Secondary battery and method for manufacturing a secondary battery

Abstract

The present invention provides a secondary battery having high energy density and excellent cycle characteristics, wherein the secondary battery has a configuration that enables assembly in a short amount of time. This secondary battery comprises: a layered body formed by winding, by folding back multiple times, a sheet having a negative electrode and separators respectively disposed on both surfaces of the negative electrode; and a plurality of positive electrodes that are respectively disposed in the gaps formed between the mutually opposing separators of the layered body.

Description

[Technical Field]

[0001] This invention relates to a secondary battery and a method for manufacturing a secondary battery. [Background technology]

[0002] In recent years, technologies that convert natural energy sources such as solar or wind power into electrical energy have attracted attention. Accordingly, various secondary batteries have been developed as energy storage devices that are highly safe and capable of storing large amounts of electrical energy.

[0003] Among these, lithium-ion batteries, which perform charging and discharging by the movement of lithium ions between the positive and negative electrodes, are known to exhibit high voltage and high energy density. A typical lithium-ion battery is the lithium-ion battery (LIB), which has active materials capable of holding lithium elements in the positive and negative electrodes, and performs charging and discharging by the exchange of lithium ions between the positive and negative electrode active materials.

[0004] Furthermore, in order to increase energy density and improve production, lithium-metal batteries (LMBs) that use lithium metal as the negative electrode active material instead of materials that can insert lithium ions, such as carbon materials, and lithium-free batteries (AFBs) that use a negative electrode consisting of a negative electrode current collector that does not have a negative electrode active material such as carbon material or lithium metal before initial charging have been developed. For example, Patent Document 1 discloses a rechargeable battery that uses a lithium metal-based electrode as the negative electrode.

[0005] Furthermore, Patent Document 2 discloses a lithium secondary battery comprising a positive electrode, a negative electrode, a separator membrane interposed between them, and an electrolyte, wherein the negative electrode has metal particles formed on a negative electrode current collector, which are moved from the positive electrode by charging, forming lithium metal on the negative electrode current collector within the negative electrode. Patent Document 2 discloses that such a lithium secondary battery can solve problems caused by the reactivity of lithium metal and problems that occur 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] Special Publication No. 2006-500755 [Patent Document 2] Special Publication No. 2019-505971 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] On the other hand, most conventional lithium-ion batteries are formed by stacking positive electrodes, negative electrodes, and separators in sequence. Manufacturing such lithium-ion batteries requires repeatedly picking up and stacking the positive electrodes, negative electrodes, and separators one by one using an arm. Therefore, the arm must repeatedly move back and forth between the location where the positive electrodes, negative electrodes, and separators are placed and the location where they will be stacked, resulting in a long manufacturing process.

[0008] Furthermore, there were lithium secondary batteries formed by alternately folding sheet-shaped separators, or laminates of sheet-shaped separators and negative electrodes, into a zigzag shape, and placing the positive electrodes between separators facing each other in the stacking direction. In such lithium secondary batteries, it is necessary to repeatedly pick up and stack at least one positive electrode at a time. Therefore, the process of the arm repeatedly moving back and forth is unavoidable, and the manufacturing process also required a long time. On the other hand, when both the positive and negative electrodes were of the wound type structure, during repeated charging and discharging, the positive and negative electrodes expanded in volume reversibly and irreversibly at the folded parts, causing problems such as cracks, fractures, and loss of active material at the folded parts.

[0009] This invention has been made in view of the above-mentioned problems, and aims to provide a secondary battery that can be assembled in a short time. [Means for solving the problem]

[0010] A secondary battery according to one embodiment of the present invention is a secondary battery comprising a laminate formed by winding a sheet having a negative electrode and separators arranged on both sides of the negative electrode in a manner that folds multiple times, and a plurality of positive electrodes arranged in each gap formed between the separators facing each other in the laminate.

