Patent Application
20240030494
The present invention relates to a lithium secondary battery.
In recent years, techniques for converting natural energy such as solar power or wind power into electric energy have attracted more attention. Accordingly, various secondary batteries have been developed as power storage devices that are safe and that can store a large amount of electric energy.
Among these, lithium secondary batteries that charge and discharge by transferring lithium ions between a positive electrode and a negative electrode are known to have a high voltage and high energy density. As a typical lithium secondary battery, a lithium ion secondary battery (LIB) that contains an active material capable of retaining a lithium element in the positive electrode and the negative electrode, and that charges and discharges by exchanging lithium ions between the positive electrode active material and the negative electrode active material is known.
For the purpose of realizing high energy density, a lithium secondary battery (lithium metal battery, LMB) that lithium metal is used as the negative electrode active material, instead of a material into which the lithium ion can be inserted, such as a carbon material, has been developed. For example, PCT Japanese Translation Patent Publication No. 2006-500755 discloses a rechargeable battery using, as a negative electrode, an electrode based on lithium metal.
For the purpose of further improving high energy density and improving productivity, or the like, a lithium secondary battery using a negative electrode that is free of a negative electrode active material such as the carbon material and the lithium metal has been developed. For example, PCT Japanese Translation Patent Publication No. 2019-505971 discloses a lithium secondary battery containing a positive electrode, a negative electrode, a separation film interposed between the electrodes, and an electrolyte, in which, in the negative electrode, metal particles are formed on a negative electrode current collector and are moved from the positive electrode by charging to form lithium metal on the negative electrode current collector in the negative electrode. PCT Japanese Translation Patent Publication No. 2019-505971 discloses that such a lithium secondary battery shows the possibility of providing a lithium secondary battery which has overcome the problem due to the reactivity of the lithium metal and the problem caused during assembly process and therefore has improved performance and service life.
When the present inventors studied batteries of the prior art including those described in the PCT Japanese Translation Patent Publication No. 2006-500755 and PCT Japanese Translation Patent Publication No. 2019-505971 mentioned above, they found that at least one of energy density or cycle characteristics was insufficient.
For example, in the lithium secondary battery that includes a negative electrode containing the negative electrode active material, due to the volume or mass occupied by the negative electrode active material, it is difficult to sufficiently increase the energy density and a capacity. In addition, even in an anode free lithium secondary battery of the prior art, that includes a negative electrode not containing a negative electrode active material, due to repeated charge and discharge, a dendritic lithium metal is likely to be formed on a surface of the negative electrode, which is likely to cause short circuiting and a decrease in capacity, resulting in insufficient cycle characteristics.
With regard to the anode free lithium secondary battery, a method has also been developed of applying a large amount of physical pressure to the battery to keep an interface between the negative electrode and a separator under high pressure, so that uneven growth is suppressed during lithium metal deposition. However, because a large mechanical mechanism is required to apply such a high level of pressure, the weight and volume of the battery increase as a whole, and the energy density decreases.
In view of these problems, it is an object of the present invention to provide a lithium secondary battery having high energy density and excellent cycle characteristics.
A lithium secondary battery according to one aspect of the present invention includes a positive electrode, a separator, a negative electrode that is free of a negative electrode active material, and an electrolytic solution. The electrolytic solution contains a fluorine solvent represented by Chemical Formulae (1) to (4).
(In the formulae, each of R10 and R20 independently represents any of a C1 to C8 alkyl group, a cycloalkyl group, an aryl group, a C1 to C8 alkyl group that is fully or partially fluorinated, a cycloalkyl group that is fully or partially fluorinated, or an aryl group that is fully or partially fluorinated.)
Such a lithium secondary battery equipped with a negative electrode that is free of a negative electrode active material has a high energy density because a lithium metal deposits on the surface of the negative electrode and charge and discharge are performed by electrolytically dissolving the resulting deposited lithium metal.
The present inventors have found that, in a lithium secondary battery that contains, in the electrolytic solution, a fluorine solvent represented by Chemical Formulae (1) to (4) described above, a solid electrolyte interphase layer (“SEI layer”) is easily formed on the surface of the negative electrode. Because the SEI layer has ionic conductivity, reactivity of lithium deposition reaction on the surface of the negative electrode, on which the SEI layer is formed, is uniform in a planar direction of the surface of the negative electrode. Therefore, in the lithium secondary battery, the growth of the dendritic lithium metal on the negative electrode is suppressed, and the cycle characteristic is excellent.
The present invention makes it possible to provide a lithium secondary battery having a high energy density and an excellent cycle characteristic.
