Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery which has a high energy density and excellent cycle characteristics. The present invention relates to a lithium secondary battery which is provided with: a positive electrode; a negative electrode that does not comprise a negative electrode active material; a separator that is arranged between the positive electrode and the negative electrode; a carbon metal composite layer that is formed on a surface of the negative electrode, the surface facing the separator; and a conductive thin film that is formed on a surface of the separator, the surface facing the negative electrode. With respect to this lithium secondary battery, the carbon metal composite layer contains a plurality of fibrous carbon materials which are randomly oriented.

Description

Technical Field

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

Background Art

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

[0003] Among them, a lithium secondary battery that performs charge and discharge by the movement of lithium ions between a positive electrode and a negative electrode is known to exhibit a high voltage and a high energy density. As a typical lithium secondary battery, there is known a lithium ion secondary battery having active materials capable of holding lithium elements in a positive electrode and a negative electrode, and performing charge and discharge by the transfer of lithium ions between the positive electrode active material and the negative electrode active material.

[0004] In addition, for the purpose of realizing a higher energy density, a lithium secondary battery using lithium metal has been developed in place of a material such as a carbon-based material into which lithium elements can be inserted as a negative electrode active material. For example, Patent Document 1 discloses a lithium secondary battery including an ultrathin lithium metal anode in order to achieve a volume energy density exceeding 1000 Wh / L and / or a mass energy density exceeding 350 Wh / kg during discharge at a rate of at least 1C at room temperature. Patent Document 1 discloses that in such a lithium secondary battery, charging is performed by direct precipitation of additional lithium metal on the lithium metal as the negative electrode active material.

[0005] In addition, for the purpose of further increasing the energy density and improving productivity, etc., lithium secondary batteries that do not use a negative electrode active material have been developed. For example, Patent Document 2 discloses a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed therebetween, and an electrolyte, wherein the negative electrode has metal particles formed on a negative electrode current collector and lithium metal is formed on the negative electrode current collector in the negative electrode by being moved from the positive electrode during charging. Patent Document 2 discloses that such a lithium secondary battery can solve problems due to the reactivity of lithium metal and problems occurring during the assembly process, and can provide a lithium secondary battery with improved performance and lifespan.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, when the present inventors carefully studied conventional batteries including those described in the above patent documents, it was found that at least one of the energy density and cycle characteristics was insufficient.

[0008] For example, in a lithium secondary battery including a negative electrode having a negative electrode active material, it is difficult to make the energy density and capacity sufficiently high due to the volume and mass occupied by the negative electrode active material. Also, for an anode-free type lithium secondary battery including a negative electrode without a negative electrode active material, in conventional ones, dendritic lithium metal is likely to be formed on the negative electrode surface by repeating charge and discharge, and short circuits and capacity degradation are likely to occur, so the cycle characteristics are insufficient.

[0009] In addition, in an anode-free type lithium secondary battery, a method has been developed to apply a large physical pressure to the battery to keep the interface between the negative electrode and the separator at a high pressure in order to suppress discrete (non-uniform) growth during lithium metal deposition. However, since such application of high pressure requires a large mechanical mechanism, the overall battery has a large mass and volume, and the energy density decreases.

[0010] The present invention has been made in view of the above problems, and an object thereof is to provide a lithium secondary battery having a high energy density and excellent cycle characteristics.

Means for Solving the Problems

[0011] A lithium secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode having no negative electrode active material, a separator disposed between the positive electrode and the negative electrode, a carbon-metal composite layer formed on a surface of the negative electrode facing the separator, and a conductive thin film formed on a surface of the separator facing the negative electrode, and the carbon-metal composite layer includes a plurality of fibrous carbon materials randomly oriented.

[0012] Since such a lithium secondary battery has no negative electrode active material, compared with a lithium secondary battery having a negative electrode active material, the volume and mass of the entire battery are small, and the energy density is inherently high. Such a battery is charged and discharged by lithium metal depositing on the surface of the negative electrode and the deposited lithium metal being electrochemically dissolved.

[0013] In addition, such a carbon-metal composite layer has a high and uniform electrical conductivity due to the fibrous carbon materials being intertwined with each other to form a three-dimensional network structure, and can make the potential on the negative electrode surface uniform. Furthermore, since the carbon-metal composite layer as a whole has carbon materials that can serve as the starting points for lithium metal precipitation, there are more starting points for lithium metal precipitation than in the case of a negative electrode that is a metal electrode, and non-uniform growth of lithium metal in the lithium secondary battery is suppressed. Also, in the above lithium secondary battery, by providing a conductive thin film on the surface of the separator facing the negative electrode, a potential is applied to the deposited lithium metal from both the negative electrode side and the conductive thin film side. Therefore, in such a lithium secondary battery, non-uniform reactions of lithium metal are further suppressed, and uniform lithium metal is likely to precipitate on the negative electrode surface. That is, the growth of dendritic lithium metal on the negative electrode is suppressed, and the cycle characteristics of the lithium secondary battery become excellent.

[0014] Instead of the above separator, a solid electrolyte may be used. According to such an embodiment, since the lithium secondary battery can be made into a solid battery, a lithium secondary battery with higher safety can be obtained.

[0015] The average fiber diameter of the above fibrous carbon material is preferably 2 nm or more and 500 nm or less. According to such an embodiment, since the three-dimensional network structure of the fibrous carbon material is more likely to be formed, the lithium secondary battery becomes more excellent in cycle characteristics.

[0016] The average of the ratio of the fiber length to the fiber diameter of the above fibrous carbon material is preferably 20 or more and 5000 or less. According to such an embodiment, since the three-dimensional network structure of the fibrous carbon material is more likely to be formed, the lithium secondary battery becomes more excellent in cycle characteristics.

[0017] The above fibrous carbon material may be at least one selected from the group consisting of single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon nanofibers.

[0018] The occupied volume ratio of the fibrous carbon material in the carbon-metal composite layer is preferably 0.1% or more and 50.0% or less. According to such an aspect, the electric field generated on the surface of the negative electrode becomes more uniform, and the growth of dendritic lithium metal on the negative electrode is further suppressed.

[0019] The thickness of the carbon-metal composite layer is preferably 5 nm or more and 5000 nm or less. According to such an aspect, the electric field generated on the surface of the negative electrode becomes more uniform, and the growth of dendritic lithium metal on the negative electrode is further suppressed.

[0020] The carbon-metal composite layer preferably contains at least one metal selected from the group consisting of Sn, Zn, Bi, Ag, In, Pb, and Al. According to such an aspect, the affinity of the carbon-metal composite layer with lithium is further improved, so that the deposited lithium metal on the negative electrode is further suppressed from peeling off.

[0021] The negative electrode is preferably an electrode made of at least one selected from the group consisting of Cu, Ni, Ti, Fe, and other metals that do not react with Li, and alloys thereof, and stainless steel (SUS). According to such an aspect, it is not necessary to use highly flammable lithium metal during manufacturing, so it is more excellent in safety and productivity. In addition, since such a negative electrode is stable, the cycle characteristics of the secondary battery are further improved.

[0022] A lithium secondary battery including a negative electrode having no negative electrode active material has no lithium metal formed on the surface of the negative electrode before initial charging. Therefore, since it is not necessary to use highly flammable lithium metal during the manufacture of the lithium secondary battery, it is excellent in safety and productivity.

[0023] The lithium secondary battery preferably has an energy density of 350 Wh / kg or more.

[0024] The positive electrode may have a positive electrode active material.

[0025] The above conductive thin film may be a thin film made of carbon, a thin film made of a metal or an alloy, or a laminated film thereof.

[0026] The film thickness of the above conductive thin film is preferably 1 μm or less. According to such an aspect, the ion conductivity of the separator tends to be sufficiently maintained.

Advantages of the Invention

[0027] According to the present invention, it is possible to provide a lithium secondary battery having a high energy density and excellent cycle characteristics.

Brief Description of the Drawings

[0028]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0029] Hereinafter, embodiments of the present invention (hereinafter referred to as "the present embodiments") will be described in detail with reference to the drawings as necessary. In the drawings, the same elements will be denoted by the same reference numerals, and redundant descriptions will be omitted. Also, the positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings unless otherwise specified. Furthermore, the dimensional ratios in the drawings are not limited to the illustrated ratios.

[0030] [First Embodiment of the Present Invention] (Lithium secondary battery) FIG. 1 is a schematic cross-sectional view of a lithium secondary battery according to the first embodiment of the present invention. As shown in FIG. 1, the lithium secondary battery 100 according to the first embodiment of the present invention includes a positive electrode 110, a negative electrode 140 having no negative electrode active material, a separator 120 disposed between the positive electrode 110 and the negative electrode 140, and a carbon-metal composite layer 130 formed on a surface of the negative electrode 140 facing the separator 120. A conductive thin film (not shown in FIG. 1) is formed on a surface of the separator 120 facing the negative electrode 140. Hereinafter, each component of the lithium secondary battery 100 will be described.