[0011] The secondary battery with the above configuration can be manufactured by winding the positive electrodes without stacking them one by one. Therefore, it can be assembled in a shorter time compared to conventional secondary batteries that require the process of stacking each positive electrode individually. Furthermore, since there is virtually no positive electrode active material on the end face of the positive electrode opposite the folded portion, reversible and irreversible volume expansion due to repeated charging and discharging does not occur at the folded portion of the winding structure. This makes it possible to suppress problems such as cracks and breaks occurring at the folded portion.

[0012] In the above secondary battery, preferably, it is a secondary battery in which the negative electrode does not have a negative electrode active material. In such a battery without a negative electrode active material, since the negative electrode is substantially formed on the negative electrode current collector facing the positive electrode or within the opposing negative electrode buffer function layer, there is no negative electrode active material on the negative electrode current collector within the folded portion. Therefore, compared with a configuration having a negative electrode active material, the weight of the battery can be reduced, and the weight energy density can be further increased.

[0013] In the above secondary battery, preferably, the sheet includes buffer function layers respectively disposed between the negative electrode and the separators disposed on both sides of the negative electrode.

[0014] According to the secondary battery having the above configuration, by adopting a configuration provided with buffer function layers, the volume expansion of the cell can be effectively suppressed. Thereby, it is possible to suppress the stress caused by the volume expansion of the cell from being applied to the folded portion of the sheet, and it is possible to suppress the occurrence of defects such as breakage or fracture of the folded portion.

[0015] In the above secondary battery, preferably, the buffer function layer has a higher porosity than the separator.

[0016] In the above secondary battery, preferably, the porosity of the buffer function layer is 50% or more and 97% or less.

[0017] In the above secondary battery, preferably, it further includes an electrolytic solution in which the separator is immersed.

[0018] In the above secondary battery, preferably, the negative electrode is formed of at least one selected from the group consisting of metals that do not react with Cu, Ni, Ti, Fe, and Li, alloys thereof, and stainless steel.

[0019] In the above secondary battery, preferably, the positive electrode is disposed at a distance of 0.01 mm or more and 5 mm or less from the folded portion of the sheet in the laminate.

[0020] Preferably, in the above secondary battery, the average thickness of the negative electrode is 1 μm or more and 10 μm or less.

[0021] In the above secondary battery, preferably, the buffering layer has an average thickness of 100 μm or less, a porosity of 50% or more, and a weight of 10 mg / cm² per unit area. 2 The following applies:

[0022] A method for manufacturing a secondary battery according to one embodiment of the present invention includes the steps of: preparing a sheet having a negative electrode without a negative electrode active material and separators arranged on both sides of the negative electrode; arranging a plurality of positive electrodes on the sheet along the longitudinal direction of the sheet; and winding the sheet on which the positive electrodes are arranged so that a positive electrode is placed in each gap formed between the separators facing each other.

[0023] The secondary battery with the above configuration can be manufactured by winding a sheet on which the positive electrodes are arranged, without stacking the positive electrodes one by one. Therefore, it can be assembled in a shorter time compared to conventional secondary batteries that require the process of stacking the positive electrodes one by one. Furthermore, since the manufactured secondary battery employs a negative electrode without negative electrode active material, irreversible volume expansion of the cell, including the positive electrode, negative electrode, and separator, can be suppressed even after repeated use. As a result, stress caused by the volume expansion of the cell can be suppressed at the folded portion of the sheet in the laminate. This prevents defects such as damage or breakage at the folded portion.

