Lithium secondary battery

Abstract

Provided is a lithium secondary battery having high energy density and excellent cycle characteristics or safety. The present invention relates to a lithium secondary battery having a positive electrode, a negative electrode having no negative electrode active material, and an electrolyte, the electrolyte containing a lithium salt, a fluorine compound having a monovalent group represented by formula (A) or formula (B), and a chain ether compound having no fluorine atom and having a branched chain.

Description

Technical Field

[0001] This invention relates to lithium secondary batteries. Background Technology

[0002] In recent years, technologies that convert natural energy such as sunlight or wind power into electrical energy have attracted much attention. Along with this, various types of rechargeable batteries have been developed as energy storage devices that are highly safe and capable of storing a large amount of electrical energy.

[0003] Among these, secondary batteries that are known to perform charging and discharging by moving metal ions between the positive and negative electrodes exhibit high voltage and high energy density; lithium-ion secondary batteries are a typical example. As a typical lithium-ion secondary battery, an active material capable of retaining lithium is introduced into both the positive and negative electrodes, and charging and discharging are performed by giving and receiving lithium ions between the positive and negative electrode active materials. Furthermore, as a secondary battery that does not use an active material at the negative electrode, lithium metal secondary batteries have been developed that retain lithium by depositing lithium metal on the surface of the negative electrode.

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

[0005] Furthermore, Patent Document 2 discloses a lithium secondary battery comprising a positive electrode, a negative electrode, a separator sandwiched between them, and an electrolyte. In this battery, metal particles are formed on a negative electrode current collector at the negative electrode, and these particles move from the positive electrode during charging, forming lithium metal on the negative electrode current collector within the negative electrode. Patent Document 2 discloses a lithium secondary battery that solves problems caused by the reactivity of lithium metal and issues arising during assembly, providing a lithium secondary battery with improved performance and lifespan.

[0006] Existing technical documents

[0007] Patent documents

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

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

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

[0011] For example, typical secondary batteries that charge and discharge by giving and receiving metal ions between the positive and negative electrode active materials have insufficient energy density. Furthermore, existing lithium metal secondary batteries, as described in the aforementioned patent literature, which retain lithium by depositing lithium metal on the negative electrode surface, are prone to forming dendritic lithium metal on the negative electrode surface due to repeated charge and discharge cycles, leading to short circuits and capacity reduction. As a result, cycle performance is inadequate.

[0012] Furthermore, in lithium secondary batteries, as described in the aforementioned patent documents, where lithium metal is deposited on the surface of the negative electrode to retain lithium, the solvent used for the electrolyte typically tends to have a low boiling point and high vapor pressure. Therefore, such batteries are prone to volume expansion due to the heat generated during charging. Thus, in lithium secondary batteries, safety considerations, such as battery designs with low volume expansion rates, are necessary.

[0013] The present invention was made in view of the above-mentioned problems, and its purpose is to provide a lithium secondary battery with high energy density, excellent cycle characteristics or safety.

[0014] Another embodiment of the present invention relates to a lithium secondary battery comprising a positive electrode, a negative electrode without a negative electrode active material, and an electrolyte, wherein the electrolyte comprises a lithium salt, a fluorine compound having a monovalent group represented by the following formula (A) or the following formula (B) (hereinafter, the compound is referred to simply as "fluorine compound"), and a chain ether compound having a branched chain without fluorine atoms.

[0015] [Chemical Formula 1]

[0016]

[0017] [Chemical Formula 2]

[0018]

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

[0020] Such lithium secondary batteries have a high energy density because they have a negative electrode without negative electrode active material. They are charged and discharged by the deposition of lithium metal on the surface of the negative electrode and the electrolytic dissolution of the deposited lithium metal.

[0021] Furthermore, the inventors have discovered that lithium secondary batteries containing the aforementioned fluorine compounds readily form a solid electrolyte interface layer (hereinafter also referred to as a "SEI layer") on the surface of the negative electrode. Since the SEI layer possesses ionic conductivity, the reactivity of the lithium deposition reaction on the negative electrode surface where the SEI layer forms is uniform in the planar direction of the negative electrode surface. Therefore, the aforementioned lithium secondary battery suppresses the growth of dendritic lithium metal on the negative electrode and exhibits excellent cycle characteristics. Furthermore, while the main reasons for the easy formation of the SEI layer by including the fluorine compound may not be entirely clear, the main reasons described later in the specific embodiments are taken into consideration.

[0022] Furthermore, the inventors have discovered that lithium-ion secondary batteries exhibit low volume expansion during repeated charge-discharge cycles by incorporating branched chain ether compounds without fluorine atoms into the electrolyte. The main reasons for this are considered, but are not limited to, the following aspects: Compared to unbranched compounds, branched chain ether compounds tend to have a greater electron bias within their molecules, resulting in higher polarity. Consequently, such ether compounds have higher boiling points and tend to have lower vapor pressures. Therefore, it is presumed that this contributes to the suppression of battery volume expansion and excellent safety in the lithium-ion secondary battery of the present invention. Furthermore, it is desirable that the reduced symmetry of the molecular structure improves compatibility with the aforementioned fluorine compounds.

[0023] In a lithium secondary battery according to one embodiment of the present invention, it is preferable that the aforementioned chain ether compound is a compound containing two or more but less than five ether bonds. In this manner, the solubility of the electrolyte in the electrolyte solution is improved, and the cycle characteristics of the lithium secondary battery are excellent.

[0024] In a lithium secondary battery according to one embodiment of the present invention, it is preferable that the chain ether compound has 4 or more, or 10 or less carbon atoms. In this manner, the solubility of the electrolyte in the electrolyte solution tends to increase.

[0025] In a lithium secondary battery according to one embodiment of the present invention, it is preferred that the aforementioned chain ether compound comprises at least one selected from the group consisting of 1,1-dimethoxyethane, 1,2-dimethoxypropane, 2,2-dimethoxypropane, 1,3-dimethoxybutane, 1,2-dimethoxybutane, 2,2-dimethoxybutane, 2,3-dimethoxybutane, 1,2-diethoxypropane, 1,2-diethoxybutane, 2,3-diethoxybutane, and diethoxyethane. According to this method, the lithium secondary battery exhibits superior safety and cycle characteristics.

[0026] In a lithium secondary battery according to one embodiment of the present invention, it is preferable that the branches of the aforementioned chain ether compound are unsubstituted alkyl groups having 1 to 10 carbon atoms. In this manner, the lithium secondary battery exhibits superior safety and cycle characteristics because the solubility of the electrolyte in the electrolyte is increased and the boiling point of the chain ether compound is more easily raised.

[0027] In a lithium secondary battery according to one embodiment of the present invention, it is preferable that the lithium salt is LiN(SO2F)2. This approach tends to suppress the growth of lithium metal into dendrites on the negative electrode surface.

[0028] In one embodiment of the lithium secondary battery according to the present invention, it is preferable that the electrolyte further comprises a straight-chain ether compound that does not have fluorine atoms and is not branched. In this manner, the solubility of the electrolyte in the electrolyte is increased, and the cycle characteristics of the lithium secondary battery are more excellent.

[0029] In a lithium secondary battery according to one embodiment of the present invention, it is preferable that the linear ether compound has 3 or more but less than 10 carbon atoms. In this manner, the solubility of the electrolyte in the electrolyte solution is increased, and the cycle characteristics of the lithium secondary battery are more excellent.

[0030] In a lithium secondary battery according to one embodiment of the present invention, it is preferable that the aforementioned linear ether compound is a compound containing two or more but no more than five ether bonds. In this manner, the solubility of the electrolyte in the electrolyte solution is improved, and the lithium secondary battery exhibits excellent cycle characteristics.

[0031] In a lithium secondary battery according to one embodiment of the present invention, it is preferable that the content of the aforementioned chain ether compound is 5% by volume or more and 50% by volume or less relative to the total amount of solvent components in the electrolyte. In this manner, the solubility of the electrolyte in the electrolyte is improved, and the lithium secondary battery exhibits excellent cycle characteristics.

[0032] In a lithium secondary battery according to one embodiment of the present invention, it is preferable that the content of the fluorine compound is 30% by volume or more and 95% by volume or less relative to the total amount of solvent components in the electrolyte. In this manner, the SEI layer is more easily formed, resulting in superior cycle characteristics for the lithium secondary battery.

[0033] One embodiment of the present invention relates to a lithium secondary battery comprising a positive electrode, a negative electrode without a negative electrode active material, and an electrolyte, wherein the electrolyte comprises a lithium salt, a fluorine compound having a monovalent group represented by formula (A) or formula (B) below, and an ether compound without fluorine atoms, and the content of the fluorine compound is more than 50% by volume relative to the total amount of solvent components in the electrolyte.

[0034] [Chemical Formula 3]

[0035]

[0036] [Chemical Formula 4]

[0037] (In the above formula, the wavy line represents the bonding site in the monovalent group).

[0038] Such lithium secondary batteries, as described above, have a high energy density because they have a negative electrode without a negative electrode active material, and are charged and discharged through the deposition of lithium metal on the surface of the negative electrode and the dissolution by electrolysis.

[0039] Furthermore, as mentioned above, it was found that lithium secondary batteries containing the aforementioned fluorine compound in the electrolyte at a volume percentage exceeding 50% relative to the total amount of solvent more readily form an SEI layer on the negative electrode surface. The reactivity of the lithium deposition reaction on the negative electrode surface where the SEI layer forms is uniform in the planar direction of the negative electrode surface; therefore, the aforementioned lithium secondary batteries suppress the growth of dendritic lithium metal on the negative electrode and exhibit excellent cycle characteristics.

[0040] Furthermore, the aforementioned lithium secondary battery includes an ether compound that does not contain fluorine atoms in the electrolyte. By including such an ether compound, the solubility of lithium salts in the electrolyte is further increased, and ionic conductivity is improved. Therefore, the aforementioned lithium secondary battery exhibits superior cycle characteristics.

[0041] In a lithium secondary battery according to one embodiment of the present invention, it is preferable that the fluorine compound has 3 or more but less than 10 carbon atoms. In this manner, the SEI layer is more easily formed.

[0042] Invention Effects

[0043] According to the present invention, a lithium secondary battery with high energy density, excellent cycle characteristics, or excellent safety can be provided. Attached Figure Description

[0044] Figure 1 This is a schematic cross-sectional view of a lithium secondary battery according to an embodiment of the present invention.

