Method for manufacturing lithium secondary battery

Abstract

The present invention relates to a method for manufacturing a lithium secondary battery, which can exhibit improved capacity and energy density and have excellent safety and life characteristics compared to existing lithium ion batteries and lithium metal secondary batteries.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2019-0141790 filed on November 7, 2019, in the Korean Intellectual Property Office, and Korean Patent Application No. 10-2020-0129950 filed on October 8, 2020, in the Korean Intellectual Property Office, which are hereby incorporated by reference in their entirety.

[0003] The invention relates to a method for manufacturing a lithium secondary battery. Background Art

[0004] Lithium secondary batteries developed in the early 1990s have attracted great attention because they have the advantages of higher operating voltage and much higher energy density than conventional batteries using aqueous electrolyte solutions (such as Ni-MH batteries, Ni-Cd batteries, H2SO4-Pb batteries, etc.). Recently, with the increase in demand for electric vehicles, etc., the demand for lithium secondary batteries with high energy density is also expanding.

[0005] Lithium metal secondary batteries using lithium metal as a negative electrode active material can significantly improve energy density compared to conventional lithium ion batteries using carbon-based and silicon-based active materials, and therefore, continuous research is being conducted.

[0006] However, since lithium metal is highly reactive and easily reacts with moisture and oxygen in the air, the assembly process is complicated, and there are problems with lithium metal growing into dendrites and causing battery short circuits, as well as lithium metal dissolution and precipitation reactions occurring during operation.

[0007] Therefore, there are ongoing efforts in the art to develop lithium secondary batteries having improved safety and lifespan characteristics while having higher capacity than conventional lithium ion batteries. Summary of the invention

[0008] Technical issues

[0009] The object of the present invention is to provide a lithium secondary battery which can achieve higher energy density and has better safety and life characteristics than conventional lithium ion batteries.

[0010] Technical Solution

[0011] According to an embodiment of the present invention, there is provided a method for manufacturing a lithium secondary battery, comprising the following steps:

[0012] Assembling a battery comprising the following components: a negative electrode, wherein a negative electrode active material layer is coated on a negative electrode current collector; a positive electrode, wherein a positive electrode active material layer is coated on a positive electrode current collector; a separator; and an electrolyte, and

[0013] charging the battery,

[0014] wherein, in the step of assembling the battery, the ratio of the negative electrode loading to the positive electrode loading (N / P ratio) is 0.01 to 0.99, and

[0015] Wherein, in the step of charging the battery, charging is performed in a manner corresponding to the positive electrode load. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram illustrating changes in the negative electrode in the method for manufacturing a lithium secondary battery of the present invention. DETAILED DESCRIPTION

[0017] The terms used herein are only used to describe exemplary embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless they have absolutely opposite meanings in the context. It should be understood that the terms "include", "comprise" and "have" used herein are intended to specify the presence of the features, quantities, steps, constituent elements or combinations thereof, but it should be understood that they do not exclude the possibility of the presence or increase of one or more other features, quantities, steps, constituent elements or combinations thereof.

[0018] Since the present invention can be modified in various forms and can have various embodiments, the following exemplary embodiments are described in detail. However, this is not intended to limit the content of the present invention to specific embodiments, and the present invention should be interpreted as including various changes, equivalents and substitutes within the technical scope and spirit of the present invention.

[0019] The present invention relates to a method for manufacturing a lithium secondary battery, which exhibits high energy density and has excellent safety and life characteristics compared with conventional lithium ion batteries and lithium metal secondary batteries.

[0020] According to one embodiment of the present invention, there is provided a method for manufacturing a lithium secondary battery, comprising the following steps:

[0021] Assemble the battery including the following components:

[0022] A negative electrode, wherein a negative electrode active material layer is coated on a negative electrode current collector;

[0023] A positive electrode, wherein a positive electrode active material layer is coated on a positive electrode current collector;

[0024] Diaphragm; and

[0025] Electrolytes, and

[0026] charging the battery,

[0027] wherein, in the step of assembling the battery, the ratio of the negative electrode loading to the positive electrode loading (N / P ratio) is 0.01 to 0.99, and

[0028] Wherein, in the step of charging the battery, charging is performed in a manner corresponding to the positive electrode load.

