Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery having a high energy density and excellent cycling characteristics. It relates to a lithium secondary battery including a positive electrode, a separator, a negative electrode without a negative electrode active material, and an electrolyte solution, wherein the electrolyte solution contains at least any one of the compounds represented by formulas (1) to (4) as a solvent.

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 power 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 power.

[0003] Among them, a lithium secondary battery that performs charge and discharge by moving lithium ions between a positive electrode and a negative electrode is known to exhibit a high voltage and a high energy density. As a typical lithium secondary battery, a lithium ion secondary battery is known that has active materials capable of holding lithium elements at the positive electrode and the negative electrode, and performs charge and discharge by giving and receiving lithium ions between the positive electrode active material and the negative electrode active material.

[0004] In addition, for the purpose of achieving a higher energy density, a lithium secondary battery has been developed that uses lithium metal as the negative electrode active material instead of a material such as a carbon-based material into which lithium elements can be inserted. For example, in Patent Document 1, a lithium secondary battery having an ultrathin lithium metal anode is disclosed in order to achieve 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. Patent Document 1 discloses that in this lithium secondary battery, charging is performed by directly depositing lithium metal further on the lithium metal as the negative electrode active material.

[0005] In addition, for the purpose of further improving the high energy density, productivity, etc., a lithium secondary battery that does not use a negative electrode active material has been developed. For example, 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 the assembly process, 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 inventors of the present invention have studied in detail existing batteries such as the battery described in the above patent document, and found that at least one of the energy density and the cycle characteristics is insufficient.

[0011] For example, in a lithium secondary battery having a negative electrode with a negative electrode active material, it is difficult to sufficiently increase the energy density and the capacity due to the volume and mass occupied by the negative electrode active material. In addition, in an anode-free type lithium secondary battery having a negative electrode without a negative electrode active material in the existing mode, dendritic lithium metal is likely to be formed on the surface of the negative electrode due to repeated charge and discharge, and short circuit and capacity reduction are likely to occur, so 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 the precipitation of lithium metal, 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 completed 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 without a negative electrode active material, and an electrolytic solution, and the electrolytic solution contains at least any one of the compounds represented by formulas (1) to (4) as a solvent.

[0015] [Chemical formula 1]

[0016]

[0017]

[0018]

[0019]

[0020] (In the formula, R 1 ~R 4 represent hydrogen, a halogen, or a fluorine-substituted or partially substituted or unsubstituted hydrocarbon group, n is 1 or more, and R represents a fluorine-substituted or partially substituted or unsubstituted hydrocarbon group).

[0021] Such a lithium secondary battery has a high energy density because it has a negative electrode without a negative electrode active material, and 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.

[0022] In addition, the inventors of the present invention have found that in a lithium secondary battery containing the above compounds (1) to (4) as a solvent in the electrolyte, 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 with respect to the surface direction of the negative electrode. Therefore, the above 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 a compound as a solvent is not necessarily clear, the main reasons described later in the specific embodiments can be considered.

[0023] The content of the compound is preferably 30% by volume or more with respect to the total amount of the solvent components of the above electrolyte. According to this method, since the SEI layer is more easily formed, the lithium secondary battery is more excellent in cycle characteristics.

[0024] The above electrolyte may further contain a fluorine solvent. According to this method, since the solubility of the electrolyte in the electrolyte tends to be further increased or the SEI layer is more easily formed, the lithium secondary battery is more excellent in cycle characteristics.

[0025] The above electrolyte may further contain a non-fluorine solvent. According to this method, since the solubility of the electrolyte in the electrolyte tends to be further increased or the SEI layer is more easily formed, the lithium secondary battery is more excellent in cycle characteristics.

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

[0027] The above 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). According to this method, since highly flammable lithium metal does not need to be used during manufacturing, the 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 lithium secondary battery is preferably one in which no lithium foil is formed on the surface of the negative electrode before the initial charge. According to this method, since highly flammable lithium metal does not need to be used during manufacturing, the safety and productivity are more excellent.