[0045] Figure 2 This is a schematic cross-sectional view of the use of a lithium secondary battery according to an embodiment of the present invention. Detailed Implementation

[0046] Hereinafter, embodiments of the present invention (hereinafter referred to as "this embodiment") will be described in detail with reference to the accompanying drawings. Furthermore, in the drawings, the same reference numerals are used to label the same elements, and repeated descriptions are omitted. Moreover, unless otherwise specified, positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings. Furthermore, the scale of the drawings is not limited to the scale shown in the illustrations.

[0047] [First Implementation Method]

[0048] (Lithium-ion rechargeable battery)

[0049] Figure 1 This is a schematic cross-sectional view of the lithium secondary battery involved in this embodiment. Figure 1 As shown, the lithium secondary battery 100 of this embodiment includes a positive electrode 120, a negative electrode 140 without negative electrode active material, a separator 130 disposed between the positive electrode 120 and the negative electrode 140, and a separator 130 in… Figure 1 The electrolyte is not shown in the figure. The positive electrode 120 has a positive current collector 110 on the surface opposite to the surface opposite to the separator 130.

[0050] The following describes the structure of the lithium secondary battery 100.

[0051] (negative electrode)

[0052] The negative electrode 140 does not contain a negative electrode active material. In this specification, "negative electrode active material" refers to a substance that generates electrode reactions in the negative electrode, namely oxidation and reduction reactions. Specifically, lithium metal and host materials of lithium elements (lithium ions or lithium metal) are listed as negative electrode active materials in this embodiment. A host material of lithium elements refers to a substance provided to retain lithium ions or lithium metal on the negative electrode. There are no particular limitations on such retaining mechanisms; examples include intercalation, alloying, and absorption by metal clusters, with intercalation being typical.

[0053] Since the lithium secondary battery of this embodiment does not have negative electrode active material at the negative electrode before the initial charging of the battery, charging and discharging are carried out by depositing lithium metal on the negative electrode and electrolyzing the deposited lithium metal. Therefore, compared with lithium secondary batteries with negative electrode active material, the lithium secondary battery of this embodiment has a smaller overall volume and mass due to the reduction in the volume and mass of the negative electrode active material, and thus, in principle, a higher energy density.

[0054] In this embodiment of the lithium secondary battery 100, before the initial charging of the battery, the negative electrode 140 does not have negative electrode active material. Lithium metal is deposited on the negative electrode during battery charging, and the deposited lithium metal is electrolytically dissolved during battery discharge. Therefore, in this embodiment of the lithium secondary battery, the negative electrode functions as a negative electrode current collector.

[0055] When the lithium secondary battery 100 of this embodiment is compared with lithium-ion batteries (LIB) and lithium metal batteries (LMB), it differs in the following aspects.

[0056] In a lithium-ion battery (LIB), the negative electrode has a host material containing lithium elements (lithium ions or lithium metal). Lithium elements are filled into this material during battery charging, and the lithium elements are released through the host material to discharge the battery. The LIB differs from the lithium secondary battery 100 of this embodiment in that the negative electrode contains a host material containing lithium elements.

[0057] Lithium metal batteries (LMBs) are manufactured by having an electrode with lithium metal on its surface, or by using a single lithium metal cell as the negative electrode. That is, the difference between an LMB and the lithium secondary battery 100 of this embodiment is that the negative electrode of an LMB has lithium metal as the negative electrode active material immediately after assembly, i.e., before the initial charging of the battery. While LMBs use electrodes containing highly flammable and reactive lithium metal in their manufacturing, the lithium secondary battery 100 of this embodiment offers superior safety and manufacturability because it uses a negative electrode without lithium metal.

[0058] In this specification, "not having negative electrode active material" means that the negative electrode 140 does not have or substantially does not have negative electrode active material. "Substantially not having negative electrode active material" means that the content of negative electrode active material in the negative electrode 140 is less than 10% by mass relative to the total negative electrode. The content of negative electrode active material in the negative electrode relative to the total negative electrode 140 is preferably less than 5.0% by mass, but can be less than 1.0% by mass, less than 0.1% by mass, or less than 0.0% by mass. By having the negative electrode 140 not having negative electrode active material or the content of negative electrode active material in the negative electrode 140 being within the above range, the energy density of the lithium secondary battery 100 is increased.

[0059] In this specification, "before initial charging" refers to the state of the battery from assembly to the first charging. Furthermore, "at the end of discharge" refers to the state of the battery with a voltage of 1.0V to 3.8V, preferably 1.0V to 3.0V.

[0060] In this specification, "a lithium secondary battery having a negative electrode without negative electrode active material" means that, before the initial charging of the battery, the negative electrode 140 does not have negative electrode active material. Therefore, the phrase "a negative electrode without negative electrode active material" can be translated as "a negative electrode without negative electrode active material before the initial charging of the battery," "regardless of the charging state of the battery, it does not have negative electrode active material other than lithium metal, and before the initial charging, it does not have a lithium metal negative electrode current collector," or "before the initial charging, it does not have a lithium metal negative electrode current collector," etc. Furthermore, "a lithium secondary battery having a negative electrode without negative electrode active material" can also be translated as an anode-free lithium battery, a zero-anode lithium battery, or a negative electrode-free lithium battery.

[0061] In this embodiment, the negative electrode 140 has a content of negative electrode active material other than lithium metal of 10% by mass or less relative to the total negative electrode, which is preferably 5.0% by mass or less, but can be 1.0% by mass or less, 0.1% by mass or less, 0.0% by mass or less, or even 0% by mass, regardless of the charging state of the battery.

[0062] Furthermore, in this embodiment, before initial charging, the lithium metal content of the negative electrode 140 is 10% by mass or less relative to the overall negative electrode, preferably 5.0% by mass or less, but can be 1.0% by mass or less, 0.1% by mass or less, 0.0% by mass or less, or even 0% by mass.

[0063] In this embodiment, when the battery voltage is 1.0V or higher and 3.5V or lower, the lithium metal content relative to the negative electrode 140 can be 10% by mass or lower (preferably 5.0% by mass or lower, or 1.0% by mass or lower); when the battery voltage is 1.0V or higher and 3.0V or lower, the lithium metal content relative to the negative electrode 140 can be 10% by mass or lower (preferably 5.0% by mass or lower, or 1.0% by mass or lower); or, when the battery voltage is 1.0V or higher and 2.5V or lower, the lithium metal content relative to the negative electrode 140 can also be 10% by mass or lower (preferably 5.0% by mass or lower, or 1.0% by mass or lower).

[0064] Furthermore, in the lithium secondary battery 100 of this embodiment, the mass M of lithium metal deposited on the negative electrode when the battery voltage is 3.0V is... 3.0 The mass M of lithium metal deposited on the negative electrode relative to the battery voltage of 4.2V. 4.2 The ratio M 3.0 / M 4.2 Preferably, it is 40% or less; more preferably, it is 38% or less; and even more preferably, it is 35% or less. Ratio M 3.0 / M4.2 It can be 1.0% or higher, 2.0% or higher, 3.0% or higher, or 4.0% or higher.

[0065] Examples of negative electrode active materials used in this embodiment include lithium metal and alloys containing lithium metal, carbon-based materials, metal oxides and metals alloyed with lithium, and alloys containing such metals. The carbon-based materials are not particularly limited, and examples include graphene, graphite, hard carbon, mesoporous carbon, carbon nanotubes, and carbon nanotubes. The metal oxides are not particularly limited, and examples include titanium oxide compounds, tin oxide compounds, and cobalt oxide compounds. Metals alloyed with lithium include silicon, germanium, tin, lead, aluminum, and gallium.

[0066] As the negative electrode 140 in this embodiment, there are no particular limitations if it does not have a negative electrode active material but can be used as a current collector. Examples include at least one composition 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). Preferably, at least one composition selected from the group consisting of Cu, Ni and their alloys, and stainless steel (SUS) is used. When such a negative electrode is used, the energy density and productivity of the battery tend to be better. Furthermore, when SUS is used in the negative electrode, various conventionally known substances can be used as the type of SUS. The aforementioned negative electrode materials can be used alone or in combination of two or more. In addition, in this specification, "metals that do not react with Li" refers to metals that do not react with lithium ions or lithium metal under the operating conditions of a lithium secondary battery.

[0067] The capacity of the negative electrode 140 is sufficiently small relative to the capacity of the positive electrode 120, for example, it may be less than 20%, less than 15%, less than 10%, or less than 5%. Furthermore, the capacities of the positive electrode 120 and the negative electrode 140 can be measured using existing known methods.

[0068] 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 even more preferably 6 μm or more and 15 μm or less. In this manner, the volume occupied by the negative electrode 140 in the secondary battery 100 is reduced, thus further improving the energy density of the lithium secondary battery 100.

[0069] (electrolyte)

[0070] The electrolyte contains electrolyte and solvent, and is a solution with ion conductivity, serving as a conductive path for lithium ions. The electrolyte can be immersed in the separator 130, or it can be sealed together with the laminate of the positive electrode 120, separator 130, and negative electrode 140 in a sealed container.

[0071] The electrolyte contains lithium salt, a fluorine compound having a monovalent group represented by formula (A) or formula (B) below, and an ether compound without fluorine atoms, wherein the content of the fluorine compound is more than 50% by volume relative to the total amount of the solvent components of the electrolyte.

[0072] [Chemical Formula 5]

[0073]

[0074] [Chemical Formula 6]

[0075]

[0076] In the above formula, the wavy line represents the bonding site in the monovalent group.

[0077] Generally, in anode-free lithium secondary batteries with an electrolyte, an SEI layer is formed on the surface of the negative electrode by decomposing the solvent in the electrolyte. In lithium secondary batteries, the SEI layer suppresses further decomposition of components in the electrolyte, as well as the irreversible reduction of lithium ions and the generation of gases caused by this. Furthermore, since the SEI layer has ion conductivity, the reactivity of the lithium metal deposition reaction becomes more uniform in the planar direction on the negative electrode surface where the SEI layer forms. Therefore, promoting the formation of the SEI layer is very important because it improves the performance of anode-free lithium secondary batteries. The inventors have discovered that in lithium secondary batteries containing the aforementioned fluorine 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, resulting in improved cycle characteristics. The main reasons for this are not necessarily clear, but the following are considered as key factors.