[0029] In traditional lithium-ion batteries, carbon-based materials capable of lithium insertion / extraction, such as artificial graphite, natural graphite, and hard carbon, have been widely used as negative electrode active materials. Among them, graphite is mainly used in commercial batteries due to its stable structure, low electronic chemical reactivity, and excellent lithium ion storage capacity, but its theoretical capacity is about 372 mAh / g, which limits its application in high-capacity batteries.

[0030] Therefore, as high-capacity negative electrode materials, silicon and tin that can react with lithium to form alloys are being studied as alternative materials, but these materials have the problem that their volume changes significantly when lithium ions are inserted / desorbed.

[0031] On the other hand, as for lithium metal, which can theoretically achieve the highest capacity, it is highly reactive with moisture and oxygen, and is difficult to handle due to its brittleness, so there is a problem of poor processability in the process of manufacturing lithium metal electrodes. Furthermore, the lithium metal electrode composed of a current collector and a lithium metal film may cause such a phenomenon: during the lithium plating and dissolution process during battery operation, lithium metal is precipitated in a dendrite due to the uneven electron density on the electrode surface. In this case, the lithium metal precipitated in a dendrite (lithium dendrite) cannot be used as an active material, and when the dendrite lithium continues to grow, the problem of causing a battery short circuit will occur.

[0032] Therefore, in the present invention, a negative electrode assembly battery including a conventional negative electrode active material (excluding lithium metal) is used, wherein the ratio of the negative electrode loading amount to the positive electrode loading amount, i.e., the N / P ratio, is set to a ratio lower than that of an ordinary battery, and then charged with a positive electrode loading amount higher than the negative electrode loading amount, thereby continuously performing the intercalation of the negative electrode active material and the plating of the lithium metal, thereby manufacturing a lithium secondary battery. In the lithium secondary battery manufactured in this way, the negative electrode active material previously included in the battery assembly process and the lithium metal plated due to charging are both used as negative electrode active materials, and therefore, compared with conventional lithium ion batteries and lithium metal secondary batteries, the lithium secondary battery exhibits improved safety and life characteristics, while having high capacity.

[0033] That is, according to the manufacturing method of the present invention, the negative electrode of the lithium secondary battery finally manufactured has a structure including a negative electrode active material layer coated on the current collector and a lithium metal layer formed on the negative electrode active material layer during the charging process. However, in the step of assembling the battery, since the lithium metal is not treated separately, it is not necessary to shield moisture and oxygen, and therefore, the assembly process is very simple. In addition, when lithium metal is plated, since the specific surface area of ​​the negative electrode active material layer as the plating surface is very large, the current density and overvoltage are significantly reduced compared with the lithium metal film. Therefore, the lithium metal will not grow in a dendritic manner and can be evenly plated on the surface of the negative electrode active material layer. Therefore, the plated lithium metal can be used as an active material.

[0034] Hereinafter, the present invention will be described in detail.

[0035] In the manufacturing method of the lithium secondary battery of the present invention, first, a battery including the following components is assembled: a negative electrode, wherein a negative electrode active material layer is coated on a negative electrode current collector; a positive electrode, wherein a positive electrode active material layer is coated on a positive electrode current collector; a separator; and an electrolyte. At this time, the ratio of the negative electrode loading to the positive electrode loading (N / P ratio) is set to 0.01 to 0.99.

[0036] The current collectors of the positive and negative electrodes are not particularly limited as long as they have conductivity and do not cause chemical changes in the battery.

[0037] For example, the positive electrode current collector may include stainless steel, aluminum, nickel, titanium, fired carbon, or a material formed by surface-treating the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, or the like.

[0038] In addition, the negative electrode current collector may include copper, stainless steel, aluminum, nickel, titanium, fired carbon, or a material formed by surface-treating the surface of copper or stainless steel with carbon, nickel, titanium, silver, or the like.

[0039] The current collector may be formed in various forms, such as a film, a sheet, a foil, a mesh, a porous body, a foam body, and a non-woven fabric structure. The thickness of the current collector may be 3 to 500 micrometers.