[0024] In the above method for manufacturing a secondary battery, preferably, the sheet comprises a buffering layer disposed between the negative electrode and the separators disposed on both sides of the negative electrode. [Effects of the Invention]

[0025] According to the present invention, a secondary battery that can be assembled in a short time can be provided. [Brief explanation of the drawing]

[0026] [Figure 1] This is a cross-sectional view of a secondary battery according to Embodiment 1. [Figure 2] This figure shows the manufacturing process of a secondary battery according to Embodiment 1. [Figure 3] This figure shows the manufacturing process of a secondary battery according to Embodiment 1. [Figure 4] This is a flowchart showing the manufacturing process of a secondary battery according to Embodiment 1. [Figure 5] This is a cross-sectional view of a secondary battery according to Embodiment 2. [Figure 6] This diagram shows the structure of the buffering layer. [Figure 7] This figure shows the manufacturing process of a secondary battery according to Embodiment 2. [Figure 8] This is a cross-sectional view of a secondary battery according to Comparative Example 1. [Figure 9] This is a cross-sectional view of a secondary battery according to Comparative Example 2. [Figure 10] This is a cross-sectional view of the secondary battery according to Comparative Example 3. [Modes for carrying out the invention]

[0027] The embodiments of the present invention will be described in detail below, with reference to the drawings as necessary. In the drawings, the same elements will be denoted by the same reference numerals, and redundant explanations will be omitted. Furthermore, unless otherwise specified, positional relationships such as up, down, left, and right will be based on the positional relationships shown in the drawings. Moreover, the dimensional ratios in the drawings are not limited to those shown.

[0028] [Embodiment 1] (Configuration of a secondary battery) Figure 1 is a schematic cross-sectional view of an anode-free secondary battery 100 according to Embodiment 1. As shown in Figure 1, the secondary battery 100 according to Embodiment 1 comprises a laminate 150 and a plurality of positive electrodes 140. The laminate 150 is formed by winding a sheet 130 so as to be folded multiple times. The sheet 130 has a negative electrode 120 that does not have a negative electrode active material, and a first separator 110a and a second separator 110b arranged on both sides of the negative electrode 120. The plurality of positive electrodes 140 are each arranged in the gaps formed between the separators facing each other in the laminate 150. In this specification, the first separator 110a and the second separator 110b may be collectively referred to as separator 110.

[0029] (Laminated structure) The laminate 150 is formed by winding a sheet 130 so that it is repeatedly folded at the folded portion 160. In the flat portion 170 between the folded portions 160, the sheet 130 extends in a flat shape without bending. The flat portions 170 extend substantially parallel to each other.

[0030] The number of layers in the laminate 150 refers to the number of times the sheet 130 is folded, and corresponds to the number of folded portions 160. For example, as shown in Figure 1, a laminate 150 formed by folding the sheet 130 four times has five flat portions 170 and four folded portions 160, and its number of layers is four. The number of layers in the laminate 150 also corresponds to the number of positive electrodes 140 arranged. The number of layers in the secondary battery 100 is the same as the number of layers in the laminate 150 contained within the secondary battery 100.

[0031] In the secondary battery 100, the number of layers is two or more. That is, the secondary battery 100 comprises a laminate 150 having three flat sections 170 and two folded sections 160. The number of layers of the secondary battery 100 is preferably three or more, more preferably five or more, and even more preferably ten or more. The capacity of the secondary battery 100 is further improved when the number of layers of the secondary battery 100 is within the above range. There is no particular upper limit to the number of layers of the secondary battery 100, but the number of layers may be 50 or less, 40 or less, or 30 or less. Productivity is further improved when the number of layers of the secondary battery 100 is within the above range.

[0032] (Negative electrode) The secondary battery 100 of this embodiment, which is an anode-free lithium secondary battery (hereinafter also referred to as "anode-free battery" or "AFB"), has a negative electrode 120 made of a negative electrode current collector that does not have a negative electrode active material.

[0033] The negative electrode active material is a substance that causes an electrode reaction, i.e., an oxidation reaction and a reduction reaction, at the negative electrode 120. Specifically, examples of the negative electrode active material in this embodiment include lithium metal and a host material for lithium elements (lithium ions or lithium metal). A host material for lithium elements means a substance provided to retain lithium ions or lithium metal at the negative electrode 120. Such retention mechanisms include, for example, intercalation, alloying, and intercalation of metal clusters, with intercalation being the most typical example.