Embodiments of the present invention (“embodiments”) will now be described with reference to the drawings when necessary. In the drawings, identical elements are designated by the same reference numbers, and redundant descriptions of these elements have been omitted. Positional relationships such as up, down, left, and right are based on the positional relationship shown in the drawings unless otherwise specified. The dimensional ratios shown in the drawings are not limited to the depicted ratios.
Lithium Secondary Battery
Hereinafter, each configuration of the lithium secondary battery 100 will be described.
Negative Electrode
The negative electrode 130 is free of a negative electrode active material. In the present specification, the “negative electrode active material” is a substance that causes an electrode reaction, that is, an oxidation reaction and a reduction reaction at the negative electrode. Specifically, examples of the negative electrode active material include lithium metal and a host material for a lithium element (lithium ions or lithium metal). The host material for the lithium element means a material provided to retain the lithium ions or the lithium metal in the negative electrode. Examples of retaining mechanisms include, but are not limited to, intercalation, alloying, and occlusion of metallic clusters, and intercalation is typically used.
In the lithium secondary battery in the embodiment, because the negative electrode is free of a negative electrode active material before initial charging of the battery, charge and discharge are performed by depositing lithium metal on the negative electrode and electrolytically dissolving the deposited lithium metal. Therefore, in the lithium secondary battery in the embodiment, the volume occupied by the negative electrode active material and the mass of the negative electrode active material are reduced as compared with a lithium secondary battery containing the negative electrode active material, and the volume and mass of the entire battery are small, so that the energy density is high in principle.
In the lithium secondary battery in the embodiment, the negative electrode does not contain a negative electrode active material before initial charging of the battery, lithium metal is deposited on the negative electrode when the battery is charged, and the deposited lithium metal is electrolytically dissolved when the battery is discharged. Therefore, in the lithium secondary battery in the embodiment, the negative electrode is substantially free of the negative electrode active material even at the end of discharging of the battery. Therefore, in the lithium secondary battery in the embodiment, the negative electrode acts as a negative electrode current collector.
In a case where the lithium secondary battery in the embodiment is compared with a lithium ion battery (LIB) and a lithium metal battery (LMB), the following points are different.
In the lithium ion battery (LIB), a negative electrode contains a host material for a lithium element (lithium ions or lithium metal), this material is filled with the lithium element when the battery is charged, and the host material releases the lithium element, thereby discharging the battery. The LIB is different from the lithium secondary battery in the embodiment in that the negative electrode contains the host material for the lithium element.
The lithium metal battery (LMB) is produced by using, as a negative electrode, an electrode having lithium metal on its surface or a single lithium metal. That is, the LMB is different from the lithium secondary battery in the embodiment in that the negative electrode contains the lithium metal, that is the negative electrode active material, immediately after assembling the battery, that is, before initial charging of the battery. The LMB uses the electrode containing lithium metal having high flammability and reactivity in its production. However, because the lithium secondary battery in the embodiment uses the negative electrode free of lithium metal, the lithium secondary battery in the embodiment is more safe and productive.
In the present specification, the negative electrode “free of a negative electrode active material” means the negative electrode does not contain the negative electrode active material or does not substantially contain the negative electrode active material. The fact that the negative electrode does not substantially contain the negative electrode active material means the amount of the negative electrode active material in the negative electrode is 10% by mass or less relative to the overall mass of the negative electrode. The amount of negative electrode active material in the negative electrode relative to the overall mass of the negative electrode is preferably 5.0% by mass or less relative to the overall mass of the negative electrode, and may be 1.0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less. Since the negative electrode does not contain the negative electrode active material or the amount of the negative electrode active material in the negative electrode is within the above-described range, the energy density of the lithium secondary battery 100 is high.
In the present specification, the “lithium metal is deposited on the negative electrode” means the lithium metal is deposited on the surface of the negative electrode or on at least one surface of a solid electrolyte interphase (SEI) layer formed on the surface of the negative electrode, which will be described later. For example, in
In the present specification, the “before initial charging” of the battery means a state from the time when the battery is assembled to the time when the battery is first charged. In addition, “at the end of discharging” of the battery means a state in which the battery voltage is 1.0 V or more and 3.8 V or less, preferably 1.0 V or more and 3.0 V or less.
In the present specification, the “lithium secondary battery including a negative electrode free of a negative electrode active material” means the negative electrode is free of the negative electrode active material before initial charging of the battery or at the end of discharging of the battery. Therefore, the phrase “negative electrode free of a negative electrode active material” may be rephrased as “negative electrode free of a negative electrode active material before initial charging of the battery or at the end of discharging of the battery”, “negative electrode that does not contain a negative electrode active material other than lithium metal regardless of the state of charge of the battery and does not contain the lithium metal before initial charging or at the end of discharging”, “negative electrode current collector free of lithium metal before initial charging or at the end of discharging”, or the like. In addition, the “lithium secondary battery including a negative electrode free of a negative electrode active material” may be rephrased as an anode free lithium battery, a zero anode lithium battery, or an anodeless lithium battery.