[0040] The negative electrode active material layer includes a negative electrode active material, a binder and optionally a conductive material.

[0041] As the negative electrode active material, a carbon-based active material, an alloy of lithium metal, and / or a material capable of reversibly forming a lithium-containing compound by reacting with lithium ions may be used, except for lithium metal.

[0042] As carbon-based active material, crystalline carbon, amorphous carbon or its combination can be used. The example of crystalline carbon can include graphite, such as natural graphite or artificial graphite, and graphite can be in the form of amorphous, plane, flaky, spherical or fibrous. The example of amorphous carbon can be soft carbon (low temperature calcined carbon), hard carbon, mesophase pitch-based carbon and calcined coke etc.

[0043] The lithium metal alloy may be an alloy of lithium (Li) and a metal selected from the group consisting of sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), germanium (Ge) and tin (Sn).

[0044] Examples of materials capable of reversibly forming lithium-containing compounds by reacting with lithium ions may be silicon-based active materials and tin-based active materials. Specific examples thereof may include Si, SiOx (0<x<2), Si-C complex, Si-Q alloy (Q is an alkali metal, an alkaline earth metal, an element of Group 13 to 16, a transition metal, a rare earth element, or a combination thereof, but not Si), Sn, SnO2, Sn-C complex, Sn-R (R is an alkali metal, an alkaline earth metal, an element of Group 13 to 16, a transition metal, a rare earth element, or a combination thereof, but not Sn), etc. Specific elements of Q and R may include Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po or combinations thereof.

[0045] In addition, lithium titanium oxide (LTO) can be used as a negative electrode active material. Lithium titanium oxide can be composed of the formula Li a Ti b O4 (0.5≤a≤3, 1≤b≤2.5), specifically Li 0.8 Ti 2.2 O4、Li 2.67 Ti 1.33 O4, LiTi2O4, Li 1.33 Ti 1.67 O4, or Li 1.14 Ti 1.71 O4, etc., but not limited thereto.

[0046] The content of the above-mentioned negative electrode active material is preferably 70% to 99.5% by weight, or 80% to 99% by weight, based on the total weight of the negative electrode active material layer (i.e., the negative electrode active material layer in the battery assembly step before forming the lithium metal layer, the same below). When the content of the negative electrode active material is less than 70% by weight, it may be disadvantageous in terms of energy density, and when the content of the negative electrode active material exceeds 99.5% by weight, there may be a problem of insufficient binder and peeling of the active material. However, in the case of a lithium metal alloy, since it can be used in the form of foil on a current collector by electroplating without a binder, it can be used at 100% by weight.

[0047] The binder is used to attach the active material particles to each other well and also to attach the active material to the current collector well. Typical examples thereof include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylate-butadiene rubber, epoxy resin, nylon, etc., but are not limited thereto.

[0048] The conductive material is used to impart conductivity to the electrode, and any conductive material can be used without particular limitation as long as it has electronic conductivity without causing chemical changes in the battery. Specific examples thereof include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, and carbon fiber; metal-based materials, for example, metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or mixtures thereof can be used.

[0049] The positive electrode active material layer includes a positive electrode active material, a binder, and an optional conductive material. At this time, as the binder and the conductive material, the above-mentioned binders and conductive materials can be used.

[0050] As the positive electrode active material, a compound known in the art as a material capable of reversibly inserting and extracting lithium ions can be used without particular limitation. Specifically, the positive electrode active material may include a lithium composite metal oxide including at least one metal such as cobalt, manganese, nickel or aluminum, and lithium.

[0051] Lithium composite metal oxides may include lithium-manganese-based oxides (such as LiMnO2, LiMn2O4, etc.), lithium-cobalt-based oxides (such as LiCoO2, etc.), lithium-nickel-based oxides (such as LiNiO2, etc.), lithium-nickel-manganese-based oxides (such as LiNi 1-Y Mn Y O2 (where 0<Y<1), LiMn 2-z Ni zO4 (where 0 < z < 2, etc.), lithium-nickel-cobalt-based oxides (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1, etc.), lithium-manganese-cobalt-based oxides (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2, etc.), lithium-nickel-manganese-cobalt-based oxides (such as Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2), etc.), or lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li(Ni p2 Co q2 Mn r3 M s2 )O2 (where M is at least one selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are atomic fractions of each independent element, 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, p2 + q2 + r3 + s2 = 1), etc.), and may include any one or more than two of these compounds.

