lithium secondary batteries

Abstract

The present invention provides a lithium secondary battery with excellent cycle characteristics. In one embodiment, the lithium secondary battery comprises a positive electrode and a negative electrode without a negative electrode active material, wherein the negative electrode is made of Cu having a maximum surface roughness Rz of 1.0 μm or less and an orientation plane of the (200) plane or the (220) plane.

Description

Technical Field

[0001] The present invention relates to a lithium secondary battery. Background Art

[0002] In recent years, technologies that convert natural energy such as sunlight and wind power into electrical energy have attracted much attention. Consequently, various secondary batteries have been developed as safe and energy-storage devices capable of storing large amounts of electrical energy.

[0003] Among them, secondary batteries that are known to be charged and discharged by moving metal ions between the positive electrode and the negative electrode exhibit high voltage and high energy density. Typically, lithium-ion secondary batteries are known. As a typical lithium-ion secondary battery, there is a lithium-ion secondary battery that is charged and discharged by introducing an active material capable of retaining lithium elements into the positive electrode and the negative electrode, and giving and receiving lithium ions between the positive electrode active material and the negative electrode active material. In addition, as a secondary battery that does not use an active material at the negative electrode, a lithium metal secondary battery has been developed that retains lithium elements by precipitating lithium metal on the surface of the negative electrode.

[0004] For example, Patent Document 1 discloses a high-energy-density, high-output lithium metal anode secondary battery having a volumetric energy density exceeding 1000 Wh / L and / or a mass energy density exceeding 350 Wh / kg when discharged at a rate of at least 1 C at room temperature. Patent Document 1 discloses that an extremely thin lithium metal anode is used to realize such a lithium metal anode secondary battery.

[0005] Furthermore, Patent Document 2 discloses a lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed therebetween, and an electrolyte, wherein the negative electrode forms metal particles on a negative electrode current collector. During charging, the metal particles migrate from the positive electrode to form lithium metal on the negative electrode current collector within the negative electrode. Patent Document 2 discloses that such a lithium secondary battery solves problems caused by the reactivity of lithium metal and problems that arise during assembly, thereby providing a lithium secondary battery with improved performance and lifespan.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application No. 2019-517722

[0009] Patent Document 2: Japanese Patent Application No. 2019-537226 Summary of the Invention

[0010] However, the present inventors have studied conventional batteries including the batteries described in the above-mentioned patent documents in detail and have found that at least one of energy density and cycle characteristics is insufficient.

[0011] For example, typical secondary batteries that charge and discharge by giving and accepting metal ions between the positive and negative active materials have insufficient energy density. Furthermore, existing lithium metal secondary batteries, such as those described in the aforementioned patent documents, that retain lithium by precipitating lithium metal on the negative electrode surface, are prone to forming dendritic lithium metal on the negative electrode surface due to repeated charge and discharge, which can easily cause short circuits and reduced capacity. Consequently, the cycle characteristics are insufficient.

[0012] In addition, in lithium metal secondary batteries, methods have been developed to suppress the discrete growth of lithium metal during precipitation by applying significant physical pressure to the battery to maintain a high pressure at the interface between the negative electrode and the separator. However, since applying this high pressure requires a large mechanical mechanism, the overall weight and volume of the battery increase, and the energy density decreases.

[0013] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a lithium secondary battery having high energy density and excellent cycle characteristics.

[0014] A lithium secondary battery according to one embodiment of the present invention comprises a positive electrode and a negative electrode having no negative electrode active material. The negative electrode is made of Cu having a maximum surface roughness Rz of 1.0 μm or less and an orientation plane of (200) or (220).

[0015] Such a lithium secondary battery has a high energy density because it has a negative electrode without a negative electrode active material. The charge and discharge are performed by the precipitation of lithium metal on the surface of the negative electrode and the electrolytic dissolution of the precipitated lithium metal. Furthermore, in a lithium secondary battery according to one embodiment of the present invention, the negative electrode is made of Cu having a maximum surface roughness Rz of 1.0 μm or less and an orientation plane of (200) or (220). This improves the cycle characteristics of the secondary battery.

[0016] Furthermore, the lithium secondary battery according to one embodiment of the present invention forms the negative electrode Cu by rolling, thereby enabling a structure using Cu having a (200) or (220) orientation plane suitable for the negative electrode.

[0017] The lithium secondary battery is a lithium secondary battery in which charge and discharge are performed by the deposition of lithium metal on the surface of the negative electrode and the dissolution of the deposited lithium metal. This configuration has a higher energy density.

