Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery having a high energy density and excellent cycling characteristics. A lithium secondary battery is provided, which includes a positive electrode, a separator, a negative electrode without a negative electrode active material, and an electrolytic solution. The electrolytic solution contains, as a solvent, a compound having at least one of a monovalent group represented by the formula (A) and a monovalent group represented by the formula (B).

Description

Technical Field

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

[0002] In recent years, technologies for converting natural energy such as sunlight or wind power into electric energy have attracted much attention. Along with this, various secondary batteries have been developed as power storage devices that are highly safe and can store a large amount of electric energy.

[0003] Among them, it is known that a secondary battery that performs charge and discharge by moving metal ions between a positive electrode and a negative electrode exhibits a high voltage and a high energy density. Typically, a lithium ion secondary battery is known. As a typical lithium ion secondary battery, a lithium ion secondary battery is cited in which an active material capable of holding lithium element is introduced into a positive electrode and a negative electrode, and charge and discharge are performed by 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 in which lithium metal is deposited on the surface of the negative electrode to hold lithium element.

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

[0005] In addition, in Patent Document 2, a lithium secondary battery is disclosed in which, in a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed therebetween, and an electrolyte, the negative electrode forms metal particles on a negative electrode current collector, and lithium metal is formed on the negative electrode current collector in the negative electrode by charging and moving from the positive electrode. Patent Document 2 discloses that such a lithium secondary battery solves problems caused by the reactivity of lithium metal and problems occurring during assembly, and can provide a lithium secondary battery with improved performance and lifespan.

[0006] Prior Art Documents

[0007] Patent Documents

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

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

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

[0011] For example, the energy density of a typical secondary battery that charges and discharges by giving and receiving metal ions between a positive electrode active material and a negative electrode active material is insufficient. In addition, in a conventional lithium metal secondary battery that holds lithium element by depositing lithium metal on the surface of the negative electrode as described in the above patent document, dendritic lithium metal is likely to form on the surface of the negative electrode due to repeated charge and discharge, and short circuit and capacity reduction are likely to occur. As a result, the cycle characteristics are insufficient.

[0012] In addition, in a lithium metal secondary battery, a method has also been developed in which a large physical pressure is applied to the battery to suppress the discrete growth during lithium metal deposition, and the interface between the negative electrode and the separator is maintained at a high pressure. However, since a large mechanical mechanism is required for applying such a high pressure, the weight and volume of the battery as a whole increase, and the energy density decreases.

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

[0014] A lithium secondary battery according to an embodiment of the present invention includes a positive electrode, a separator, a negative electrode having no negative electrode active material, and an electrolytic solution. The electrolytic solution contains, as a solvent, a compound having at least one of a monovalent group represented by the following formula (A) and a monovalent group represented by the following formula (B) (hereinafter, this compound will also be simply referred to as "fluorinated alkyl compound").

[0015] [Chemical formula 1]

[0016]

[0017] [Chemical formula 2]

[0018]

[0019] (In the formula, the wavy line indicates the bonding site in the monovalent group).

[0020] Such a lithium secondary battery has a high energy density because it has a negative electrode having no negative electrode active material and charges and discharges by depositing lithium metal on the surface of the negative electrode and electrolytically dissolving the deposited lithium metal.

[0021] In addition, the inventors of the present invention have found that in a lithium secondary battery having the above-mentioned fluorinated alkyl compound as a solvent in an electrolytic solution, a solid electrolyte interface layer (hereinafter, also referred to as "SEI layer") is likely to be formed on the surface of the negative electrode. Since the SEI layer has ion conductivity, the reactivity of the lithium precipitation reaction in the surface of the negative electrode where the SEI layer is formed is uniform in the plane direction of the negative electrode surface. Therefore, the above-mentioned lithium secondary battery suppresses the growth of dendritic lithium metal on the negative electrode and has excellent cycle characteristics. In addition, although the main reason for the easy formation of the SEI layer by containing the fluorinated alkyl compound as a solvent is not necessarily clear, the main reasons described later in the specific embodiments can be considered.

[0022] A lithium secondary battery according to an embodiment of the present invention preferably contains a fluorinated alkyl compound having an ether bond as a solvent in an electrolytic solution. According to such a mode, since the solubility of the electrolyte in the electrolytic solution is further increased and the SEI layer is more easily formed, the cycle characteristics of the lithium secondary battery are more excellent.

[0023] The above-mentioned electrolytic solution preferably contains two or more kinds of fluorinated alkyl compounds. For example, the electrolytic solution contains at least two selected from the group consisting of an ether compound having both a monovalent group represented by the above formula (A) and a monovalent group represented by the above formula (B), an ether compound having a monovalent group represented by the above formula (A) and not having a monovalent group represented by the above formula (B), and an ether compound not having a monovalent group represented by the above formula (A) and having a monovalent group represented by the above formula (B). According to such a mode, since the SEI layer is more easily formed, the lithium secondary battery is more excellent in cycle characteristics.

[0024] The above-mentioned electrolytic solution preferably further contains a fluoroalkyl ether compound not having either a monovalent group represented by the above formula (A) or a monovalent group represented by the above formula (B). According to such a mode, the solubility of the electrolyte in the electrolytic solution tends to be further increased or the SEI layer is more easily formed, so the lithium secondary battery is more excellent in cycle characteristics.

[0025] The content of the fluorinated alkyl compound is preferably 40% by volume or more with respect to the total amount of the solvent components of the above-mentioned electrolytic solution. According to such a mode, since the SEI layer is more easily formed, the lithium secondary battery is more excellent in cycle characteristics.

[0026] The above-mentioned lithium secondary battery is preferably a lithium secondary battery in which lithium metal is precipitated on the surface of the negative electrode and the precipitated lithium is dissolved to perform charge and discharge. According to such a mode, the energy density is further increased.

[0027] The above-mentioned negative electrode is preferably an electrode composed of at least one selected from the group consisting of Cu, Ni, Ti, Fe, metals that do not react with Li other than these, and their alloys, and stainless steel (SUS). In this way, since highly flammable lithium metal does not need to be used during manufacturing, safety and productivity are more excellent. In addition, since such a negative electrode is stable, the cycle characteristics of the lithium secondary battery are further improved.

[0028] The above-mentioned lithium secondary battery is preferably such that no lithium foil is formed on the surface of the above-mentioned negative electrode before initial charging. In this way, since highly flammable lithium metal does not need to be used during manufacturing, safety and productivity are more excellent.