In the negative electrode in the embodiment, regardless of the state of charge of the battery, the amount of the negative electrode active material other than lithium metal is 10% by mass or less relative to the overall mass of the negative electrode, preferably 5.0% by mass or less, and may be 1.0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less.
In addition, in the negative electrode in the embodiment, before initial charging or at the end of discharging, the amount of lithium metal is 10% by mass or less relative to the overall mass of the negative electrode, preferably 5.0% by mass or less, and may be 1.0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less. In the negative electrode, before initial charging or at the end of discharging, the amount of lithium metal is preferably 10% by mass or less relative to the overall mass of the negative electrode (among these, the amount of lithium metal is preferably 5.0% by mass or less relative to the overall mass of the negative electrode, and may be 1.0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less).
In the lithium secondary battery in the embodiment, in a case where the battery voltage is 1.0 V or more and 3.5 V or less, the amount of lithium metal may be 10% by mass or less relative to the overall mass of the negative electrode (preferably 5.0% by mass or less, and may be 1.0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less); in a case where the battery voltage is 1.0 V or more and 3.0 V or less, the amount of lithium metal may be 10% by mass or less relative to the overall mass of the negative electrode (preferably 5.0% by mass or less, and may be 1.0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less); or in a case where the battery voltage is 1.0 V or more and 2.5 V or less, the amount of lithium metal may be 10% by mass or less relative to the overall mass of the negative electrode (preferably 5.0% by mass or less, and may be 1.0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less).
In the lithium secondary battery in the embodiment, a ratio M3.0/M4.2 of a mass M3.0 of lithium metal deposited on the negative electrode in a state in which the battery voltage is 3.0 V to a mass M4.2 of lithium metal deposited on the negative electrode in a state in which the battery voltage is 4.2 V is preferably 20% or less, more preferably 15% or less, and even more preferably 10% or less. The ratio M3.0/M4.2 may be 8.0% or less, 5.0% or less, 3.0% or less, or 1.0% or less.
Examples of the negative electrode active material in the embodiment include lithium metal, alloys containing lithium metal, carbon-based substances, metal oxides, metals alloyed with lithium, and alloys containing the metals. The carbon-based substance is not particularly limited and examples thereof include graphene, graphite, hard carbon, mesoporous carbon, carbon nanotube, and carbon nanohorn. Examples of metal oxides include, but are not limited to, titanium oxide-based compounds, tin oxide-based compounds, and cobalt oxide-based compounds. Examples of metals alloyed with lithium include silicon, germanium, tin, lead, aluminum, and gallium.
There are no particular restrictions on the negative electrode in the embodiment as long as it does not contain a negative electrode active material and can be used as a current collector. Examples of the negative electrode in the embodiment include at least one selected from the group consisting of metals such as Cu, Ni, Ti, Fe, and other metals that do not react with Li, alloys of these metals, and stainless steel (SUS). When SUS is used as the negative electrode, any well-known type of SUS can be used. The negative electrode materials mentioned above may be either singly or in combinations of two or more. In the present specification, the “metal that does not react with Li” refers to a metal that does not react with lithium ions or lithium metal to form an alloy under the operating conditions of the lithium secondary battery.
The negative electrode in the embodiment is preferably at least one selected from the group consisting of Cu, Ni, Ti, Fe, alloys of these metals, and stainless steel (SUS), and more preferably at least one selected from the group consisting of Cu, Ni, alloys of these metals, and stainless steel (SUS). The negative electrode is even more preferably Cu, Ni, alloys of these metals, or stainless steel (SUS). When this negative electrode is used, the energy density and productivity of the battery tend to be further improved.
The average thickness of the negative electrode in the embodiment 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. Because the volume occupied by the negative electrode in the battery is reduced in this aspect of the present invention, the energy density of the battery is further improved.
Positive Electrode
The positive electrode 120 is not particularly limited insofar as it has a positive electrode active material and is a positive electrode commonly used in a lithium secondary battery, and a known material can be selected as needed, depending on the use of the lithium secondary battery. Because the positive electrode 120 has a positive electrode active material, the stability and the output voltage are high.
In the present specification, the “positive electrode active material” is a substance that causes an electrode reaction, that is, an oxidation reaction and a reduction reaction at the positive electrode. Specifically, examples of the positive electrode active material in the embodiment include a host material for a lithium element (typically, lithium ions).