[0029] The above 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 EMBODIMENTS

[0034] Hereinafter, 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 drawings, the same reference numerals are assigned to the same elements, and redundant 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 of 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 that does not have 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, that is, it does not have an active material that serves as a host for lithium and lithium. Therefore, the lithium secondary battery 100 has a smaller volume and mass as a whole than a lithium secondary battery having a negative electrode with a negative electrode active material, and has a higher energy density in principle. Here, the lithium secondary battery 100 is charged and discharged by the precipitation of lithium metal on the negative electrode 140 and the electrolytic dissolution of the precipitated lithium metal.

[0040] In the present embodiment, "the precipitation of lithium metal on the negative electrode" means that lithium metal precipitates 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 precipitates on the surface of the SEI layer, but the precipitation position is not limited thereto.

[0041] In this specification, the "negative electrode active material" refers to a material for holding lithium ions or lithium metal in the negative electrode 140, and can also be alternatively referred to as a host material for lithium element (typically lithium metal). As such a holding mechanism, there is no particular limitation, and examples include insertion, alloying, and absorption of metal clusters, etc., and typically it is insertion.

[0042] As such a negative electrode active material, there is no particular limitation, and examples include lithium metal and alloys containing lithium metal, carbonaceous materials, metal oxides, and metals alloyed with lithium and alloys containing such metals, etc. As the above carbonaceous materials, there is no particular limitation, and examples include graphene, graphite, hard carbon, mesoporous carbon, carbon nanotubes, and carbon nanohorns, etc. 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, etc. As the above metals alloyed with lithium, examples include silicon, germanium, tin, lead, aluminum, and gallium.

[0043] In this specification, the negative electrode "not having a negative electrode active material" means that the content of the negative electrode active material in the negative electrode is 10 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 mass% or less, 1.0 mass% or less, 0.1 mass% or less, and can also be 0.0 mass% or less. By the negative electrode not having a negative electrode active material, or the content of the negative electrode active material in the negative electrode being within the above range, the energy density of the lithium secondary battery 100 becomes high.

[0044] More specifically, regardless of the charging state of the battery, the content of the negative electrode active material other than lithium metal in the negative electrode 140 is 10 mass% or less with respect to the entire negative electrode, preferably 5.0 mass% or less, can be 1.0 mass% or less, can also be 0.1 mass% or less, and can also be 0.0 mass% or less. In addition, before the initial charging and / or at the end of discharge of the negative electrode 140, the content of lithium metal is 10 mass% or less with respect to the entire negative electrode, preferably 5.0 mass% or less, can be 1.0 mass% or less, can also be 0.1 mass% or less, and can also be 0.0 mass% or less.

[0045] Therefore, the "lithium secondary battery having a negative electrode not having a negative electrode active material" can also be alternatively referred to as an anode-free secondary battery, a zero-anode secondary battery, or a negative electrode-free secondary battery. In addition, the "lithium secondary battery having a negative electrode not having a negative electrode active material" can also be alternatively referred to as "a lithium secondary battery having a negative electrode not having a negative electrode active material other than lithium metal and not having lithium metal before the initial charging and / or at the end of discharge", "a lithium secondary battery having a negative electrode current collector not having lithium metal before the initial charging and / or at the end of discharge".

[0046] In this specification, the battery being "before initial charging" refers to the state of the battery from assembly until the first charging. In addition, the battery being "at the end of discharge" refers to the state where the voltage of the battery is 1.0 V or more and 3.8 V or less.

[0047] In addition, in the lithium secondary battery 100, the mass M of lithium metal deposited on the negative electrode when the voltage of the battery is in the state of 3.0 V 3.0 relative to the mass M of lithium metal deposited on the negative electrode when the voltage of the battery is in the state of 4.2 V 4.2 of the ratio M 3.0 / M 4.2 is preferably 20% or less, more preferably 15% or less, and still more preferably 10% or less.

[0048] In a typical lithium secondary battery, the capacity of the negative electrode (capacity of the negative electrode active material) is set to be of the same level as the capacity of the positive electrode (capacity of the positive electrode active material). However, in the lithium secondary battery 100, since the negative electrode 140 does not have a negative electrode active material that serves as a host material for lithium elements, it is not necessary to specify its capacity. Therefore, the lithium secondary battery 100 is not limited by the charging capacity of the negative electrode, and thus can, in principle, improve the energy density.