[0078] It is believed that during the charging of the lithium secondary battery 100, especially during the initial charging, not only lithium ions but also the aforementioned fluorine compound, which serves as a solvent, is reduced on the negative electrode. Then, due to the substitution of the portion represented by formula (A) and the portion represented by formula (B) of the fluorine compound with multiple fluorine atoms, the oxygen atoms are highly reactive, and it is speculated that part or all of the portion represented by formula (A) and the portion represented by formula (B) easily detaches. As a result, it is speculated that since part or all of the portion represented by formula (A) and the portion represented by formula (B) adsorbs onto the negative electrode surface during the charging of the lithium secondary battery 100, and this adsorbed portion serves as the starting point for the formation of an SEI layer, the lithium secondary battery 100 easily forms an SEI layer. However, the main reasons are not limited to the above.

[0079] Furthermore, it was surprisingly found that the SEI layer formed in the lithium secondary battery 100 containing the aforementioned fluorine compound exhibits higher ion conductivity compared to the SEI layer formed in existing lithium secondary batteries. This is believed to be because the fluorine content of the formed SEI layer increases due to the substitution of the portions represented by formula (A) and formula (B) above with fluorine, thereby increasing or expanding the migration path of lithium ions in the SEI layer. However, the main reason is not limited to this.

[0080] Therefore, although the lithium secondary battery 100 readily forms an SEI layer, it exhibits low internal resistance and excellent rate performance. In other words, the lithium secondary battery 100 possesses excellent cycle characteristics and rate performance. Furthermore, "rate performance" refers to the ability to charge and discharge at high currents, and excellent rate performance is known to occur when the battery's internal resistance is low.

[0081] Furthermore, in the portions represented by formula (A) and formula (B) above, a portion of the hydrogen atoms in the alkyl group of the fluorine compound is not substituted with fluorine atoms. Therefore, since the lithium secondary battery 100 containing the above-mentioned fluorine compound can increase the concentration of electrolyte in the electrolyte, it can further improve the cycle characteristics and rate performance.

[0082] Furthermore, in this specification, the term "containing fluorine compounds" in an electrolyte means that the solvent in the electrolyte contains fluorine compounds. That is, in the application environment of lithium secondary batteries, fluorine compounds can be used to dissolve the electrolyte and form an electrolyte in a solution phase, or they can be a liquid, either as monomers of the compound or as a mixture of other compounds. The same applies to the term "containing ether compounds" in an electrolyte.

[0083] In this embodiment, the fluorine compound is not particularly limited to compounds having a monovalent group represented by formula (A) or formula (B) above, and examples include compounds having an ether bond (hereinafter referred to as "ether compound"), compounds having an ester bond, and compounds having a carbonate bond. From the viewpoint of improving the solubility of the electrolyte in the electrolyte and facilitating the formation of the SEI layer, the fluorine compound is preferably an ether compound.

[0084] Ether compounds that are fluorinated compounds 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 solvents"), ether compounds having a monovalent group represented by formula (A) but not a monovalent group represented by formula (B) (hereinafter also referred to as "second fluorinated solvents"), and ether compounds not having a monovalent group represented by formula (A) but having a monovalent group represented by formula (B) (hereinafter also referred to as "third fluorinated solvents"), etc.

[0085] Examples of first fluorinated solvents include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyldiethoxymethane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyldiethoxypropane. From the viewpoint of effectively and reliably achieving the effects of the aforementioned fluorinated compounds, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is preferred as the first fluorinated solvent.

[0086] Examples of second fluorinated solvents include 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, and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether. From the viewpoint of effectively and reliably achieving the effects of the aforementioned fluorinated compounds, 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 as second fluorinated solvents.

[0087] Examples of third fluorinated solvents include 2,2,3,3-tetrafluoropropyl difluoromethyl ether, 2,2,3,3-tetrafluoropropyl trifluoromethyl ether, 2,2,3,3-tetrafluoropropyl monofluoromethyl ether, and 2,2,3,3-tetrafluoropropyl methyl ether. From the viewpoint of effectively and reliably achieving the effects of the aforementioned fluorinated compounds, 2,2,3,3-tetrafluoropropyl difluoromethyl ether is preferred as a third fluorinated solvent.

[0088] In fluorine compounds, the ratio of the number of fluorine atoms to the total number of hydrogen atoms (F / (H+F)) is not particularly limited and can be 0.1 or more, 0.2 or more, 0.3 or more, 0.4 or more, 0.5 or more, or 0.6 or more. Furthermore, the above ratio (F / (H+F)) can also be, for example, 0.9 or less, 0.8 or less, or 0.7 or less.

[0089] The number of carbon atoms in the fluorinated compound is not particularly limited, provided that it exhibits sufficient solubility relative to the electrolyte; for example, it can be 3 or more and 15 or less. From the viewpoint of further improving the solubility of the electrolyte in the electrolyte solution, the number of carbon atoms in the fluorinated compound is preferably 4 or more, 5 or more, or 6 or more. Furthermore, from the same viewpoint, the number of carbon atoms in the fluorinated compound is preferably 14 or less, 12 or less, 10 or less, or 8 or less.

[0090] The electrolyte may contain at least one fluorinated compound. From the viewpoint of increasing the solubility of the electrolyte in the electrolyte and facilitating the formation of the SEI layer, the electrolyte preferably contains two or more fluorinated compounds. Similarly, the electrolyte preferably contains at least one selected from the group consisting of a first fluorinated solvent, a second fluorinated solvent, and a third fluorinated solvent; more preferably, it contains at least two selected from the group consisting of a first fluorinated solvent, a second fluorinated solvent, and a third fluorinated solvent; and even more preferably, it contains all three fluorinated solvents simultaneously.

[0091] The electrolyte, as a solvent, may also contain fluorinated compounds other than fluorinated compounds. There are no particular limitations on such compounds; examples include fluoroalkyl ether compounds (hereinafter also referred to as "quaternary fluorine solvents") that do not have either the monovalent group represented by formula (A) or the monovalent group represented by formula (B). From the viewpoint of further increasing the solubility of the electrolyte in the electrolyte and facilitating the formation of the SEI layer, the electrolyte preferably contains a quadrary fluorine solvent.

[0092] In addition, "fluorinated alkyl ether compound" means an ether compound having a fluorinated alkyl group, "fluorinated alkyl group" means an alkyl group with at least one hydrogen atom replaced by fluorine, and "compound without fluorine atom" means a compound without a fluorinated alkyl group.

[0093] In the fourth fluorinated solvent, the ratio of the number of fluorine atoms to the total number of hydrogen atoms (F / (H+F)) is not particularly limited and can be 0.1 or more, 0.2 or more, or 0.5 or more. Furthermore, the fourth fluorinated solvent preferably has a perfluorinated alkyl group and an unsubstituted alkyl group. Examples of perfluorinated alkyl groups include straight-chain or branched substances with 1 to 10 carbon atoms, preferably straight-chain substances with 2 to 6 carbon atoms. Examples of unsubstituted alkyl groups include straight-chain or branched substances with 1 to 5 carbon atoms, preferably straight-chain substances with 1 to 3 carbon atoms, such as methyl or ethyl groups.

[0094] Examples of fourth fluorinated solvents 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 aforementioned fluorinated compounds, methyl nonafluorobutyl ether, ethyl nonafluorobutyl ether, and 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-trifluoromethylpentane are preferred as fourth fluorinated solvents.

[0095] The electrolyte contains an ether compound that does not have fluorine atoms (hereinafter referred to as "ether secondary solvent"). By including the ether secondary solvent, the solubility of the electrolyte in the electrolyte is further improved, thus enhancing the ionic conductivity of the electrolyte. As a result, the lithium secondary battery 100 exhibits excellent cycle characteristics.

[0096] As an ether co-solvent, there are no particular limitations on ether compounds that do not have fluorine atoms, and examples include triethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,1-dimethoxyethane, diethylene glycol dimethyl ether, tetrahydrofuran, tetrahydropyran, dioxane, 4-methyl-1,3-dioxane, oxetane, and epoxide. From the viewpoint of further improving the solubility of the electrolyte in the electrolyte solution, compounds having two, three, four, five, six, seven, or eight ether bonds are preferred as ether co-solvents, and 1,2-dimethoxyethane, diethylene glycol dimethyl ether, and triethylene glycol dimethyl ether are more preferred.

[0097] The carbon number of the ether subsolvent is not particularly limited, and for example, it is 2 or more and 20 or less. From the viewpoint of further improving the solubility of the electrolyte in the electrolyte solution, the carbon number of the ether subsolvent is preferably 3 or more, 4 or more, 5 or more, or 6 or more. Furthermore, from the same viewpoint, the carbon number of the ether subsolvent is preferably 15 or less, 12 or less, 10 or less, 9 or less, or 7 or less.

[0098] The electrolyte may also contain compounds without fluorine atoms (hereinafter also referred to as "non-ether solvents") other than the aforementioned ether solvents. There are no particular limitations on non-ether solvents, and examples include 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. Non-ether solvents may also have at least one group selected from the group consisting of carbonate, carbonyl, ketone, and ester groups.

[0099] As a solvent for the electrolyte, the aforementioned fluorinated compounds, fluorinated compounds other than those mentioned above, ether auxiliary solvents, and non-ether auxiliary solvents can be freely combined. A single fluorinated solvent or two or more solvents can also be used in combination. In the electrolyte, compounds without fluorine atoms can be used alone as an ether auxiliary solvent, or the ether auxiliary solvent can be further combined with two or more ether auxiliary solvents or non-ether auxiliary solvents.

[0100] The content of fluorine compounds in the electrolyte exceeds 50% by volume relative to the total solvent content of the electrolyte. With the fluorine compound content within the above range, the SEI layer is easily formed, resulting in excellent cycle characteristics for the lithium secondary battery 100. The fluorine compound content within the above range is not particularly limited, but is preferably 51% by volume or more, 55% by volume or more, 60% by volume or more, 65% by volume or more, 70% by volume or more, 75% by volume or more, 80% by volume or more, 85% by volume or more, or 90% by volume or more relative to the total solvent content of the electrolyte. The upper limit of the fluorine compound content is not particularly limited; the fluorine compound content can be less than 100% by volume, and can be 99% by volume or less, 98% by volume or less, 95% by volume or less, 90% by volume or less, 85% by volume or less, 80% by volume or less, 70% by volume or less, or even 60% by volume or less. With the fluorine compound content within the above range, the solubility of the electrolyte in the electrolyte is further improved.