Examples of positive electrode active materials include, but are not limited to, metal oxides and metal phosphates. The metal oxides are not particularly limited and examples thereof include cobalt oxide-based compounds, manganese oxide-based compounds, and nickel oxide-based compounds. Examples of metal phosphates include, but are not limited to, iron phosphate-based compounds and cobalt phosphate-based compounds. Typical examples of positive electrode active materials include LiCoO2, LiNixCoyMnzO (x+y+z=1), LiNixMnyO (x+y=1), LiNiO2, LiMn2O4, LiFePO, LiCoPO, LiFeOF, LiNiOF, and TiS2. The positive electrode active materials mentioned above may be used either singly or in combinations of two or more.
The positive electrode 120 may contain components other than the positive electrode active material. Such a component is not particularly limited, and examples thereof include known conductive aids, binders, polymer electrolytes, and inorganic solid electrolytes.
Examples of conductive aids that can be used in the positive electrode 120 include, but are not limited to, carbon black, single-wall carbon nanotubes (SWCNT), multi-wall carbon nanotubes (MWCNT), carbon nanofibers (CF), and acetylene black. Examples of binders include, but are not limited to, polyvinylidene fluoride, polytetrafluoroethylene, styrene butadiene rubber, acrylic resins, and polyimide resins.
The amount of the positive electrode active material in the positive electrode 120 may be, for example, 50% by mass or more and 100% by mass or less relative to the overall mass of the positive electrode 120. The amount of the conductive aid in the positive electrode 120 may be, for example, 0.5% by mass or more and 30% by mass or less relative to the overall mass of the positive electrode 120. The amount of the binder in the total amount of the positive electrode 120 may be, for example, 0.5% by mass or more and 30% by mass or less. The total amount of the solid polymer electrolyte and the inorganic solid electrolyte may be, for example, 0.5% by mass or more and 30% by mass or less relative to the overall mass of the positive electrode 120.
Positive Electrode Current Collector
The positive electrode current collector 110 is disposed on one side of the positive electrode 120. The positive electrode current collector 110 is not particularly limited as long as it is a conductor that does not react with lithium ions in the battery. Examples of such a positive electrode current collector include aluminum.
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 aspect of the present invention, because the volume occupied by the positive electrode current collector 110 in the lithium secondary battery 100 is reduced, the energy density of the lithium secondary battery 100 is further improved.
Separator
The separator 140 is the component that separates the positive electrode 120 and the negative electrode 130 to prevent short circuiting of the battery, while maintaining conductivity of the lithium ions serving as the charge carrier between the positive electrode 120 and the negative electrode 130. The separator 140 consists of a material that does not have electronic conductivity and that does not react with lithium ions. The separator 140 also plays a role in retaining the electrolytic solution. Although the material itself constituting the separator does not have ionic conductivity, the separator retains the electrolytic solution so that the lithium ions are conducted through the electrolytic solution. There are no particular restrictions on the separator 140 as long as it can play this role. The separator 140 can be composed of a porous organic film, preferably composed of a porous polymer film, for example, a polyethylene (PE) film, a polypropylene (PP) film, or a laminated structure thereof.
The separator 140 may be coated with a separator coating layer. The separator coating layer can be applied to one or both sides of the separator 140. The separator coating layer is not particularly limited as long as it does not react with lithium ions. The separator coating layer preferably bonds the separator 140 to the adjacent layer firmly. Examples of such a separator coating layer include, but are not limited to, for example, a layer containing a binder such as polyvinylidene fluoride (PVDF), styrene butadiene rubber and carboxymethyl cellulose (SBR-CMC) mixtures, polyacrylic acid (PAA), lithium polyacrylate (Li-PAA), polyimide (PI), polyamideimide (PAI), and aramids. The separator coating layer may contain inorganic particles such as silica, alumina, titania, zirconia, magnesium oxide, magnesium hydroxide, and lithium nitrate in the above-described binder. The separator 140 may be a separator having no separator coating layer, or a separator having the separator coating layer.
The average thickness of the separator 140 is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less. Because the volume occupied by the separator 140 in the lithium secondary battery 100 is reduced in this aspect of the present invention, the energy density of the lithium secondary battery 100 is further improved. The average thickness of the separator 140 is also preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more. In this aspect of the present invention, the positive electrode 120 and the negative electrode 130 can be separated more reliably, and short circuiting of the battery can be further suppressed.