[0049] As the negative electrode 140, there is no particular limitation as long as it does not have a negative electrode active material but can be used as a current collector. 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 can be used alone or in combination of two or more. In addition, in this specification, "a metal that does not react with Li" refers to a metal that does not alloy with lithium ions or lithium metal under the operating conditions of the lithium secondary battery.

[0050] The negative electrode 140 is preferably composed of at least one selected from the group consisting of Cu, Ni, Ti, Fe, and their alloys, and stainless steel (SUS), and more preferably composed of at least one selected from the group consisting of Cu, Ni, and their alloys, and stainless steel (SUS). The negative electrode 140 is more preferably Cu, Ni, their alloys, or stainless steel (SUS). If such a negative electrode is used, the energy density and productivity of the battery tend to be more excellent.

[0051] The negative electrode 140 is an electrode that does not contain lithium metal. Therefore, highly flammable and reactive lithium metal does not need to be used during manufacturing, so the lithium secondary battery 100 has excellent safety, productivity, and cycle characteristics.

[0052] 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 still more 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.

[0053] (Electrolyte solution)

[0054] 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 together with the laminate of the positive electrode 120, the separator 130, and the negative electrode 140 in a closed container.

[0055] The solvent contains at least any one of the compounds represented by the formulas (1) to (4).

[0056] [Chemical formula 2]

[0057]

[0058]

[0059]

[0060]

[0061] (In the formula, R 1 ~R 4 represents hydrogen, a halogen, or a fluorine-substituted or partially substituted or unsubstituted hydrocarbon group, n is 1 or more, and R represents a fluorine-substituted or partially substituted or unsubstituted hydrocarbon group).

[0062] Generally, in an anode-free type lithium secondary battery having an electrolyte solution, by decomposing a solvent or the like in the electrolyte solution, 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 solution and the reduction of irreversible lithium ions and the generation of gas caused thereby. In addition, since the SEI layer has ionic 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 type lithium secondary battery. The inventors of the present invention found that in a lithium secondary battery containing the above compound as a solvent, an SEI layer is easily formed on the surface of the negative electrode, and the growth of dendritic lithium metal on the negative electrode is suppressed. As a result, the cycle characteristics are improved. The main reason is not necessarily clear, but the following main reasons are considered.

[0063] It is considered that during the charging of the lithium secondary battery 100, especially during the initial charging, not only lithium ions but also the above-mentioned compound as a solvent are reduced on the negative electrode. Moreover, due to the substitution of multiple fluorines for the moiety represented by the following formula (A) in the compounds (1) to (4) and the moiety represented by the following formula (B), the reactivity of the oxygen atom is high, and it is speculated that part or all of the moiety represented by formula (A) and the moiety represented by formula (B) are liable to be detached. As a result, it is speculated that during the charging of the lithium secondary battery 100, part or all of the moiety represented by formula (A) and the moiety represented by formula (B) are adsorbed on the negative electrode surface, and the adsorbed part serves as a starting point to generate the SEI layer, so the lithium secondary battery 100 is liable to form the SEI layer. However, the main reason is not limited to the above.

[0064] [Chemical formula 3]

[0065]

[0066] In addition, it was surprisingly found that the SEI layer formed in the lithium secondary battery 100 containing any one of the above-mentioned compounds (1) to (4) has higher ionic conductivity than the SEI layer formed in the existing lithium secondary battery. This is considered because the fluorine substitution of the moiety represented by the above formula (A) and the moiety represented by the above formula (B) increases the fluorine content rate of the formed SEI layer, and the movement path of lithium ions in the SEI layer increases or expands. However, the main reason is not limited to this.

[0067] Therefore, although the lithium secondary battery 100 is liable to form the 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.

[0068] In addition, in this specification, "comprising 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 liquid, and furthermore, as long as it can dissolve the electrolyte to prepare an electrolytic solution in a solution phase.

[0069] The compounds (1) to (4) used in the embodiment of the present invention are classified into the compound (1) which is an ether having 1,1,2,2-tetrafluoroethyl on the skeleton, the compound (2) which is an ester having 1,1,2,2-tetrafluoroethyl on the skeleton, the compound (3) which is an ether having 2,2,3,3-tetrafluoropropyl on the skeleton, and the compound (4) which is an ester having 2,2,3,3-tetrafluoropropyl on the skeleton.