[0101] When the electrolyte contains two or more fluorinated compounds from the first fluorinated solvent, the second fluorinated solvent, or the third fluorinated solvent, the content of the first fluorinated solvent is not particularly limited, and relative to the total amount of the aforementioned fluorinated compounds, it can be 5% or more by volume or 10% or more by volume, or it can be less than 50% by volume, less than 40% by volume, less than 30% by volume, less than 20% by volume, or less than 15% by volume.

[0102] In the above cases, the content of the second fluorinated solvent is not particularly limited. Relative to the total amount of the above fluorinated compounds, it can be 50% or more by volume, 60% or more by volume, 65% or more by volume, 70% or more by volume, or 75% or more by volume, or it can be less than 95% by volume, less than 90% by volume, or less than 85% by volume.

[0103] In the above cases, the content of the third fluorinated solvent is not particularly limited. Relative to the total amount of the above fluorinated compounds, it can be 0% by volume, more than 5% by volume, or more than 10% by volume, or less than 50% by volume, less than 40% by volume, less than 30% by volume, less than 20% by volume, or less than 15% by volume.

[0104] Furthermore, when the electrolyte contains two or more primary fluorinated solvents, the content of the primary fluorinated solvent is calculated using these total amounts. Similarly, the content is calculated when the electrolyte contains two or more secondary or tertiary fluorinated solvents.

[0105] When the electrolyte contains a tetrafluoromethane solvent, the content of the tetrafluoromethane solvent is not particularly limited, but it is preferably more than 0%, 5%, 10%, or 15% by volume relative to the total solvent content of the electrolyte. By keeping the content of the tetrafluoromethane solvent within the above range, the solubility of the electrolyte in the electrolyte tends to be higher, or the SEI layer is easier to form. There is no particular upper limit to the content of the tetrafluoromethane solvent; the content of the tetrafluoromethane solvent relative to the total solvent content of the electrolyte can be less than 40%, less than 35%, less than 30%, less than 25%, less than 20%, or less than 15% by volume.

[0106] The content of the ether auxiliary solvent in the electrolyte is not particularly limited, but relative to the total amount of solvent components in the electrolyte, it is preferably 5% by volume or more, 10% by volume or more, 15% by volume or more, 20% by volume or more, 25% by volume or more, 30% by volume or more, or 35% by volume or more. The content of the ether auxiliary solvent can be less than 50% by volume, 45% by volume, 40% by volume or less, 35% by volume or less, 30% by volume or less, 25% by volume or less, 20% by volume or less, 15% by volume or less, or even less than 10% by volume. With the ether auxiliary solvent within the above range, the cycle characteristics of the battery tend to be further improved.

[0107] When the electrolyte contains non-ether auxiliary solvents, the content of the non-ether auxiliary solvents is not particularly limited, and can be 0% or more, 5% or more, 10% or more, or 15% or more relative to the total amount of solvent components in the electrolyte. Furthermore, the content of non-ether auxiliary solvents can be less than 50% by volume, less than 40% by volume, less than 30% by volume, less than 25% by volume, less than 20% by volume, less than 15% by volume, or less than 10% by volume.

[0108] In this embodiment, the types of compounds that can be used as solvents are illustrated in the following tables along with the structural formulas. Table 1 illustrates substances that can be used as the aforementioned fluorinated compounds and other fluorinated compounds. Furthermore, Table 2 illustrates substances that can be used as ether secondary solvents. However, the types of compounds that can be used as solvents are not limited to these.

[0109] [Table 1]

[0110]

[0111] [Table 2]

[0112]

[0113] The lithium salts included in the electrolyte are not particularly limited, but include LiI, LiCl, LiBr, LiF, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2CF3CF3)2, LiBF2(C2O4), LiB(O2C2H4)2, LiB(O2C2H4)F2, LiB(OCOCF3)4, LiNO3, and Li2SO4. From the viewpoint of achieving superior energy density and cycle characteristics in the lithium secondary battery 100, LiN(SO2F)2 and LiBF2(C2O4) are preferred lithium salts. Furthermore, when the electrolyte contains at least one of LiN(SO2F)2, LiPF6, and LiBF2(C2O4), it further promotes the formation and growth of the SEI layer on the negative electrode surface, leading to a lithium secondary battery 100 with even better cycle characteristics. In addition, the aforementioned lithium salts can be used alone or in combination with two or more.

[0114] The electrolyte, as an electrolyte solution, may also contain salts other than lithium salts. Examples of such salts include salts of Na, K, Ca, and Mg.

[0115] The concentration of lithium salt in the electrolyte is not particularly limited, but is preferably 0.5 M or more, more preferably 0.7 M or more, even more preferably 0.9 M or more, and even more preferably 1.0 M or more. When the lithium salt concentration is within the above range, the SEI layer is more easily formed, and the internal resistance tends to decrease further. In particular, lithium secondary batteries 100 containing fluorine compounds as solvents can improve cycle characteristics and rate performance by increasing the concentration of lithium salt in the electrolyte. The upper limit of the lithium salt concentration is not particularly limited; the lithium salt concentration can be 10.0 M or less, 5.0 M or less, or 2.0 M or less.

[0116] The lithium secondary battery of this embodiment may also contain electrolyte or electrolyte components in a state other than liquid. For example, it can be configured as a battery in which electrolyte is contained in a solid or semi-solid (gel-like) component by adding electrolyte when preparing the separator described later. In addition, the electrolyte may also be an electrolyte solution.

[0117] Furthermore, the presence of fluorine compounds and ether byproducts in the electrolyte can be determined using various methods known to exist. Examples of such methods include mass analysis methods such as NMR measurement, HPLC-MS, and IR measurement.

[0118] (Solid electrolyte interface layer)

[0119] It is presumed that in the lithium secondary battery 100, a solid electrolyte interphase (SEI) layer is formed on the surface of the negative electrode 140 through charging, especially the initial charging. However, the lithium secondary battery 100 may not have an SEI layer. The formed SEI layer is presumably an organic compound containing at least one of the portions represented by formula (A) and formula (B) above, derived from the aforementioned fluorine compound. For example, it may also contain other lithium-containing inorganic compounds and lithium-containing organic compounds.

[0120] There are no particular limitations on the types of organic and inorganic compounds containing lithium, provided they are substances contained in a known SEI layer. This is not intended to be limiting, but examples of lithium-containing organic compounds include alkyl lithium carbonate, lithium alkoxides, and lithium alkyl esters, while examples of lithium-containing inorganic compounds include LiF, Li₂CO₃, Li₂O, LiOH, lithium borate compounds, lithium phosphate compounds, lithium sulfate compounds, lithium nitrate compounds, lithium nitrite compounds, and lithium sulfite compounds.

[0121] Since the lithium secondary battery 100 contains fluorine compounds as a solvent, it promotes the formation of the SEI layer. Because the SEI layer has ion conductivity, the reactivity of the lithium deposition reaction on the negative electrode surface where the SEI layer forms is uniform in the planar 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, resulting in excellent cycle characteristics.

[0122] The typical average thickness of the SEI layer is between 1 nm and 10 μm. When an SEI layer is formed on the lithium secondary battery 100, lithium metal deposited during battery charging can be deposited at either the interface between the negative electrode 140 and the SEI layer or at the interface between the SEI layer and the separator.

[0123] (positive electrode)

[0124] As for the positive electrode 120, there are generally no special limitations if it is used in a lithium secondary battery. Depending on the application of the lithium secondary battery, known materials can be appropriately selected. 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.

[0125] In this specification, "positive electrode active material" refers to the material used in a battery to retain lithium elements (typically lithium ions) at the positive electrode; in other words, it can be considered the host material for lithium elements (typically lithium ions). Such positive electrode active materials are not particularly limited, but examples include metal oxides and metal phosphates. As for the aforementioned metal oxides, there are no particular limitations, but examples include cobalt oxide compounds, manganese oxide compounds, and nickel oxide compounds. As for the aforementioned metal phosphates, there are no particular limitations, but examples include iron phosphate compounds and cobalt phosphate compounds. Typical positive electrode active materials include LiCoO2 and LiNi. x Co y Mn Z O(x+y+z=1), LiNi x Mn y O2 (x+y=1), LiNiO2, LiMn2O4, LiFePO4, LiCoPO4, FeF3, LiFeOF, LiNiOF and TiS2.

[0126] The aforementioned positive electrode active material may be used alone or in combination with two or more. The positive electrode 120 may also contain components other than the aforementioned positive electrode active material. Such components are not particularly limited, but include, for example, known conductive additives, binders, solid polymer electrolytes, and inorganic solid electrolytes.

[0127] As a conductive additive in the positive electrode 120, there are no particular limitations, and examples include carbon black, single-walled carbon nanotubes (SW-CNTs), multi-walled carbon nanotubes (MW-CNTs), carbon nanofibers, and acetylene black. Furthermore, as a binder, there are no particular limitations, and examples include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylic resin, and polyimide resin.

[0128] The content of the positive electrode active material in the positive electrode 120 can be, for example, 50% by mass or more and 100% by mass or less relative to the overall positive electrode 120. The content of the conductive additive can also be, for example, 0.5% by mass or more and 30% by mass or less relative to the overall positive electrode 120. The content of the binder can also be, for example, 0.5% by mass or more and 30% by mass or less relative to the overall positive electrode 120. The combined content of the solid polymer electrolyte and the inorganic solid electrolyte can also be, for example, 0.5% by mass or more and 30% by mass or less relative to the overall positive electrode 120.

[0129] (Positive current collector)

[0130] A positive current collector 110 is formed on one side of the positive electrode 120. The positive current collector 110 is not particularly limited as long as it is a conductor that does not react with lithium ions in the battery. Aluminum is an example of such a positive current collector.

[0131] 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 even more preferably 6 μm or more and 15 μm or less. In this manner, the volume occupied by the positive electrode current collector 110 in the lithium secondary battery 100 is reduced, thus further improving the energy density of the lithium secondary battery 100.