Electrolytic Solution
The lithium secondary battery 100 contains an electrolytic solution. In the lithium secondary battery 100, the separator 140 may be impregnated with the electrolytic solution or the electrolytic solution may be sealed together with a laminate of the positive electrode current collector 110, the positive electrode 120, the separator 140, and the negative electrode 130 inside a sealed container. The electrolytic solution is a solution that contains an electrolyte and a solvent and has ionic conductivity, and acts as a conductive path for lithium ions. Therefore, in the aspect including the electrolytic solution, an internal resistance of the battery is further reduced, and the energy density, capacity, and cycle characteristics are further improved.
In an anode free lithium secondary battery containing the electrolytic solution, a solid electrolyte interphase layer (SEI layer) is formed on a surface of the negative electrode or the like by decomposing the solvent or the like in the electrolytic solution. Due to the SEI layer in the lithium secondary battery, further decomposition of components in the electrolytic solution, irreversible reduction of lithium ions caused by the decomposition, generation of gas, and the like are suppressed. In addition, because the SEI layer has ionic conductivity, reactivity of lithium metal deposition reaction on the surface of the negative electrode, on which the SEI layer is formed, is uniform in a planar direction of the surface of the negative electrode. When a specific fluorine solvent is used in the lithium secondary battery 100, the SEI layer is easily formed on the surface of the negative electrode, and the growth of dendritic lithium metal on the negative electrode is further suppressed. As a result, the cycle characteristics tend to be further improved.
In the present specification, a compound “contained as a solvent” means, in the usage environment of lithium secondary batteries, it is sufficient that the compound alone or a mixture of the compound with other compounds is a liquid, and furthermore, it is sufficient that the electrolyte can be dissolved to form an electrolytic solution in a solution phase.
In the embodiment, the electrolytic solution contains a fluorine solvent represented by Chemical Formulae (1) to (4).
In the formulae, each of R10 and R20 independently represents any of a C1 to C8 alkyl group, a cycloalkyl group, an aryl group, a C1 to C8 alkyl group that is fully or partially fluorinated, a cycloalkyl group that is fully or partially fluorinated, or an aryl group that is fully or partially fluorinated.
R10 is preferably a C1 to C6 alkyl group, a cycloalkyl group, an aryl group, or a C1 to C6 alkyl group that is fully or partially fluorinated, and it preferably includes one or more CF3 groups and more preferably includes one to three CF3 groups. R10 is preferably selected from a methyl group, an ethyl group, an n-propyl group, or a 2-propyl group. R10 may be selected from a methyl group that is fully or partially fluorinated, an ethyl group that is fully or partially fluorinated, an n-propyl group that is fully or partially fluorinated, or a 2-propyl group that is fully or partially fluorinated.
R20 is preferably a C1 to C6 alkyl group, a cycloalkyl group, an aryl group, or a C1 to C6 alkyl group that is fully or partially fluorinated, and it preferably includes one or more CF3 groups and more preferably includes one to three CF3 groups. R20 may be trifluoroethyl or hexafluoroisopropyl.
As the fluorinated ether solvent of Chemical Formula (1), any of compounds represented by Chemical Formulae (11) to (15) can be used. As the fluorinated ether solvent of Chemical Formula (1), a fluorinated ether solvent having a branched chain is particularly preferable, and for example, a fluorinated ether solvent represented by Chemical Formula (11) or (12) below is preferably used.
In the fluorinated diether solvent of Chemical Formula (2), R10 and R20 may be the same. As the fluorinated diether solvent of Chemical Formula (2), any of fluorinated diether solvents represented by Chemical Formulae (21) and (22) can be used.
As the fluorinated ester solvent of Chemical Formula (3), any of fluorinated ester solvents represented by Chemical Formulae (31) to (34) can be used.
As the fluorine solvent of Chemical Formula (4), any of fluorinated carbonate solvents represented by Chemical Formulae (41) to (44) can be used.
The electrolytic solution preferably further contains a fluorine solvent consisting of a fluorinated alkyl compound having at least one of a group represented by Formula (A) or Formula (B).
(In the formulae, a wavy line represents a bonding site in a monovalent group.)
Examples of such a fluorinated alkyl compound include a compound having an ether bond (which will hereinafter be called “ether compound”), a compound having an ester bond, and a compound having a carbonate bond. From the standpoint of further improving solubility of the electrolyte in the electrolytic solution and from the standpoint that the SEI layer is more easily formed, the fluorinated alkyl compound is preferably an ether compound.
Examples of the ether compound as the fluorinated alkyl compound include an ether compound having both the monovalent group represented by Formula (A) and the monovalent group represented by Formula (B) (which will hereinafter also be called “fluorine solvent AB”), an ether compound that has the monovalent group represented by Formula (A) and does not have the monovalent group represented by Formula (B) (which will hereinafter also be called “fluorine solvent A”), and an ether compound that does not have the monovalent group represented by Formula (A) and has the monovalent group represented by Formula (B) (which will hereinafter also be called “fluorine solvent B”).