[0018] The lithium secondary battery preferably has no lithium foil formed on the surface of the negative electrode before initial charging. This configuration eliminates the need for using highly flammable lithium metal during manufacturing, resulting in improved safety and productivity.

[0019] The lithium secondary battery preferably has an energy density of 350 Wh / kg or more.

[0020] Effects of the Invention

[0021] According to the present invention, a lithium secondary battery having high energy density and excellent cycle characteristics can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic cross-sectional view of the lithium secondary battery according to the first embodiment.

[0023] Figure 2 This is a schematic cross-sectional view illustrating the use of the lithium secondary battery according to the first embodiment.

[0024] Figure 3 It is a schematic cross-sectional view of a lithium secondary battery according to the second embodiment.

[0025] Figure 4 is a graph showing an example of X-ray analysis measurement results of Cu used for the negative electrode. DETAILED DESCRIPTION

[0026] Below, an embodiment of the present invention (hereinafter referred to as the "present embodiment") will be described in detail with reference to the accompanying drawings as needed. In the accompanying drawings, identical elements are denoted by the same reference numerals, and duplicate descriptions are omitted. Furthermore, positional relationships, such as up and down, left and right, are based on those shown in the accompanying drawings unless otherwise specified. Furthermore, the dimensional ratios in the accompanying drawings are not limited to those shown.

[0027] [First embodiment]

[0028] (Lithium secondary battery)

[0029] Figure 1 : is a schematic cross-sectional view of the lithium secondary battery according to the first embodiment. Figure 1 As shown, the lithium secondary battery 100 of the first embodiment includes a positive electrode 120, a negative electrode 130 having no negative electrode active material, and a separator 140 disposed between the positive electrode 120 and the negative electrode 130. The positive electrode 120 has a positive electrode current collector 110 on the surface opposite to the surface facing the separator 140.

[0030] (negative electrode)

[0031] Negative electrode 130 does not contain a negative electrode active material. Lithium secondary batteries with negative electrodes containing negative electrode active materials have difficulty increasing their energy density due to the presence of the negative electrode active material. On the other hand, since the lithium secondary battery 100 of this first embodiment has a negative electrode 130 without a negative electrode active material, this problem does not arise. Specifically, the lithium secondary battery 100 of this first embodiment achieves high energy density because charging and discharging are performed by the deposition of lithium metal on the negative electrode 130 and the electrolytic dissolution of the deposited lithium metal.

[0032] In this embodiment, "lithium metal is deposited on the surface of the negative electrode" means that lithium metal is deposited on at least one of the surface of the negative electrode and the surface of a solid electrolyte interface (SEI) layer formed on the surface of the negative electrode, which will be described later. Therefore, in a lithium secondary battery, lithium metal may be deposited on the surface of the negative electrode 130 (the interface between the negative electrode 130 and the separator 140), for example.

[0033] In this specification, "negative electrode active material" refers to a substance used to retain lithium ions or lithium metal (hereinafter also referred to as "carrier metal") that will become charge carriers in the negative electrode 130 in the battery. In other words, it can be said that the host material of the carrier metal is used as such a retaining mechanism, and there are no special restrictions, for example, embedding, alloying and absorption of metal clusters are listed.

[0034] Such negative electrode active materials are not particularly limited, and examples thereof include carbon-based substances, metal oxides, and metals or alloys. Examples of the above-mentioned carbon-based substances are not particularly limited, and examples thereof include graphene, graphite, hard carbon, mesoporous carbon, carbon nanotubes, and carbon nanohorns. Examples of the above-mentioned metal oxides are not particularly limited, and examples thereof include titanium oxide compounds, tin oxide compounds, lithium oxide compounds, and cobalt oxide compounds. Examples of the above-mentioned metals or alloys are not particularly limited as long as they can be alloyed with a carrier metal, and examples thereof include silicon, germanium, tin, lead, aluminum, gallium, lithium, and alloys thereof.

[0035] As the negative electrode 130, Cu (copper) without a negative electrode active material is used. The Cu of the negative electrode 130 of this embodiment uses Cu that has been subjected to a known surface treatment by rolling a copper plate into a foil so that the orientation surface becomes a (200) surface or a (220) surface. In addition, the Cu used has a maximum surface roughness height Rz of 1.0 μm or less. In addition, the maximum surface roughness height Rz represents the height difference between the highest position and the lowest position on the Cu surface. Although the detailed mechanism of action is unknown, it has been confirmed that by controlling the surface roughness Rz by adopting a specific orientation surface (200) surface or (220) surface of Cu, as shown in the examples described later, the cycle characteristics of the secondary battery can be improved.