[0029] The above-mentioned lithium secondary battery preferably has an energy density of 350 Wh / kg or more.

[0030] Advantages of the Invention

[0031] According to the present invention, it is possible to provide a lithium secondary battery having a high energy density and excellent cycle characteristics. Brief Description of the Drawings

[0032] Figure 1 is a schematic cross-sectional view of a lithium secondary battery according to an embodiment of the present invention.

[0033] Figure 2 is a schematic cross-sectional view of the use of a lithium secondary battery according to an embodiment of the present invention. Detailed Description of the Embodiment

[0034] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail with reference to the drawings as needed. In the drawings, the same reference numerals are assigned to the same elements, and repeated descriptions are omitted. In addition, the positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings unless otherwise specified. Moreover, the dimensional ratios in the drawings are not limited to the ratios shown.

[0035] [The Present Embodiment]

[0036] (Lithium Secondary Battery)

[0037] Figure 1 is a schematic cross-sectional view of a lithium secondary battery according to the present embodiment. As Figure 1 shown, the lithium secondary battery 100 of the present embodiment includes a positive electrode 120, a negative electrode 140 without a negative electrode active material, a separator 130 disposed between the positive electrode 120 and the negative electrode 140, and an electrolytic solution (not shown) in Figure 1 . The positive electrode 120 has a positive electrode current collector 110 on the surface opposite to the surface facing the separator 130.

[0038] (Negative Electrode)

[0039] The negative electrode 140 does not have a negative electrode active material. In a lithium secondary battery having a negative electrode with a negative electrode active material, it is difficult to increase the energy density due to the presence of the negative electrode active material. On the other hand, since the lithium secondary battery 100 of the present embodiment has a negative electrode 140 that does not have a negative electrode active material, such a problem does not occur. That is, in the lithium secondary battery 100 of the present embodiment, since charging and discharging are performed by lithium metal depositing on the negative electrode 140 and the deposited lithium metal being electrochemically dissolved, the energy density is high.

[0040] In the present embodiment, "lithium metal depositing on the negative electrode" means that lithium metal deposits at least at 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. In the lithium secondary battery of the present embodiment, it is considered that lithium metal mainly deposits on the surface of the SEI layer, but the deposition position is not limited thereto.

[0041] In the present specification, the "negative electrode active material" refers to a material in the battery that holds lithium ions or lithium metal (hereinafter, also referred to as "carrier metal"), which becomes a charge carrier, on the negative electrode 140. In other words, it can be said that it is a host material for the carrier metal. As such a holding mechanism, there is no particular limitation, and examples include insertion, alloying, and absorption of metal clusters, typically insertion. In addition, lithium metal itself is not included in the negative electrode active material.

[0042] As such a negative electrode active material, there is no particular limitation, and examples include carbonaceous materials, metal oxides, and metals or alloys. As the above carbonaceous materials, there is no particular limitation, and examples include graphene, graphite, hard carbon, mesoporous carbon, carbon nanotubes, and carbon nanohorns. As the above metal oxides, there is no particular limitation, and examples include titanium oxide-based compounds, tin oxide-based compounds, and cobalt oxide-based compounds. As the above metal or alloy, if it can be alloyed with the carrier metal, there is no particular limitation, and examples include silicon, germanium, tin, lead, aluminum, gallium, and alloys containing them.

[0043] As the negative electrode 140, if it does not have a negative electrode active material but can be used as a current collector, there is no particular limitation, and examples include at least one selected from the group consisting of Cu, Ni, Ti, Fe, and other metals that do not react with Li and their alloys, and stainless steel (SUS). In addition, when SUS is used in the negative electrode, various known substances can be used as the type of SUS. The above-mentioned negative electrode materials are used alone or in combination of two or more. In addition, in the present specification, the "metal that does not react with Li" refers to a metal that does not react with lithium ions or lithium metal to form an alloy under the operating conditions of the lithium secondary battery.

[0044] The negative electrode 140 is preferably an electrode that does not contain lithium. In this way, since highly flammable lithium metal does not need to be used during manufacturing, the safety and productivity of the lithium secondary battery 100 are more excellent. From the same viewpoint and from the viewpoint of improving the stability of the negative electrode 140, among them, the negative electrode 140 is more preferably at least one selected from the group consisting of Cu, Ni, these alloys, and stainless steel (SUS). From the same viewpoint, the negative electrode 140 is more preferably composed of Cu, Ni, or an alloy composed of these, and particularly preferably composed of Cu or Ni.

[0045] 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 with respect to the entire negative electrode. 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, and may also be 0.0% by mass or less. In addition, the lithium secondary battery 100 having a negative electrode that does not have a negative electrode active material means that the lithium secondary battery 100 is a non-aqueous anode lithium battery, a zero anode lithium battery, or a non-negative electrode lithium battery in the generally used sense.

[0046] The capacity of the negative electrode 140 is sufficiently smaller than the capacity of the positive electrode 120, and can be, for example, 20% or less, 15% or less, 10% or less, or 5% or less. In addition, the capacities of the positive electrode 120 and the negative electrode 140 can be measured by existing well-known methods.

[0047] The average thickness of the negative electrode 140 is preferably 4 μm or more and 20 μm or less, more preferably 5 μm or more and 18 μm or less, and further preferably 6 μm or more and 15 μm or less. In this way, since the volume occupied by the negative electrode 140 in the lithium secondary battery 100 is reduced, the energy density of the lithium secondary battery 100 is further increased.

[0048] (Electrolyte solution)

[0049] The electrolyte solution contains an electrolyte and a solvent, is a solution having ionic conductivity, and functions as a conduction path for lithium ions. The electrolyte solution can infiltrate the separator 130, or can be sealed in a closed container together with the laminate of the positive electrode 120, the separator 130, and the negative electrode 140.

[0050] The electrolyte solution contains, as a solvent, a fluorinated alkyl compound having at least one of a monovalent group represented by the following formula (A) and a monovalent group represented by the following formula (B).

[0051] [Chemical formula 3]

[0052]

[0053] [Chemical formula 4]

[0054]

[0055] Among them, in the formula, the wavy line indicates the bonding site in the monovalent group.