[0034] In an anode-free battery, the negative electrode 120 does not have negative electrode active material before initial charging. In an anode-free battery, after initial charging, lithium metal is deposited on the negative electrode 120, and charging and discharging occur as the deposited lithium metal is electrolytically dissolved. Therefore, an anode-free battery has the advantage of having a higher energy density in principle because the volume and mass of the negative electrode active material are reduced, resulting in a smaller overall volume and mass of the battery.

[0035] In this specification, "substantially free of negative electrode active material" means that at the end of discharge, the thickness of the negative electrode active material layer in the negative electrode 120 is 25 μm or less. The content of negative electrode active material in the negative electrode 120 is preferably 20 μm or less, more preferably 15 μm or less, and may be 0 μm, relative to the entire negative electrode 120. By substantially free of negative electrode active material, the negative electrode 120 can have a high energy density not only by weight but also by volume.

[0036] Furthermore, in the anode-free battery described herein, "before initial charging" means the state from when the battery is assembled until the first charge is performed. Also, "at the end of discharge" means the state in which the open-circuit voltage of the battery is 2.5V or higher and 3.6V or lower.

[0037] Furthermore, in the anode-free battery of this embodiment, the mass M of lithium metal deposited on the negative electrode 120 when the battery voltage is 4.2V 4.2 The mass M of lithium metal deposited on the negative electrode 120 when the battery voltage is 3.0V is... 3.0 Ratio M 3.0 / M 4.2 The ratio M is preferably 40% or less, more preferably 38% or less, and even more preferably 35% or less. 3.0 / M 4.2 It may be 1.0% or more, 2.0% or more, 3.0% or more, or 4.0% or more.

[0038] Examples of negative electrode active materials include lithium metal and alloys containing lithium metal, carbon-based materials, metal oxides, and metals that alloy with lithium and alloys containing such metals. The carbon-based materials are not particularly limited and include, for example, graphene, graphite, hard carbon, and carbon nanotubes. The metal oxides are not particularly limited and include, for example, titanium oxide compounds and cobalt oxide compounds. Examples of metals that alloy with lithium include silicon, germanium, tin, lead, aluminum, and gallium.

[0039] The negative electrode 120 of the anode-free battery is not particularly limited as long as it does not have a negative electrode active material and can be used as a current collector. Examples include at least one selected from the group consisting of Cu, Ni, Ti, Fe, and other metals that do not react with Li, alloys thereof, and stainless steel (SUS). Preferably, it consists of at least one selected from the group consisting of Cu, Ni, alloys thereof, and stainless steel (SUS). Using such a negative electrode 120 tends to result in even better energy density and productivity of the battery. The above-mentioned negative electrode materials can be used individually or in combination of two or more. In this specification, "metals that do not react with Li" means metals that do not react with lithium ions or lithium metal to form alloys under the operating conditions of a lithium secondary battery.

[0040] The average thickness of the negative electrode 120 of the anode-free battery is not particularly limited, and is, for example, 1.0 μm or more and 30 μm or less. From the viewpoint of reducing the volume occupied by the negative electrode 120 in the anode-free battery and improving the energy density, the average thickness of the negative electrode 120 is preferably 2.0 μm or more and 20 μm or less, more preferably 2.0 μm or more and 18 μm or less, and even more preferably 3.0 μm or more and 15 μm or less.

[0041] The negative electrode 120 of the anode-free battery in this embodiment does not have a negative electrode active material coating layer. If a negative electrode active material coating layer is present, there is a risk that the negative electrode 120 may peel off or fall off at the folded portion for winding, which could cause malfunctions inside the battery.