[0036] The orientation plane of Cu is defined as: when the wavelength of the characteristic X-ray is set to When the crystal structure of rolled Cu is evaluated by X-ray diffraction measurement (XRD), the orientation plane corresponding to the position where the highest peak intensity is obtained is determined. That is, the orientation plane of Cu is determined based on which plane the peak intensity is obtained at, the (111) plane, the (200) plane, or the (220) plane. For example, when Figure 4 In such a measurement result, peaks P1, P2, and P3 correspond to the (111), (200), and (220) orientation planes, respectively, and the peak P2 corresponding to the (200) plane has the highest peak intensity. Therefore, it can be said that the orientation plane of Cu that obtained this measurement result is the (200) plane.

[0037] In this specification, “the negative electrode does not have a negative electrode active material” means that the content of the negative electrode active material in the negative electrode is 10% by mass or less relative to the negative electrode as a whole. The content of the negative electrode active material in the negative electrode is preferably 5.0% by mass or less, 1.0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less relative to the negative electrode as a whole. In addition, the lithium secondary battery 100 having a negative electrode without a negative electrode active material means that the lithium secondary battery 100 is an anode-free lithium secondary battery, a zero anode lithium secondary battery, or a negative electrode-free lithium secondary battery in the generally used sense. An anode-free lithium secondary battery refers to a battery system that does not contain lithium metal and negative electrode active material at all, and is composed only of a collector on the negative electrode side.

[0038] Furthermore, in a typical lithium-ion secondary battery, the capacity of the negative electrode active material in the negative electrode is set to be the same as that of the positive electrode. Therefore, if the capacity of the negative electrode active material in the negative electrode 130 is smaller than that of the positive electrode 120, for example, by 20%, 15%, 10%, or 5%, this also means that "the negative electrode does not contain the negative electrode active material."

[0039] The average thickness of the negative electrode 130 is preferably 4 μm to 20 μm, more preferably 5 μm to 18 μm, and even more preferably 6 μm to 15 μm. In this manner, the volume occupied by the negative electrode 130 in the lithium secondary battery 100 is reduced, thereby further improving the energy density of the lithium secondary battery 100.

[0040] (positive electrode)

[0041] Since the positive electrode 120 contains a positive electrode active material, the lithium secondary battery 100 has excellent stability and a high output voltage. In this specification, "positive electrode active material" refers to a substance used to maintain lithium ions at the positive electrode, or in other words, a host material for lithium ions. Such positive electrode active materials are not particularly limited, and examples include metal oxides and metal phosphates. Examples of the above-mentioned metal oxides are not particularly limited, and examples include cobalt oxide compounds, manganese oxide compounds, and nickel oxide compounds. Examples of the above-mentioned metal phosphates are not particularly limited, and examples include iron phosphate compounds and cobalt phosphate compounds. Typical positive electrode active materials include LiCoO2, LiNi x Co y Mn Z O(x+y+z=1), LiNi x Mn y O2 (x+y=1), LiNiO2, LiMn2O4, LiFePO4, LiCoPO4, LiFeOF, LiNiOF, and TiS2. The positive electrode active materials described above are used alone or in combination of two or more.

[0042] The positive electrode 120 may also contain components other than the above-mentioned positive electrode active material. Such components are not particularly limited, and examples thereof include known conductive additives, binders, solid polymer electrolytes, and inorganic solid electrolytes.

[0043] The conductive additive in the positive electrode 120 is not particularly limited, and examples thereof include carbon black, single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), carbon nanofibers (CF), and acetylene black. Furthermore, the binder is not particularly limited, and examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylic resin, and polyimide resin.

[0044] The content of the positive electrode active material in the positive electrode 120 may be, for example, 50% by mass or more and 100% by mass or less relative to the total positive electrode 120. The content of the conductive additive may be, for example, 0.5% by mass or more and 30% by mass or less relative to the total positive electrode 120. The content of the binder may be, for example, 0.5% by mass or more and 30% by mass or less relative to the total positive electrode 120. The total content of the solid polymer electrolyte and the inorganic solid electrolyte may be, for example, 0.5% by mass or more and 30% by mass or less relative to the total positive electrode 120.

[0045] (Positive electrode current collector)

[0046] A positive electrode current collector 110 is formed on one side of the positive electrode 120. The positive electrode current collector 110 is not particularly limited as long as it is a conductor that does not react with lithium ions in the battery. Examples of such a positive electrode current collector include aluminum.

[0047] The average thickness of the positive electrode current collector 110 is preferably 4 μm to 20 μm, more preferably 5 μm to 18 μm, and even more preferably 6 μm to 15 μm. This reduces the volume occupied by the positive electrode current collector 110 in the lithium secondary battery 100, further improving the energy density of the lithium secondary battery 100.