[0052] Furthermore, in order to provide a sufficient amount of lithium ions to the negative electrode during the charging process after assembling the battery, a Ni-rich cathode material (NCM811, NCM911) or a Li-rich cathode material (over-lithiated layered oxide: xLi2MnO3·(1 - x)LiMO2, where 0 < x < 1) can be used. Specifically, Li[Li 0.2 Mn 0.54 Ni 0.13 Co 0.13 O2 is preferably used.

[0053] In addition, the loading amount per unit area of the positive electrode active material layer is preferably 4 to 15 mAh / cm 2 , 5 to 10 mAh / cm 2 , or 5 to 8 mAh / cm 2 . When the above loading amount range is satisfied, a sufficient amount of lithium ions can be provided from the positive electrode active material to the negative electrode during the charging step of the battery.

[0054] Meanwhile, the positive electrode active material layer of the present invention may further include an irreversible additive to provide a sufficient amount of lithium ions to the negative electrode. The irreversible additive refers to a material that is irreversible after lithium ions are desorbed during the initial charging process of the battery, that is, it will not re-absorb the desorbed lithium ions.

[0055] At this time, the lithium ions may remain in the irreversible additive converted into an irreversible phase, and the remaining lithium ions are reversibly adsorbed and desorbed, but the lithium ions released during the initial charging process will not desorb into an irreversible compensating additive during the subsequent discharge process, but will be plated on the negative electrode.

[0056] The irreversible additive is not particularly limited as long as it is a compound having the above-mentioned effect. Specifically, it can be selected from Li 7 / 3 Ti 5 / 3 O4、Li 2.3 6S 7.7 , Li2NiO2, Li2CuO2, Li6CoO4, Li5FeO4, Li6MnO4, Li2MoO3, Li3N, Li2O, LiOH and Li2CO3. Among them, in terms of stable life performance and energy density, at least one selected from the group consisting of Li2NiO2, Li6CoO4 and Li3N can be preferably used.

[0057] The content of the irreversible additive is preferably less than 10% by weight of the total weight of the positive electrode active material layer. When the content of the irreversible additive exceeds 10% by weight of the total weight of the positive electrode active material layer, gelation problems may occur during the preparation of the positive electrode slurry. The irreversible additive is a material selectively used to provide a greater amount of lithium ions to the negative electrode, but there is no lower limit.

[0058] The method for manufacturing the negative electrode and the positive electrode is not particularly limited. For example, they can be manufactured by mixing an active material, a binder, an optional conductive material and / or an irreversible additive in an organic solvent to prepare an active material slurry, and coating the prepared active material slurry on a current collector and drying it, and optionally performing compression molding on the current collector.

[0059] As the organic solvent, preferably, those solvents in which the active material, binder, conductive material and irreversible additive can be uniformly dispersed and easily evaporated are used. Specifically, N-methylpyrrolidone, acetonitrile, methanol, ethanol, tetrahydrofuran, water and isopropanol can be cited, but are not limited thereto.

[0060] At the same time, the N / P ratio (= negative electrode loading / positive electrode loading) of the negative electrode and the positive electrode is 0.01 to 0.99. In this way, during the initial assembly of the electrode, when the negative electrode loading is significantly lower than the positive electrode loading, during the battery charging process, lithium ions can be supplied from the positive electrode and plated on the negative electrode active material layer, and a lithium metal layer is formed on the negative electrode active material layer, thereby obtaining a high-capacity negative electrode.

[0061] The separator separates the negative electrode from the positive electrode and provides a channel for the movement of lithium ions. Any separator can be used without particular limitation, as long as it is generally used as a separator of a lithium secondary battery. That is, a separator having excellent electrolyte impregnation ability and low resistance to the migration of electrolyte ions can be used. For example, the separator can be selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFE) or a combination thereof, and can be in the form of a nonwoven or woven fabric.