[0042] The negative electrode 120 of an anode-free battery may be coated with a compound (hereinafter also referred to as "negative electrode coating agent") containing an aromatic ring in which two or more elements selected from the group consisting of N, S, and O are independently bonded to at least a portion of the surface facing the positive electrode 140. It is presumed that the negative electrode coating agent can be retained on the negative electrode 120 by at least one element selected from the group consisting of N, S, and O coordinating with a metal atom constituting the negative electrode 120. Furthermore, it is expected that using a negative electrode 120 coated with a negative electrode coating agent will suppress the non-uniform deposition reaction of lithium metal on its surface, thereby inhibiting the growth of lithium metal deposited on the negative electrode 120 in a dendrite-like manner.

[0043] The negative electrode coating agent is not particularly limited as long as it is a compound containing an aromatic ring in which two or more elements selected from the group consisting of N, S, and O are independently bonded, that is, a compound having a structure in which two or more N, S, or O elements are independently bonded to the aromatic ring. Examples of aromatic rings include aromatic hydrocarbons such as benzene, naphthalene, azulene, anthracene, and pyrene, as well as heteroaromatic compounds such as furan, thiophene, pyrrole, imidazole, pyrazole, pyridine, pyridazine, pyrimidine, and pyrazine. Among these, aromatic hydrocarbons are preferred, benzene and naphthalene are more preferred, and benzene is even more preferred.

[0044] Furthermore, it is preferable that the negative electrode coating agent has one or more nitrogen atoms bonded to the aromatic ring. More preferably, the negative electrode coating agent is a compound having a structure in which a nitrogen atom is bonded to the aromatic ring, and in addition to the nitrogen atom, one or more elements selected from the group consisting of N, S, and O are independently bonded to it. When a compound in which a nitrogen atom is bonded to the aromatic ring is used as a negative electrode coating agent, the battery's cycle characteristics tend to improve even further.

[0045] Specifically, examples of negative electrode coating agents include at least one selected from the group consisting of benzotriazole, benzimidazole, benzimidazole thiol, benzoxazole, benzoxazole thiol, benzothiazole, and mercaptobenzothiazole, as well as their derivatives. Among these, at least one selected from the group consisting of benzotriazole, benzimidazole, benzoxazole, and mercaptobenzothiazole, as well as their derivatives, is preferred as the negative electrode coating agent. In such embodiments, the electrical connection between the electronegative electrode and the lithium ions coordinated with the negative electrode coating agent becomes even better, and the battery's cycle characteristics tend to improve further.

[0046] The negative electrode coating agent only needs to be applied to at least a portion of the surface of the negative electrode 120 that faces the positive electrode 140. That is, it is sufficient for 10% or more of the surface area of ​​the negative electrode 120 to be covered by the negative electrode coating agent, preferably 20% or more, more preferably 40% or more, even more preferably 60% or more, and even more preferably 80% or more of the surface to be covered by the negative electrode coating agent.

[0047] (Separator) The separator 110 of an anode-free battery is not particularly limited as long as it has the function of physically and / or electrically isolating the positive electrode 140 and the negative electrode 120, and ensuring the ionic conductivity of lithium ions. Examples of such materials include insulating porous materials, polymer electrolytes, gel electrolytes, and inorganic solid electrolytes, and typically at least one selected from the group consisting of insulating porous materials, polymer electrolytes, and gel electrolytes. Furthermore, the separator 110 may be a single material or a combination of two or more materials.

[0048] As the separator 110 of the anode-free battery, an insulating porous material, a polymer electrolyte, or a gel electrolyte can be used individually or in combination of two or more. When an insulating porous material is used alone as the separator, the secondary battery 100 must also include an electrolyte.

[0049] When the separator 110 includes an insulating porous member, the member exhibits ionic conductivity when an ionic conductive substance is filled into the pores of the member. Therefore, in this embodiment, for example, the electrolyte of this embodiment and a gel electrolyte containing the electrolyte of this embodiment are filled into the member.

[0050] The materials constituting the insulating porous member are not particularly limited, and examples include insulating polymer materials, specifically polyethylene (PE) and polypropylene (PP). That is, the separator may be a porous polyethylene (PE) film, a porous polypropylene (PP) film, or a laminated structure thereof.