[0031] (Negative electrode) The negative electrode 140 has no negative electrode active material, that is, it has no active material that serves as a host for lithium metal and lithium (lithium metal or ions). Therefore, the lithium secondary battery 100 has a smaller volume and mass of the entire battery and a higher energy density in principle compared to a lithium secondary battery having a negative electrode with a negative electrode active material. Here, in the lithium secondary battery 100, charge and discharge are performed by lithium metal depositing on the negative electrode 140 and the deposited lithium metal electrolytically eluting.

[0032] In this specification, "lithium metal deposits on the surface of the negative electrode" means that lithium metal deposits on at least one of the surface of the negative electrode, the surface of the carbon-metal composite layer formed on the surface of the negative electrode, and the surface of the solid electrolyte interface (SEI) layer described later formed on the surface of the negative electrode and / or the carbon-metal composite layer. In the lithium secondary battery of the present embodiment, it is considered that lithium metal mainly deposits on the surface of the carbon-metal composite layer or the surface of the SEI layer formed on the surface of the carbon-metal composite layer, but the deposition location is not limited to these. Therefore, in the lithium secondary battery 100, lithium metal may deposit, for example, on the surface of the negative electrode 140 (the interface between the surface of the negative electrode 140 and the carbon-metal composite layer 130), or on the surface of the carbon-metal composite layer 130 (the interface between the carbon-metal composite layer 130 and the separator 120).

[0033] In this specification, the "negative electrode active material" is a material that causes an electrode reaction, i.e., an oxidation reaction and a reduction reaction, in the negative electrode. Specifically, examples of the negative electrode active material of the present embodiment include lithium metal and host materials for lithium elements (lithium ions or lithium metal). The host material for lithium elements means a material provided to hold lithium ions or lithium metal in the negative electrode. The mechanism for such holding is not particularly limited, and examples include intercalation, alloying, and occlusion of metal clusters, and typically, intercalation and alloying.

[0034] Examples of such negative electrode active materials are not particularly limited, and include, for example, lithium metal and alloys containing lithium metal, carbon-based materials, metal oxides, and metals alloyed with lithium or alloys containing such metals. Examples of the carbon-based materials are not particularly limited, and include, for example, graphene, graphite, hard carbon, mesoporous carbon, carbon nanotubes, and carbon nanohorns. Examples of the metal oxides are not particularly limited, and include, for example, titanium oxide-based compounds, tin oxide-based compounds, and cobalt oxide-based compounds. Examples of the metals alloyed with lithium include, for example, silicon, germanium, tin, lead, aluminum, and gallium.

[0035] In this specification, when the negative electrode "does not have a negative electrode active material", it means that the content of the negative electrode active material in the negative electrode is 10% by mass or less with respect to the entire negative electrode. The content of the negative electrode active material in the negative electrode is preferably 5.0% by mass or less, may be 1.0% by mass or less, may be 0.1% by mass or less, and may be 0.0% by mass or less with respect to the entire negative electrode. When the negative electrode does not have a negative electrode active material or the content of the negative electrode active material in the negative electrode is within the above range, the energy density of the lithium secondary battery 100 becomes high. Note that the content of the negative electrode active material being 0.0% by mass or less means that the negative electrode active material is not measured with two significant figures.

[0036] More specifically, regardless of the state of charge of the battery, the content of the negative electrode active material other than lithium metal in the negative electrode 140 is 10% by mass or less, preferably 5.0% by mass or less, and may be 1.0% by mass or less, 0.1% by mass or less, or even 0.0% by mass with respect to the entire negative electrode. Further, in the negative electrode 140, before the initial charge and / or at the end of discharge, the content of lithium metal is 10% by mass or less, preferably 5.0% by mass or less, and may be 1.0% by mass or less, 0.1% by mass or less, or even 0.0% by mass with respect to the entire negative electrode.

[0037] In the negative electrode 140, before the initial charge and at the end of discharge, the content of lithium metal may be 10% by mass or less (preferably 5.0% by mass or less, and may be 1.0% by mass or less, 0.1% by mass or less, or even 0.0% by mass) with respect to the entire negative electrode; before the initial charge or at the end of discharge, the content of lithium metal may be 10% by mass or less (preferably 5.0% by mass or less, and may be 1.0% by mass or less, 0.1% by mass or less, or even 0.0% by mass) with respect to the entire negative electrode; before the initial charge, the content of lithium metal may be 10% by mass or less (preferably 5.0% by mass or less, and may be 1.0% by mass or less, 0.1% by mass or less, or even 0.0% by mass) with respect to the entire negative electrode; or at the end of discharge, the content of lithium metal may be 10% by mass or less (preferably 5.0% by mass or less, and may be 1.0% by mass or less, 0.1% by mass or less, or even 0.0% by mass) with respect to the entire negative electrode.

[0038] As used herein, the term "lithium secondary battery comprising a negative electrode without negative electrode active material" means that the negative electrode has no negative electrode active material before the initial charging of the battery or at the end of discharging. Therefore, the phrase "negative electrode without negative electrode active material" may be equivalently expressed as "negative electrode having no negative electrode active material before the initial charging of the battery or at the end of discharging", "negative electrode having no negative electrode active material other than lithium metal regardless of the state of charge of the battery and having no lithium metal before the initial charging or at the end of discharging", or "negative electrode current collector having no lithium metal before the initial charging or at the end of discharging", etc. Further, the "lithium secondary battery comprising a negative electrode without negative electrode active material" may be equivalently expressed as an anode-free lithium battery, a zero anode lithium battery, or an anode-less lithium battery. From this perspective, it can be said that the lithium secondary battery of the present embodiment has a different configuration from that of a conventional lithium ion battery (LIB) or a lithium metal battery (LMB). Here, the lithium ion battery means a lithium battery containing a host material for holding a lithium element in the negative electrode in the negative electrode, and the lithium metal battery means a lithium battery having a lithium metal foil in the negative electrode before the initial charging (at the time of battery assembly).

[0039] As used herein, the battery being "before the initial charging" means the state from when the battery is assembled until the first charging. Also, the battery being "at the end of discharging" means the state where the voltage of the battery is 1.0 V or more and 3.8 V or less (preferably 1.0 V or more and 3.0 V or less).

[0040] When the voltage of the lithium secondary battery 100 is 1.0 V or more and 3.5 V or less, the lithium metal content may be 10% by mass or less with respect to the entire negative electrode (preferably 5.0% by mass or less, may be 1.0% by mass or less, may be 0.1% by mass or less, or may be 0.0% by mass or less); when the voltage of the battery is 1.0 V or more and 3.0 V or less, the lithium metal content may be 10% by mass or less with respect to the entire negative electrode (preferably 5.0% by mass or less, may be 1.0% by mass or less, may be 0.1% by mass or less, or may be 0.0% by mass or less); or when the voltage of the battery is 1.0 V or more and 2.5 V or less, the lithium metal content may be 10% by mass or less with respect to the entire negative electrode (preferably 5.0% by mass or less, may be 1.0% by mass or less, may be 0.1% by mass or less, or may be 0.0% by mass or less).

[0041] Further, in the lithium secondary battery 100, the mass M of lithium metal deposited on the negative electrode 140 when the voltage of the battery is 4.2 V 4.2 with respect to the mass M of lithium metal deposited on the negative electrode 140 when the voltage of the battery is 3.0 V 3.0 The ratio M 3.0 / M 4.2 is preferably 20% or less, more preferably 15% or less, and still more preferably 10% or less. The ratio M 3.0 / M 4.2 may be 8.0% or less, may be 5.0% or less, may be 3.0% or less, or may be 1.0% or less.

[0042] In a typical lithium secondary battery, the capacity of the negative electrode (the capacity of the negative electrode active material) is set to be approximately the same as the capacity of the positive electrode (the capacity of the positive electrode active material). However, in the lithium secondary battery 100, since lithium metal is deposited on the negative electrode 140 and the deposited lithium metal is electrochemically dissolved and discharged, it is not necessary to define the capacity of the negative electrode. Therefore, since the lithium secondary battery 100 is not restricted by the charging capacity of the negative electrode, the energy density can be increased in principle. In the lithium secondary battery 100, a carbon-metal composite layer 130 is formed on the surface of the negative electrode 140. Although the carbon-metal composite layer may contain a metal and / or a carbon material that can react with lithium, its capacity is sufficiently small compared to that of the positive electrode. Therefore, the lithium secondary battery 100 can be said to "include a negative electrode having no negative electrode active material".

[0043] The total capacity of the negative electrode 140 and the carbon-metal composite layer 130 is sufficiently small compared to the capacity of the positive electrode 110. For example, it may be 20% or less, 15% or less, 10% or less, or 5% or less. The capacities of the positive electrode 110, the negative electrode 140, and the carbon-metal composite layer 130 can be measured by conventionally known methods.