[0132] (Separator)

[0133] The separator 130 is a component used to prevent short circuits in the battery by isolating the positive electrode 120 from the negative electrode 140, and to ensure the ionic conductivity of lithium ions, which act as charge carriers between the positive and negative electrodes 120 and 140. That is, the separator 130 has the functions of isolating the positive electrode 120 from the negative electrode 140 and ensuring the ionic conductivity of lithium ions. Furthermore, the separator 130 also serves to retain the electrolyte. There are no particular limitations on the separator 130 in fulfilling the above functions; examples include porous components with insulating properties, polymer electrolytes, and gel electrolytes.

[0134] When the separator comprises a porous component with insulating properties, the component exhibits ionic conductivity by filling the pores of the component with an ionicly conductive material. Examples of filling materials include electrolytes, polymer electrolytes, and gel electrolytes, which will be described later.

[0135] The separator 130 can be used alone or in combination with one or more porous components, polymer electrolytes or gel electrolytes that have insulating properties.

[0136] The materials used to constitute the aforementioned insulating porous component are not particularly limited, and examples include insulating polymer materials, specifically polyethylene (PE) and polypropylene (PP). That is, the separator 130 can be a porous polyethylene (PE) film, a porous polypropylene (PP) film, or a laminated structure thereof.

[0137] The polymer electrolyte or gel electrolyte in separator 130 is not particularly limited if it is a material generally used in lithium secondary batteries, and known materials can be appropriately selected. The polymer constituting the polymer electrolyte or gel electrolyte is not particularly limited, and examples include resins having ethylene oxide units on the main chain and / or side chains, such as ethylene oxide (PEO), acrylic resins, vinyl resins, ester resins, nylon resins, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polysiloxane, polyphosphazene, polymethyl methacrylate, polyamide, polyimide, aromatic polyamide, polylactic acid, polyethylene, polystyrene, polyurethane, polypropylene, polybutene, polyacetal, polysulfone, and polytetrafluoroethylene. One or more of the above-mentioned polymers can be used alone. Furthermore, the polymer electrolyte and gel electrolyte can contain the same components as the electrolyte described above.

[0138] The separator 130 may also be covered by a separator cover layer. The separator cover layer may cover both sides of the separator 130 or only one side. There are no particular limitations on whether the separator cover layer is a component that does not react with lithium ions; preferably, it is a layer that can firmly bond the separator 130 to layers adjacent to it. Such separator cover layers are not particularly limited, and examples include adhesives containing polyvinylidene fluoride (PVDF), styrene-butadiene rubber and carboxymethyl cellulose composites (SBR-CMC), polyacrylic acid (PAA), lithium polyacrylate (Li-PAA), polyimide (PI), polyamide-imide (PAI), and aramid fibers. Inorganic particles such as silica, alumina, titanium dioxide, zirconium oxide, magnesium oxide, magnesium hydroxide, and lithium nitrate may also be added to the separator cover layer in the aforementioned adhesives.

[0139] The average thickness of the separator 130 is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less. In this manner, the energy density of the lithium secondary battery 100 is further improved because the volume occupied by the separator 130 in the lithium secondary battery 100 is reduced. Furthermore, the average thickness of the separator 130 is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more. In this manner, the positive electrode 120 and the negative electrode 140 can be more reliably isolated, and short circuits in the battery can be more effectively suppressed.

[0140] (Use of lithium secondary batteries)

[0141] exist Figure 2The diagram illustrates one usage method of the lithium secondary battery according to this embodiment. The lithium secondary battery 200 has a positive terminal 210 and a negative terminal 220 respectively connected to the positive current collector 110 and the negative terminal 140 for connecting the lithium secondary battery 200 to an external circuit. The lithium secondary battery 200 is charged and discharged by connecting the negative terminal 220 to one end of the external circuit and the positive terminal 210 to the other end of the external circuit.

[0142] 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 to the positive terminal 210 through an external circuit. It is presumed that during the initial charging, a solid electrolyte interphase (SEI) layer is formed on the surface of the negative electrode 140 (the interface between the negative electrode 140 and the separator 130), but the lithium secondary battery 200 may also lack an SEI layer. During charging, lithium metal deposition occurs at the interfaces of the negative electrode 140 and the SEI layer, the negative electrode 140 and the separator 130, and / or the SEI layer and the separator 130.

[0143] Regarding the charged lithium secondary battery 200, the lithium secondary battery 200 discharges when the positive terminal 210 and the negative terminal 220 are connected. As a result, lithium metal deposition occurs at the negative electrode via electrolytic dissolution. In the case where an SEI layer is formed on the lithium secondary battery 200, lithium metal deposition occurs at at least one of the interfaces between the negative electrode 140 and the SEI layer and / or the interface between the SEI layer and the separator 130.

[0144] (Manufacturing method of lithium secondary batteries)

[0145] As such Figure 1 The method for manufacturing the lithium secondary battery 100 shown is not particularly limited if it is a method that can manufacture a lithium secondary battery having the above-described structure. Examples of such methods are as follows.

[0146] The positive current collector 110 and the positive electrode 120 are manufactured, for example, as follows. The aforementioned positive electrode active material, a known conductive additive, and a known binder are mixed to obtain a positive electrode mixture. The proportions, for example, relative to the total positive electrode mixture, are: positive electrode active material 50% to 99% by mass, conductive additive 0.5% to 30% by mass, and binder 0.5% to 30% by mass. The obtained positive electrode mixture is coated onto one side of a metal foil (e.g., Al foil) having a specified thickness (e.g., 5 μm to 1 mm) and stamped. The resulting molded body is then stamped to a specified size by a blanking process to obtain the positive current collector 110 and the positive electrode 120.

[0147] Next, the above-mentioned negative electrode material, such as a metal foil of 1 μm or more but less than 1 mm (e.g., electrolytic Cu foil), is cleaned with a solvent containing aminosulfonic acid, then punched into a specified size, further ultrasonically cleaned with ethanol, and then dried to obtain negative electrode 140.

[0148] Next, a separator 130 having the above-described configuration is prepared. The separator 130 can be manufactured using existing known methods or commercially available separators can be used.

[0149] Next, an electrolyte is prepared by dissolving an electrolyte such as a lithium salt in a solution obtained by mixing at least one of the above-mentioned fluorine compounds and an ether auxiliary solvent, as well as the above-mentioned fourth fluorine solvent and / or a non-ether auxiliary solvent as needed. The mixing ratio of the solvent and electrolyte can be appropriately adjusted so that the types of each solvent and electrolyte, as well as their contents or concentrations in the electrolyte, are within the above-mentioned ranges.

[0150] The positive current collector 110, separator 130, and negative electrode 140, which form the positive electrode 120 as described above, are laminated in this order with the positive electrode 120 and separator 130 facing each other to obtain a laminate. By sealing the obtained laminate together with the electrolyte in a sealed container, a lithium secondary battery 100 can be obtained. There is no particular limitation on the sealed container, and a laminated film is an example.

[0151] [Second Implementation Method]

[0152] (Lithium-ion rechargeable battery)

[0153] The lithium secondary battery of the second embodiment is the same as the lithium secondary battery 100 of the first embodiment, and includes a positive current collector, a positive electrode, a negative electrode without negative electrode active material, and a separator disposed between the positive electrode and the negative electrode. Furthermore, the lithium secondary battery of the second embodiment, like the lithium secondary battery 100, includes an electrolyte.

[0154] Except for the points described later, the configuration and preferred manner of the positive current collector, positive electrode, separator, and negative electrode are the same as those of the lithium secondary battery 100 of the first embodiment. With respect to these configurations, the lithium secondary battery of the second embodiment achieves the same effect as the lithium secondary battery of the first embodiment or provides further performance.

[0155] The lithium secondary battery of the second embodiment differs from the lithium secondary battery of the first embodiment in that the electrolyte has the composition described in detail below.

[0156] (electrolyte)

[0157] The electrolyte in the lithium secondary battery of the second embodiment includes a lithium salt, a fluorine compound having a monovalent group represented by formula (A) or formula (B) below, and a chain-like ether compound having a branched chain but no fluorine atom.

[0158] The lithium secondary battery of the second embodiment has superior safety due to the presence of such an electrolyte.

[0159] [Chemical Formula 7]

[0160]

[0161] [Chemical Formula 8]

[0162]

[0163] In the above formula, the wavy line represents the bonding site in the monovalent group.

[0164] The electrolyte of this embodiment contains a branched chain ether compound that does not have fluorine atoms. Compared to unbranched linear ether compounds, branched chain ether compounds tend to have higher boiling points and lower vapor pressures, and tend to have lower volatility. This is believed to be because, compared to unbranched ether compounds, branched chain ether compounds tend to have a greater intramolecular electron bias, thus tending to have higher polarity. Therefore, the lithium secondary battery of this embodiment also suppresses volume expansion during repeated charge and discharge, resulting in superior safety. However, the main reasons are not limited to those described above.

[0165] Furthermore, the branched chain ether compounds, lacking fluorine atoms, are compounds with ether bonds. These ether compounds adequately enhance the lithium-ion conductivity of the electrolyte, tending to increase the solubility of the electrolyte. Therefore, the electrolyte of this embodiment exhibits high lithium-ion conductivity, resulting in superior cycle and rate characteristics for the lithium secondary battery. Moreover, the branched chain ether compounds tend to have strong coordination bonds with the electrolyte and low intramolecular electron density; by including such compounds, the affinity for the aforementioned fluorine compounds increases, and the compatibility in the electrolyte is also improved.

[0166] Furthermore, in the lithium secondary battery of this embodiment, the electrolyte contains a fluorine compound having a monovalent group represented by formula (A) or formula (B) above. As described above, such a fluorine compound readily forms an SEI layer during the charging of the lithium secondary battery, especially during the initial charging, resulting in excellent cycle characteristics and rate characteristics for the lithium secondary battery.

[0167] Therefore, the lithium secondary battery of this embodiment, by combining multiple solvent components that represent the above-mentioned characteristics, not only has excellent cycle characteristics and rate characteristics, but also excellent safety.

[0168] In this specification, "branch" refers to a group other than hydrogen atoms bonded to the longest molecular chain (composed of carbon and oxygen atoms) within the molecular structure. Furthermore, "having a branch" means having at least one such branch within the molecular structure; compounds with branches exhibit a branched structure. On the other hand, unbranched chain-like compounds in this embodiment are represented as "straight-chain" compounds. Additionally, in this specification, sometimes a branched compound among chain-like ether compounds without fluorine atoms is specifically designated as a "chain-like ether compound," and an unbranched compound is specifically designated as a "straight-chain ether compound."