Examples of the fluorine solvent AB include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTFE), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl diethoxymethane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl diethoxypropane. From the standpoint of effectively and reliably exhibiting the effects of fluorinated alkyl compound mentioned above, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is preferable as the fluorine solvent AB.
Examples of the fluorine solvent A include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFEE), methyl-1,1,2,2-tetrafluoroethyl ether, ethyl-1,1,2,2-tetrafluoroethyl ether, propyl-1,1,2,2-tetrafluoroethyl ether, 1H,1H,5H-perfluoropentyl-1,1,2,2-tetrafluoroethyl ether, and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether. From the standpoint of effectively and reliably exhibiting the effects of fluorinated alkyl compound mentioned above, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, methyl-1,1,2,2-tetrafluoroethyl ether, ethyl-1,1,2,2-tetrafluoroethyl ether, or 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether is preferable as the fluorine solvent A.
Examples of the fluorine solvent B include difluoromethyl-2,2,3,3-tetrafluoropropyl ether, trifluoromethyl-2,2,3,3-tetrafluoropropyl ether, fluoromethyl-2,2,3,3-tetrafluoropropyl ether, and methyl-2,2,3,3-tetrafluoropropyl ether. From the standpoint of effectively and reliably exhibiting the effects of fluorinated alkyl compound mentioned above, difluoromethyl-2,2,3,3-tetrafluoropropyl ether is preferable as the fluorine solvent B.
The electrolytic solution may contain other fluorine solvents. Examples of these fluorine solvents include methyl nonafluorobutyl ether, ethyl nonafluorobutyl ether, 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-trifluoromethylpentane, methyl-2,2,3,3,3-pentafluoropropyl ether, 1,1,2,3,3,3-hexafluoropropylmethyl ether, ethyl-1,1,2,3,3,3-hexafluoropropyl ether, and tetrafluoroethyltetrafluoropropyl ether.
The fluorine solvents mentioned above may be used either singly or in combinations of two or more.
The amount of the fluorine solvent described above (total of fluorine solvents) is not particularly limited, but is 5% by volume or more, 10% by volume or more, 20% by volume or more, or 30% by volume or more relative to the overall amount of solvent components in the electrolytic solution, preferably 40% by volume or more, more preferably 50% by volume or more, even more preferably 60% by volume or more, and still more preferably 70% by volume or more. When the amount of the fluorine solvent is within the above-described range, the SEI layer is more easily formed, so that the cycle characteristics of the battery tend to be further improved. The upper limit of the amount of the fluorine solvent is not particularly limited, and the amount of the fluorine solvent may be 100% by volume or less, 95% by volume or less, 90% by volume or less, or 80% by volume or less relative to the overall amount of the solvent components in the electrolytic solution.
The proportion when the fluorine solvent represented by Chemical Formulae (1) to (4) and the fluorine solvent consisting of a compound having at least one of a monovalent group represented by Formula (A) or a monovalent group represented by Formula (B) are combined is 1:10 to 10:1, preferably 1:5 to 5:1, 1:3 to 3:1, 1:2 to 2:1, or 1:1.
It is preferable that the electrolytic solution is a non-aqueous electrolytic solution and the solvent includes a non-fluorine solvent, and it is more preferable to include an ether solvent as the non-fluorine solvent, and it is still more preferable to include a polyether solvent. Examples of such a non-fluorine solvent include, but are not limited to, dimethoxyethane (DME), 1,2-dimethoxypropane (DMP), triethylene glycol dimethyl ether (TGM), diethylene glycol dimethyl ether (DGM), tetraethylene glycol dimethyl ether (TetGM), dimethyl ether, acetonitrile, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, chloroethylene carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, trimethyl phosphate, and triethyl phosphate.
The amount of the non-fluorine solvent (total of non-fluorine solvents) is not particularly limited, but is 5% by volume or more relative to the overall amount of the solvent components in the electrolytic solution, preferably 10% by volume or more, more preferably 15% by volume or more, and even more preferably 20% by volume or more, and preferably 30% by volume or less.
There are no particular restrictions on the electrolyte which is contained in the electrolytic solution insofar as it is a salt. Examples of the electrolyte include salts of Li, Na, K, Ca, and Mg. As the electrolyte, a lithium salt is preferred. The lithium salt is not particularly limited, and LiN(SO2F)2 (FSI) can be used. In addition to FSI, examples of the lithium salt include LiI, LiCl, LiBr, LiF, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2CF3)2, LiN(SO2CF3CF3)2, LiBF2(C2O4), LiB(C2O4)2, LiB(O2C2H4)2, LiB(O2C2H4)F2, LiB(OCOCF3)4, LiNO3, Li2SO4, Li[PF2(C2O4)2] (lithium difluorobisoxarate phosphate; LiDODFP), and LiPO2F2. The lithium salts mentioned above may be used either singly or in combinations of two or more.