[0048] (Separator)

[0049] The separator 140 prevents battery short circuits by isolating the positive electrode 120 from the negative electrode 130 and ensures ionic conductivity for lithium ions, which serve as charge carriers between the positive and negative electrodes 120 and 130. It is made of a material that is non-electronically conductive and non-reactive with lithium ions. Separator 140 also holds the electrolyte. The separator 140 is not particularly limited as long as it fulfills these functions; examples include porous polyethylene (PE) film, polypropylene (PP) film, or a laminated structure thereof.

[0050] The separator 140 may also be covered by a separator covering layer. The separator covering layer may cover both sides of the separator 140 or only one side. The separator covering layer has ion conductivity and is not particularly limited if it is a component that does not react with lithium ions. It is preferably capable of firmly bonding the separator 140 to the layer adjacent to the separator 140. Such a separator covering layer is not particularly limited, and examples thereof include adhesives such as polyvinylidene fluoride (PVDF), a composite material of styrene-butadiene rubber and carboxymethyl cellulose (SBR-CMC), polyacrylic acid (PAA), lithium polyacrylate (Li-PAA), polyimide (PI), polyamideimide (PAI), and aramid. The separator covering layer may also be formed by adding inorganic particles such as silica, alumina, titanium dioxide, zirconium oxide, magnesium oxide, magnesium hydroxide, and lithium nitrate to the above-mentioned adhesive. In addition, the separator 140 is a separator including a separator covering layer.

[0051] The average thickness of the separator 140 is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less. In this manner, the volume occupied by the separator 140 in the lithium secondary battery 100 is reduced, thereby further improving the energy density of the lithium secondary battery 100. Furthermore, the average thickness of the separator 140 is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more. In this manner, the positive electrode 120 and the negative electrode 130 can be more reliably isolated, and battery short circuits can be further suppressed.

[0052] (Electrolyte)

[0053] The lithium secondary battery 100 may also include an electrolyte. The electrolyte may impregnate the separator 140, or a product obtained by enclosing the electrolyte together with the lithium secondary battery 100 may be used as a finished product. The electrolyte, containing an electrolyte and a solvent, is an ionically conductive solution that acts as a conductive path for lithium ions. Therefore, the internal resistance of the lithium secondary battery 100 including the electrolyte is further reduced, and the energy density, capacity, and cycle characteristics are further improved.

[0054] If the electrolyte is a salt, there is no particular limitation, and examples include salts of Li, Na, K, Ca, and Mg. As the electrolyte, a lithium salt is preferably used. As the lithium salt, there is no particular limitation, and examples include LiI, LiCl, LiBr, LiF, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2CF3CF3)2, LiB(O2C2H4)2, LiB(O2C2H4)F2, LiB(OCOCF3)4, LiNO3, and Li2SO4. From the perspective of better energy density, capacity, and cycle characteristics of the lithium secondary battery 100, LiN(SO2F)2 is preferred as the lithium salt. In addition, one of the above lithium salts is used alone or two or more are used in combination.

[0055] The solvent is not particularly limited, and examples thereof include dimethyl ether, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, chloroethylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, trifluoromethylpropylene carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, nonafluorobutyl methyl ether, nonafluorobutyl ethyl ether, tetrafluoroethyl tetrafluoropropyl ether, trimethyl phosphate, and triethyl phosphate. These solvents may be used alone or in combination of two or more.

[0056] (Use of lithium secondary batteries)

[0057] exist Figure 2 1 shows one way to use the lithium secondary battery of the first embodiment. The lithium secondary battery 100 has a positive terminal 220 and a negative terminal 230 connected to the positive electrode current collector 110 and the negative electrode 130, respectively, for connecting the lithium secondary battery 100 to an external circuit. The lithium secondary battery 100 is charged and discharged by connecting the negative terminal 230 to one end of the external circuit and the positive terminal 220 to the other end of the external circuit.

[0058] The lithium secondary battery 100 is charged by applying a voltage between the positive terminal 220 and the negative terminal 230 such that a current flows from the negative terminal 230 to the positive terminal 220 through an external circuit. Charging the lithium secondary battery 100 deposits lithium metal at the interface between the negative electrode 130 and the separator 140 .

[0059] During initial charging of the lithium secondary battery 100, a solid electrolyte interface layer (SEI layer) 210 is formed at the interface between the negative electrode 130 and the separator 140. The SEI layer 210 formed is not particularly limited and may include, for example, an inorganic compound containing lithium or an organic compound containing lithium. The SEI layer typically has an average thickness of 1 nm to 10 μm.