[0070] The contents of compounds (1) to (4) in the electrolyte are not particularly limited, and are preferably 30% by volume or more, more preferably 40% by volume or more, still more preferably 50% by volume or more, still more preferably 60% by volume or more, and even more preferably 70% by volume or more, based on the total amount of the solvent components of the electrolyte. Since it is easier to form the SEI layer when the content of the compound is within the above range, the lithium secondary battery 100 has more excellent cycle characteristics. The upper limit of the content of the compound is not particularly limited, and the content of the compound can be 100% by volume or less, 95% by volume or less, 90% by volume or less, or 80% by volume or less.

[0071] The electrolyte may contain a fluorine solvent other than compounds (1) to (4). The fluorine solvent refers to a solvent composed of a compound having an alkyl group substituted by fluorine. As the fluorine solvent, an ether or ester having a structure of formula (A) or (B) is more preferred. Examples of such fluorine solvents include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 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, 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, methyl nonafluorobutyl ether, ethyl nonafluorobutyl ether, 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.

[0072] The electrolyte may further contain a non-fluorine solvent as a co-solvent. The non-fluorine solvent refers to a solvent composed of a compound not having an alkyl group substituted by fluorine. Examples of the non-fluorine solvent include 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 improving the solubility of the electrolyte in the electrolyte, an ether compound or an ester compound is preferred as the co-solvent.

[0073] As the solvent of the electrolyte, the above compounds (1) to (4) and the co-solvent can be freely combined and used, and compounds (1) to (4) can also be used alone or in combination of two or more. In the electrolyte, the co-solvent may or may not be included.

[0074] When the electrolytic solution 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 electrolytic solution, it is preferably greater than 0% by volume, more preferably 5% by volume or more, and still more preferably 10% by volume or more. By the content of the co-solvent being within the above range, it tends to further increase the solubility of the electrolyte in the electrolytic solution. The content of the co-solvent can be 20% by volume or more, and can also be 30% by volume or more. In addition, the content of the co-solvent can be 60% by volume or less, 50% by volume or less, 40% by volume or less, 30% by volume or less.

[0075] As the electrolyte contained in the electrolytic solution, if it is a salt, there is no particular limitation, and examples include salts of Li, Na, K, Ca, and Mg, etc. 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, 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 etc. From the viewpoint that the energy density and cycle characteristics of the lithium secondary battery 100 are more excellent, as the lithium salt, LiN(SO 2 F) 2 and LiBF 2 (C 2 O 4 ) are preferred. In addition, when the electrolytic solution contains LiN(SO 2 F) 2 and LiBF 2 (C 2 O4 ) When at least one or more of them are used, the formation and growth of the SEI layer on the negative electrode surface are further promoted, and it tends to be possible to obtain a lithium secondary battery 100 with more excellent cycle characteristics. In addition, the above lithium salts are used alone or in combination of two or more.

[0076] The concentration of the electrolyte in the electrolyte is not particularly limited, and is 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 easily formed, and in addition, the internal resistance tends to be further reduced. In particular, since the lithium secondary battery 100 containing the compounds (1) to (4) as solvents can increase the concentration of the electrolyte in the electrolyte, 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.

[0077] In addition, the inclusion of the compounds (1) to (4) in the electrolyte can be determined by various conventionally known methods. As such methods, for example, NMR measurement methods, mass analysis methods such as HPLC-MS, and IR measurement methods are listed.

[0078] (Solid electrolyte interface layer)

[0079] 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, especially 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 containing at least one of the part represented by the above formula (A) and the part represented by the above formula (B) derived from any one of the above compounds (1) to (4). For example, it may also contain a lithium-containing inorganic compound and a lithium-containing organic compound other than these.

[0080] 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 the 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. 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.

[0081] Since the lithium secondary battery 100 contains compounds (1) to (4) 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 negative electrode surface where the SEI layer is formed is uniform in the plane direction of the negative electrode surface. 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.

[0082] 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 precipitated by charging the battery can precipitate at the interface between the negative electrode 140 and the SEI layer, or at the interface between the SEI layer and the separator.