The concentration of the electrolyte in the electrolytic solution is not particularly limited, but is preferably 0.2 M or more, more preferably 0.7 M or more, still more preferably 0.9 M or more, and even more preferably 1.0 M or more. When the concentration of the electrolyte is within the above-described range, the SEI layer is more easily formed and the internal resistance tends to be further reduced. The upper limit of the concentration of the electrolyte is not particularly limited, and the concentration of the electrolyte may be 10.0 M or less, 5.0 M or less, or 2.5 M or less.
Use of Lithium Secondary Battery
The lithium secondary battery 200 is charged by applying voltage between the positive electrode terminal 220 and the negative electrode terminal 210 so that current flows from the negative electrode terminal 210 to the positive electrode terminal 220 via the external circuit. By charging the lithium secondary battery 200, the lithium metal is deposited on the negative electrode.
In the lithium secondary battery 200, the solid electrolyte interphase layer (SEI layer) may be formed on the surface of the negative electrode 130 (interface between the negative electrode 130 and the separator 140) by a first charging (initial charging) after the battery is assembled. The SEI layer to be formed is not particularly limited, but it may contain, for example, a lithium-containing inorganic compound or a lithium-containing organic compound. The typical average thickness of the SEI layer is 1 nm or more and 10 μm or less.
When the positive electrode terminal 220 and the negative electrode terminal 210 are connected to the charged lithium secondary battery 200, the lithium secondary battery 200 is discharged. As a result, the deposition of the lithium metal generated on the negative electrode is electrolytically dissolved.
Lithium Secondary Battery Production Method
There are no particular restrictions on the method used to produce the lithium secondary battery 100 shown in
First, the positive electrode 120 is prepared by a known production method or by purchasing a commercially available one. The positive electrode 120 may be produced in the following manner. Such a positive electrode active material as mentioned above, a known conductive aid, and a known binder are mixed together to obtain a positive electrode mixture. The mixing ratio may be, for example, 50% by mass or more and 99% by mass or less of the positive electrode active material, 0.5% by mass or more and 30% by mass or less of the conductive aid, and 0.5% by mass or more and 30% by mass or less of the binder relative to the entire mass of the positive electrode mixture. The resulting positive electrode mixture is applied to one side of a metal foil (for example, Al foil) as a positive electrode current collector, that has a predetermined thickness (for example, 5 μm or more and 1 mm or less), and then press-molded. The resulting molded material is punched to a predetermined size to obtain a positive electrode 120 formed on a positive electrode current collector 110.
Next, the negative electrode 130 can be prepared by washing the negative electrode material described above, such as a metal foil (for example, an electrolytic Cu foil) having a thickness of 1 μm or more and 1 mm or less, with a solvent containing sulfamic acid.
Next, the separator 140 with the configuration described above is prepared. The separator 140 may be produced using any method common in the art, or a commercially available one may be used. The electrolytic solution may be prepared by dissolving the above-described electrolyte (typically, a lithium salt) in the above-described solvent.
The positive electrode current collector 110 on which the positive electrode 120 is formed, the separator 140, and the negative electrode 130 described above are laminated in this order to obtain a laminate as shown in
In the present specification, “the energy density is high” and “high energy density” mean the capacity is high relative to the total volume or total mass of the battery. This is preferably 800 Wh/L or more or 350 Wh/kg or more, more preferably 900 Wh/L or more or 400 Wh/kg or more, and even more preferably 1000 Wh/L or more or 450 Wh/kg or more.
In the present specification, “excellent cycle characteristics” means that the rate of decline in battery capacity is low before and after the number of times of charge and discharge cycles that can be expected during normal use. In other words, when comparing a first discharge capacity after initial charging to a discharge capacity after the number of times of charge and discharge cycles that can be expected during normal use, the discharge capacity after the charge and discharge cycles has not declined significantly relative to the first discharge capacity after the initial charging. Here, “the number of times that can be expected during normal use” can be, for example, 20 times, 30 times, 50 times, 70 times, 100 times, 300 times, or 500 times, depending on the application for the lithium secondary battery. In addition, the “discharge capacity after the charge and discharge cycles not declining significantly relative to the first discharge capacity after the initial charging” depends on the application for the lithium secondary battery. For example, it may be 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, or 85% or more of the first discharge capacity after the initial charging.