[0051] The separator 110 may be covered (coated) with a separator coating layer. The coating layer may cover both sides of the separator, or only one side. From the viewpoint of improving the cycle characteristics of the secondary battery 100 in this embodiment, it is preferable to cover both sides of the separator 110. In this embodiment, the separator coating layer is a uniformly continuous film-like coating layer, for example, a uniformly continuous film-like coating layer covering an area of ​​50% or more of the surface of the separator 110.

[0052] The separator coating layer is not particularly limited, but is preferably one containing a binder such as polyvinylidene fluoride (PVDF), a composite material of styrene-butadiene rubber and carboxymethylcellulose (SBR-CMC), and polyacrylic acid (PAA). The separator coating layer may also have inorganic particles such as silica, alumina, titania, zirconia, and magnesium hydroxide added to the binder.

[0053] The average thickness of the separator, including the separator coating layer, is not particularly limited, and is, for example, 3.0 μm or more and 40 μm or less. In the secondary battery 100, from the viewpoint of reliably isolating the positive electrode 140 and the negative electrode 120 while reducing the volume occupied by the separator 110 in the battery, the average thickness of the separator is preferably 5.0 μm or more and 30 μm or less, more preferably 7.0 μm or more and 10 μm or less, and even more preferably 10 μm or more and 20 μm or less.

[0054] (positive electrode) As shown in Figure 1, in the secondary battery 100, the positive electrodes 140 are each positioned in the gaps formed by the folding and winding of the sheet 130. More specifically, the positive electrodes 140 are each positioned between adjacent planar portions 170 in the stacking direction Z. As shown in Figure 1, the positive electrodes 140 face either the first separator 110a or the second separator 110b.

[0055] Since the positive electrode 140 is positioned between the adjacent planar portions 170 as described above, the positive electrode 140 faces the negative electrode 120 on both sides via the first separator 110a or the second separator 110b.

[0056] As shown in Figure 1, the positive electrode 140 is positioned such that it is spaced between 0.01 mm and 5.00 mm from the winding end 180, which is the end of the folded portion 160 in the laminate 150. That is, the distance d between the positive electrode 140 and the winding end 180 is preferably between 0.01 mm and 5.00 mm. The winding end 180 is the point in the cross-section of the secondary battery 100 that is closest to the positive electrode 140 within the folded portion 160. The distance d between the positive electrode 140 and the winding end 180 is the distance between the winding end 180 and the end face of the positive electrode 140 at the position opposite the winding end 180. Since the time required to position the positive electrode 140 is reduced by a distance d of 0.01 mm or more, the productivity of the secondary battery 100 is further improved. Furthermore, when the distance d is 5.00 mm or less, the area in contact between the positive electrode 140 and the negative electrode 120 is further increased, thereby further improving the energy density and capacity of the secondary battery 100. From a similar viewpoint, the distance d is more preferably 0.05 mm or more and 4.00 mm or less, and even more preferably 0.10 mm or more and 3.00 mm or less.

[0057] The distance d between the positive electrode 140 and the winding end 180 can be measured as follows. First, the secondary battery 100 is cut by a plane parallel to the stacking direction Z and perpendicular to at least one folded portion 160. The resulting cut surface is observed visually, using an optical microscope, or an electron microscope, and the distance d between the positive electrode 140 and the winding end 180 is measured for at least two or more positive electrodes 140. The distance d between the positive electrode 140 and the winding end 180 can be determined by calculating the arithmetic mean of the measurement results.

[0058] The positive electrode 140 of the anode-free type battery is not particularly limited as long as it is generally used in a lithium secondary battery, and known materials can be appropriately selected according to the use of the lithium secondary battery. From the viewpoint of improving the stability and output voltage of the battery, it is preferable that the positive electrode 140 has a positive electrode active material. When the positive electrode 140 has a positive electrode active material, typically, lithium ions are filled and desorbed in the positive electrode active material by charging and discharging the battery.