[0062] For example, a polyolefin-based polymer separator such as polyethylene, polypropylene, or a separator including a coating containing a ceramic component or a polymer material to ensure heat resistance or mechanical strength can be used. The separator can be used as a single layer or multilayer structure. In one embodiment, as the separator, a separator prepared by coating a ceramic coating material containing ceramic particles and an ion binder polymer on both surfaces of a polyolefin-based polymer substrate can be used.

[0063] As the electrolyte, an electrolyte solution including a lithium salt and a nonaqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, or the like, which is generally used for lithium secondary batteries, can be used.

[0064] The electrolyte solution contains a non-aqueous organic solvent and a lithium salt.

[0065] The nonaqueous organic solvent serves as a medium that enables the movement of ions participating in the electrochemical reaction of the battery.

[0066] The non-aqueous organic solvent may include carbonates, esters, ethers, ketones, alcohols or aprotic solvents. Examples of carbonate solvents may include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. Examples of ester solvents may include methyl acetate, ethyl acetate, n-propyl acetate, 1,1-dimethylethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decalactone, valerolactone, methylhydroxyvalerolactone and caprolactone, etc. Examples of ether solvents may include dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc. Examples of ketone solvents may include cyclohexanone, etc. Examples of alcohol solvents may include ethanol and isopropanol, etc. Examples of aprotic solvents may include nitriles such as R-CN (wherein R is a C2 to C20 linear, branched or cyclic hydrocarbon group, a double bond aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolane, and the like.

[0067] The non-aqueous organic solvent can be used alone or in a mixture of two or more thereof. When the solvent is used in a combination of two or more, the mixing ratio can be appropriately controlled according to the desired battery performance, which is widely known by those skilled in the art.

[0068] In addition, when a carbonate solvent is used, it is advantageous to use a mixture of a cyclic carbonate and a chain carbonate. In this case, the cyclic carbonate and the chain carbonate are mixed in a volume ratio of 1:1 to 1:9, so that the performance of the electrolyte can be advantageously exhibited.

[0069] In addition to the carbonate-based solvent, the non-aqueous organic solvent may further include an aromatic hydrocarbon-based organic solvent. Here, the carbonate-based solvent and the aromatic hydrocarbon-based organic solvent may be mixed in a volume ratio of 1:1 to 30:1.

[0070] The non-aqueous electrolyte may further contain vinylene carbonate or ethylene carbonate compounds to increase the service life of the battery.

[0071] Representative examples of ethylene carbonate compounds may include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, and fluoroethylene carbonate, etc. When vinylene carbonate or ethylene carbonate compounds are further used, their usage amount may be appropriately adjusted to increase the service life.

[0072] Lithium salts are dissolved in non-aqueous organic solvents and serve as a lithium ion supply source in the battery, thereby enabling the basic operation of lithium secondary batteries and facilitating the movement of lithium ions between the positive electrode and the negative electrode. Representative examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2) (wherein x and y are natural numbers), LiCl, LiI, LiB(C2O4)2 (lithium bis(oxalato)borate; LiBOB) or a combination thereof, which are included as supporting electrolytic salts. The concentration of the lithium salt is preferably used in the range of 0.1 to 2.0 M. When the concentration of the lithium salt is included in this range, the electrolyte has sufficient conductivity and viscosity, thereby exhibiting excellent electrolyte performance and effectively moving lithium ions.

[0073] Among these electrolytes, if it is an electrolyte containing a large amount of FEC (fluoroethylene carbonate), that is, an electrolyte known to be suitable for lithium metal batteries, a highly concentrated electrolyte, an electrolyte containing a fluorinated ether dilution solvent, such as TTE (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether), OTE (1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether), then the inhibitory effect on the side reaction between the electrolyte solution and the lithium plating layer is the greatest, which can be said to be the most preferred example of this experiment. That is to say, in the present invention, it is preferred to use an electrolyte that inhibits the side reaction of the electrolyte.

[0074] Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate polymers, polyagitated lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing ion dissociative groups.

[0075] Examples of inorganic solid electrolytes include nitrides, halides and sulfates of lithium (Li), such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH and Li3PO4-Li2S-SiS2.