[0056] Generally, in an anode-free lithium secondary battery having an electrolyte, by decomposing a solvent in the electrolyte or the like, an SEI layer is formed on the surface of the negative electrode or the like. In the lithium secondary battery, the SEI layer suppresses further decomposition of the components in the electrolyte and irreversible reduction of lithium ions and generation of gas caused thereby. In addition, since the SEI layer has ion conductivity, the reactivity of the lithium metal precipitation reaction becomes uniform in the surface direction of the negative electrode surface where the SEI layer is formed. Therefore, promoting the formation of the SEI layer is very important because it improves the performance of the anode-free lithium secondary battery. The inventors of the present invention have found that in a lithium secondary battery containing the above-mentioned fluorinated alkyl compound as a solvent, an SEI layer is easily formed on the negative electrode surface, suppressing the growth of dendritic lithium metal on the negative electrode, and as a result, the cycle characteristics are improved. The main reason is not necessarily clear, but the following main reasons are considered.

[0057] It is considered that during charging of the lithium secondary battery 100, especially during initial charging, not only lithium ions but also the above-mentioned fluorinated alkyl compound as a solvent are reduced on the negative electrode. Then, due to the replacement of a part represented by the above formula (A) and a part represented by the above formula (B) in the fluorinated alkyl compound with multiple fluorines, the reactivity of the oxygen atom is high, and it is speculated that a part or all of the part represented by the above formula (A) and the part represented by the above formula (B) are easily detached. As a result, it is speculated that during charging of the lithium secondary battery 100, since a part or all of the part represented by the above formula (A) and the part represented by the above formula (B) are adsorbed on the negative electrode surface and the adsorbed part is used as a starting point to generate an SEI layer, the lithium secondary battery 100 easily forms an SEI layer. However, the main reason is not limited to the above.

[0058] In addition, it has been surprisingly found that the SEI layer formed in the lithium secondary battery 100 containing the above-mentioned fluorinated alkyl compound has higher ion conductivity than the SEI layer formed in the existing lithium secondary battery. This is because the fluorine content of the formed SEI layer becomes higher due to the replacement of the part represented by the above formula (A) and the part represented by the above formula (B) with fluorine, and the movement path of lithium ions in the SEI layer increases or expands. However, the main reason is not limited to this.

[0059] Therefore, although the lithium secondary battery 100 easily forms an SEI layer, the internal resistance of the battery is low and the rate performance is excellent. That is, the lithium secondary battery 100 has excellent cycle characteristics and rate performance. In addition, "rate performance" means the performance capable of charging and discharging with a large current, and it is known that the rate performance is excellent when the internal resistance of the battery is low.

[0060] In addition, in the part represented by the above formula (A) and the part represented by the above formula (B) of the fluorinated alkyl compound, the hydrogen atoms of a part of the alkyl group are not substituted with fluorine atoms. Therefore, since the lithium secondary battery 100 containing the above fluorinated alkyl compound as a solvent can increase the concentration of the electrolyte in the electrolyte, the cycle characteristics and the rate performance can be further improved.

[0061] In addition, in this specification, "containing as a solvent" for a compound means that in the usage environment of the lithium secondary battery, as long as the compound monomer or a mixture with other compounds is a liquid, and further as long as it can dissolve the electrolyte to prepare an electrolyte in a solution phase.

[0062] In the embodiment of the present invention, the fluorinated alkyl compound is not particularly limited, and examples include compounds having an ether bond (hereinafter, referred to as "ether compounds"), compounds having an ester bond, and compounds having a carbonate bond. From the viewpoints of further increasing the solubility of the electrolyte in the electrolyte and more easily forming an SEI layer, the fluorinated alkyl compound is preferably an ether compound.

[0063] As the ether compound which is a fluorinated alkyl compound, examples include ether compounds having both a monovalent group represented by formula (A) and a monovalent group represented by formula (B) (hereinafter, also referred to as "first fluorinated solvent"), ether compounds having a monovalent group represented by formula (A) and not having a monovalent group represented by formula (B) (hereinafter, also referred to as "second fluorinated solvent"), and ether compounds not having a monovalent group represented by formula (A) and having a monovalent group represented by formula (B) (hereinafter, also referred to as "third fluorinated solvent").

[0064] As the first fluorinated solvent, examples include 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether (TTFE), 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl diethoxymethane, and 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl diethoxypropane. From the viewpoint of effectively and reliably achieving the effects of the above fluorinated alkyl compound, as the first fluorinated solvent, 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether is preferred.

[0065] As the second fluorinated solvent, for example, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl methyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, 1,1,2,2-tetrafluoroethyl propyl ether, 1H,1H,5H-perfluoropentyl-1,1,2,2-tetrafluoroethyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, etc. are listed. From the viewpoint of effectively and reliably achieving the effects of the above-mentioned fluorinated alkyl compounds, as the second fluorinated solvent, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl methyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether are preferred.

[0066] As the third fluorinated solvent, for example, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, 2,2,3,3-tetrafluoropropyl trifluoromethyl ether, 2,2,3,3-tetrafluoropropyl monofluoromethyl ether, 2,2,3,3-tetrafluoropropyl methyl ether, etc. are listed. From the viewpoint of effectively and reliably achieving the effects of the above-mentioned fluorinated alkyl compounds, as the third fluorinated solvent, 2,2,3,3-tetrafluoropropyl difluoromethyl ether is preferred.

[0067] The electrolyte may contain at least one fluorinated alkyl compound. From the viewpoints of further increasing the solubility of the electrolyte in the electrolyte and more easily forming the SEI layer, the electrolyte preferably contains two or more fluorinated alkyl compounds. From the same viewpoints, the electrolyte preferably contains at least one selected from the group consisting of the first fluorinated solvent, the second fluorinated solvent, and the third fluorinated solvent, more preferably contains at least two selected from the group consisting of the first fluorinated solvent, the second fluorinated solvent, and the third fluorinated solvent, and still more preferably contains at least three selected from the group consisting of the first fluorinated solvent, the second fluorinated solvent, and the third fluorinated solvent.

[0068] The electrolyte may also contain a compound other than the fluorinated alkyl compound as a solvent. Such a compound is not particularly limited, and for example, a fluoroalkyl ether compound that does not have any of the monovalent groups represented by formula (A) and the monovalent groups represented by formula (B) (hereinafter, also referred to as "fourth fluorinated solvent") and a compound that does not have a fluorine-substituted alkyl group (hereinafter, referred to as "sub-solvent") are listed. From the viewpoints of further increasing the solubility of the electrolyte in the electrolyte and more easily forming the SEI layer, the electrolyte preferably contains the fourth fluorinated solvent. In addition, "fluoroalkyl ether compound" means an ether compound having a fluorine-substituted alkyl group, and "fluorine-substituted alkyl" means an alkyl group in which at least one hydrogen atom is substituted by fluorine.