[0060] When the SEI layer 210 is formed on the lithium secondary battery 100 , lithium metal deposited during charging of the lithium secondary battery 100 may be deposited at the interface between the negative electrode 130 and the SEI layer 210 or at the interface between the SEI layer 210 and the separator 140 .

[0061] The charged lithium secondary battery 100 is discharged when the positive terminal 220 and the negative terminal 230 are connected. Lithium metal precipitates at least at the interface between the negative electrode 130 and the SEI layer 210 and / or the interface between the SEI layer 210 and the separator 140 and is electrolytically dissolved.

[0062] (Method for Manufacturing Lithium Secondary Battery)

[0063] As Figure 1 The method for manufacturing the lithium secondary battery 100 as shown is not particularly limited as long as it is a method that can manufacture a lithium secondary battery having the above-described structure, and examples thereof include the following methods.

[0064] First, prepare the positive electrode 120 using a known method or by purchasing a commercial product. The positive electrode 120 is manufactured, for example, as follows. The above-mentioned positive electrode active material, a known conductive aid, and a known binder are mixed to obtain a positive electrode mixture. The mixing ratio can also be, for example, relative to the above-mentioned positive electrode mixture as a whole, such that the positive electrode active material is 50% by mass or more and 99% by mass, the conductive aid is 0.5% by mass or more and 30% by mass, and the binder is 0.5% by mass or more and 30% by mass. The obtained positive electrode mixture is applied to one side of a metal foil (for example, Al foil) serving as a positive electrode collector having a specified thickness (for example, 5 μm or more and 1 mm or less) and stamped into shape. The obtained molded body is punched into a specified size by punching to obtain the positive electrode 120.

[0065] Next, the negative electrode material, for example, a Cu foil having a thickness of 1 μm to 1 mm, is subjected to rust prevention treatment and then used as the negative electrode 130 .

[0066] Next, the separator 140 having the above-mentioned structure is prepared. The separator 140 may be manufactured by a conventionally known method, or a commercially available separator may be used.

[0067] The positive electrode 120, separator 140, and negative electrode 130 obtained as described above are sequentially stacked to form a laminate. The obtained laminate is sealed in a sealed container together with an electrolyte solution to obtain the lithium secondary battery 100. The sealed container is not particularly limited, and examples thereof include laminate films.

[0068] [This second embodiment]

[0069] (Lithium secondary battery)

[0070] Figure 3 : is a schematic cross-sectional view of a lithium secondary battery according to the second embodiment. Figure 3 As shown, the lithium secondary battery 300 of the second embodiment includes a positive electrode current collector 110, a positive electrode 120 formed on a single surface of the positive electrode current collector 110 and having a positive electrode active material, a negative electrode 130 not having a negative electrode active material, and a solid electrolyte 310 disposed between the positive electrode 120 and the negative electrode 130. The structures of the positive electrode current collector 110, the positive electrode 120, and the negative electrode 130 and their preferred embodiments are the same as those of the lithium secondary battery 100 of the first embodiment, except for the aspects described below. The lithium secondary battery 300 achieves the same effects as the lithium secondary battery 100.

[0071] (Solid Electrolyte)

[0072] Generally speaking, batteries with liquid electrolytes tend to experience varying physical pressure from the electrolyte on the negative electrode surface due to the oscillation of the liquid. On the other hand, since the lithium secondary battery 300 includes a solid electrolyte 310, the pressure applied by the solid electrolyte 310 to the surface of the negative electrode 130 becomes more uniform, further aligning the shape of the carrier metal deposited on the surface of the negative electrode 130. This further suppresses the dendritic growth of the carrier metal deposited on the surface of the negative electrode 130, resulting in improved cycle characteristics for the lithium secondary battery 300.

[0073] As the solid electrolyte 310, generally speaking, if it is used in a lithium secondary battery, there is no particular limitation, and a known material can be appropriately selected according to the purpose of the lithium secondary battery 300 and the type of carrier metal. The solid electrolyte 310 preferably has ion conductivity and does not have electronic conductivity. By having ion conductivity and not having electronic conductivity, the solid electrolyte 310 can further suppress the internal resistance of the obtained lithium secondary battery 300 and further suppress the occurrence of short circuits inside the lithium secondary battery 300. As a result, the energy density, capacity and cycle characteristics of the lithium secondary battery 300 are more excellent.