[0044] The negative electrode 140 is not particularly limited as long as it has no negative electrode active material and can be used as a current collector. For example, it includes at least one selected from the group consisting of Cu, Ni, Ti, Fe, and other metals that do not react with Li, alloys thereof, and stainless steel (SUS). When SUS is used for the negative electrode 140, various conventionally known types of SUS can be used. The above-described negative electrode materials are used alone or in combination of two or more. In this specification, the "metal that does not react with Li" means a metal that does not alloy by reacting with lithium ions or lithium metal under the operating conditions of the lithium secondary battery.

[0045] The negative electrode 140 is preferably made of at least one selected from the group consisting of Cu, Ni, Ti, Fe, and their alloys, and stainless steel (SUS), and more preferably, made of at least one selected from the group consisting of Cu, Ni, and their alloys, and stainless steel (SUS). The negative electrode 140 is even more preferably Cu, Ni, their alloys, or stainless steel (SUS). When such a negative electrode is used, the energy density and productivity of the battery tend to be further improved.

[0046] The negative electrode 140 is an electrode that does not contain lithium metal. Therefore, since it is not necessary to use highly flammable and reactive lithium metal during manufacturing, the lithium secondary battery 100 is excellent in safety, productivity, and cycle characteristics.

[0047] The average thickness of the negative electrode 140 is preferably 4 μm or more and 20 μm or less, more preferably 5 μm or more and 18 μm or less, and even more preferably 6 μm or more and 15 μm or less. According to such an aspect, since the volume occupied by the negative electrode 140 in the lithium secondary battery 100 decreases, the energy density of the lithium secondary battery 100 is further improved.

[0048] (Carbon metal composite layer) Figure 2 is a schematic cross-sectional view showing one aspect of the deposition of lithium metal on the surface of the negative electrode in a lithium secondary battery. Figure 2(A) shows the deposition mode of lithium metal on the surface of the negative electrode in a conventional lithium secondary battery, and Figure 2(B) shows the deposition mode of lithium metal on the surface of the negative electrode in the lithium secondary battery of this embodiment. As shown in FIG. 2(A), in a conventional lithium secondary battery, it is difficult for lithium metal 210 deposited on the surface of the negative electrode 140 to grow uniformly in the plane direction. As a result, the lithium metal deposited on the negative electrode surface tends to grow in a dendrite shape, and the cycle characteristics of the battery deteriorate. On the other hand, as shown in FIG. 1, in the lithium secondary battery 100 of the first embodiment, a carbon-metal composite layer 130, which is a composite layer containing a carbon material and a metal material, is formed on the surface of the negative electrode 140. The carbon-metal composite layer 130 contains a plurality of fibrous carbon materials randomly oriented as carbon materials. In such a lithium secondary battery of the present embodiment, as shown in FIG. 2(B), in the carbon-metal composite layer 130, the fibrous carbon materials 220 are intertwined with each other to form a three-dimensional network structure. The fibrous carbon material 220 having the three-dimensional network structure is considered to make the electric conductivity of the entire carbon-metal composite layer 130 high and uniform, and to make the electric field generated on the surface of the carbon-metal composite layer 130 uniform in the plane direction. Further, the carbon-metal composite layer as a whole has a carbon material that can be a starting point for lithium metal precipitation. As a result, the reactivity of the lithium metal precipitation reaction becomes more uniform regardless of the location on the surface of the carbon-metal composite layer 130. Therefore, as shown in FIG. 2(B), in the lithium secondary battery of the present embodiment, lithium metal 210 that has grown uniformly in the plane direction is deposited on the surface of the carbon-metal composite layer 130, and it is considered that the growth of lithium metal in a dendrite shape is suppressed. However, the factors contributing to the excellent cycle characteristics of the lithium secondary battery of the present embodiment are not limited to those described above. In FIG. 2(B), the lithium metal 210 may be deposited at the interface between the negative electrode 140 and the carbon-metal composite layer 130.

[0049] In this specification, "the growth of lithium metal in a dendritic form is suppressed" means that the lithium metal formed on the surface of the negative electrode is suppressed from becoming dendritic due to charge and discharge of the lithium secondary battery or repetition thereof. In other words, it means inducing the lithium metal formed on the surface of the negative electrode due to charge and discharge of the lithium secondary battery or repetition thereof to grow in a non-dendritic form. Here, "non-dendritic" is not particularly limited, but is typically plate-like, valley-like, or hill-like.

[0050] The fibrous carbon material contained in the carbon-metal composite layer 130 is not particularly limited as long as it is a material known as a fibrous carbon material among those skilled in the art. From the viewpoint of facilitating the formation of a three-dimensional network structure of the fibrous carbon material, the average fiber diameter of the fibrous carbon material is preferably 2 nm or more and 500 nm or less. From the same viewpoint, the average fiber diameter of the fibrous carbon material is more preferably 5 nm or more and 300 nm or less, still more preferably 5 nm or more and 100 nm or less, and even more preferably 7 nm or more and 80 nm or less.

[0051] The average fiber diameter of the fibrous carbon material can be measured by a known measurement method. For example, it can be measured by a scanning electron microscope (SEM) or a transmission electron microscope (TEM). More specifically, before the carbon-metal composite layer is formed, the fibrous carbon material used for the production of the carbon-metal composite layer is observed by SEM or TEM, and the fiber diameter of the fibrous carbon material can be measured visually or by image analysis software from the obtained image. The average fiber diameter is calculated by calculating the average (arithmetic mean) of the fiber diameters obtained as described above, and the number n of fibers to be measured is 3 or more, preferably 5 or more, and more preferably 10 or more. The measurement of the average fiber diameter of the fibrous carbon material described above may be performed by observing the fibrous carbon material in the carbon-metal composite layer after formation. When observing the fibrous carbon material in the carbon-metal composite layer after formation, the following may be done. For example, the lithium secondary battery 100 may be cut in the thickness direction, and the carbon-metal composite layer 130 on the exposed cut surface may be observed by SEM or TEM. Alternatively, after disassembling the lithium secondary battery 100 into its respective components, the surface of the carbon-metal composite layer 130 may be etched with a focused ion beam (FIB) to expose the inside of the carbon-metal composite layer 130, and the exposed surface may be observed by SEM or TEM. Since the fibrous carbon material in the layer after the formation of the carbon-metal composite layer forms a three-dimensional network structure, the SEM or TEM image of the exposed surface includes fibrous carbon materials extending in a direction perpendicular to the image and / or fibrous carbon materials extending in a direction parallel to the image. Therefore, the average diameter can be calculated by extracting a plurality (preferably at least 3 or more as described above) of such fibrous carbon materials from the SEM or TEM image.

[0052] The fibrous carbon material having an average fiber diameter within the above range can be manufactured by a known manufacturing method and can also be obtained commercially. When obtaining the fibrous carbon material commercially, the fibrous carbon material having an average fiber diameter within the above range can be obtained by referring to the public information of the manufacturer. After obtaining it, it is preferable to measure the average fiber diameter by the above method.

[0053] The length of the fibrous carbon material is not particularly limited. However, from the viewpoint that the three-dimensional network structure of the fibrous carbon material is more likely to be formed, it is preferably defined by the ratio of the fiber length to the fiber diameter of the fibrous carbon material (hereinafter also referred to as the "aspect ratio"). From the same viewpoint, the average aspect ratio of the fibrous carbon material is preferably 20 or more and 5000 or less, more preferably 100 or more and 4000 or less, still more preferably 300 or more and 3000 or less, and particularly preferably 400 or more and 2500 or less.

[0054] The length of the fibrous carbon material can be measured by known measurement methods. For example, it can be measured by a scanning electron microscope (SEM) or a transmission electron microscope (TEM). More specifically, the same method as when measuring the fiber diameter of the fibrous carbon material may be used. (When observing the fibrous carbon material after the formation of the carbon-metal composite layer, since the fibrous carbon material in the layer forms a three-dimensional network structure, the SEM or TEM image of the exposed surface includes the fibrous carbon material extending in a direction parallel to the image. Therefore, the average length can be calculated by extracting a plurality of such fibrous carbon materials from the SEM or TEM image.) The average of the ratio (aspect ratio) of the fiber length to the fiber diameter of the fibrous carbon material can be obtained by measuring the fiber diameter and fiber length of each fibrous carbon material by the above method and then calculating the ratio to obtain the aspect ratio, and further calculating the arithmetic mean of the calculated aspect ratios. Alternatively, the average aspect ratio of the fibrous carbon material may be obtained by calculating the ratio (average fiber length / average fiber diameter) of the values after calculating the average fiber diameter and average fiber length of the fibrous carbon material by the above method.