[0069] In the fourth fluorinated solvent, the ratio of the number of fluorine atoms to the total of the number of hydrogen atoms and the number of fluorine atoms (F / (H + F)) is not particularly limited, and may be 0.1 or more, may be 0.2 or more, or may be 0.5 or more. Further, the fourth fluorinated solvent preferably has a perfluoroalkyl group and an unsubstituted alkyl group. Examples of the perfluoroalkyl group include linear or branched substances having 1 to 10 carbon atoms, and preferably linear substances having 2 to 6 carbon atoms. Examples of the unsubstituted alkyl group include linear or branched substances having 1 to 5 carbon atoms, and preferably linear substances having 1 to 3 carbon atoms, and include a methyl group or an ethyl group.

[0070] Examples of the fourth fluorinated solvent include methyl nonafluorobutyl ether, ethyl nonafluorobutyl ether, 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-trifluoromethylpentane, 2,2,3,3,3-pentafluoropropyl methyl ether, 1,1,2,3,3,3-hexafluoropropyl methyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, and tetrafluoroethyl tetrafluoropropyl ether. From the viewpoint of effectively and reliably achieving the effects of the above-mentioned fluorinated alkyl compounds, as the fourth fluorinated solvent, methyl nonafluorobutyl ether, ethyl nonafluorobutyl ether, and 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-trifluoromethylpentane are preferred.

[0071] As the co-solvent, if it is a compound having no fluorine-substituted alkyl group, it is not particularly limited, and examples thereof include ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, dimethoxyethane, diethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate, chloroethylene carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, trimethyl phosphate, and triethyl phosphate. From the viewpoint of further increasing the solubility of the electrolyte in the electrolyte solution, as the co-solvent, a diether compound having two ether bonds is preferred, and ethylene glycol dimethyl ether and triethylene glycol dimethyl ether are more preferred.

[0072] As the solvent of the electrolyte solution, the above-mentioned fluorinated alkyl compound and the co-solvent can be freely combined and used, and the fluorinated alkyl compound can also be used alone or in combination of two or more. In the electrolyte solution, the co-solvent may or may not be included.

[0073] The content of the fluoroalkyl compound in the electrolyte is not particularly limited. Relative to the total amount of the solvent components of the electrolyte, it is preferably 40% by volume or more, more preferably 50% by volume or more, further preferably 60% by volume or more, and even more preferably 70% by volume or more. By having the content of the fluoroalkyl compound within the above range, since the SEI layer is more easily formed, the lithium secondary battery 100 is more excellent in cycle characteristics. The upper limit of the content of the fluoroalkyl compound is not particularly limited. The content of the fluoroalkyl compound can be 100% by volume or less, can be 95% by volume or less, can be 90% by volume or less, and can also be 80% by volume or less.

[0074] When the electrolyte contains a tetrafluorinated solvent, the content of the tetrafluorinated solvent is not particularly limited. Relative to the total amount of the solvent components of the electrolyte, it is preferably more than 0% by volume, more preferably 5% by volume or more, and further preferably 8% by volume or more. By having the content of the tetrafluorinated solvent within the above range, the solubility of the electrolyte in the electrolyte tends to be further increased or the SEI layer is more easily formed. The upper limit of the content of the tetrafluorinated solvent is not particularly limited. The content of the tetrafluorinated solvent can be 20% by volume or less, or can be 15% by volume or less.

[0075] When the electrolyte contains a co-solvent, the content of the co-solvent is not particularly limited. Relative to the total amount of the solvent components of the electrolyte, it is preferably more than 0% by volume, more preferably 5% by volume or more, and further preferably 10% by volume or more. By having the content of the co-solvent within the above range, the solubility of the electrolyte in the electrolyte tends to be further increased. The content of the co-solvent can be 20% by volume or more, or can be 30% by volume or more. In addition, the content of the co-solvent can be 60% by volume or less, can be 50% by volume or less, and can also be 40% by volume or less.

[0076] In the present embodiment, the types and structural formulas of the compounds that can be used as solvents are illustrated in the following table. Substances that can be used as fluorinated solvents are illustrated in Table 1. In addition, substances that can be used as co-solvents are illustrated in Table 2. However, the types of compounds that can be used as solvents are not limited thereto.

[0077] [Table 1]

[0078]

[0079] [Table 2]

[0080]

[0081] The electrolyte contained in the electrolyte solution is not particularly limited if it is a salt, and examples thereof include salts of Li, Na, K, Ca, and Mg. As the electrolyte, lithium salts are preferably used. As the lithium salts, there are no particular limitations, and examples thereof include LiI, LiCl, LiBr, LiF, and LiBF. 4 、LiPF 6 、LiAsF 6 、LiSO 3 CF 3 、LiN(SO 2 F) 2 、LiN(SO 2 CF 3 ) 2 、LiN(SO 2 CF 3 CF 3 ) 2 , LiBF 2 (C 2 O 4 )、LiB(O 2 C 2 H 4 ) 2 、LiB(O 2 C 2 H 4 )F 2 、LiB(OCOCF 3 ) 4 、LiNO 3 and Li 2 SO 4 From the viewpoint of achieving better energy density and cycle characteristics of the lithium secondary battery 100, the lithium salt is preferably LiN(SO 2 F) 2 and LiBF 2 (C 2 O 4 ). In addition, when the electrolyte contains LiN(SO 2 F) 2 and LiBF 2 (C 2 O 4 ), the formation and growth of the SEI layer on the negative electrode surface are further promoted, and a lithium secondary battery 100 with better cycle characteristics tends to be obtained. In addition, the above lithium salts are used alone or in combination of two or more.

[0082] The concentration of the electrolyte in the electrolytic solution is not particularly limited, preferably 0.5 M or more, more preferably 0.7 M or more, further preferably 0.9 M or more, and even more preferably 1.0 M or more. When the concentration of the electrolyte is within the above range, the SEI layer is more likely to form, and furthermore, the internal resistance tends to be further reduced. In particular, since the lithium secondary battery 100 containing a fluorinated alkyl compound as a solvent can increase the concentration of the electrolyte in the electrolytic solution, the cycle characteristics and rate performance can be further improved. The upper limit of the concentration of the electrolyte is not particularly limited, and the concentration of the electrolyte can be 10.0 M or less, 5.0 M or less, or 2.0 M or less.