[0076] There is no particular limitation on the manufacturing method of the lithium secondary battery. In one example, the lithium secondary battery is manufactured by stacking the positive electrode, the separator and the negative electrode in sequence to prepare an electrode assembly, placing the electrode assembly in a battery box, and injecting an electrolyte solution into the upper part of the battery box, and sealing it with a cover plate and a gasket. When a solid electrolyte is contained as an electrolyte, the solid electrolyte can be located between the positive electrode and the negative electrode to replace the separator.

[0077] Next, the assembled battery is overcharged by designing the positive electrode load to be higher than the negative electrode load, thereby manufacturing a final lithium secondary battery.

[0078] Figure 1 It is a schematic diagram illustrating changes in the negative electrode in the method for manufacturing a lithium secondary battery of the present invention.

[0079] When the battery is initially assembled, the negative electrode includes a negative electrode current collector 10 and a negative electrode active material layer 21. Thereafter, when the assembled battery is overcharged, the lithium desorbed from the positive electrode active material layer is first intercalated with the active material contained in the negative electrode active material layer. After the intercalation is fully completed, lithium is plated on the pores and surface of the negative electrode active material layer. Since the negative electrode active material layer has a very large specific surface area, low current density, and low overvoltage for lithium plating reaction, during the plating process of lithium metal, the lithium metal will not grow in a dendrite shape, and uniform plating may occur. Therefore, the plated lithium metal can be used as a second negative electrode active material.

[0080] After completing the overcharging step, the negative electrode 100 of the lithium secondary battery finally manufactured includes a current collector 10, a negative electrode active material layer 22 for inserting lithium, and a lithium metal layer 23. The negative electrode active material and lithium metal included in the negative electrode active material layer 22 and the lithium metal layer 23 can be used as active materials of the negative electrode.

[0081] The overcharging step may be performed by charging once or by continuously charging twice or more. The charging method may use various charging methods such as constant current-constant voltage mode (CC-CV mode), constant current mode (CC mode), constant voltage mode (CV mode) and constant power mode (CP mode) charging without particular limitation.

[0082] When the overcharge step is performed by charging once, the charging (lithium insertion) of the negative electrode active material and the plating of lithium metal occur continuously. As a charging method, an appropriate method among the various charging methods described above can be selected. However, when charging is performed only in a manner that the capacity of the positive electrode is higher than that of the negative electrode, overcharging occurs to the negative electrode, and therefore, a lithium plating layer may be formed after charging the negative electrode active material. At this time, when the overcharge step is performed by charging twice in succession, one charge is performed to the extent that the active material included in the negative electrode active material layer is charged for the first time, and then a second charge may be performed so that lithium metal can be continuously plated. At this time, the conditions for the first charge and the second charge may be set differently in consideration of the characteristics of each step.

[0083] Specifically, two consecutive charges can be performed by the following process, wherein the first charge is performed with the initial load corresponding to the negative electrode active material layer, and in the second charge, the charge is performed corresponding to the remaining positive electrode load, thereby performing two consecutive charges. At this time, if high-speed charging is performed in the step of electroplating lithium metal, the side reaction of the electrolyte may become serious and the internal resistance may increase. Therefore, it is preferred that the current during the secondary charge is lower than that during the primary charge.

[0084] The thickness of the lithium metal layer generated after the overcharging step may be 1% to 5000%, or 1% to 200% of the thickness of the negative electrode active material layer. By satisfying such a range, high capacity and long life can be achieved.

[0085] Below, described preferred embodiment of the present invention, comparative example and the experimental example for evaluating them. However, it is obvious to those skilled in the art that these examples are only for illustrative purposes, various changes and modifications can be made within the scope and spirit of this description, and it is self-evident that such modifications and changes fall within the scope of the appended claims.

[0086] <Example 1>

[0087] Cathode Manufacturing

[0088] LiNi used as positive electrode active material 0.6 Co 0.2 Mn 0.2 O2, a conductive material (carbon black) and a binder (PVdF) were added to NMP (N-methyl-2-pyrrolidone) and mixed in a weight ratio of 94:3:3 to prepare a positive electrode mixture.