[0055] The fibrous carbon material with the aspect ratio within the above range can be manufactured by known manufacturing methods and can also be obtained commercially. When obtaining the fibrous carbon material commercially, by referring to the public information of the manufacturer, a fibrous carbon material with the aspect ratio within the above range can be obtained. After obtaining it, it is preferable to measure the aspect ratio by the above method.

[0056] Preferable specific examples of the fibrous carbon material contained in the carbon-metal composite layer 130 include single-walled carbon nanotubes (hereinafter also referred to as "SWCNT"), multi-walled carbon nanotubes (hereinafter also referred to as "MWCNT"), and carbon nanofibers (hereinafter also referred to as "CF"). Among them, vapor-grown carbon nanofibers (hereinafter also referred to as "VGCF") are preferably used as the carbon nanofibers. The above fibrous carbon materials are used alone or in combination of two or more.

[0057] The content of the fibrous carbon material in the carbon-metal composite layer 130 is not particularly limited, but it is preferably in the range where the occupied volume ratio of the fibrous carbon material in the carbon-metal composite layer is 0.1% or more and 50.0% or less. When the occupied volume ratio of the fibrous carbon material is 0.1% or more, the three-dimensional network structure of the fibrous carbon material tends to be more easily formed. When the occupied volume ratio of the fibrous carbon material is 50.0% or less, the lithium metal affinity of the surface of the carbon-metal composite layer tends to be further improved. From the same perspective, the occupied volume ratio of the fibrous carbon material in the carbon-metal composite layer is more preferably 1.0% or more and 40.0% or less, still more preferably 2.0% or more and 35.0% or less, even more preferably 2.5% or more and 30.0% or less, and particularly preferably 3.0% or more and 20.0% or less.

[0058] The occupied volume ratio of the fibrous carbon material in the carbon-metal composite layer can be measured by a known measurement method. For example, the lithium secondary battery 100 is cut in the thickness direction, and the carbon-metal composite layer 130 on the exposed cut surface can be measured by observing it with SEM or TEM. Alternatively, after the lithium secondary battery 100 is disassembled into each component, the surface of the carbon-metal composite layer 130 is etched with a focused ion beam (FIB) to expose the inside of the carbon-metal composite layer 130, and the exposed surface can be measured by observing it with SEM or TEM. More specifically, the SEM image or TEM image obtained as described above is subjected to binary analysis using image analysis software to measure the occupied area ratio of the fibrous carbon material on the measurement surface, and the obtained occupied area ratio of the fibrous carbon material can be used as the occupied volume ratio of the fibrous carbon material in the carbon-metal composite layer. Incidentally, the occupied volume ratio of the fibrous carbon material in the carbon-metal composite layer can be controlled, for example, by using the method for producing the carbon-metal composite layer described later.

[0059] The amount of the fibrous carbon material supported on the negative electrode surface is not particularly limited, but per 1 cm of the negative electrode 2Preferably, it is 0.1 μg or more, more preferably 0.2 μg or more, and still more preferably 0.3 μg or more. When the loading amount of the fibrous carbon material is within the above range, the three-dimensional network structure of the fibrous carbon material tends to be more easily formed. Also, the loading amount of the fibrous carbon material is preferably 10 mg / cm 2 or less, more preferably 5 mg / cm 2 or less, still more preferably 1 mg / cm 2 or less, even more preferably 100 μg / cm 2 or less, and even more preferably 50 μg / cm 2 or less, particularly preferably 10 μg / cm 2 or less. When the loading amount of the fibrous carbon material is within the above range, the lithium metal affinity of the surface of the carbon metal composite layer tends to be further improved. Note that the loading amount of the fibrous carbon material can be measured by a conventionally known method. For example, it can be obtained by measuring the mass of the negative electrode before and after loading the fibrous carbon material and finding the difference therebetween.

[0060] The carbon metal composite layer 130 contains a metal. By the carbon metal composite layer 130 containing a metal, the surface of the carbon metal composite layer becomes more excellent in affinity with lithium metal compared to the case where the carbon metal composite layer consists only of the fibrous carbon material, and it is possible to suppress the deposited lithium metal on the negative electrode surface from peeling off. Generally, in a lithium secondary battery in which charge and discharge are performed by lithium metal depositing on the negative electrode surface and the deposited lithium electrolytically eluting, when the deposited lithium metal peels off, the capacity of the battery decreases, that is, it is known that the peeling of the deposited lithium metal deteriorates the cycle characteristics of the lithium secondary battery. Therefore, by the carbon metal composite layer 130 containing a metal, it is possible to suppress the deposited lithium metal on the negative electrode surface from peeling off, and the cycle characteristics of the lithium secondary battery become more excellent.

[0061] From the perspective of further improving the lithium metal affinity on the surface of the carbon-metal composite layer, the carbon-metal composite layer 130 preferably contains at least one metal selected from the group consisting of Sn, Zn, Bi, Ag, In, Pb, and Al. From a similar perspective, the carbon-metal composite layer 130 more preferably contains at least one metal selected from the group consisting of Sn, Zn, Ag, Bi, and Al.

[0062] The thickness of the carbon-metal composite layer 130 is not particularly limited, but is preferably 5 nm or more, more preferably 10 nm or more, and still more preferably 15 nm or more. When the thickness of the carbon-metal composite layer is within the above range, the effects of the carbon-metal composite layer 130 described above tend to be effectively and surely exhibited. Also, the thickness of the carbon-metal composite layer is preferably 5000 nm or less, more preferably 3000 nm or less, still more preferably 1000 nm or less, even more preferably 500 nm or less, even more preferably 300 nm or less, and particularly preferably 100 nm or less. When the thickness of the carbon-metal composite layer is within the above range, the internal electrical resistance of the lithium secondary battery further decreases, so the lithium secondary battery tends to have a higher energy density and better cycle characteristics.

[0063] The thickness of the carbon-metal composite layer can be measured by known measurement methods. For example, the lithium secondary battery 100 can be cut in the thickness direction, and the carbon-metal composite layer 130 on the exposed cut surface can be observed by SEM or TEM for measurement.

[0064] (Positive electrode) The positive electrode 110 is not particularly limited as long as it is generally used in lithium secondary batteries, and known materials can be appropriately selected depending on the application of the lithium secondary battery. From the perspective of improving the stability and output voltage of the lithium secondary battery, the positive electrode 110 preferably has a positive electrode active material.

[0065] As used herein, the "positive electrode active material" means a material for retaining lithium ions in the positive electrode 110, and may also be referred to as a host material for lithium ions.

[0066] Such positive electrode active materials are not particularly limited, and examples thereof include 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. The metal phosphates are not particularly limited, and examples thereof include iron phosphate-based compounds and cobalt phosphate-based compounds. Typical positive electrode active materials include LiCoO2, LiNi x Co y Mn z O(x + y + z = 1), LiNi x Mn y O(x + y = 1), LiNiO2, LiMn2O4, LiFePO, LiCoPO, LiFeOF, LiNiOF, and TiS2. The positive electrode active materials as described above are used alone or in combination of two or more.

[0067] The positive electrode 110 may contain components other than the above positive electrode active material. Such components are not particularly limited, and examples thereof include known conductive aids, binders, solid polymer electrolytes, and inorganic solid electrolytes.

[0068] The conductive aids in the positive electrode 110 are not particularly limited, and examples thereof include carbon black, single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), carbon nanofibers (CF), and acetylene black. The binders are not particularly limited, and examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylic resin, and polyimide resin.

[0069] In the positive electrode 110, the content of the positive electrode active material may be, for example, 50% by mass or more and 100% by mass or less with respect to the entire positive electrode 110. The content of the conductive assistant may be, for example, 0.5% by mass to 30% by mass or less with respect to the entire positive electrode 110. The content of the binder may be, for example, 0.5% by mass to 30% by mass or less with respect to the entire positive electrode 110. The total content of the solid polymer electrolyte and the inorganic solid electrolyte may be, for example, 0.5% by mass to 30% by mass or less with respect to the entire positive electrode 110.

[0070] (Positive electrode current collector) A positive electrode current collector may be disposed on one side of the positive electrode 110. The positive electrode current collector 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.

[0071] The average thickness of the positive electrode current collector is preferably 4 μm or more and 20 μm or less, more preferably 5 μm or more and 18 μm or less, and still more preferably 6 μm or more and 15 μm or less. According to such an aspect, since the volume occupied by the positive electrode current collector in the lithium secondary battery 100 decreases, the energy density of the lithium secondary battery 100 is further improved.

[0072] (Separator) The separator 120 is a member for preventing the battery from short-circuiting by separating the positive electrode 110 and the negative electrode 140, and ensuring the ionic conductivity of lithium ions, which are charge carriers between the positive electrode 110 and the negative electrode 140. It has no electronic conductivity and is composed of a material that does not react with lithium ions. In addition, the separator 120 also plays a role of holding the electrolyte solution. Although the material itself constituting the separator has no ionic conductivity, when the separator holds the electrolyte solution, lithium ions are conducted through the electrolyte solution. The separator 120 is not limited as long as it plays the above role. For example, it is composed of a porous polyethylene (PE) film, a polypropylene (PP) film, or a laminated structure thereof.