[0083] In addition, the inclusion of a fluorinated alkyl compound in the electrolytic solution can be determined by various conventionally known methods. As such methods, for example, NMR measurement, mass analysis methods such as HPLC-MS, and IR measurement are listed.

[0084] (Solid electrolyte interface layer)

[0085] It is presumed that in the lithium secondary battery 100, a solid electrolyte interface layer (SEI layer) is formed on the surface of the negative electrode 140 by charging, particularly initial charging, but the lithium secondary battery 100 may not have an SEI layer. The formed SEI layer is presumed to contain an organic compound including at least one of the part represented by the above formula (A) and the part represented by the above formula (B) derived from the above fluorinated alkyl compound. For example, it may also contain a lithium-containing inorganic compound and a lithium-containing organic compound other than these.

[0086] As the lithium-containing organic compound and the lithium-containing inorganic compound, there is no particular limitation as long as they are substances contained in a conventionally known SEI layer. Without intending to limit, as the lithium-containing organic compound, organic compounds such as alkyl lithium carbonate, lithium alkoxide, and alkyl ester lithium are listed, and as the lithium-containing inorganic compound, LiF, Li 2 CO 3 、Li 2 O, LiOH, lithium borate compounds, lithium phosphate compounds, lithium sulfate compounds, lithium nitrate compounds, lithium nitrite compounds, and lithium sulfite compounds are listed.

[0087] Since the lithium secondary battery 100 contains a fluorinated alkyl compound as a solvent, the formation of the SEI layer is promoted. Since the SEI layer has ion conductivity, the reactivity of the lithium precipitation reaction on the surface of the negative electrode where the SEI layer is formed is uniform in the surface direction of the negative electrode. Therefore, the growth of dendritic lithium metal on the negative electrode of the lithium secondary battery 100 is suppressed, and the cycle characteristics become excellent.

[0088] As a typical average thickness of the SEI layer, it is 1 nm or more and 10 μm or less. When the SEI layer is formed on the lithium secondary battery 100, the lithium metal deposited by charging the battery can be deposited at the interface between the negative electrode 140 and the SEI layer, or at the interface between the SEI layer and the separator.

[0089] (Positive electrode)

[0090] As the positive electrode 120, generally speaking, there is no particular limitation if it is used for a lithium secondary battery, and known materials can be appropriately selected according to the use of the lithium secondary battery. From the viewpoint of improving the stability and output voltage of the lithium secondary battery 100, the positive electrode 120 preferably has a positive electrode active material.

[0091] In this specification, the "positive electrode active material" refers to a substance that holds lithium element (typically lithium ion) in the positive electrode in the battery, and can also be paraphrased as a host substance of lithium element (typically lithium ion). As such a positive electrode active material, there is no particular limitation, and examples include metal oxides and metal phosphates. As the above metal oxides, there is no particular limitation, and examples include cobalt oxide compounds, manganese oxide compounds, and nickel oxide compounds. As the above metal phosphates, there is no particular limitation, and examples include iron phosphate compounds and cobalt phosphate compounds. As typical positive electrode active materials, LiCoO 2 、LiNi x Co y Mn Z O(x + y + z = 1), LiNi x Mn y O 2 (x + y = 1), LiNiO 2 、LiMn 2 O 4 、LiFePO 4 、LiCoPO 4 、FeF 3 、LiFeOF, LiNiOF, and TiS 2 。

[0092] One kind of the above positive electrode active materials is used alone or two or more kinds are used in combination. The positive electrode 120 may also contain components other than the above positive electrode active material. As such components, there is no particular limitation, and examples include known conductive aids, binders, solid polymer electrolytes, and inorganic solid electrolytes.

[0093] As the conductive additive in the positive electrode 120, there is no particular limitation, and examples thereof include carbon black, single-walled carbon nanotubes (SW-CNT), multi-walled carbon nanotubes (MW-CNT), carbon nanofibers, and acetylene black. In addition, as the binder, there is no particular limitation, and examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylic resin, and polyimide resin.

[0094] The content of the positive electrode active material in the positive electrode 120 may also be, for example, 50% by mass or more and 100% by mass or less with respect to the entire positive electrode 120. The content of the conductive additive may also be, for example, 0.5% by mass or more and 30% by mass or less with respect to the entire positive electrode 120. The content of the binder may also be, for example, 0.5% by mass or more and 30% by mass or less with respect to the entire positive electrode 120. The total content of the solid polymer electrolyte and the inorganic solid electrolyte may also be, for example, 0.5% by mass or more and 30% by mass or less with respect to the entire positive electrode 120.

[0095] (Positive electrode current collector)

[0096] 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.

[0097] The average thickness of the positive electrode current collector 110 is preferably 4 μm or more and 20 μm or less, more preferably 5 μm or more and 18 μm or less, and further preferably 6 μm or more and 15 μm or less. In this way, since the volume occupied by the positive electrode current collector 110 in the lithium secondary battery 100 is reduced, the energy density of the lithium secondary battery 100 is further increased.

[0098] (Separator)

[0099] The separator 130 is a component for preventing battery short circuit by isolating the positive electrode 120 from the negative electrode 140 and ensuring the ionic conductivity of lithium ions that are charge carriers between the positive electrode 120 and the negative electrode 140, and is made of a material that does not have electronic conductivity and does not react with lithium ions. In addition, the separator 130 also undertakes the function of holding the electrolyte. The separator 130 is not particularly limited as long as it undertakes the above functions, and examples thereof include those made of a porous polyethylene (PE) film, a polypropylene (PP) film, or a laminated structure thereof.

[0100] The separator 130 may also be covered with a separator coating. The separator coating may cover both sides of the separator 130 or only one side. The separator coating has ionic 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 130 to the layer adjacent to the separator 130. Such a separator coating is not particularly limited, and examples include adhesives such as composites of polyvinylidene fluoride (PVDF), styrene-butadiene rubber and carboxymethyl cellulose (SBR-CMC), polyacrylic acid (PAA), lithium polyacrylate (Li-PAA), polyimide (PI), polyamideimide (PAI), and aramid. Inorganic particles such as silica, alumina, titanium dioxide, zirconia, magnesia, magnesium hydroxide, and lithium nitrate may also be added to the above adhesives for the separator coating. In addition, the separator 130 is a separator including a separator coating.

