0101] Li x Ni a M1 b M2 c O 2+z

[0102] In chemical formula 1-1, M1 may contain Co, Mn, and / or Al. M2 may contain the aforementioned auxiliary elements. In chemical formula 1-1, the elements can be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b+c≤0.4, and -0.5≤z≤0.1.

[0103] The positive electrode active material may further include coating elements or doping elements. For example, elements that are substantially the same as or similar to the auxiliary elements described above can be used as coating elements or doping elements. For example, a single element or a combination of two or more elements described above can be used as coating elements or doping elements.

[0104] The coating element or dopant element may exist on the surface of the lithium-nickel metal oxide particles or penetrate through the surface of the lithium-nickel metal oxide particles and be contained in the bonding structure represented by chemical formula 1 or chemical formula 1-1.

[0105] The positive electrode active material may contain nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, NCM-based lithium oxide with increased nickel content can be used.

[0106] Ni can be provided as a transition metal related to the power and capacity of lithium secondary batteries. Therefore, as described above, by using a high-content (High-Ni) composition for the positive electrode active material, a high-capacity positive electrode and a high-capacity lithium secondary battery can be provided.

[0107] However, with increasing Ni content, the long-term storage stability and lifetime stability of the cathode or secondary battery may relatively decrease, and side reactions with the electrolyte may also increase. However, according to an exemplary embodiment, conductivity can be maintained by including Co, while lifetime stability and capacity retention characteristics can be improved by including Mn.

[0108] The Ni content in the NCM-based lithium oxide (e.g., the mole fraction of nickel in the total moles of nickel, cobalt, and manganese) can be 0.6 or more, 0.7 or more, or 0.8 or more. In some embodiments, the Ni content can be 0.8 to 0.95, 0.82 to 0.95, 0.83 to 0.95, 0.84 to 0.95, 0.85 to 0.95, or 0.88 to 0.95.

[0109] In some embodiments, the positive electrode active material may further comprise lithium cobalt oxide-based active material, lithium manganese oxide-based active material, lithium nickel oxide-based active material, or lithium iron phosphate (LFP)-based active material (e.g., LiFePO4).

[0110] In some embodiments, the positive electrode active material may include, for example, a Mn-rich based active material having a chemical structure or crystal structure represented by Chemical Formula 2, a Li rich layered oxide (LLO) / Over Lithiated Oxide (OLO) based active material, and a Co-less based active material.

[0111] [Chemical Formula 2]

[0112] p[Li2MnO3]·(1-p)[Li q JO2]

[0113] In Chemical Formula 2, 0 < p < 1, 0.9 ≤ q ≤ 1.2, and J may include at least one element selected from Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

[0114] The positive electrode active material layer 320 may further include a binder and / or a conductive material. The binder may include an organic-based binder such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, nitrile rubber (NBR), polybutadiene rubber (BR), styrene-butadiene-styrene block copolymer (SBS), or a water-based binder such as styrene-butadiene rubber (SBR), and may be used together with a thickener such as carboxymethyl cellulose (CMC). In one embodiment, a PVDF-based binder may be used as the binder for the positive electrode.

[0115] The conductive material may be included to promote electron migration / conductivity between the active material particles. For example, the conductive material may include a carbon-based conductive material such as graphite, carbon black, graphene, carbon nanotubes, vapor-grown carbon fiber (VGCF), and / or a metal-based conductive material including perovskite materials such as tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3.

[0116] The positive electrode 300 may be formed by coating a positive electrode composition (e.g., in the form of a slurry) including the positive electrode active material on the positive electrode current collector 310, followed by drying and calendaring to form the positive electrode active material layer 320. The positive electrode composition may be prepared by dispersing the positive electrode active material, the binder, the conductive material, etc. in a solvent.

[0117] When the positive current collector 310 is omitted, the positive electrode composition can be heated and molded using a mold to form the positive electrode active material layer 320.

[0118] The negative electrode 200 may include a lithium metal layer. Figure 2 The diagram shows a case where the negative electrode 200 is composed of a lithium metal layer, but the negative electrode 200 may also include a negative electrode current collector and a lithium metal layer disposed on the negative electrode current collector.

[0119] In an exemplary embodiment, a multilayer separator 100 may be provided such that the first coating 20 is adjacent to the lithium metal layer of the negative electrode 200, and the second coating 30 is adjacent to the positive electrode 300. Therefore, the nitrate compound contained in the first coating 20 decomposes on the surface of the negative electrode 200, thereby forming a nitrogen-based solid electrolyte interface layer.

[0120] The nitrogen-based solid electrolyte interface layer can be disposed between the lithium metal layer of the negative electrode 200 and the first coating layer 20. Therefore, lithium can be induced to deposit uniformly on the surface of the lithium metal layer, thereby improving the battery's lifespan characteristics.

[0121] The nitrogen-based solid electrolyte interface layer can be formed after the formation and charging / discharging of the lithium secondary battery. For example, the nitrogen-based solid electrolyte interface layer can be formed by charging and discharging after the lithium secondary battery is formed by assembling the positive electrode 300, the negative electrode 200, and the multilayer separator 100.