[0169] The branched chain ether compound (chain ether compound) without fluorine atoms used in this embodiment is not particularly limited, and examples include 1,1-dimethoxyethane, 1,2-dimethoxypropane, 2,2-dimethoxypropane, 1,3-dimethoxybutane, 1,2-dimethoxybutane, 2,2-dimethoxybutane, 2,3-dimethoxybutane, 1,2-diethoxypropane, 1,2-diethoxybutane, 2,3-diethoxybutane, and diethoxyethane. Among these, 1,2-dimethoxypropane, 2,2-dimethoxypropane, and 2,2-dimethoxybutane are more preferred.

[0170] From the viewpoint of achieving the effect of chain ether compounds in electrolytes more effectively and reliably, it is preferable that the chain ether compound has one, two, three, four, five, six, seven or eight ether bonds.

[0171] The number of carbon atoms in the chain ether compound is not particularly limited as long as it exhibits sufficient solubility relative to the electrolyte, for example, it can be 4 or more and 15 or less. From the viewpoint of further improving the solubility of the electrolyte in the electrolyte solution, the number of carbon atoms in the chain ether compound is preferably 4 or more, 5 or more, 6 or more, or 7 or more. Furthermore, from the same viewpoint, the number of carbon atoms in the chain ether compound is preferably 14 or less, 12 or less, 10 or less, 9 or less, or 8 or less.

[0172] Furthermore, the number of carbon atoms in the branches of the chain ether compound is not particularly limited, for example, it is 1 to 15. From the viewpoint of improving the solubility of the electrolyte in the electrolyte and the safety of the lithium secondary battery, the number of carbon atoms in the branches of the chain ether compound is preferably 1 or more, 2 or more, 3 or more, or 4 or more. Also, from the same viewpoint, the number of carbon atoms in the branches of the chain ether compound is preferably 14 or less, 12 or less, 10 or less, 9 or less, or 8 or less.

[0173] In the second embodiment, the same fluorine compound as that used in the first embodiment is listed as the fluorine compound. From the viewpoint of further increasing the solubility of the electrolyte in the electrolyte and making it easier to form an SEI layer, the fluorine compound is the same as described above, and preferably an ether compound.

[0174] Furthermore, the number of carbon atoms in the fluorine compound and the fluorine atom content ratio (F / (H+F)) are not particularly limited as long as they exhibit sufficient solubility relative to the electrolyte; examples include the same quantities as in the first embodiment. Moreover, from the same viewpoint as in the first embodiment, such quantities are preferred.

[0175] As with the above, first fluorinated solvent, second fluorinated solvent, and third fluorinated solvent are listed as fluorinated compounds. Furthermore, the preferred manner and effects of the compounds in the first fluorinated solvent, second fluorinated solvent, and third fluorinated solvent are the same as those of the electrolyte in the first embodiment.

[0176] The electrolyte of the second embodiment contains at least one fluorinated compound. From the viewpoint of further increasing the solubility of the electrolyte in the electrolyte and facilitating the formation of the SEI layer, the electrolyte preferably contains two or more fluorinated compounds. Similarly, the electrolyte preferably contains at least one selected from the group consisting of a first fluorinated solvent, a second fluorinated solvent, and a third fluorinated solvent; more preferably, it contains at least two selected from the group consisting of a first fluorinated solvent, a second fluorinated solvent, and a third fluorinated solvent. The electrolyte of the second embodiment particularly preferably contains either a first fluorinated solvent or a second fluorinated solvent.

[0177] When the electrolyte contains two or more first fluorinated solvents, second fluorinated solvents, and third fluorinated solvents, the preferred content of each solvent in the total amount of fluorinated compound is the same as that in the electrolyte of the first embodiment.

[0178] In this embodiment, the electrolyte preferably further comprises a linear ether compound (straight-chain ether compound) that does not have fluorine atoms and is not branched. Since the solubility of the electrolyte in the electrolyte is further improved by including a linear ether compound, the ionic conductivity of the electrolyte is high. As a result, the lithium secondary battery of this embodiment has superior cycle characteristics.

[0179] The linear ether compound is not particularly limited, and examples include ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,3-dimethoxypropane, and 1,4-dimethoxybutane. From the viewpoint of further improving the solubility of the electrolyte in the electrolyte solution, compounds having one, two, three, four, five, six, seven, or eight ether bonds are preferred as linear ether compounds, and 1,2-dimethoxyethane is more preferred.

[0180] The number of carbon atoms in the straight-chain ether compound is not particularly limited as long as it demonstrates sufficient solubility relative to the electrolyte; for example, it can be 2 or more and 15 or less. From the viewpoint of further improving the solubility of the electrolyte in the electrolyte solution, the number of carbon atoms in the straight-chain ether compound is preferably 3 or more, 4 or more, 5 or more, or 6 or more. Furthermore, from the same viewpoint, the number of carbon atoms in the chain ether compound is preferably 7 or less, 9 or less, 10 or less, 11 or less, or 13 or less.

[0181] Furthermore, the electrolyte in the second embodiment may contain at least one chain-like ether compound. From the viewpoint of further improving the solubility of the electrolyte in the electrolyte, the electrolyte contains at least one chain-like ether compound, and more preferably at least one straight-chain ether compound.

[0182] The electrolyte may also contain compounds (solvents) other than the aforementioned chain ether compounds, straight-chain ether compounds, and fluorine compounds. Such compounds are not particularly limited, but examples include compounds having fluorine atoms without any monovalent group represented by formula (A) and formula (B) (quaternary fluorine solvents), ether compounds having a cyclic structure (hereinafter also referred to as "cyclic ether compounds"), and compounds without fluorine atoms and ether bonds (non-ether secondary solvents).

[0183] In the second embodiment, the same compounds as those in the first embodiment are listed as the fourth fluorinated solvent and the non-ether auxiliary solvent, respectively.

[0184] In this embodiment, there is no particular limitation on the cyclic ether compound, and examples include tetrahydrofuran, tetrahydropyran, dioxane, 4-methyl-1,3-dioxane, oxetane, epoxide, 1,2-bis(methoxycarbonyloxy)ethane, 1,2-bis(ethoxycarbonyloxy)ethane, and 1,2-bis(ethoxycarbonyloxy)propane.

[0185] There is no particular limitation on the number of carbon atoms in cyclic ether compounds, for example, it can be 3 to 40. Furthermore, there is no particular limitation on the number of ether bonds in cyclic ether compounds, for example, it can be 1 to 8.

[0186] The content of the chain ether compound in the electrolyte of this embodiment is not particularly limited. However, relative to the total amount of solvent components in the electrolyte, it is preferable that the content is more than 0% by volume, 0.1% by volume or more, 5% by volume or more, 10% by volume or more, 15% by volume or more, or 20% by volume or more. The upper limit of the chain ether compound content is not particularly limited; the content can be less than 60% by volume, less than 55% by volume, less than 50% by volume, less than 45% by volume, less than 40% by volume, or less than 35% by volume. With the chain ether compound content within the above ranges, the lithium secondary battery exhibits excellent safety and tends to have improved battery cycle characteristics.

[0187] The content of the linear ether compound in the electrolyte of this embodiment is not particularly limited. However, relative to the total amount of solvent components in the electrolyte, it is preferable that the content is 0% by volume, more than 0% by volume, 0.1% by volume or more, 1% by volume or more, 2% by volume or more, 3% by volume or more, 5% by volume or more, 10% by volume or more, 15% by volume or more, or 20% by volume or more. The upper limit of the linear ether compound content is not particularly limited; the content can be 60% by volume or less, 50% by volume or less, 40% by volume or less, 30% by volume or less, 25% by volume or less, or 20% by volume or less. By ensuring the linear ether compound content is within the above range, the cycle characteristics of the battery tend to be further improved.

[0188] In the second embodiment, the content of fluorine compounds in the electrolyte is not particularly limited. Preferably, relative to the total amount of solvent components in the electrolyte, it is 30% by volume or more, 35% by volume or more, 40% by volume or more, 45% by volume or more, 50% by volume or more, 55% by volume or more, 60% by volume or more, 65% by volume or more, or 70% by volume or more. The upper limit of the fluorine compound content is not particularly limited; the fluorine compound content can be less than 100% by volume, 98% by volume or less, 95% by volume or less, 90% by volume or less, 85% by volume or less, or even 80% by volume or less. With the fluorine compound content within the above range, the cycle characteristics of the battery tend to be further improved.

[0189] The content of the tetrafluoromethane solvent in the electrolyte of the second embodiment is not particularly limited. For example, relative to the total amount of solvent components in the electrolyte, it can be 0% by volume, more than 0% by volume, 5% by volume or more, 10% by volume or more, or 15% by volume or more. The upper limit of the tetrafluoromethane solvent content is not particularly limited. The content of the tetrafluoromethane solvent relative to the total amount of solvent components in the electrolyte can be 40% by volume or less, 35% by volume or less, 30% by volume or less, 25% by volume or less, 20% by volume or less, or 15% by volume or less.

[0190] The content of cyclic ether compounds and non-ether auxiliary solvents in the electrolyte of this embodiment is not particularly limited. For example, the content of cyclic ether compounds and non-ether auxiliary solvents relative to the total amount of solvent components in the electrolyte can be 0% by volume, more than 0% by volume, 5% by volume or more, 10% by volume or more, or 15% by volume or more. Furthermore, the total content of cyclic ether compounds and non-ether auxiliary solvents can also be 50% by volume or less, 40% by volume or less, 30% by volume or less, 25% by volume or less, 20% by volume or less, 15% by volume or less, or 10% by volume or less. Moreover, the electrolyte of this embodiment may contain both cyclic ether compounds and non-ether auxiliary solvents, or only either one.

[0191] The fluorinated solvents listed in Table 1 and the ether solvents listed in Table 2 can be used in the second embodiment. However, the types of compounds that can be used as solvents are not limited thereto.

[0192] The lithium salt included in the electrolyte of the second embodiment is the same lithium salt included in the electrolyte of the first embodiment. From the viewpoint of having superior energy density and cycle characteristics of lithium secondary batteries, LiN(SO2F)2 is preferred as the lithium salt in this embodiment. Furthermore, one or more lithium salts may be used alone or in combination.