[0083] (Positive electrode)

[0084] As the positive electrode 120, generally, if it is used for a lithium secondary battery, there is no particular limitation, 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.

[0085] In this specification, the "positive electrode active material" refers to a material that holds lithium element (typically lithium ion) in the positive electrode in the battery, and can also be paraphrased as a host material for 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 a typical positive electrode active material, 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 .

[0086] The positive electrode active material as described above is used alone or in combination of two or more. The positive electrode 120 may also contain components other than the above positive electrode active material. Such components are not particularly limited, and examples include known conductive aids, binders, solid polymer electrolytes, and inorganic solid electrolytes.

[0087] The conductive aid in the positive electrode 120 is not particularly limited, and examples include carbon black, single-walled carbon nanotubes (SW-CNT), multi-walled carbon nanotubes (MW-CNT), carbon nanofibers, and acetylene black. In addition, the binder is not particularly limited, and examples include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylic resin, and polyimide resin.

[0088] 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 aid 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.

[0089] (Positive electrode current collector)

[0090] 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.

[0091] 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.

[0092] (Separator)

[0093] 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 include those composed of a porous polyethylene (PE) film, a polypropylene (PP) film, or a laminated structure thereof.

[0094] The separator 130 can also be covered with a separator coating layer. The separator coating layer can cover both sides of the separator 130 or only one side. The separator coating layer 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. There is no particular limitation on such a separator coating layer, and examples include adhesives such as a composite material 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. The separator coating layer can also add inorganic particles such as silica, alumina, titanium dioxide, zirconia, magnesia, magnesium hydroxide, and lithium nitrate to the above adhesives. In addition, the separator 130 is a separator including a separator coating layer.

[0095] 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 circuit can be more effectively suppressed.

[0096] (Use of lithium secondary battery)

[0097] In Figure 2 One usage 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 the external circuit and connecting the positive terminal 210 to the other end of the external circuit.

[0098] 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 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.

[0099] 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 the interface between the SEI layer and the separator 130.

[0100] (Method for manufacturing a lithium secondary battery)

[0101] 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.

[0102] 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, 50% by mass or more and 99% by mass or less for the positive electrode active material, 0.5% by mass or more and 30% by mass or less for the conductive assistant, and 0.5% by mass or more and 30% by mass or less for the binder, based on the entire positive electrode mixture. 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 stamped. The obtained molded body is blanked into a predetermined size to obtain the positive electrode current collector 110 and the positive electrode 120.

[0103] Next, after cleaning the above negative electrode material, such as a metal foil (e.g., electrolytic Cu foil) of 1 μm or more and 1 mm or less, 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.

[0104] 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.

[0105] Next, a solution obtained by mixing at least one of the above compounds (1) to (4) and, if necessary, the above co-solvent is used as a solvent, and an electrolyte such as a lithium salt is dissolved in the solution to prepare an electrolytic solution. The mixing ratio of the solvent and the electrolyte may be appropriately adjusted so that the types of the respective solvents and electrolytes and the contents or concentrations in the electrolytic solution are within the above ranges.

[0106] 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 an electrolytic solution in a hermetic container, the lithium secondary battery 100 can be obtained. The hermetic container is not particularly limited, and examples include a laminated film.

[0107] [Modification Example]

[0108] 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.

[0109] For example, in the lithium secondary battery 100 of the present embodiment, each component may not be a laminate, but may be fixed at a distance, and an electrolytic solution may be filled therebetween.

[0110] In addition, for example, in the lithium secondary battery 100, an auxiliary component that assists in 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.

[0111] 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 further 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 relative to the capacity of the positive electrode 120, for example, 20% or less, 15% or less, 10% or less, or 5% or less.

[0112] Before initial charging of the lithium secondary battery of the present embodiment, no lithium foil is formed between the separator and the negative electrode. When no lithium foil is formed between the separator and the negative electrode before initial charging of the lithium secondary battery of the present embodiment, since highly flammable lithium metal does not need to be used during manufacturing, a lithium secondary battery with more excellent safety and productivity can be obtained.