[0122] The embodiments of the present invention will be further described below with reference to specific experimental examples. The embodiments and comparative examples included in the experimental examples are only for illustrating the present invention and are not intended to limit the scope of the claims. Various changes and modifications can be made to the embodiments within the scope of the present invention and the technical concept, which will be obvious to those skilled in the art, and such changes and modifications also fall within the scope of the claims.

[0123] Example 1

[0124] Manufacturing of multilayer diaphragms

[0125] A solution of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved at a concentration of 0.5 M was prepared in a mixed solvent containing ethylene carbonate, ethyl methyl carbonate, and tetrahydrofuran in a volume ratio of 25:50:25. A first coating composition was prepared by adding 100 parts by weight of poly(vinylidene fluoride-co-hexafluoropropylene) and 10 parts by weight of LiNO3 to 100 parts by weight of the solution.

[0126] A solution of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved at a concentration of 0.5 M was prepared in a mixed solvent containing ethylene carbonate, ethyl methyl carbonate, and tetrahydrofuran in a volume ratio of 25:50:25. A second coating composition was prepared by adding 100 parts by weight of poly(vinylidene fluoride-co-hexafluoropropylene) to 100 parts by weight of the solution.

[0127] The first coating composition is coated on the surface of a release substrate (PVDF-HFP solution), and a porous substrate (e-PTFE) is laminated on the coated first coating composition.

[0128] The second coating composition is coated onto the porous substrate, then dried once at room temperature for 1 hour, and then dried a second time at 60°C for 12 hours. The release liner is then peeled off to produce a multilayer diaphragm. At this point, the porous substrate has a thickness of 6 μm, the first layer has a thickness of 10 μm, and the second layer has a thickness of 8 μm.

[0129] Manufacturing of secondary batteries

[0130] A positive electrode slurry was prepared by adding 95% by weight of lithium cobalt oxide as the positive electrode active material, 2.5% by weight of polyvinylidene fluoride (PVDF) as a binder, and 2.5% by weight of Super-P as a conductive material to N-methylpyrrolidone (NMP). The positive electrode slurry was then uniformly coated onto an aluminum substrate with a thickness of 12 μm, and subsequently dried and calendered to manufacture the positive electrode.

[0131] Lithium foil is used as the negative electrode.

[0132] The positive and negative electrodes are cut (notching) to specified dimensions, and the multilayer separator is placed in between and stacked to manufacture an electrode assembly. At this stage, the electrode assembly is manufactured such that the first coating of the multilayer separator is adjacent to the negative electrode.

[0133] The electrode assembly is placed in a soft package, and the three sides except for the electrolyte injection surface are sealed. The portion with the tabs is then included within the sealed portion. Electrolyte is injected through the electrolyte injection surface, and the electrolyte injection surface is sealed. The battery is then immersed for at least 12 hours to manufacture a 250mAh lithium secondary battery.

[0134] A solution containing 0.8 M LiTFSI, 0.2 M LiDFBP, and 0.05 M LiPF6 dissolved in a mixed solvent of fluoroethylene carbonate (FEC) / ethyl methyl carbonate (EMC) (1 / 3, volume ratio) was used as the electrolyte.

[0135] Example 2

[0136] The multilayer separator and secondary battery were manufactured using the same method as in Example 1, except that AgNO3 was used instead of LiNO3.

[0137] Example 3

[0138] The multilayer separator and secondary battery were manufactured using the same method as in Example 1, except that 15 parts by weight of LiNO3 were added when preparing the first coating composition.

[0139] Example 4

[0140] The multilayer separator and secondary battery were manufactured using the same method as in Example 1, except that 5 parts by weight of LiNO3 were added when preparing the first coating composition.

[0141] Example 5

[0142] The multilayer separator and secondary battery were manufactured using the same method as in Example 1, except that the thickness of the second coating was changed to 15 μm.

[0143] Example 6

[0144] The multilayer separator and secondary battery are manufactured using the same method as in Example 1, except that a first coating composition is used to form the first coating and the second coating.

[0145] Comparative Example 1

[0146] The multilayer separator and secondary battery are manufactured using the same method as in Example 1, except that a second coating composition is used to form the first and second coatings.

[0147] Comparative Example 2

[0148] The multilayer separator and secondary battery were manufactured using the same method as in Example 1, except that the first coating composition of Example 1 was coated onto the surface of the release substrate (PVDF-HFP solution), and a porous substrate (e-PTFE) was stacked on the coated first coating composition. After drying at room temperature for 1 hour, drying at 60°C for 12 hours, the release substrate was peeled off.

[0149] Experimental Example 1: Evaluation of Capacity Retention

[0150] The lithium secondary batteries of the examples and comparative examples were subjected to 200 charge-discharge cycles at 25°C and a cycle voltage range of 3-4.3V, consisting of charging (CC-CV 0.33C, 0.05C cut-off) and discharging (CC 1C) as one cycle, and the capacity retention rate was measured. The discharge capacity after 200 cycles, relative to the discharge capacity after one cycle, was calculated as the capacity retention rate (%), and the capacity retention rate relative to the number of cycles is shown in Table 1 below.