[0089] The prepared cathode mixture was coated on an aluminum foil with a thickness of 20 μm and a loading capacity of 4.5 mAh / cm 2 , and then roll-pressed and dried to make the positive electrode.

[0090] Negative electrode manufacturing

[0091] Artificial graphite used as a negative electrode, a conductive material (carbon black), and a binder (PVdF) were added to NMP (N-methyl-2-pyrrolidone) at a weight ratio of 95:3:2 and mixed to produce a negative electrode mixture.

[0092] The resulting negative electrode mixture has a 3.0 mAh / cm 2 The negative electrode was coated on a 20 μm thick copper foil, followed by roll pressing and drying.

[0093] Manufacturing of secondary batteries

[0094] A separator (DB307B, BA1 SRS composition, thickness: 15 μm, cloth 7 μm, total coating thickness 8 μm, coating thickness of each surface SRS is 4 μm) was inserted between the negative electrode and the positive electrode, and laminated under a linear pressure of 1 kgf / mm to manufacture an electrode assembly. The electrode assembly was then placed in a pouch battery box, and a non-aqueous electrolyte was added thereto, in which propylene carbonate and dimethyl carbonate were mixed in a volume ratio of 2:8 and contained 3.8M LiFSI as a lithium salt to manufacture a pouch lithium secondary battery.

[0095] Charging process

[0096] Lithium secondary battery at 0.2C condition at 4.5mAh / cm 2 Charge the positive electrode load.

[0097] <Example 2>

[0098] A lithium secondary battery was manufactured in the same manner as in Example 1, except that during the charging process of Example 1, the manufactured lithium secondary battery was charged at 3 mAh / cm 2 The negative electrode loading was charged at 0.2C and then the remaining 1.5 mAh / cm 2 Recharge at 0.1C.

[0099] <Example 3>

[0100] A lithium secondary battery was manufactured in the same manner as in Example 1, except that during the positive electrode manufacturing process of Example 1, LiNi used as the positive electrode active material was used. 0.6 Co 0.2 Mn 0.2 O2, irreversible additive (Li2NiO2), conductive material (carbon black) and binder (PVdF) were added to NMP (N-methyl-2-pyrrolidone) in a weight ratio of 88:2:5:4 to produce a positive electrode mixture.

[0101] <Example 4>

[0102] A lithium secondary battery was manufactured in the same manner as in Example 1, except that during the charging process of Example 1, the manufactured lithium secondary battery was charged at 3 mAh / cm 2 The negative electrode loading was charged at 0.2C and then the remaining 1.5 mAh / cm 2 Recharge at 0.3C.

[0103] <Comparative Example 1>

[0104] A lithium secondary battery was manufactured in the same manner as in Example 1, except that during the charging process of Example 1, the manufactured lithium secondary battery was charged at 3 mAh / cm2 The negative electrode loading was charged at 0.2C.

[0105] <Comparative Example 2>

[0106] A lithium secondary battery was manufactured in the same manner as in Example 1, except that a 60 μm lithium metal negative electrode was used as the negative electrode active material in Example 1.

[0107] <Experimental Example 1>

[0108] In Examples 1 to 3 and Comparative Example 1, the loading amounts of the positive electrode and the negative electrode were measured in the following manner.

[0109] To measure the positive electrode loading, the positive electrode was punched with a coin to 1.6 cm 2 Then, a coin half-cell evaluation was performed using a Li metal electrode as the counter electrode to measure the expression capacity. The positive electrode loading can be calculated by dividing the measured expression capacity by the positive electrode area. Then, the negative electrode was punched out to 1.6 cm with a coin. 2 , a coin half-cell evaluation was performed using a Li metal electrode as the counter electrode to measure the expressed capacity. The negative electrode loading can be calculated by dividing the measured expressed capacity by the area of ​​the negative electrode. The negative electrode loading was divided by the calculated positive electrode loading, as shown in Table 1 below.