[0101] The average thickness of the separator 130 is preferably 20 μm or less, more preferably 18 μm or less, and further preferably 15 μm or less. In this way, since the volume occupied by the separator 130 in the lithium secondary battery 100 is reduced, the energy density of the lithium secondary battery 100 is further increased. In addition, the average thickness of the separator 130 is preferably 5 μm or more, more preferably 7 μm or more, and further preferably 10 μm or more. In this way, the positive electrode 120 and the negative electrode 140 can be more reliably isolated, and battery short circuits can be more effectively suppressed.

[0102] (Use of lithium secondary battery)

[0103] In Figure 2 a use mode of the lithium secondary battery of this embodiment is shown. Positive and negative terminals 210 and 220 for connecting the lithium secondary battery 200 to an external circuit are respectively joined to the positive electrode current collector 110 and the negative electrode 140 of the lithium secondary battery 200. The lithium secondary battery 200 is charged and discharged by connecting the negative terminal 220 to one end of an external circuit and connecting the positive terminal 210 to the other end of the external circuit.

[0104] Between the positive terminal 210 and the negative terminal 220, the lithium secondary battery 200 is charged by applying a voltage such that a current flows from the negative terminal 220 through the external circuit to the positive terminal 210. It is speculated that a solid electrolyte interface layer (SEI layer) is formed on the surface of the negative electrode 140 (the interface between the negative electrode 140 and the separator 130) by the initial charging of the lithium secondary battery 200, but the lithium secondary battery 200 may not have an SEI layer. By charging the lithium secondary battery 200, lithium metal precipitation occurs at the interface between the negative electrode 140 and the SEI layer, the interface between the negative electrode 140 and the separator 130, and / or the interface between the SEI layer and the separator 130.

[0105] Regarding the charged lithium secondary battery 200, when the positive terminal 210 and the negative terminal 220 are connected, the lithium secondary battery 200 discharges. As a result, the precipitation electrolytic dissolution of lithium metal occurs on the negative electrode. When an SEI layer is formed on the lithium secondary battery 200, the precipitation electrolytic dissolution of lithium metal occurs at least at the interface between the negative electrode 140 and the SEI layer and / or at the interface between the SEI layer and the separator 130.

[0106] (Method for manufacturing a lithium secondary battery)

[0107] As Figure 1 Regarding the method for manufacturing the lithium secondary battery 100 as shown above, there is no particular limitation as long as it is a method capable of manufacturing a lithium secondary battery having the above-described configuration. For example, the following methods are listed.

[0108] The positive electrode current collector 110 and the positive electrode 120 are manufactured as follows, for example. The above positive electrode active material, a known conductive assistant, and a known binder are mixed to obtain a positive electrode mixture. The mixing ratio may be, for example, with respect to the entire positive electrode mixture, the positive electrode active material is 50% by mass or more and 99% by mass or less, the conductive assistant is 0.5% by mass or more and 30% by mass or less, and the binder is 0.5% by mass or more and 30% by mass or less. The obtained positive electrode mixture is coated on one side of a metal foil (e.g., Al foil) serving as a positive electrode current collector having a predetermined thickness (e.g., 5 μm or more and 1 mm or less) and is press-molded. The obtained molded body is blanked into a predetermined size to obtain the positive electrode current collector 110 and the positive electrode 120.

[0109] Next, after cleaning the above negative electrode material, for example, a metal foil of 1 μm or more and 1 mm or less (e.g., electrolytic Cu foil) with a solvent containing sulfamic acid, it is blanked into a predetermined size, further ultrasonically cleaned with ethanol, and then dried to obtain the negative electrode 140.

[0110] Next, the separator 130 having the above-described configuration is prepared. The separator 130 can be manufactured by a conventionally known method or a commercially available separator can be used.

[0111] Next, a solution obtained by mixing at least one of the above fluorinated alkyl compounds and, if necessary, the above-mentioned fourth fluorinated solvent and / or a co-solvent is used as a solvent, and an electrolyte such as a lithium salt is dissolved in this solution to prepare an electrolytic solution. The mixing ratio of the solvent and the electrolyte may be appropriately adjusted so that the types of each solvent and electrolyte and the content or concentration in the electrolytic solution are within the above ranges.

[0112] The current collector 110 of the positive electrode 120, the separator 130, and the negative electrode 140 obtained as described above are laminated in this order with the positive electrode 120 facing the separator 130 to obtain a laminate. By sealing the obtained laminate together with the electrolyte in a hermetically sealed container, the lithium secondary battery 100 can be obtained. The hermetically sealed container is not particularly limited, and examples include a laminated film.

[0113] [Modification Example]

[0114] The above-described embodiment is an example for explaining the present invention and is not intended to limit the present invention only to this embodiment. The present invention can be variously modified without departing from its gist.

[0115] For example, in the lithium secondary battery 100 of the present embodiment, each component may not be formed as a laminate, but may be fixed at a distance from each other, and the electrolyte may be filled therebetween.

[0116] In addition, for example, in the lithium secondary battery 100, an auxiliary component that assists the precipitation and / or dissolution of lithium metal during charge and discharge may be disposed between the separator 130 and the negative electrode 140. Examples of such an auxiliary component include a component containing a metal alloyed with lithium metal, and for example, it may be a metal layer formed on the surface of the negative electrode 140. Examples of such a metal layer include a layer containing at least one selected from the group consisting of Si, Sn, Zn, Bi, Ag, In, Pb, Sb, and Al. The average thickness of the metal layer may also be, for example, 5 nm or more and 500 nm or less.

[0117] According to the manner in which the lithium secondary battery 100 has the above-described auxiliary component, since the affinity between the negative electrode and the lithium metal precipitated on the negative electrode is further improved, the peeling of the lithium metal precipitated on the negative electrode is more suppressed, and the cycle characteristics tend to be further improved. In addition, although the auxiliary component can contain a metal alloyed with lithium metal, its capacity is sufficiently small compared to the capacity of the positive electrode. In a typical lithium ion secondary battery, the capacity of the negative electrode active material of the negative electrode is set to be approximately the same as the capacity of the positive electrode. However, since the capacity of this auxiliary component is sufficiently small compared to the capacity of the positive electrode, the lithium secondary battery 100 having such an auxiliary component can be referred to as "a negative electrode having no negative electrode active material". Therefore, the capacity of the auxiliary component is sufficiently small compared to the capacity of the positive electrode 120, for example, 20% or less, 15% or less, 10% or less, or 5% or less.