[0074] The solid electrolyte 310 is not particularly limited, and examples thereof include materials containing resins and lithium salts. Examples of such resins are not particularly limited, and examples thereof include resins having ethylene oxide units in the main chain and / or side chain, acrylic resins, vinyl resins, ester resins, nylon resins, polysiloxanes, polyphosphonitriles, polyvinylidene fluoride, polymethyl methacrylate, polyamides, polyimides, aramids, polylactic acid, polyethylene, polystyrene, polyurethane, polypropylene, polybutylene, polyacetal, polysulfone, and polytetrafluoroethylene. These resins may be used alone or in combination of two or more.

[0075] The lithium salt contained in the solid electrolyte 310 is not particularly limited, and examples thereof include LiI, LiCl, LiBr, LiF, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2CF3CF3)2, LiB(O2C2H4)2, LiB(O2C2H4)F2, LiB(OCOCF3)4, LiNO3, and Li2SO4. The above lithium salts may be used alone or in combination of two or more.

[0076] Generally, the ratio of the resin to the lithium salt in the solid electrolyte is determined by the ratio of oxygen atoms in the resin to lithium atoms in the lithium salt ("Li" / "O"). In solid electrolyte 310, the resin to lithium salt content ratio ("Li" / "O") is preferably adjusted to a value of 0.02 to 0.20, more preferably 0.03 to 0.15, and even more preferably 0.04 to 0.12.

[0077] Solid electrolyte 310 may also contain components other than the resin and lithium salt described above. Such components are not particularly limited, and examples include solvents and salts other than lithium salts. Examples of salts other than lithium salts are not particularly limited, and examples include salts of Na, K, Ca, and Mg.

[0078] The solvent is not particularly limited, and examples thereof include the solvents exemplified in the electrolyte solution that may be included in the lithium secondary battery 100 .

[0079] The average thickness of the solid electrolyte 310 is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less. According to this method, since the volume occupied by the solid electrolyte 310 in the lithium secondary battery 300 is reduced, the energy density of the lithium secondary battery 300 is further improved. In addition, the average thickness of the solid electrolyte 310 is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more. According to this method, the positive electrode 120 and the negative electrode 130 can be more reliably isolated, and the battery short circuit can be further suppressed.

[0080] In this specification, "solid electrolyte" refers to a substance including a gel electrolyte. The gel electrolyte is not particularly limited, and examples thereof include substances containing a polymer, an organic solvent, and a lithium salt. The polymer in the gel electrolyte is not particularly limited, and examples thereof include copolymers of polyethylene and / or polyethylene oxide, polyvinylidene fluoride, and copolymers of polyvinylidene fluoride and hexafluoropropylene.

[0081] It is worth noting that in Figure 3 In the case of the lithium secondary battery 300, a solid electrolyte interface layer (SEI) may also be formed on the surface of the negative electrode 130. The SEI layer formed is not particularly limited, and may include, for example, an inorganic compound containing lithium and an organic compound containing lithium. The typical average thickness of the SEI layer is greater than 1 nm and less than 10 μm. When the SEI layer is formed in the lithium secondary battery 300, the lithium metal precipitated by charging may be precipitated at the interface between the negative electrode 130 and the SEI layer, or may be precipitated at the interface between the SEI layer and the solid electrolyte 310.

[0082] (Method for Manufacturing Secondary Battery)

[0083] The lithium secondary battery 300 can be manufactured in the same manner as the method for manufacturing the lithium secondary battery 100 according to the first embodiment, except that a solid electrolyte is used instead of a separator.

[0084] The method for manufacturing the solid electrolyte 310 is not particularly limited as long as it is a method for obtaining the above-mentioned solid electrolyte 310, and can be, for example, as follows. A resin and a lithium salt that have been used for solid electrolytes (for example, the above-mentioned resin and lithium salt that can be contained in the solid electrolyte 310) are dissolved in an organic solvent. The obtained solution is cast on a molding substrate to a predetermined thickness, thereby obtaining the solid electrolyte 310. Here, the mixing ratio of the resin and the lithium salt can be determined by the ratio ([Li] / [O]) of the oxygen atoms possessed by the resin to the lithium atoms possessed by the lithium salt, as described above. The above-mentioned ratio ([Li] / [O]) is, for example, not less than 0.02 and not more than 0.20. In addition, as an organic solvent, there is no particular limitation, and acetonitrile can also be used, for example. As a molding substrate, there is no particular limitation, and PET film or glass substrate can also be used, for example.

[0085] The above-described embodiment is an example for explaining the present invention, and the present invention is not limited to the embodiment. Various modifications are possible without departing from the spirit of the present invention.

[0086] For example, in the lithium secondary battery 100 of the first embodiment and the lithium secondary battery 300 of the second embodiment, the positive electrode 120 may be formed on both sides of the positive electrode current collector 110. In this case, the lithium secondary battery includes two positive electrodes 120, two negative electrodes 130 arranged to face each positive electrode 120, and two separators 140 or solid electrolytes 310 respectively arranged between the positive electrodes 120 and the negative electrodes 130. This method can further increase the capacity of the lithium secondary battery.