[0113] Terminals for connection to an external circuit may also be mounted on the positive electrode current collector and / or the negative electrode of the lithium secondary battery 100. For example, metal terminals 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 the joining method, known methods in the art may be used, or ultrasonic welding may be used, for example.

[0114] In addition, in this specification, "high energy density" or "being of 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.

[0115] In addition, in this specification, "excellent cycle characteristics" means that the reduction rate of the battery capacity is low before and after charge-discharge cycles for the number of times conceivable in normal use. That is, it means that when comparing the first discharge capacity after initial charging with the capacity after charge-discharge cycles for the number of times conceivable in normal use, the capacity after charge-discharge cycles hardly decreases relative to the first discharge capacity after initial charging. Here, the "number of times conceivable in normal use" means, depending on the application for which the lithium secondary battery is used, for example, 30 times, 50 times, 70 times, 100 times, 300 times, or 500 times. In addition, "the capacity after charge-discharge cycles hardly decreases relative to the first discharge capacity after initial charge-discharge" means, depending on the application for which the lithium secondary battery is used, for example, the capacity after charge-discharge cycles is 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, or 85% or more relative to the first discharge capacity after initial charge-discharge.

[0116] Examples

[0117] Hereinafter, the present invention will be described more specifically using examples and comparative examples of the present invention. The present invention is not limited by any of the following examples.

[0118] [Example 1]

[0119] A lithium secondary battery was fabricated as follows.

[0120] First, a 10-μm electrolytic Cu foil is cleaned with a solvent containing sulfamic acid and then cut into a specified size (45 mm × 45 mm). Further, it is ultrasonically cleaned with ethanol and then dried. After that, the Cu foil is degreased, cleaned with pure water, and then immersed in an electroplating bath containing Sb ions. The surface of the Cu foil is electroplated by keeping the Cu foil horizontal and stationary, and 100 nm thick Sb is electroplated as a metal layer on the surface of the Cu foil. The Cu foil is taken out from the electroplating bath, cleaned with ethanol, and then cleaned with pure water. The Cu foil coated with the Sb thin film is used as the negative electrode.

[0121] Next, the positive electrode is fabricated. 96 parts by mass of LiNi 0.85 Co 0.12 Al 0.03 O 2 as the positive electrode active material, 2 parts by mass of carbon black as the conductive additive, and 2 parts by mass of polyvinylidene fluoride (PVDF) as the binder are mixed and then coated on one side of a 12-μm Al foil and stamped into shape. The obtained shaped body is cut into a specified size (40 mm × 40 mm) by blanking to obtain the positive electrode.

[0122] 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 is prepared.

[0123] The electrolyte is prepared as follows. In Example 1, only 1,1,2,2-tetrafluoroethoxymethoxyethane was used as the first solvent (main solvent). LiN(SO 2 F) 2 was dissolved in this solvent at a molar concentration of 1.25 M as the electrolyte to obtain the electrolyte.

[0124] The positive electrode current collector formed by the positive electrode obtained as above, the separator, and the negative electrode are 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 are 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 is injected into the outer package. By sealing the outer package, a lithium secondary battery is obtained.

[0125] [Examples 2 - 24]

[0126] A lithium secondary battery is obtained in the same manner as in Example 1 except that the electrolyte is prepared using the solvents and electrolytes (lithium salts) listed in Table 1.

[0127] In addition, in Table 1, "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, "DME" represents dimethoxyethane, "TGM" represents triethylene glycol dimethyl ether, and "EA" represents ethyl acetate.

[0128] In addition, in Table 1, the first solvent (main solvent) is a compound corresponding to the above compounds (1) to (4), the second solvent is a non-fluorinated solvent as a co-solvent, and the third solvent is a fluorinated solvent other than compounds (1) to (4) as a co-solvent. In addition, in the table, the co-solvent and its type are recorded by volume %, and each lithium salt and its type are recorded by molar concentration (M). In addition, the content of the first solvent as the main solvent is not recorded in the table, but the content of the first solvent corresponds to the content (the remaining part) obtained by subtracting the contents of the second solvent and the third solvent from 100%.

[0129] [Comparative Examples 1-2]

[0130] A lithium secondary battery was obtained in the same manner as in Example 1 except that the electrolytic solution was prepared using the solvents described in Table 1. In addition, in Comparative Examples 1 and 2, solvents not containing compounds (1) to (4) were used.