[0110]

Table 1

[0111] Load ratio Example 1 0.67 Example 2 0.67 Example 3 0.60 Example 4 0.67 Comparative Example 1 0.67

[0112] <Experimental Example 2>

[0113] After the lithium secondary batteries charged in Examples 1 to 4 and Comparative Examples 1 to 2 were fully discharged at 0.5C, the measured charge / discharge capacity values ​​are shown in Table 2. Similarly, under the same charge / discharge conditions, cycle evaluation was performed at room temperature until the 200th cycle, and the capacity retention rate was calculated and shown in Table 2. (The value calculated from the discharge capacity after 200 cycles / the discharge capacity after 1 cycle) × 100 is expressed as the capacity retention rate (%))

[0114]

Table 2

[0115]

[0116] Referring to Table 2, it was confirmed that in the case of Examples 1 to 4, charging was performed so that the intercalation of the negative electrode active material and the plating of lithium metal were continuously generated, so that the initial charge / discharge capacity was high, thereby manufacturing a high-capacity battery. In addition, it was confirmed that when the capacity retention rate of the 200th cycle was examined, only charging as much as the load amount of the negative electrode was performed, so that it showed the same life characteristics as Comparative Example 1 in which only the negative electrode active material was intercalated, and also showed better life characteristics than Comparative Example 2 in which only lithium metal was used as the negative electrode active material and charging was performed only by lithium plating.

[0117] On the other hand, it was confirmed that Comparative Example 1 in which charging was performed at the negative electrode loading amount had good life characteristics, but the basic capacity was significantly smaller than that of the examples of the present invention.

[0118] In addition, it can be seen that Example 2, which was charged twice, showed a more excellent life characteristic than Example 1, which was charged once. However, since Example 4, which was charged a second time with a large current, had a life similar to that of Example 1, charging a second time with a low current when charging twice was the most excellent in terms of life characteristic.

[0119] Industrial Applicability

[0120] According to the present invention, a lithium secondary battery can be manufactured that can exhibit improved capacity and energy density and has excellent safety and life characteristics compared to existing lithium ion batteries and lithium metal secondary batteries.

Claims

1. A method for manufacturing a lithium secondary battery, comprising the following steps: Assembling a battery, the battery comprising: A negative electrode, wherein a negative electrode active material layer is coated on a negative electrode current collector; A positive electrode, wherein a positive electrode active material layer is coated on a positive electrode current collector; Diaphragm; and Electrolytes, and charging the battery, Wherein, in the step of assembling the battery, the ratio of the negative electrode loading to the positive electrode loading, i.e., the N / P ratio, is 0.01 to 0.99, wherein, in the step of charging the battery, charging is performed in a manner corresponding to the positive electrode load, wherein the battery charging step is performed twice in succession, and Wherein, in the step of charging the battery, the charging current of the second charging is lower than the charging current of the first charging, and In the step of charging the battery, lithium is plated on the pores and the surface of the negative electrode active material layer, thereby forming a lithium metal layer on the negative electrode active material layer.

2. The method for manufacturing a lithium secondary battery according to claim 1, wherein: The thickness of the lithium metal layer is 1% to 5000% of the thickness of the negative electrode active material layer.

3. The method for manufacturing a lithium secondary battery according to claim 1, wherein: The positive electrode active material layer has a loading capacity per unit area of ​​4 mAh / cm 2 Up to 15mAh / cm 2 .

4. The method for manufacturing a lithium secondary battery according to claim 1, wherein: The positive electrode active material layer further comprises at least one irreversible additive selected from the group consisting of Li 7 / 3 Ti 5 / 3 O4、Li 2.3 6S 7.7 、Li2NiO2、Li2CuO 2、 The group consisting of Li6CoO4, Li5FeO4, Li6MnO4, Li2MoO3, Li3N, Li2O, LiOH and Li2CO3.

5. The method for manufacturing a lithium secondary battery according to claim 4, wherein: The content of the irreversible additive is 10 wt % or less of the total weight of the positive electrode active material layer.

6. The method for manufacturing a lithium secondary battery according to claim 1, wherein: The negative electrode active material is at least one selected from the following substances: a carbon-based active material; an alloy of lithium and a metal selected from the group consisting of sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, radium, aluminum, germanium and tin; a silicon-based active material; a tin-based active material; and lithium titanium oxide.

Images