[0087] The lithium secondary battery of this embodiment may also be a lithium solid secondary battery. According to this embodiment, since no electrolyte solution is used, the problem of electrolyte leakage does not occur, and the safety of the battery is further improved.

[0088] The lithium secondary battery of this embodiment may also include a current collector configured to contact the negative electrode. Such a current collector is not particularly limited, and examples thereof include current collectors that can be used for negative electrode materials. In addition, when the lithium secondary battery does not include a negative electrode current collector, the negative electrode itself functions as the current collector.

[0089] The lithium secondary battery 100 may also have terminals for connecting to an external circuit installed on the positive electrode current collector and the negative electrode. For example, a metal terminal (e.g., Al, Ni, etc.) with a thickness of 10 μm to 1 mm may be bonded to one or both of the positive electrode current collector and the negative electrode. The bonding method may be conventionally known methods, such as ultrasonic welding.

[0090] In addition, in this specification, "high energy density" or "high energy density" means high capacity per unit total volume or total mass, preferably 800Wh / L or 350Wh / kg or more, more preferably 900Wh / L or 400Wh / kg or more, and further preferably 1000Wh / L or 450Wh / kg or more.

[0091] In addition, in this specification, "excellent cycle characteristics" means that the rate of reduction of battery capacity is low before and after the number of charge and discharge cycles that can be imagined in normal use. That is, it means that when comparing the first discharge capacity after the initial charge with the capacity after the number of charge and discharge cycles that can be imagined in normal use, the capacity after the charge and discharge cycle has almost no reduction relative to the first discharge capacity after the initial charge. Here, "the number of times that can be imagined in normal use" refers to different uses of the lithium secondary battery, for example, 30 times, 50 times, 70 times, 100 times, 300 times or 500 times. In addition, "the capacity after the charge and discharge cycle has almost no reduction relative to the first discharge capacity after the initial charge and discharge" means that according to the use of the lithium secondary battery, for example, the capacity after the charge and discharge cycle is 75% or more, 80% or more, or 85% or more relative to the first discharge capacity after the initial charge and discharge.

[0092] Example

[0093] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[0094] [Manufacturing of lithium secondary batteries]

[0095] Each step related to the production of a lithium secondary battery is performed as follows.

[0096] (Preparation of negative electrode)

[0097] A 10 μm Cu foil prepared in accordance with the following examples was used as a negative electrode and subjected to rust prevention treatment and the like.

[0098] (Preparation of separators)

[0099] As a separator, a separator of a predetermined size (50 mm×50 mm) was prepared in which both surfaces of a 12 μm polyethylene microporous membrane were coated with 2 μm polyvinylidene fluoride (PVDF).

[0100] (Production of positive electrode)

[0101] 96 parts by mass of LiNi as the positive electrode active material 0.85 Co 0.12 Mn 0.03A mixture of O2, 2 parts by mass of carbon black as a conductive additive, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder was applied to one side of a 12μm thick aluminum foil, serving as the positive electrode current collector, and then press-formed. The resulting molded body was punched out to a specified size (40 mm x 40 mm) to obtain the positive electrode.

[0102] (Battery Assembly)

[0103] As an electrolyte, a 4M dimethoxyethane (DME) solution of LiN(SO2F)2(LFSI) was prepared. Then, a laminate was obtained by laminating the positive electrode, the separator and the negative electrode in sequence. It is worth noting that when a buffer functional layer is formed on the separator, the laminate is implemented in a manner that the buffer functional layer is opposite to the negative electrode, and when a metal layer is formed on the negative electrode, the laminate is implemented in a manner that the metal layer is opposite to the separator. Furthermore, after ultrasonically welding a 100μm Al terminal and a 100μm Ni terminal to the positive electrode and the negative electrode, respectively, the outer packaging body of the laminate was inserted. Then, the electrolyte obtained as described above was injected into the above-mentioned outer packaging body. By sealing the outer packaging body, a lithium secondary battery was obtained.

[0104] [Example 1]

[0105] Cu having a (200) orientation plane formed by rolling and a maximum surface roughness height Rz of 0.3 μm was used as a negative electrode, and a secondary battery including a positive electrode, a separator, and a negative electrode was prepared.

[0106] [Example 2]

[0107] Cu having a (200) orientation plane formed by rolling and a maximum surface roughness height Rz of 0.7 μm was used as a negative electrode, and a secondary battery including a positive electrode, a separator, and a negative electrode was prepared.