[0131] As follows, the characteristics of the lithium secondary batteries produced in each of the Examples and Comparative Examples were evaluated.

[0132] [Capacity and Cycle Characteristics]

[0133] In an environment at a temperature of 25 °C, the produced 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"). Then, it was charged at 13.6 mA in CC mode until the voltage reached 4.2 V, and then discharged at 20.4 mA in CC mode until the voltage reached 3.0 V in a cycle. For each example, the capacity obtained from the initial discharge (hereinafter referred to as "initial capacity") is shown in Table 1. In addition, for each example, the number of cycles when the discharge capacity becomes 80% of the initial capacity (referred to as "cycle" in the table) is shown in Table 1.

[0134] [DC Resistance (DCR)]

[0135] The fabricated lithium secondary battery was CC charged at 5.0 mA up to 4.2 V and then CC discharged at 30 mA, 60 mA, and 90 mA for 30 seconds, respectively. In addition, at this time, the lower limit voltage was set to 2.5 V, but this set value was not actually reached. Further, between each discharge and discharge, it was CC charged again at 5.0 mA up to 4.2 V, and after the charging was completed, the next CC discharge was carried out. The current value I and the voltage drop V obtained as described above were plotted, and the DC resistance (DCR) (unit: Ω) was obtained from the slope of the I-V characteristics obtained by linearly approximating each point.

[0136] [Table 1]

[0137]

[0138] In Table 1, "Non-" means not having the corresponding component.

[0139] From Table 1, it was found that Examples 1 to 24 containing the compounds represented by Formulas (1) to (4) as solvents had a much higher number of cycles and excellent cycle characteristics compared to Comparative Examples 1 and 2 which were not so. Further, it was found that Examples 1 to 24 had a lower DC resistance value compared to the DC resistance values predicted from the very high cycle characteristics and had a DC resistance value equivalent to those of Comparative Examples 1 and 2. From this, it can be seen that Examples 1 to 24 have excellent cycle characteristics and excellent rate performance.

[0140] Industrial Applicability

[0141] Since the lithium secondary battery of the present invention has a high energy density and excellent cycle characteristics, it has industrial applicability as a power storage device used in various applications.

[0142] Explanation of Reference Numerals

[0143] 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 without a negative electrode active material, and an electrolyte solution. The electrolyte solution contains, as a solvent, at least any one of the compounds represented by formulas (1) to (4), but does not contain fluoroethylene carbonate (FEC). [Chemical formula 1] In formula (1), R 1 ~R 4 represent hydrogen, a halogen, or a hydrocarbon group that is fluorine-substituted or partially fluorine-substituted or unsubstituted, n is 1 or more, and R represents an unsubstituted hydrocarbon group, In formulas (2) to (4), R 1 ~R 4 represents hydrogen, a halogen, or a hydrocarbon group that is fluorine-substituted or partially fluorine-substituted or unsubstituted, n is 1 or more, and R represents a hydrocarbon group that is fluorine-substituted or partially fluorine-substituted or unsubstituted.

2. The lithium secondary battery according to claim 1. Wherein the content of the compound represented by formulas (1) to (4) is 30% by volume or more relative to the total amount of the solvent components of the electrolyte solution.

3. The lithium secondary battery according to claim 1 or 2. Wherein the electrolyte solution further contains a fluorinated solvent other than the compound represented by formulas (1) to (4) and fluoroethylene carbonate (FEC).

4. The lithium secondary battery according to claim 1 or 2. Wherein the electrolyte solution further contains a non-fluorinated solvent.

5. The lithium secondary battery according to claim 1 or 2. 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.

6. The lithium secondary battery according to claim 1 or 2. Wherein the negative electrode is an electrode composed of at least one selected from the group consisting of Cu, Ni, Ti, Fe, and metals and their alloys that do not react with Li other than these, and stainless steel SUS.

7. The lithium secondary battery according to claim 1 or 2. Wherein before the initial charging, no lithium foil is formed on the surface of the negative electrode.

8. The lithium secondary battery according to claim 1 or 2. Wherein the energy density is 350 Wh / kg or more.

Images