The embodiments described above are provided merely to explain the present invention and are not intended to limit the present invention to the embodiments. Various modifications are possible without departing from the scope and spirit of the present invention. For example, an additional functional layer may be inserted between the laminate of the positive electrode current collector, the positive electrode, the separator, and the negative electrode.
The following is a more detailed description of the present invention with reference to examples and comparative examples. The present invention is not limited in any way by these examples.
A lithium secondary battery was produced as follows.
First, an electrolytic Cu foil having a thickness of 8 μm was washed with a solvent containing sulfamic acid, and then washed with water. Subsequently, the electrolytic Cu foil was immersed in a solution containing 1H-benzotriazole as a negative electrode coating agent, dried, and further washed with water to obtain a Cu foil coated with the negative electrode coating agent. The resulting Cu foil was punched to a predetermined size (45 mm×45 mm) to obtain a negative electrode.
As a separator, a separator having a thickness of 16 μm and a predetermined size (50 mm×50 mm), in which both sides of a 12 μm-thick polyethylene microporous film were coated with a 2 μm-thick polyvinylidene fluoride (PVdF), was prepared.
A positive electrode was produced as follows. A mixture of 96 parts by mass LiNi0.85CO0.12Al0.03O2 positive electrode active material, 2 parts by mass carbon black conductive aid, and 2 parts by mass polyvinylidene fluoride (PVdF) binder was applied to one side of 12 μm-thick Al foil positive electrode current collector, and press-molded. The resulting molded material was punched to a predetermined size (40 mm×40 mm) to obtain a positive electrode formed on the positive electrode current collector.
LiN(SO2F)2 (FSI) as a lithium salt was dissolved in a mixed solvent of a fluorine solvent and a non-fluorine solvent to prepare an electrolytic solution including 1.2 M FSI solution. Table 1 of
The positive electrode current collector on which the positive electrode was formed, the separator, and the negative electrode obtained as described above were laminated in this order such that the positive electrode faced the separator to obtain a laminate. A 100 μm-thick Al terminal and a 100 μm-thick Ni terminal were connected to the positive electrode current collector and the negative electrode by ultrasonic welding, respectively, and then the laminate was inserted into a laminated outer casing. Next, the electrolytic solution prepared as described above was injected into the outer casing. The outer casing was then sealed to obtain a lithium secondary battery.
Evaluation of Cycle Characteristics
The cycle characteristics of the lithium secondary batteries produced in the examples were evaluated as follows.
With respect to the produced lithium secondary battery (cell at 25° C. and 32 mAh), a cycle of CC-charging at a charging rate of 0.1 C and then CC-discharging at a discharging rate of 0.3 C was repeated at a temperature of 25° C. For the examples, the number of cycles (referred to as “Number of cycles” in the table) when the discharge capacity reached 80% of the initial capacity is shown in Table 1 of
In Table 1 of
As Comparative Example 1, a lithium secondary battery was produced in the same manner as in Examples, except that LiPF6 as a lithium salt was dissolved in a mixed solvent of ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (30% by volume/70% by volume), and an electrolytic solution containing 1 M LiPF6 solution was used, and the cycle characteristics of the lithium secondary battery was evaluated. The number of cycles of Comparative Example 1 was 2 times.
As Comparative Example 2, a lithium secondary battery was produced in the same manner as in Examples, except that LiN(SO2F)2 (FSI) as a lithium salt was dissolved in a mixed solvent of ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (30% by volume/70% by volume), and an electrolytic solution containing 1 M FSI solution was used, and the cycle characteristics of the lithium secondary battery was evaluated. The number of cycles of Comparative Example 2 was 3 times.
As shown in Table 1 of
Next, electrolytic solutions were prepared by using a mixed solvent that contained a mixture of the fluorinated ether solvent and other fluorine solvents and contained a mixture of the non-fluorine solvents, and the cycle characteristics of lithium secondary batteries with these electrolytic solutions were evaluated in the same manner as described above. Table 2 of
In Table 2 of
Next, electrolytic solutions were prepared by changing the proportion (capacity ratio) of the solvents in the mixed system of the fluorine solvent, and the cycle characteristics of lithium secondary batteries with these electrolytic solutions were evaluated in the same manner as described above. Table 3 of
As shown in Table 3 of
The lithium secondary battery of the present invention has a high energy density and an excellent cycle characteristic so that it has industrial applicability as a power storage device to be used for various uses.
This application is a continuation of International Application No. PCT/JP2021/014610, entitled “LITHIUM SECONDARY BATTERY”, filed on Apr. 6, 2021, the entire contents of which are incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP21/14610 | Apr 2021 | US |
| Child | 18480013 | US |