[0108] [Example 3]

[0109] Cu having a (200) orientation plane formed by rolling and a maximum surface roughness height Rz of 0.7 μm was used as a negative electrode, and a secondary battery including a positive electrode, a separator, and a negative electrode was prepared.

[0110] [Example 4]

[0111] Cu having a (200) orientation plane formed by rolling and a maximum surface roughness height Rz of 1.0 μm was used as a negative electrode, and a secondary battery including a positive electrode, a separator, and a negative electrode was prepared.

[0112] [Comparative Example 1]

[0113] Cu having a (200) orientation plane formed by rolling and a maximum surface roughness height Rz of 1.2 μm was used as a negative electrode, and a secondary battery including a positive electrode, a separator, and a negative electrode was prepared.

[0114] [Comparative Example 2]

[0115] An electrolytic copper foil having a (111) orientation plane formed by electroplating and a maximum surface roughness height Rz of 0.8 μm was used as a negative electrode to produce a secondary battery including a positive electrode, a separator, and a negative electrode.

[0116] [Comparative Example 3]

[0117] An electrolytic copper foil having a (111) orientation plane formed by electroplating and a maximum surface roughness height Rz of 1.5 μm was used as a negative electrode to produce a secondary battery including a positive electrode, a separator, and a negative electrode.

[0118] [Evaluation of energy density and cycle characteristics]

[0119] The cycle characteristics of the lithium secondary batteries produced in each of the Examples and Comparative Examples were evaluated as follows.

[0120] The prepared lithium secondary battery was repeatedly heated at 0.2 mAh / cm 2 After charging to 4.2V (initial charge), the 2 The discharge was continued until the voltage reached 3.0 V (initial discharge). Then, the discharge was repeated 49 times at 1.0 mAh / cm2 at a temperature of 25°C. 2 After charging to 4.2V, the 2 The charge and discharge cycle is performed until the voltage becomes 3.0V. For any embodiment and comparative example, the capacity (initial capacity) obtained from the initial discharge is 60mAh. When the initial charge and discharge cycle is counted as the first cycle, the ratio of the discharge capacity obtained from the discharge in the fiftieth cycle of the charge and discharge cycle to the discharge capacity obtained from the discharge in the second cycle of the charge and discharge cycle is calculated as the capacity retention rate (%), which is used as an indicator of the cycle characteristics. The higher the capacity retention rate, the better the cycle characteristics. The capacity retention rates in each example are shown in Table 1.

[0121] [Table 1]

[0122] Sample number Preparation method Rz(μm) Orientation plane Capacity retention rate (%) Example 1 Calendering 0.3 200 90 Example 2 Calendering 0.7 200 90 Example 3 Calendering 0.7 220 90 Example 4 Calendering 1.0 200 88 Comparative Example 1 Calendering 1.2 200 70 Comparative Example 2 electrolysis 0.8 111 70 Comparative Example 3 electrolysis 1.5 111 71

[0123] As shown in Table 1, the lithium secondary batteries of Examples 1 to 4 exhibited high capacity retention rates of 88% to 90%. On the other hand, the lithium secondary batteries of Comparative Examples 1 to 3 exhibited capacity retention rates of 70% to 71%. This indicates that Examples 1 to 4, which used Cu having a (200) or (220) orientation and a maximum surface roughness height Rz of 1.0 μm or less, exhibited higher capacity retention rates and superior cycle characteristics compared to Comparative Examples 1 to 3.

[0124] Industrial Applicability

[0125] The lithium secondary battery of the present invention has high energy density and excellent cycle characteristics, and therefore has industrial applicability as a power storage device used in various applications.

[0126] Description of Reference Numerals

[0127] 100, 300…lithium secondary battery; 110…positive electrode current collector; 120…positive electrode; 130…negative electrode; 140…separator; 210…solid electrolyte interface layer (SEI layer); 220…positive electrode terminal; 230…negative electrode terminal; 310…solid electrolyte.

Claims

1. A lithium secondary battery comprising a positive electrode and a negative electrode having no negative electrode active material, The negative electrode is Cu having a maximum surface roughness height Rz of 1.0 μm or less and an orientation plane of a (200) plane or a (220) plane, and the Cu is formed by rolling.

2. The lithium secondary battery according to claim 1, wherein Charging and discharging are performed by the deposition of lithium metal on the surface of the negative electrode and the dissolution of the deposited lithium metal.

3. The lithium secondary battery according to claim 1, wherein Before initial charge, no lithium foil is formed on the surface of the negative electrode.

4. The lithium secondary battery according to claim 1, wherein The energy density is above 350Wh / kg.

Images