[0193] In addition, the electrolyte can also contain salts other than lithium salts. Examples of such salts include salts of Na, K, Ca, and Mg.

[0194] The concentration of lithium salt in the electrolyte of the second embodiment is not particularly limited, but is preferably 0.5 M or more, more preferably 0.7 M or more, even more preferably 0.9 M or more, and even more preferably 1.0 M or more. With the lithium salt concentration within the above range, the SEI layer is more easily formed, and the internal resistance tends to decrease further. In particular, since lithium secondary batteries containing fluorine compounds as solvents can increase the concentration of lithium salt in the electrolyte, the cycle characteristics and rate performance can be further improved. The upper limit of the lithium salt concentration is not particularly limited, and the lithium salt concentration can be 10.0 M or less, 5.0 M or less, or 2.0 M or less.

[0195] Furthermore, the presence of fluorine compounds, chain ether compounds, and / or straight-chain ether compounds in the electrolyte can be determined using various methods known to exist. Examples of such methods include mass analysis methods such as NMR measurement, HPLC-MS, and IR measurement.

[0196] (Manufacturing method of lithium secondary batteries)

[0197] The lithium secondary battery of the second embodiment can be manufactured in the same way as the lithium secondary battery of the first embodiment, except for the electrolyte preparation process.

[0198] Furthermore, in the preparation of the electrolyte, a solution obtained by mixing at least one of the aforementioned chain ether compounds and at least one of the aforementioned fluorine compounds with one or more compounds selected as needed from the group consisting of the aforementioned straight-chain ether compounds, tetrafluorinated solvents, cyclic ether compounds, and non-ether auxiliary solvents is used as a solvent. An electrolyte such as a lithium salt is dissolved in this solution to prepare the electrolyte. The mixing ratio of the solvent and electrolyte can be appropriately adjusted so that the types of each solvent and electrolyte, as well as their content or concentration in the electrolyte, fall within the aforementioned ranges.

[0199] [Variation Example]

[0200] The above-described embodiments are illustrative of the present invention and are not intended to limit the present invention to these embodiments only. The present invention can be modified in various ways as long as it does not depart from its spirit.

[0201] For example, in the lithium secondary battery of this embodiment, the constituent elements may not be laminated, but rather spaced at fixed distances and filled with electrolyte.

[0202] Furthermore, for example, in the lithium secondary battery of this embodiment, an auxiliary component that assists in the deposition and / or dissolution of lithium metal during charging and discharging may be disposed between the separator and the negative electrode. Examples of such auxiliary components include those containing a metal alloyed with lithium metal, such as a metal layer formed on the surface of the negative electrode. Examples of such metal layers include those 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 be, for example, 5 nm or more and 500 nm or less.

[0203] By incorporating the aforementioned auxiliary components into a lithium-ion secondary battery, the affinity between the negative electrode and the lithium metal deposited on it is further enhanced, thus suppressing the stripping of lithium metal deposited on the negative electrode and improving cycle performance. Furthermore, the auxiliary components may contain metals alloyed with lithium metal, but their 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 is set to be the same as that of the positive electrode. However, since the capacity of this auxiliary component is sufficiently small compared to the capacity of the positive electrode, a lithium-ion secondary battery with such an auxiliary component can be described as having a "negative electrode without 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, less than 20%, less than 15%, less than 10%, or less than 5%.

[0204] The lithium secondary battery of this embodiment may also have a current collector on the surface of the negative electrode, arranged in contact with the negative electrode. There are no particular limitations on such a current collector; examples include current collectors that can be used with the negative electrode material. Furthermore, in the case where the lithium secondary battery does not have a positive electrode current collector and a negative electrode current collector, the positive and negative electrodes themselves function as current collectors.

[0205] In this embodiment of the lithium secondary battery, a portion or all of the surface of the negative electrode, opposite to the separator, can be coated with a coating agent. Examples of negative electrode coating agents include benzotriazole (BTA), imidazole (IM), and triazine thiol (TAS) and their derivatives. For example, after cleaning the aforementioned negative electrode material, the negative electrode coating agent can be applied by immersing the material in a solution containing the negative electrode coating agent (e.g., a solution containing 0.01% to 10% by volume) and drying it in air. It is presumed that by applying the above-mentioned compound, uneven deposition of lithium metal on the surface of the negative electrode can be suppressed, and the growth of lithium metal deposited on the negative electrode into dendrites can be suppressed.

[0206] The lithium secondary battery of this embodiment can also have terminals for connection to an external circuit installed on the positive current collector and / or the negative electrode. For example, metal terminals (e.g., Al, Ni, etc.) with a diameter of 10 μm or more but less than 1 mm can be joined to one or both of the positive current collector and the negative electrode. As a joining method, existing known methods can be used, or, for example, ultrasonic welding can be used.

[0207] Furthermore, in this specification, "high energy density" or "high energy density" means a high capacity per unit of total volume or total mass, preferably 800Wh / L or more or 350Wh / kg or more, more preferably 900Wh / L or more or 400Wh / kg or more, and even more preferably 1000Wh / L or more or 450Wh / kg or more.

[0208] Furthermore, in this specification, "excellent cycle characteristics" means that the rate of capacity reduction of the battery is low before and after a number of charge-discharge cycles conceivable in normal use. That is, it means that when comparing the first discharge capacity after the initial charge with the capacity after a number of charge-discharge cycles conceivable in normal use, the capacity after charge-discharge cycles is almost negligible compared to the first discharge capacity after the initial charge. Here, "number of cycles conceivable in normal use" refers to, for example, 30, 50, 70, 100, 300, or 500 cycles, depending on the intended use of the lithium-ion battery. Furthermore, "the capacity after charge-discharge cycles is almost negligible compared to the first discharge capacity after the initial charge-discharge cycle" means, depending on the intended use of the lithium-ion battery, for example, that 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 of the capacity after the initial charge-discharge cycle.

[0209] In this specification, the numerical ranges described as preferred ranges can also be replaced by any combination of the described upper and lower limits. For example, if a parameter is preferably 50 or more, more preferably 60 or more, more preferably 100 or less, or more preferably 90 or less, the parameter can also be any one of 50 or more and 100 or less, 50 or more and 90 or less, 60 or more and 100 or less, or 60 or more and 90 or less.

[0210] Example

[0211] The present invention will now be described in more detail using embodiments and comparative examples. The present invention is not limited to the following experimental examples.

[0212] [Experimental Example 1]

[0213] [Example 1]

[0214] The lithium secondary battery of Example 1 is prepared as follows.

[0215] (Preparation of the negative electrode)

[0216] First, a 10μm electrolytic Cu foil is cleaned with a solvent containing aminosulfonic acid and then cut into a specified size (45mm×45mm). The Cu foil obtained by further ultrasonic cleaning with ethanol and drying is used as the negative electrode.

[0217] (The production of the positive electrode)

[0218] Next, the positive electrode is prepared. 96 parts by mass of LiNi will be used as the positive electrode active material. 0.85 Co 0.12 Al 0.03The mixture of O2, 2 parts by mass of carbon black as a conductive additive, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder is coated onto one side of a 12μm Al foil and stamped. The resulting molded body is then cut to a specified size (40mm × 40mm) to obtain the positive electrode.

[0219] (Preparation of the separator)

[0220] As a separator, a separator of a specified size (50mm × 50mm) is prepared by coating both sides of a 12μm polyethylene microporous membrane with 2μm polyvinylidene fluoride (PVDF).

[0221] (Electrolyte preparation)

[0222] The electrolyte was prepared as follows: The two solvents were mixed with 60 vol% 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 40 vol% dimethyl ether. The electrolyte was obtained by dissolving LiN(SO₂F)₂ in the resulting mixture at a molar concentration of 1.3 M.

[0223] (Battery assembly)

[0224] The positive current collector, separator, and negative electrode, formed by the positive electrode obtained as described above, are laminated in this order with the positive electrode and separator facing each other to obtain a laminate. Then, 100μm Al terminals and 100μm Ni terminals are ultrasonically welded onto the positive current collector and negative electrode, respectively, and an outer casing of the laminate is inserted. Next, the electrolyte obtained as described above is injected into the outer casing. By sealing the outer casing, a lithium secondary battery is obtained.

[0225] [Examples 2-62]

[0226] The lithium secondary battery was obtained in the same manner as in Example 1, except that the solvent and electrolyte (lithium salt) used to prepare the electrolyte were the same as those described in Tables 3-6.

[0227] In addition, in Tables 3-7, “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-trifluoromethylpentane. In addition, "DME" stands for 1,2-dimethoxyethane, "DGM" stands for diethylene glycol dimethyl ether, and "TGM" stands for triethylene glycol dimethyl ether. Regarding lithium salts used as electrolytes, "FSI" stands for LiN(SO₂F)₂ and "LiDFOB" stands for LiBF₂(C₂O₄).

[0228] Furthermore, in Tables 3-7, each solvent is classified as a first fluorinated solvent, a second fluorinated solvent, a third fluorinated solvent, a fourth fluorinated solvent, or a secondary solvent including ether-based and non-ether-based secondary solvents, according to the above definitions. In the tables, for example, the first fluorinated solvent is referred to as "first". Furthermore, in the tables, the content of each solvent and its type is stated as volume % (%), and the concentration of each lithium salt and its type is stated as volume molar concentration (M). For example, Example 1 means that it contains 60 volume % TTFE and 40 volume % DME as a solvent, and 1.3 M FSI as an electrolyte.

[0229] [Comparative Examples 1-2]

[0230] The lithium secondary battery was obtained in the same manner as in Example 1, except that the solvent and electrolyte (lithium salt) used to prepare the electrolyte were as described in Table 7. Furthermore, Comparative Examples 1 and 2 did not contain fluorine compounds, and only contained secondary solvents as solvents.

[0231] [Evaluation of Cyclic Characteristics]

[0232] The cycle characteristics of the lithium secondary batteries manufactured in the various embodiments and comparative examples are evaluated as follows.