[0073] Separator 120 may be coated with a separator coating layer. The separator coating layer may cover both sides of the separator 120 or may cover only one side. The separator coating layer is not particularly limited as long as it has ion conductivity and does not react with lithium ions, but it is preferably one that can firmly bond the separator 120 and the layer adjacent to the separator 120. Such separator coating layers are not particularly limited, but examples include those containing binders such as polyvinylidene fluoride (PVDF), a composite of styrene butadiene rubber and carboxymethyl cellulose (SBR-CMC), polyacrylic acid (PAA), lithium polyacrylate (Li-PAA), polyimide (PI), polyamideimide (PAI), and aramid. Inorganic particles such as silica, alumina, titania, zirconia, magnesium oxide, magnesium hydroxide, and lithium nitrate may be added to the above binders for the separator coating layer. Note that the separator 120 may be a separator without a separator coating layer or a separator with a separator coating layer.

[0074] The average thickness of the separator 120 is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less. According to such an aspect, since the volume occupied by the separator 120 in the lithium secondary battery 100 decreases, the energy density of the lithium secondary battery 100 is further improved. Also, the average thickness of the separator 120 is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more. According to such an aspect, the positive electrode 110 and the negative electrode 140 can be more reliably isolated, and the occurrence of a short circuit in the battery can be further suppressed.

[0075] (Conductive thin film) The conductive thin film is formed on the surface of the separator 120 facing the negative electrode 140. That is, the conductive thin film is provided at the interface between the separator 120 and the carbon-metal composite layer 130. By providing such a thin film having conductivity on the surface of the separator, while maintaining the ionic conductivity of the separator 120 sufficiently high, the potential on the surface of the separator can be made uniform, and lithium metal can be deposited uniformly on the negative electrode synergistically with the carbon-metal composite layer.

[0076] The conductive thin film is not particularly limited as long as it is a thin film having conductivity, but preferably, it is a thin film made of a metal or an alloy, a thin film made of carbon, or a laminated film thereof. When the above materials are used as the conductive thin film, the irreversible uptake of lithium ions into the conductive thin film is suppressed, and the cycle characteristics of the battery tend to be further improved.

[0077] The metal forming the conductive thin film or the metal element contained in the alloy is not particularly limited. When an element forming an alloy with lithium is used, it is preferable to form a thin film serving as a base on the separator side with a metal or an alloy that does not form an alloy with lithium, or the above carbon thin film, and then form a thin film with a metal or an alloy that forms an alloy with lithium thereon. Examples of the metal and alloy that do not form an alloy with lithium include Cu, Ni, Fe, Mn, Ti, Cr, and stainless steel. Examples of the metal and alloy that form an alloy with lithium include Si, Sn, Al, In, Zn, Ag, Bi, Pb, Sb, and alloys containing these elements.

[0078] As the thin film made of carbon, those made of carbon are preferable, and examples of such thin films include diamond-like carbon (DLC) thin films. The thin film made of carbon may be laminated with the thin film made of a metal or an alloy on the separator, and may further be patterned in-plane. 3 Those made of carbon are preferable, and examples of such thin films include diamond-like carbon (DLC) thin films. The thin film made of carbon may be laminated with the thin film made of a metal or an alloy on the separator, and may further be patterned in-plane.

[0079] The film thickness of the conductive thin film is preferably 1 μm or less. By having the film thickness of the conductive thin film be 1 μm or less, the ion conductivity of the separator 120 can be maintained even higher. The film thickness of the conductive thin film is, for example, preferably set to 0.9 μm, 0.8 μm, 0.7 μm, 0.6 μm, 0.5 μm, 0.4 μm, 0.3 μm, 0.2 μm, 0.1 μm (100 nm), 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, 20 nm, 10 nm, 8 nm, 5 nm, or a value between these. Examples of the preferable range of the film thickness are, for example, 5 nm or more and 200 nm or less, or 8 nm or more and 100 nm or less. When the conductive thin film has a laminated structure of multiple layers, it is preferable that the total of the film thicknesses is within the above range. Note that the thickness of the conductive thin film can be measured by a known measurement method. For example, it can be measured by cutting the lithium secondary battery 100 or the separator on which the conductive thin film is formed in the thickness direction and observing the conductive thin film on the exposed cut surface by SEM or TEM.

[0080] Incidentally, there is also a method of forming a coating film composed of carbonaceous particles and a binder component on the separator in order to impart conductivity to the separator surface. However, such a method is not preferable because the binder component acts to hinder conductivity, lithium ions are irreversibly taken into such a coating film, and it is difficult to uniformly form the coating film on the separator surface with a thickness of 1 μm or less. In the present embodiment, even when a thin film made of carbon is used as the conductive thin film, it is clearly distinguished from a coating film composed of carbonaceous particles and a binder component in that it contains no such binder component and consists only of carbon. The thin film made of carbon can achieve lower resistance and a uniform film thickness while reducing the film thickness as compared with a coating film (carbon coat layer) in which carbonaceous particles are dispersed in a binder component.

[0081] (Electrolyte solution) The lithium secondary battery 100 preferably further includes an electrolyte solution. The electrolyte solution may be impregnated into the separator 120, or a product in which the electrolyte solution is encapsulated together with the lithium secondary battery 100 may be used as a finished product. The electrolyte solution contains an electrolyte and a solvent, is a solution having ionic conductivity, and acts as a conduction path for lithium ions. Therefore, the lithium secondary battery 100 having the electrolyte solution has a further reduced internal resistance and further improved energy density, capacity, and cycle characteristics.

[0082] The electrolyte is not particularly limited as long as it is a salt, and examples thereof include salts of Li, Na, K, Ca, and Mg. Among them, as the electrolyte, a lithium salt is preferably used. The lithium salt is not particularly limited, and examples thereof include LiI, LiCl, LiBr, LiF, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2CF3CF3)2, LiB(O2C2H4)2, LiB(O2C2H4)F2, LiB(OCOCF3)4, LiNO3, and Li2SO4. From the viewpoint of further excellent energy density, capacity, and cycle characteristics of the lithium secondary battery 100, LiN(SO2F)2 is preferable as the lithium salt. The above lithium salts may be used alone or in combination of two or more.

[0083] The solvent is not particularly limited, and examples thereof include dimethoxyethane, dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, chloroethylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, trifluoromethylpropylene carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, nonafluorobutyl methyl ether, nonafluorobutyl ethyl ether, tetrafluoroethyl tetrafluoropropyl ether, trimethyl phosphate, and triethyl phosphate. The above solvents may be used alone or in combination of two or more.

[0084] (Use of Lithium Secondary Battery) FIG. 2 shows one usage mode of the lithium secondary battery of this embodiment. The lithium secondary battery 300 has a positive electrode current collector 310 disposed on a surface opposite to the surface of the lithium secondary battery 100 facing the separator 120 of the positive electrode 110.

[0085] For the lithium secondary battery 300, a positive electrode terminal 330 and a negative electrode terminal 340 for connecting the lithium secondary battery 300 to an external circuit are respectively joined to the positive electrode current collector 310 and the negative electrode 140. The lithium secondary battery 300 is charged and discharged by connecting the negative electrode terminal 340 to one end of the external circuit and the positive electrode terminal 330 to the other end of the external circuit.

[0086] During the initial charging of the lithium secondary battery 300, a solid electrolyte interface layer (SEI layer) 320 may be formed at the interface between the carbon metal composite layer 130 and the conductive thin film formed on the separator 120. The SEI layer 320 to be formed is not particularly limited, and may include, for example, an inorganic compound containing lithium, an organic compound containing lithium, and the like. The typical average thickness of the SEI layer is 1 nm or more and 10 μm or less.

[0087] The lithium secondary battery 300 is charged by applying a voltage between the positive electrode terminal 330 and the negative electrode terminal 340 such that current flows from the negative electrode terminal 340 through the external circuit to the positive electrode terminal 330. By charging the lithium secondary battery 300, precipitation of lithium metal occurs on the surface of the negative electrode. Note that the precipitation of the lithium metal occurs at least at one of the interface between the negative electrode 140 and the carbon metal composite layer 130, the interface between the carbon metal composite layer 130 and the SEI layer 320, and the interface between the SEI layer 320 and the separator 120.

[0088] When the positive electrode terminal 330 and the negative electrode terminal 340 are connected to the charged lithium secondary battery 300, the lithium secondary battery 300 is discharged. Thereby, the precipitation of lithium metal generated on the surface of the negative electrode is electrolytically eluted.