[0118] Before initial charging of the lithium secondary battery according to this embodiment, a lithium foil may or may not be formed between the separator and the negative electrode. When a lithium foil is not formed between the separator and the negative electrode before initial charging of the lithium secondary battery according to this embodiment, since highly flammable lithium metal does not need to be used during manufacturing, the lithium secondary battery has more excellent safety and productivity.

[0119] The lithium secondary battery 100 may or may not have a current collector disposed in contact with the negative electrode on the surface of 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 have a positive electrode current collector and a negative electrode current collector, the positive electrode and the negative electrode themselves function as current collectors, respectively.

[0120] The lithium secondary battery 100 may also have terminals for connecting to an external circuit mounted on the positive electrode current collector and / or the negative electrode. For example, metal terminals having a thickness of 10 μm or more and 1 mm or less (e.g., Al, Ni, etc.) may be joined to one or both of the positive electrode current collector and the negative electrode. As a joining method, a conventionally known method may be used, or ultrasonic welding may be used, for example.

[0121] In addition, in this specification, "high energy density" or "having a high energy density" means a high capacity per unit total volume or total mass, preferably 800 Wh / L or more or 350 Wh / kg or more, more preferably 900 Wh / L or more or 400 Wh / kg or more, and further preferably 1000 Wh / L or more or 450 Wh / kg or more.

[0122] In addition, in this specification, "excellent cycle characteristics" means that the reduction rate of the battery capacity is low before and after the number of charge-discharge cycles that can be assumed in normal use. That is, it means that when comparing the first discharge capacity after initial charging with the capacity after the number of charge-discharge cycles that can be assumed in normal use, the capacity after the charge-discharge cycles is hardly reduced with respect to the first discharge capacity after initial charging. Here, the "number of times that can be assumed in normal use" means, depending on the application 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-discharge cycles is hardly reduced with respect to the first discharge capacity after initial charge-discharge" means, depending on the application of the lithium secondary battery, for example, the capacity after the charge-discharge cycles is 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, or 85% or more with respect to the first discharge capacity after initial charge-discharge.

[0123] Examples

[0124] Hereinafter, the embodiments and comparative examples of the present invention will be described in more detail. The present invention is not limited by any of the following embodiments.

[0125] [Embodiment 1]

[0126] A lithium secondary battery was fabricated as follows.

[0127] First, a 10-μm electrolytic Cu foil was cleaned with a solvent containing sulfamic acid and then cut into a specified size (45 mm × 45 mm). Further, it was ultrasonically cleaned with ethanol and then dried. After that, the Cu foil was degreased, cleaned with pure water, and then immersed in an electroplating bath containing Sb ions. The surface of the Cu foil was electroplated by keeping the Cu foil horizontal and stationary, and 100 nm-thick Sb was electroplated as a metal layer on the surface of the Cu foil. The Cu foil was taken out from the electroplating bath, cleaned with ethanol, and then cleaned with pure water. The Cu foil coated with the Sb thin film was used as the negative electrode.

[0128] Next, the positive electrode was fabricated. A mixture of 96 parts by mass of LiNi 0.85 Co 0.12 Al 0.03 O 2 , 2 parts by mass of carbon black as a conductive additive, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder was coated on one side of a 12-μm Al foil and then stamped into shape. The obtained shaped body was cut into a specified size (40 mm × 40 mm) by punching to obtain the positive electrode.

[0129] As the separator, a separator of a specified size (50 mm × 50 mm) with 2-μm polyvinylidene fluoride (PVDF) coated on both sides of a 12-μm polyethylene microporous membrane was prepared.

[0130] The electrolyte was prepared as follows. Two solvents were mixed such that 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether was 60% by volume and ethylene glycol dimethyl ether was 40% by volume. LiN(SO 2 F) 2 was dissolved in the obtained mixed solution at a molar concentration of 1.3 M to obtain the electrolyte.

[0131] The positive electrode current collector formed by the positive electrode obtained as above, the separator, and the negative electrode were laminated in this order with the positive electrode facing the separator to obtain a laminate. Further, a 100-μm Al terminal and a 100-μm Ni terminal were ultrasonically welded to the positive electrode current collector and the negative electrode, respectively, and then inserted into the outer package of the laminate. Next, the electrolyte obtained as above was injected into the outer package. By sealing the outer package, a lithium secondary battery was obtained.

[0132] [Examples 2 to 67]

[0133] A lithium secondary battery was obtained in the same manner as in Example 1, except that the electrolytic solution was prepared using the solvents and electrolytes (lithium salts) described in Tables 1 to 14.

[0134] In addition, in Tables 1 to 14, "TTFE" represents 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether, "TFEE" represents 1,1,2,2 - tetrafluoroethyl - 2,2,2 - trifluoroethyl ether, "ETFE" represents 1,1,2,2 - tetrafluoroethyl ethyl ether, "TFME" represents 1,1,2,2 - tetrafluoroethyl methyl ether, "OFTFE" represents 1H,1H,5H - octafluoropentyl - 1,1,2,2 - tetrafluoroethyl ether, "DFTFE" represents 2,2,3,3 - tetrafluoropropyl difluoromethyl ether, "NV7100" represents methyl nonafluorobutyl ether, "NV7200" represents ethyl nonafluorobutyl ether, and "NV7300" represents 1,1,1,2,2,3,4,5,5,5 - decafluoro - 3 - methoxy - 4 - trifluoromethyl pentane. In addition, "DME" represents ethylene glycol dimethyl ether, "DGM" represents diethylene glycol dimethyl ether, and "TGM" represents triethylene glycol dimethyl ether. Regarding the lithium salts used as electrolytes, "FSI" represents LiN(SO 2 F) 2 、"LiDFOB" represents LiBF 2 (C 2 O 4 ).

[0135] In addition, in Tables 1 to 14, each solvent is classified as any one of a first fluorinated solvent, a second fluorinated solvent, a third fluorinated solvent, a fourth fluorinated solvent, and a co - solvent in the above - mentioned definition, and any one of each classification is recorded. In the table, for example, the first fluorinated solvent is recorded as "First". In addition, in the table, the content of each solvent and its type are recorded in volume %, and the concentration of each lithium salt and its type are recorded in molar concentration (M). For example, Example 1 means that it contains 60 vol% TTFE and 40 vol% DME as solvents, and 1.3 M of FSI as an electrolyte.