[0233] The prepared lithium secondary battery was repeatedly charged at 3.2mA with a CC charge until the voltage reached 4.2V (initial charge) at an environment of 25°C, and then discharged at 3.2mA with a CC charge until the voltage reached 3.0V (hereinafter referred to as "initial discharge"). Next, it was charged with a CC charge at 13.6mA until the voltage reached 4.2V, and then discharged with a CC charge at 20.4mA until the voltage reached 3.0V. For each example, the capacity obtained from the initial discharge (hereinafter referred to as "initial capacity," and recorded as "capacity" in the tables) is shown in Tables 3 to 7. Furthermore, for each example, the number of cycles when the discharge capacity reached 80% of the initial capacity is shown in Tables 3 to 7 (referred to as "cycles" in the tables).

[0234] [Measurement of DC Resistance]

[0235] The fabricated lithium secondary battery was CC-charged to 4.2V at 5.0mA, and then CC-discharged for 30 seconds at 30mA, 60mA, and 90mA respectively. The lower voltage limit was set to 2.5V, but in any of the cases, it did not reach 2.5V during the 30-second discharge. Between each discharge, the battery was CC-charged again to 4.2V at 5.0mA, and then CC-discharged again after each charge 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 IV characteristic obtained by linear fitting at each point. The results are shown in Tables 3-7.

[0236] [Table 3]

[0237]

[0238] [Table 4]

[0239]

[0240] [Table 5]

[0241]

[0242] [Table 6]

[0243]

[0244] [Table 7]

[0245]

[0246] In Tables 5-7, "-" means that the ingredient is not present.

[0247] As shown in Tables 3-7, Examples 1-62, which contain fluorine compounds having monovalent groups represented by formula (A) or formula (B) below, and ether compounds without fluorine atoms as solvents, exhibit very high cycle numbers and excellent cycling characteristics compared to Comparative Examples 1 and 2, which do not exhibit these characteristics. Furthermore, Examples 1-62 show low DC resistance values ​​compared to the DC resistance values ​​predicted from the very high cycling characteristics, and have DC resistance values ​​similar to those of Comparative Examples 1 and 2. Therefore, Examples 1-62 demonstrate excellent cycling characteristics and excellent rate performance.

[0248] [Experimental Example 2]

[0249] [Examples 63-70]

[0250] The lithium secondary battery was obtained in the same manner as in Example 1, except that the solvent and electrolyte (lithium salt) used to prepare the electrolyte were prepared using the solvents and electrolytes (lithium salt) described in Table 8.

[0251] Furthermore, in Tables 8 and 9, "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, and "TFPNE" represents 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether. Additionally, "1,2-DMP" represents 1,2-dimethoxypropane, "2,2-DMP" represents 2,2-dimethoxypropane, "2,2-DMB" represents 2,2-dimethoxybutane, "DME" represents 1,2-dimethoxyethane, and "TGM" represents triethylene glycol dimethyl ether. Regarding lithium salts used as electrolytes, "FSI" represents LiN(SO₂F)₂.

[0252] Furthermore, in Tables 8 and 9, each solvent is classified as a first fluorinated solvent, a second fluorinated solvent, a chain ether compound, or a straight-chain ether compound, as defined above. In the tables, for example, the first fluorinated solvent is described as "first," and the chain ether compound as "chain." Additionally, in the tables, the content of each solvent, as it is related to its type, is described in volume % (%), and the concentration of lithium salt, as it is related to its type, is described in volume molar concentration (M). For example, this means that Example 63 contains 80 volume % TTFE and 20 volume % 1,2-DMP as a solvent, and 1.0 M FSI as an electrolyte.

[0253] [Refer to Comparative Examples 3 and 4]

[0254] The lithium secondary battery was obtained in the same manner as in Example 1, except that the solvents and lithium salts used to prepare the electrolyte were as described in Table 8. Furthermore, Comparative Examples 3 and 4 were used as examples of linear ether compounds containing only straight-chain ether compounds, without fluorine atoms and having branched chains, particularly without fluorine atoms.

[0255] [Evaluation of Cyclic Characteristics]

[0256] The cycle characteristics of the fabricated lithium secondary battery were evaluated using the same method as in Experimental Example 1.

[0257] [Measurement of volume expansion rate]

[0258] The volume expansion rate of the lithium secondary batteries produced in each embodiment and comparative example is evaluated as follows.

[0259] For each example, the thickness of the newly manufactured battery cell and the thickness of the battery cell after the 100th charge following 99 of the above charge-discharge cycles were measured using a micrometer, and the volume expansion rate accompanying the charge-discharge cycle was measured.

[0260] The volume expansion rate of the battery cell after the 100th charge (meaning the expansion ratio of the battery cell after the 100th charge relative to the thickness of the battery cell after the 100th charge) is shown as “volume expansion rate (%)” in Table 8.

[0261] [Evaluation of Rate Characteristics]

[0262] The rate characteristics of the lithium secondary batteries manufactured in the various embodiments and comparative examples are evaluated as follows.

[0263] The fabricated lithium-ion secondary battery was CC-charged to 4.2V at 3.0mA. Then, it was CC-discharged sequentially at discharge rates of 0.05C, 0.1C, 0.5C, 1.0C, 2.0C, or 3.0C. The lower limit voltage was set to 3.0V. Between each discharge, it was CC-charged again to 4.2V at 3.0mA, and after charging, CC-discharged at the next discharge rate. The ratio of the discharge capacity at 3.0C to the discharge capacity at 0.1C was used as the rate characteristic (%), which was then used as an indicator of the rate characteristic. Since the voltage drop based on internal resistance increases with increasing discharge current, and the discharge capacity tends to decrease, a higher rate characteristic value indicates a superior lithium-ion secondary battery. The results are shown in Table 8.

[0264] [Table 8]

[0265]

[0266] In Table 8, "-" means that the ingredient is not present.

[0267] As can be seen from Table 8, Examples 63-70, which explicitly used an electrolyte containing a chain-like ether compound having a branched chain but no fluorine atom and a fluorine compound having a monovalent group represented by formula (A) or the following formula (B), showed excellent cycling performance and low volume expansion rate compared to Reference Comparative Examples 3 and 4, which did not do so. That is, they showed excellent cycling performance and safety.

[0268] [Refer to Experimental Example 1]

[0269] As a reference example, a lithium-ion battery was fabricated by using a material containing 10% by mass of SiO graphite (active material: 90% by mass, conductive additive: 2% by mass, binder: 8% by mass) as the negative electrode active material supported on a 10 μm electrolytic Cu foil. The separator and positive electrode were the same as in Test Example 1. Lithium-ion batteries of Reference Examples 1 to 3 were fabricated with electrolyte compositions as shown in Table 9.

[0270] For the lithium-ion batteries prepared as Reference Examples 1-3, various measurements were performed in the same manner as in Test Example 2. The results are shown in Table 9.

[0271] [Table 9]

[0272]

[0273] In Table 9, "-" indicates that the component is not present, "NA" indicates that the value fluctuates during the measurement of the rate characteristics and the rate characteristics cannot be measured stably, and "None" indicates that no measurement is performed.

[0274] Reference Example 1 in Table 9 is a lithium-ion battery with a negative electrode containing a negative electrode active material. However, it was found that even when using an electrolyte containing a branched chain ether compound without fluorine atoms, the volume expansion rate was generally larger than that of the examples described in Table 8. Similarly, Reference Example 2, as a lithium-ion battery, used the same electrolyte as Comparative Example 4, but it was found to have fewer cycles and a larger volume expansion rate than Comparative Example 4. Furthermore, Reference Example 3, as a lithium-ion battery, used an electrolyte containing both a fluorine compound and the aforementioned chain ether compound, but compared to Examples 63-70, it was found that the capacity and cycle number were lower.

[0275] From the above examples, it is evident that the lithium secondary battery of this embodiment and the existing lithium-ion battery (LIB) with a negative electrode having a negative electrode active material teach designs that require different electrolyte compositions.

[0276] Industrial applicability

[0277] Because the lithium secondary battery of the present invention has high energy density and excellent cycle characteristics, it has industrial applicability as an energy storage device for use in a wide variety of applications.

[0278] Explanation of reference numerals in the attached figures

[0279] 100, 200… Lithium secondary battery, 110… Positive current collector, 120… Positive electrode, 130… Separator, 140… Negative electrode, 210… Positive terminal, 220… Negative terminal.

Claims

1. A lithium secondary battery comprising a positive electrode, a negative electrode without negative electrode active material, and an electrolyte, wherein the electrolyte comprises: Lithium salts; Fluorine compounds having a monovalent group represented by formula (A) or formula (B); and Chain-like ether compounds, lacking fluorine atoms but possessing branched chains. The content of the fluorine compound is between 30% and 95% by volume relative to the total solvent components of the electrolyte. The content of the chain ether compound is between 5% and 50% by volume relative to the total amount of solvent components in the electrolyte. [Chemical Formula 1] [Chemical Formula 2] In the above formula, the wavy line represents the bonding site in the monovalent group. in, The fluorine compound is 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 ether, 1,1,2,2-tetrafluoroethyl methyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, or 2,2,3,3-tetrafluoropropyl difluoromethyl ether.

2. The lithium secondary battery according to claim 1, wherein, The chain ether compound is a compound containing two or more but no more than five ether bonds.

3. The lithium secondary battery according to claim 1 or 2, wherein, The chain ether compound has 4 to 10 carbon atoms.

4. The lithium secondary battery according to claim 1 or 2, wherein, The chain ether compound comprises at least one selected from the group consisting of 1,1-dimethoxyethane, 1,2-dimethoxypropane, 2,2-dimethoxypropane, 1,3-dimethoxybutane, 1,2-dimethoxybutane, 2,2-dimethoxybutane, 2,3-dimethoxybutane, 1,2-diethoxypropane, 1,2-diethoxybutane, 2,3-diethoxybutane, and diethoxyethane.

5. The lithium secondary battery according to claim 1 or 2, wherein, The branched chain of the chain ether compound is an unsubstituted alkyl group having 1 to 10 carbon atoms.

6. The lithium secondary battery according to claim 1 or 2, wherein, The lithium salt comprises LiN(SO2F)2.

7. The lithium secondary battery according to claim 1, wherein, The electrolyte also contains a straight-chain ether compound that does not have fluorine atoms and is not branched.

8. The lithium secondary battery according to claim 7, wherein, The linear ether compound has 3 to 10 carbon atoms.

9. The lithium secondary battery according to claim 7 or 8, wherein, The straight-chain ether compound is a compound containing two or more but no more than five ether bonds.

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