[0089] In addition, in the lithium secondary battery 300 of the present embodiment, the SEI layer 320 may not be formed, or may be formed at the interface between the negative electrode 140 and the carbon metal composite layer 130.

[0090] (Method for manufacturing a lithium secondary battery) As a method for manufacturing the lithium secondary battery 100 as shown in FIG. 1, there is no particular limitation as long as it is a method capable of manufacturing a lithium secondary battery having the above-described configuration. For example, the following methods can be mentioned.

[0091] First, the positive electrode 110 is prepared by a known manufacturing method or by purchasing a commercially available product. The positive electrode 110 is manufactured, for example, as follows. The above-described positive electrode active material, a known conductive assistant, and a known binder are mixed to obtain a positive electrode mixture. The mixing ratio may be, for example, 50% by mass or more and 99% by mass or less of the positive electrode active material, 0.5% by mass to 30% by mass of the conductive assistant, and 0.5% by mass to 30% by mass of the binder with respect to the entire positive electrode mixture. The obtained positive electrode mixture is applied to one side of a metal foil (for example, an Al foil) having a thickness of 5 μm or more and 1 mm or less, and press-molded. The obtained molded body is punched into a predetermined size by punching to obtain the positive electrode 110.

[0092] Next, the separator 120 having the above-described configuration is prepared. The separator 120 may be manufactured by a conventionally known method or a commercially available product may be used.

[0093] Next, a conductive thin film is formed on one side or both sides of the separator, preferably on one side. The method for forming the conductive thin film is not particularly limited, and CVD method, PVD method, vacuum evaporation method, sputtering, electroless plating, electrolytic plating, etc. may be used. The method for forming the conductive thin film is preferably sputtering.

[0094] Next, the above-described negative electrode material, for example, a metal foil (for example, an electrolytic Cu foil) having a thickness of 1 μm or more and 1 mm or less, is washed with a solvent containing sulfamic acid, punched into a predetermined size, and further ultrasonically washed with ethanol and then dried to obtain the negative electrode 140.

[0095] Next, a carbon-metal composite layer 130 is formed on one side of the negative electrode 140. Examples of the method for forming the carbon-metal composite layer include electroless plating, electroplating, powder metallurgy, and vapor deposition methods.

[0096] As the electroless plating method, for example, a method using a plating solution containing metal ions, fibrous carbon materials, and a reducing agent can be mentioned. Specifically, methods such as immersing the negative electrode 140 in the plating solution and applying the plating solution to the negative electrode 140 by spin coating can be mentioned. In the electroless plating method, the occupied volume ratio of the fibrous carbon materials in the carbon-metal composite layer can be controlled by adjusting the concentration of the fibrous carbon materials in the plating solution.

[0097] As the electroplating method, for example, a method of electroplating the negative electrode 140 as the working electrode in an electroplating solution containing metal ions and / or fibrous carbon materials can be mentioned. The electrolysis conditions and time can be appropriately adjusted depending on the metal ions and the negative electrode 140 used. In the electroplating method, the carbon-metal composite layer may be formed at once by electroplating in an electroplating solution containing metal ions and fibrous carbon materials. Alternatively, after immersing the negative electrode in a solution containing fibrous carbon materials and depositing the charged fibrous carbon materials on the surface of the negative electrode using electrophoresis, electroplating may be performed in another solution (plating solution) containing metal ions to form the carbon-metal composite layer. In the electroplating method, the occupied volume ratio of the fibrous carbon materials in the carbon-metal composite layer can be controlled by adjusting the concentration of the fibrous carbon materials in the plating solution.

[0098] As the powder metallurgy method, for example, a method of mixing metal powder and fibrous carbon material powder, press molding, and then sintering can be mentioned. In the powder metallurgy method, the occupied volume ratio of the fibrous carbon materials in the carbon-metal composite layer can be controlled by adjusting the mixing ratio of the materials.

[0099] As a vapor deposition method, for example, there is a method of obtaining a carbon-metal composite layer by supporting a fibrous carbon material on the negative electrode 140 and then vapor-depositing a metal on the negative electrode. In the vapor deposition method, the occupied volume ratio of the fibrous carbon material in the carbon-metal composite layer can be controlled by adjusting the amount of the fibrous carbon material supported.

[0100] For any of the electroless plating method, the electrolytic plating method, the powder metallurgy method, and the vapor deposition method, after forming the carbon-metal composite layer, a denser carbon-metal composite layer may be obtained by firing the carbon-metal composite layer formed on the surface of the negative electrode. Also, two or more of the electroless plating method, the electrolytic plating method, the powder metallurgy method, and the vapor deposition method may be combined. For example, the negative electrode is immersed in a solution containing a fibrous carbon material, and the charged fibrous carbon material is deposited on the surface of the negative electrode using electrophoresis, and then the negative electrode is immersed in a plating solution containing metal ions to deposit metal by electroless plating to obtain a carbon-metal composite layer. From the viewpoint of enhancing productivity and the viewpoint that a three-dimensional network structure of the fibrous carbon material is more likely to be formed, it is preferable to form the carbon-metal composite layer by the method described in the examples. In particular, performing metal plating after depositing the fibrous carbon material on the surface of the negative electrode is also preferable from the viewpoint that the amount of the fibrous carbon material supported can be precisely controlled.

[0101] The positive electrode 110, the separator 120, and the negative electrode 140 formed with the carbon-metal composite layer 130 obtained as described above are laminated in this order so that the surface on which the conductive thin film of the carbon-metal composite layer 130 and the separator 120 is formed faces each other to obtain a laminate. The obtained laminate can be sealed in a sealed container together with an electrolytic solution to obtain a lithium secondary battery 100. The sealed container is not particularly limited, and examples thereof include a laminate film.

[0102] [Second Embodiment] (Lithium Secondary Battery) Figure 3 is a schematic cross-sectional view of a lithium secondary battery according to the second embodiment of the present invention. As shown in Figure 3, the lithium secondary battery 400 according to the second embodiment of the present invention includes a positive electrode 110, a negative electrode 140 having no negative electrode active material, a solid electrolyte 410 disposed between the positive electrode 110 and the negative electrode 140, and a carbon metal composite layer 130 formed on a surface of the negative electrode 140 facing the solid electrolyte 410. A conductive thin film (not shown in Figure 3) is formed on the surface of the solid electrolyte 410 facing the negative electrode 140. The configurations and preferred embodiments of the positive electrode 110, the carbon metal composite layer 130, the negative electrode 140, and the conductive thin film are the same as those of the lithium secondary battery 100 according to the first embodiment of the present invention, and the lithium secondary battery 400 exhibits the same effects as the lithium secondary battery 100.

[0103] (Solid electrolyte) Generally, in a battery including a liquid electrolyte, due to the shaking of the liquid, the physical pressure exerted from the electrolyte on the surface of the negative electrode tends to vary depending on the location. On the other hand, since the lithium secondary battery 400 includes the solid electrolyte 410, the pressure exerted from the solid electrolyte 410 on the surface of the negative electrode 140 becomes more uniform, and the shape of the lithium metal deposited on the surface of the negative electrode 140 can be further uniformized. That is, according to such an aspect, the growth of the lithium metal deposited on the surface of the negative electrode 140 in the form of dendrites is further suppressed, so that the cycle characteristics of the lithium secondary battery 400 become more excellent.

[0104] The solid electrolyte 410 is not particularly limited as long as it is generally used in a lithium solid secondary battery, but known materials can be appropriately selected depending on the application of the lithium secondary battery 400. The solid electrolyte 410 preferably has ionic conductivity and no electronic conductivity. Since the solid electrolyte 410 has ionic conductivity and no electronic conductivity, the internal resistance of the lithium secondary battery 400 is further reduced, and the occurrence of a short circuit inside the lithium secondary battery 400 can be further suppressed. As a result, the energy density, capacity, and cycle characteristics of the lithium secondary battery 400 become more excellent.

[0105] The solid electrolyte 410 is not particularly limited, and examples thereof include those containing a resin and a lithium salt. Such resins are not particularly limited, and examples include resins having ethylene oxide units in the main chain and / or side chain, acrylic resins, vinyl resins, ester resins, nylon resins, polysiloxanes, polyphosphazenes, polyvinylidene fluoride, polymethyl methacrylate, polyamides, polyimides, aramids, polylactic acid, polyethylene, polystyrene, polyurethanes, polypropylene, polybutylene, polyacetal, polysulfone, and polytetrafluoroethylene. The above resins are used alone or in combination of two or more.

[0106] The lithium salt contained in the solid electrolyte 410 is not particularly limited, and examples thereof include LiI, LiCl, LiBr, LiF, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2CF3CF3)2, LiB(O2C2H4)2, LiB(O2C2H4)F2, LiB(OCOCF3)4, LiNO3, and Li2SO4. The above lithium salts are used alone or in combination of two or more.