[0136] [Comparative Examples 1 to 2]

[0137] A lithium secondary battery was obtained in the same manner as in Example 1, except that the electrolytic solution was prepared using the solvents and electrolytes (lithium salts) described in Table 14. In addition, Comparative Examples 1 and 2 do not contain fluorinated alkyl compounds and contain only co - solvents as solvents.

[0138] [Evaluation of Cycling Characteristics]

[0139] The cycling characteristics of the lithium secondary batteries fabricated in each of the examples and comparative examples were evaluated as follows.

[0140] In an environment at a temperature of 25°C, the fabricated lithium secondary battery was repeatedly charged at 3.2 mA in CC mode until the voltage reached 4.2 V (initial charge), and then discharged at 3.2 mA in CC mode until the voltage reached 3.0 V (hereinafter referred to as "initial discharge"). Subsequently, a cycle of charging at 13.6 mA in CC mode until the voltage reached 4.2 V and then discharging at 20.4 mA in CC mode until the voltage reached 3.0 V was performed. For each example, the capacity obtained from the initial discharge (hereinafter referred to as "initial capacity") is shown in Tables 1 to 14. In addition, for each example, the number of cycles at which the discharge capacity becomes 80% of the initial capacity (referred to as "cycles" in the table) is shown in Table 1.

[0141] [Measurement of DC Resistance]

[0142] The fabricated lithium secondary battery was charged at 5.0 mA in CC mode to 4.2 V and then discharged at 30 mA, 60 mA, and 90 mA in CC mode for 30 seconds each. At this time, the lower limit voltage was set to 2.5 V, but in any case, the voltage did not reach 2.5 V during the 30-second discharge. In addition, between each discharge, it was charged again at 5.0 mA in CC mode to 4.2 V, and the next CC discharge was performed after the charging was completed. The current value I and voltage drop V obtained as described above were plotted, and the DC resistance (DCR) (unit: Ω) was calculated from the slope of the I-V characteristics obtained by linearly approximating each point.

[0143] [Table 3]

[0144]

[0145] [Table 4]

[0146]

[0147] [Table 5]

[0148]

[0149] [Table 6]

[0150]

[0151] [Table 7]

[0152]

[0153] [Table 8]

[0154]

[0155] [Table 9]

[0156]

[0157] [Table 10]

[0158]

[0159] [Table 11]

[0160]

[0161] [Table 12]

[0162]

[0163] [Table 13]

[0164]

[0165] [Table 14]

[0166]

[0167] [Table 15]

[0168]

[0169] [Table 16]

[0170]

[0171] In Tables 2, 3, 4, 9, 10, 12, 13, and 14, "-" means not having the component.

[0172] From Tables 1 to 14, it was found that Examples 1 to 67 containing, as a solvent, a compound having at least one of a monovalent group represented by formula (A) and a monovalent group represented by formula (B) had a much higher number of cycles and excellent cycling characteristics compared to Comparative Examples 1 and 2 which did not. In addition, it was found that Examples 1 to 67 had a lower DC resistance value compared to the DC resistance value predicted from the very high cycling characteristics, and had the same DC resistance value as that of Comparative Examples 1 and 2. From this, it can be seen that Examples 1 to 67 have excellent cycling characteristics and excellent rate performance.

[0173] Industrial Applicability

[0174] Since the lithium secondary battery of the present invention has a high energy density and excellent cycling characteristics, it has industrial applicability as a power storage device used in various applications.

[0175] Explanation of Reference Numerals

[0176] 100, 200... lithium secondary battery, 110... positive electrode current collector, 120... positive electrode, 130... separator, 140... negative electrode, 210... positive electrode terminal, 220... negative electrode terminal.

Claims

1. A lithium secondary battery comprising a positive electrode, a separator, a negative electrode having no negative electrode active material, and an electrolyte solution, wherein one or both surfaces of the separator are covered with a separator covering layer, and the separator covering layer contains at least one selected from the group consisting of a composite material of polyvinylidene fluoride, styrene-butadiene rubber and carboxymethyl cellulose, polyacrylic acid, lithium polyacrylate, polyimide, polyamideimide, and aramid; the electrolyte solution contains, as a solvent: a compound having at least one of a monovalent group represented by the following formula (A) and a monovalent group represented by the following formula (B); and an ether compound having a monovalent group represented by the following formula (A) and not having a monovalent group represented by the following formula (B), the total content of the compound and the ether compound is 60% by volume or more relative to the total amount of the solvent components of the electrolyte solution, the electrolyte solution contains, as a solvent, a diether compound having no fluorine-substituted alkyl group, [Chemical formula 1] [Chemical formula 2] In the formula, the wavy line represents the bonding site in the monovalent group.

2. The lithium secondary battery according to claim 1, wherein, the compound has an ether bond.

3. The lithium secondary battery according to claim 1, wherein, the electrolyte solution contains two or more of the compounds.

4. The lithium secondary battery according to claim 1, wherein, the electrolyte solution contains an ether compound having both a monovalent group represented by the formula (A) and a monovalent group represented by the formula (B).

5. The lithium secondary battery according to claim 4, wherein, the electrolyte solution further contains an ether compound having no monovalent group represented by the formula (A) and having a monovalent group represented by the formula (B).

6. The lithium secondary battery according to any one of claims 1 to 4, wherein, the electrolyte solution does not contain a fluoroalkyl ether compound having neither a monovalent group represented by the formula (A) nor a monovalent group represented by the formula (B), or contains 20% by volume or less of the fluoroalkyl ether compound relative to the total amount of the solvent components of the electrolyte solution.

7. The lithium secondary battery according to any one of claims 1 to 4, wherein, the lithium secondary battery is a lithium secondary battery that is charged and discharged by the precipitation of lithium metal on the surface of the negative electrode and the electrolytic dissolution of the precipitated lithium.

8. The lithium secondary battery according to any one of claims 1 to 4, wherein, the negative electrode is an electrode composed of at least one selected from the group consisting of Cu, Ni, Ti, Fe, and metals that do not react with Li other than these and their alloys, and stainless steel (SUS).

9. The lithium secondary battery according to any one of claims 1 to 4, wherein, before the initial charging, no lithium foil is formed on the surface of the negative electrode.

10. The lithium secondary battery according to any one of claims 1 to 4, wherein, the energy density is 350 Wh / kg or more.

11. The lithium secondary battery according to any one of claims 1 to 4, wherein, one or both surfaces of the separator are covered with a separator covering layer containing polyvinylidene fluoride.

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