[0151] If a short circuit or inoperability occurs before reaching 200 charge / discharge cycles, making it impossible to measure capacity retention, it is marked as "unmeasurable".

[0152] The measurement results are shown in Table 1 below and Figures 3 to 5 middle.

[0153] Figure 3 This is a graph showing the capacity retention rates according to Examples 1 to 6.

[0154] Figure 4 This is a graph showing the current and voltage according to Comparative Example 1.

[0155] Figure 5 This is a graph showing the current and voltage according to Comparative Example 2.

[0156] [Table 1]

[0157]

[0158] See Table 1 and Figures 3 to 5 It was confirmed that the comparative example experienced a short circuit or no current flow at a high rate (1C).

[0159] Furthermore, in Examples 3 and 4, where the contents of nitrate compounds were 15 parts by weight and 5 parts by weight, respectively, the capacity retention was reduced compared to Example 1 or Example 2.

[0160] In Example 5, where the thickness of the second coating is 15 μm, the capacity retention rate is reduced compared to Example 1 or Example 2.

[0161] The above description is merely an example of applying the principles of the present invention. Other configurations may be further included without departing from the scope of the present invention.

Claims

1. A multilayer separator for lithium secondary batteries, comprising: Porous substrate; A first coating is disposed on one side of the porous substrate and comprises a first adhesive, a nitrate compound, and a first lithium salt; as well as A second coating is disposed on the opposite side of the porous substrate to the surface in contact with the first coating, and comprises a second adhesive and a second lithium salt.

2. The multilayer separator for lithium secondary batteries according to claim 1, wherein, The nitrate compound includes at least one selected from lithium nitrate, cerium nitrate, and silver nitrate.

3. The multilayer separator for lithium secondary batteries according to claim 1, wherein, The content of the nitrate compound is from 5 to 15 parts by weight relative to 100 parts by weight of the first adhesive.

4. The multilayer separator for lithium secondary batteries according to claim 1, wherein, The second coating does not contain the nitrate compound.

5. The multilayer separator for lithium secondary batteries according to claim 1, wherein, The first lithium salt and the second lithium salt each independently comprise at least one selected from LiF, LiCl, LiBr, LiI, LiNO3, LiN(CN)2, LiBF4, LiClO4, LiPF6, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiCF3CF2SO3, Li(CF3SO2)2N, Li(FSO2)2N, LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, Li(SF5)3C, Li(CF3SO2)3C, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2, LiSCN, and Li(CF3CF2SO2)2N.

6. The multilayer separator for lithium secondary batteries according to claim 1, wherein, The porous substrate has a thickness of 2 μm to 20 μm, and the thicknesses of the first coating and the second coating are each independently 1 μm to 20 μm.

7. The multilayer separator for lithium secondary batteries according to claim 1, wherein, The first adhesive and the second adhesive each independently comprise at least one selected from polyethylene oxide (PEO), polytetrafluoroethylene (PTFE), polyurethane (PU), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP).

8. The multilayer separator for lithium secondary batteries according to claim 1, wherein, The first coating and the second coating each independently further comprise at least one selected from oxide-based solid electrolytes, sulfide-based solid electrolytes, and polymer electrolytes.

9. The multilayer separator for lithium secondary batteries according to claim 1, wherein, The porous substrate includes: A first region, adjacent to the first coating, and containing a first binder, a nitrate compound, and a first lithium salt within its pores; and The second region is adjacent to the second coating and contains a second adhesive and a second lithium salt within its pores.

10. A lithium secondary battery, comprising: positive electrode; The negative electrode is disposed opposite to the positive electrode and includes a lithium metal layer; as well as The multilayer separator for a lithium secondary battery according to claim 1 is disposed between the positive electrode and the negative electrode.

11. The lithium secondary battery according to claim 10, wherein, The lithium secondary battery is configured such that the first coating is adjacent to the lithium metal layer of the negative electrode, and the second coating is adjacent to the positive electrode.

12. The lithium secondary battery according to claim 11, wherein, The lithium secondary battery further includes a nitrogen-based solid electrolyte interface layer disposed between the first coating and the lithium metal layer.

13. A method for manufacturing a multilayer separator for lithium secondary batteries, comprising the following steps: A first coating composition comprising a first adhesive, a nitrate compound, a first lithium salt, and a solvent is applied to one side of a release substrate; A porous substrate is stacked on the coated first coating composition; A second coating composition comprising a second adhesive, a second lithium salt, and a solvent is applied to the porous substrate; as well as Dry it.

14. The method for manufacturing a multilayer separator for a lithium secondary battery according to claim 13, wherein, The first lithium salt in the first coating composition has a content of 0.5M to 2M, and the second lithium salt in the second coating composition has a content of 0.5M to 2M.

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