[0107] Generally, the content ratio of the resin and the lithium salt in the solid electrolyte is determined by the ratio of the lithium atoms of the lithium salt to the oxygen atoms of the resin ([Li] / [O]). In the solid electrolyte 410, the content ratio of the resin and the lithium salt is adjusted so that the above ratio ([Li] / [O]) is preferably 0.02 or more and 0.20 or less, more preferably 0.03 or more and 0.15 or less, and still more preferably 0.04 or more and 0.12 or less.

[0108] The solid electrolyte 410 may contain components other than the above resin and lithium salt. Such components are not particularly limited, and examples include solvents and salts other than lithium salts. The salts other than lithium salts are not particularly limited, and examples include salts of Li, Na, K, Ca, and Mg.

[0109] The solvent is not particularly limited, and examples thereof include those exemplified in the electrolytic solution that the secondary lithium battery 100 may contain.

[0110] The average thickness of the solid electrolyte 410 is preferably 20 μm or less, more preferably 18 μm or less, and still more preferably 15 μm or less. According to such an aspect, since the volume occupied by the solid electrolyte 410 in the secondary lithium battery 400 decreases, the energy density of the secondary lithium battery 400 is further improved. Further, the average thickness of the solid electrolyte 410 is preferably 5 μm or more, more preferably 7 μm or more, and still more preferably 10 μm or more. According to such an aspect, the positive electrode 110 and the negative electrode 140 can be more reliably separated, and the short circuit of the battery can be further suppressed.

[0111] In the present specification, the "solid electrolyte" includes a gel electrolyte. The gel electrolyte is not particularly limited, and examples thereof include those containing a polymer, an organic solvent, and a lithium salt. The polymer in the gel electrolyte is not particularly limited, and examples thereof include copolymers of polyethylene and / or polyethylene oxide, polyvinylidene fluoride, and copolymers of polyvinylidene fluoride and hexafluoropropylene.

[0112] In FIG. 3, a solid electrolyte interface layer (SEI layer) may be formed on the surface of the negative electrode 140 and / or the carbon metal composite layer 130. The formed SEI layer is not particularly limited, and may include, for example, an inorganic compound containing lithium and an organic compound containing lithium. The typical average thickness of the SEI layer is 1 nm or more and 10 μm or less.

[0113] (Method for manufacturing a secondary battery) The secondary lithium battery 400 can be manufactured in the same manner as the method for manufacturing the secondary lithium battery 100 according to the first embodiment described above, except that a solid electrolyte is used instead of the separator.

[0114] As a method for manufacturing the solid electrolyte 410, it is not particularly limited as long as it is a method for obtaining the above-described solid electrolyte 410. For example, the following method may be used. A resin conventionally used in a solid electrolyte and a lithium salt (for example, the resin and lithium salt that the solid electrolyte 410 may contain) are dissolved in an organic solvent. The obtained solution is cast onto a molding substrate so as to have a predetermined thickness, thereby obtaining the solid electrolyte 410. Here, the mixing ratio of the resin and the lithium salt may be determined by the ratio ([Li] / [O]) between the oxygen atom of the resin and the lithium atom of the lithium salt, as described above. The above ratio ([Li] / [O]) is, for example, 0.02 or more and 0.20 or less. The organic solvent is not particularly limited, and for example, acetonitrile may be used. The molding substrate is not particularly limited, and for example, a PET film or a glass substrate may be used.

[0115] As a method for forming a conductive thin film on the solid electrolyte, the same method as the method for forming a conductive thin film on the separator can be used.

[0116] [Modification Example] The above-described embodiment is an exemplification for explaining the present invention, and is not intended to limit the present invention only to the embodiment. The present invention can be variously modified without departing from the gist thereof.

[0117] For example, in the lithium secondary battery 100 of the first embodiment and the lithium secondary battery 400 of the second embodiment, carbon metal composite layers 130 may be formed on both surfaces of the negative electrode 140. In this case, the lithium secondary battery has the following order: positive electrode / separator or solid electrolyte / carbon metal composite layer / negative electrode / carbon metal composite layer / separator or solid electrolyte / positive electrode; and each component is laminated. According to such an aspect, the capacity of the lithium secondary battery can be further improved.

[0118] The lithium secondary battery of this embodiment may be a lithium solid secondary battery. According to such an aspect, since it is not necessary to use an electrolytic solution, the problem of electrolytic solution leakage does not occur, and the safety of the battery is further improved.

[0119] The lithium secondary battery of this embodiment may have a current collector disposed on the surface of the negative electrode and / or the positive electrode so as to be in contact with the negative electrode or the positive electrode. Such a current collector is not particularly limited, and examples thereof include those that can be used for the negative electrode material. When the lithium secondary battery does not have a positive electrode current collector and a negative electrode current collector, the positive electrode and the negative electrode themselves function as current collectors, respectively.

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

[0121] In this specification, "having a high energy density" or "being of a high energy density" means that the capacity per total volume or total mass of the battery is high, 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 still more preferably 1000 Wh / L or more or 450 Wh / kg or more.

[0122] In addition, in this specification, "excellent cycle characteristics" means that the rate of decrease in the capacity of the battery is low before and after the number of charge-discharge cycles that can be assumed in normal use. That is, when comparing the initial capacity with the capacity after the number of charge-discharge cycles that can be assumed in normal use, it means that the capacity after the charge-discharge cycles has hardly decreased with respect to the initial capacity. Here, the "number of times that can be assumed in normal use" depends on the application in which the lithium secondary battery is used. For example, it is 30 times, 50 times, 100 times, 300 times, 500 times, or 1000 times. Also, "the capacity after the charge-discharge cycles has hardly decreased with respect to the initial capacity" depends on the application in which the lithium secondary battery is used. For example, it means that the capacity after the charge-discharge cycles is 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, or 90% or more with respect to the initial capacity.

Example

[0123] Hereinafter, the present invention will be described more specifically using examples and comparative examples. The present invention is not limited by the following examples.

[0124] [Measurement of various physical properties of the fibrous carbon material] The average fiber diameter, aspect ratio, and occupied volume ratio in the carbon-metal composite layer of the fibrous carbon material were measured using FIB and SEM. Specifically, the surface of the carbon-metal composite layer formed on the negative electrode was etched by FIB using a gallium ion beam under the condition of an acceleration voltage of 30 kV to expose the inside of the carbon-metal composite layer. Then, using SEM, the fiber diameter, aspect ratio, and occupied volume ratio of the fibrous carbon material in the surface exposed by the etching and extending in the surface direction were measured. For the calculation of each value, image analysis software attached to SEM was used.

[0125] The average fiber diameter, average aspect ratio, and each value of the occupied volume ratio in the carbon-metal composite layer of the fibrous carbon material were determined by calculating the arithmetic mean of the results of five measurements. Since the measurement is a destructive measurement, as a sample, another sample prepared under the same production conditions as the sample used to determine the characteristics of the battery described later was used. Also, the loading amount (μg / cm 2 ) of the fibrous carbon material was determined by measuring the mass of the negative electrode before and after loading the fibrous carbon material and finding the difference therebetween.

[0126] [Example 1] A lithium secondary battery was produced as follows. First, a 10-μm electrolytic Cu foil was washed with a solvent containing sulfamic acid, punched out to a predetermined size (45 mm × 45 mm), further ultrasonically washed with ethanol, and then dried to obtain a negative electrode.

[0127] The obtained negative electrode was degreased, washed with pure water, and then immersed in a liquid bath in which a fibrous carbon material was dispersed. The charged fibrous carbon material was deposited on the surface of the negative electrode using electrophoresis. After taking out the negative electrode on which the carbon material was deposited from the liquid bath, the negative electrode was immersed in another plating bath containing zinc. By electrolytically plating the surface of the negative electrode while keeping the negative electrode stationary horizontally, zinc was plated on the surface of the negative electrode on which the fibrous carbon material was deposited, and a carbon-metal composite layer was formed on the surface of the negative electrode. The negative electrode on which the carbon-metal composite layer was formed was taken out from the plating bath, washed with ethanol, and washed with pure water. As described above, a carbon-metal composite layer was formed on one side of the negative electrode. The results of measuring each physical property value of the fibrous carbon material in the carbon-metal composite layer are shown in Table 1. A commercially available fibrous carbon material was used.

[0128] Next, a positive electrode was produced. As the positive electrode active material, LiNi 0.85 Co 0.12 Al 0.0396 parts by mass of O2, 2 parts by mass of carbon black as a conductive assistant, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder were mixed, coated on one side of a 12-μm Al foil as a positive electrode current collector, and press-molded. The obtained molded body was punched into a predetermined size (40 mm × 40 mm) by punching to obtain a positive electrode.

[0129] As a separator, a separator with a predetermined size (50 mm × 50 mm) in which both sides of a 12-μm polyethylene microporous membrane were coated with 2-μm polyvinylidene fluoride (PVDF) was prepared. On one side of this separator, a thin film of copper (Cu) as a conductive thin film was formed by sputtering. The sputtering time was adjusted so that the thickness of the thin film became 10 nm. The thickness of the conductive thin film was measured by cutting the separator on which the thin film was formed in the thickness direction and observing the exposed cut surface with SEM.

[0130] As an electrolyte, a 4M LiN(SO2F)2 (LFSI) solution in dimethoxyethane (DME) was prepared.

[0131] The positive electrode, separator, and negative electrode with a carbon metal composite layer formed on one side obtained as described above were laminated in this order so that the surface on which the carbon metal composite layer and the conductive thin film of the separator were formed faced each other to obtain a laminate. Further, after bonding 100-μm Al terminals and 100-μm Ni terminals to the positive electrode and the negative electrode by ultrasonic welding, respectively, they were inserted into a laminated exterior body. Then, the electrolyte obtained as described above was injected into the above exterior body. By sealing the exterior body, a lithium secondary battery was obtained.

[0132] [Examples 2 to 24] A lithium secondary battery was obtained in the same manner as in Example 1, except that a carbon metal composite layer containing the fibrous carbon material and metal described in Tables 1 and 2 was formed using the negative electrode of the materials described in Tables 1 and 2. The plating conditions in electroplating were appropriately adjusted according to the type of metal.

[0133] [Example 25] As a conductive thin film, a lithium secondary battery was obtained in the same manner as in Example 17, except that a 50-nm carbon (C) thin film was formed instead of the 10-nm Cu thin film. Table 2 shows the results of measuring each physical property value of the fibrous carbon material in the carbon metal composite layer.

[0134] [Comparative Example 1] A lithium secondary battery was obtained in the same manner as in Example 1, except that the carbon metal composite layer and the conductive thin film were not formed.

[0135] [Comparative Examples 2-3] A lithium secondary battery was obtained in the same manner as in Example 1, except that a metal layer made of the metal described in Table 3 was formed on the negative electrode instead of the carbon metal composite layer and the conductive thin film was not formed. The method for forming the metal layer was the same as the method for forming the carbon metal composite layer in Example 1, except that the fibrous carbon material was not used. Also, in Table 3, the thicknesses described in Comparative Examples 2-3 mean the thicknesses of the metal layers.

[0136] [Comparative Examples 4-5] In Comparative Example 4, a lithium secondary battery was obtained in the same manner as in Example 15, except that the conductive thin film was not formed. In Comparative Example 5, a lithium secondary battery was obtained in the same manner as in Example 17, except that the conductive thin film was not formed.

[0137] [Comparative Example 6] A lithium secondary battery was obtained in the same manner as in Example 1, except that the carbon metal composite layer was not formed.

[0138] [Evaluation of Energy Density and Cycle Characteristics] The energy density and cycle characteristics of the solid-state batteries fabricated in each Example and Comparative Example were evaluated as follows.

[0139] The fabricated lithium secondary battery was charged at 7 mA until the voltage reached 4.2 V, and then discharged at 7 mA until the voltage reached 3.0 V (hereinafter referred to as "initial discharge"). Subsequently, a cycle of charging at 35 mA until the voltage reached 4.2 V and then discharging at 35 mA until the voltage reached 3.0 V was repeated in an environment at a temperature of 25°C. For all examples and comparative examples, the capacity obtained from the initial discharge (hereinafter referred to as "initial capacity") was 100 mAh, and the capacity areal density was 4.0 mAh / cm 2 was. For each example, the number of cycles when the discharge capacity reached 80% of the initial capacity (i.e., 80 mAh) (referred to as "80% cycle number" in the table) is shown in Table 1.

[0140]

Table 1

[0141]

Table 2

[0142]

Table 3

[0143] In Tables 1 to 3, SWCNT, MWCNT, and VGCF respectively mean single-walled carbon nanotube, multi-walled carbon nanotube, and vapor-grown carbon nanofiber.

[0144] From Tables 1 to 3, it can be seen that Examples 1 to 25 having a carbon-metal composite layer and a conductive thin film have more cycle numbers required for the capacity to decrease from the initial capacity to 80% compared with Comparative Examples 1 to 6 having none of the configurations, indicating that Examples 1 to 25 having a carbon-metal composite layer and a conductive thin film are excellent in cycle characteristics.

[0145] In Tables 1 to 2, when Examples 1 to 3, 4 to 6, 7 to 9, 10 to 11, and 12 to 14 are compared respectively, the effects of the thickness of the carbon-metal composite layer, the aspect ratio of the fibrous carbon material, the type of the fibrous carbon material, the negative electrode material, and the loading amount of the fibrous carbon material can be understood respectively. When each example is compared, it can be said that the example with the most cycles at 80% has excellent cycle characteristics. Also, in Table 2, when Examples 15 to 20 and Examples 21 to 24 are compared respectively, the effects of the type of metal contained in the carbon-metal composite layer and the occupied volume of the fibrous carbon material can be understood respectively.

Industrial Applicability

[0146] The lithium secondary battery of the present invention has high energy density and excellent cycle characteristics, and thus has industrial applicability as a power storage device used in various applications.

Explanation of Reference Numerals

[0147] 100, 300, 400... lithium secondary battery, 110... positive electrode, 120... separator, 130... carbon-metal composite layer, 140... negative electrode, 210... lithium metal, 220... fibrous carbon material, 310... positive electrode current collector, 320... solid electrolyte interface layer, 330... positive electrode terminal, 340... negative electrode terminal, 410... solid electrolyte.

Claims

1. a positive electrode, a negative electrode current collector, a separator disposed between the positive electrode and the negative electrode current collector, a carbon-metal composite layer formed on a surface of the negative electrode current collector facing the separator, a conductive thin film formed on a surface of the separator facing the negative electrode current collector, comprising, the carbon-metal composite layer includes a plurality of fibrous carbon materials randomly oriented, on the carbon-metal composite layer, having no negative electrode active material other than lithium metal, the thickness of the carbon-metal composite layer is 5 nm or more and 1000 nm or less, a lithium secondary battery.

2. a positive electrode, a negative electrode current collector, a solid electrolyte disposed between the positive electrode and the negative electrode current collector, a conductive thin film formed on a surface of the solid electrolyte facing the negative electrode current collector, a carbon-metal composite layer formed on a surface of the negative electrode current collector facing the solid electrolyte, comprising, the carbon-metal composite layer includes a plurality of fibrous carbon materials randomly oriented, on the carbon-metal composite layer, having no negative electrode active material other than lithium metal, the thickness of the carbon-metal composite layer is 5 nm or more and 1000 nm or less, a lithium secondary battery.

3. The lithium secondary battery according to claim 1 or 2, wherein an average fiber diameter of the fibrous carbon material is 2 nm or more and 500 nm or less.

4. The lithium secondary battery according to any one of claims 1 to 3, wherein an average of a ratio of fiber length to fiber diameter of the fibrous carbon material is 20 or more and 5000 or less.

5. The lithium secondary battery according to any one of claims 1 to 4, wherein the fibrous carbon material is at least one selected from the group consisting of single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon nanofibers.

6. The lithium secondary battery according to any one of claims 1 to 5, wherein an occupied volume ratio of the fibrous carbon material in the carbon-metal composite layer is 0.1% or more and 50.0% or less.

7. The lithium secondary battery according to any one of claims 1 to 6, wherein the thickness of the carbon-metal composite layer is 5 nm or more and 500 nm or less.

8. The lithium secondary battery according to any one of claims 1 to 6, wherein the carbon-metal composite layer contains at least one metal selected from the group consisting of Sn, Zn, Bi, Ag, In, Pb, and Al.

9. The lithium secondary battery according to any one of claims 1 to 8, wherein lithium metal is deposited on at least one of the surface of the negative electrode current collector and the surface of the carbon metal composite layer, and charge and discharge are performed by electrolytic elution of the deposited lithium.

10. The lithium secondary battery according to any one of claims 1 to 9, wherein the negative electrode current collector is an electrode made of at least one selected from the group consisting of Cu, Ni, Ti, Fe, other metals that do not react with Li, alloys thereof, and stainless steel (SUS).

11. The lithium secondary battery according to any one of claims 1 to 10, wherein no lithium metal is formed on the surface of the carbon metal composite layer before initial charging.

12. The lithium secondary battery according to any one of claims 1 to 11, having an energy density of 350 Wh / kg or more.

13. The lithium secondary battery according to any one of claims 1 to 12, wherein the positive electrode has a positive electrode active material.

14. The lithium secondary battery according to any one of claims 1 to 13, wherein the conductive thin film is a thin film made of carbon, a thin film made of a metal or an alloy, or a laminated film thereof.

15. The lithium secondary battery according to any one of claims 1 to 14, wherein the thickness of the conductive thin film is 1 μm or less.

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