[0059] In addition, in this specification, the "positive electrode active material" is a substance that causes an electrode reaction, that is, an oxidation reaction and a reduction reaction, in the positive electrode 140. Specifically, examples of the positive electrode active material include host materials of lithium elements (typically, lithium ions).

[0060] Such positive electrode active materials are not particularly limited, and examples include metal oxides and metal phosphates. The above metal oxides are not particularly limited, and examples include cobalt oxide-based compounds, manganese oxide-based compounds, nickel oxide-based compounds, etc. The above metal phosphates are not particularly limited, and examples 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 Co y Al z O(x + y + z = 1), LiNi x Mn y O(x + y = 1), LiNiO2, LiMn2O4, LiFePO, LiCoPO, LiFeOF, LiNiOF, and LiTiS2. The above positive electrode active materials are used alone or in combination of two or more.

[0061] The positive electrode 140 may contain components other than the above positive electrode active material. Such components are not particularly limited, and examples include sacrificial positive electrode materials, conductive aids, binders, gel electrolytes, and polymer electrolytes.

[0062] Here, the sacrificial cathode material is a lithium-containing compound that undergoes an oxidation reaction within the charge-discharge potential range of the cathode active material, but substantially does not undergo a reduction reaction. In particular, in an anode-free battery, the cathode 140 may include a sacrificial cathode.

[0063] In particular, the positive electrode 140 may contain a gel electrolyte. In such an embodiment, the function of the gel electrolyte improves the adhesion between the positive electrode 140 and the positive electrode current collector, making it possible to attach a thinner positive electrode current collector and further improving the energy density of the battery. When attaching the positive electrode current collector to the surface of the positive electrode, a positive electrode current collector formed on release paper may be used.

[0064] The conductive additive in the positive electrode 140 is not particularly limited and includes, for example, carbon black, single-wall carbon nanotubes (SWCNTs), multi-wall carbon nanotubes (MWCNTs), and carbon nanofibers (CF). The binder is also not particularly limited and includes, for example, polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylic resin, and polyimide resin.

[0065] The gel electrolyte is not particularly limited, but examples include those containing a polymer, an organic solvent, and a lithium salt. The polymer in the gel electrolyte is not particularly limited, but examples include copolymers of polyethylene and / or polyethylene oxide, polyvinylidene fluoride, and copolymers of polyvinylidene fluoride and hexafluoropropylene. The polymer electrolyte is not particularly limited, but examples include solid polymer electrolytes mainly containing a polymer and an electrolyte, and semi-solid polymer electrolytes mainly containing a polymer, an electrolyte, and a plasticizer.

[0066] The average thickness of the positive electrode 140 is preferably 20 μm to 100 μm, more preferably 30 μm to 80 μm, and even more preferably 40 μm to 70 μm. However, the average thickness of the positive electrode 140 can be appropriately adjusted according to the desired battery capacity.

[0067] A positive electrode current collector is placed on one side of the positive electrode 140 of the anode-free battery. 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. For example, aluminum can be used as such a positive electrode current collector. Note that a positive electrode current collector is not required, in which case the positive electrode 140 itself acts as a current collector. The positive electrode current collector acts to transfer electrons to and from the positive electrode 140 (especially the positive electrode active material) and is in physical and / or electrical contact with the positive electrode.

[0068] In an anode-free battery, the average thickness of the positive electrode current collector is preferably 1.0 μm to 15 μm, more preferably 2.0 μm to 10 μm, and even more preferably 3.0 μm to 6.0 μm. According to such an embodiment, the volume occupied by the positive electrode current collector in the anode-free battery is reduced, thereby further improving the energy density of the anode-free battery.

[0069] The content of the positive electrode active material in the positive electrode 140 may be, for example, 50% by mass or more and 100% by mass or less relative to the entire positive electrode 140. The content of the conductive additive may be, for example, 0.5% by mass and 30% by mass or less relative to the entire positive electrode 140. The content of the binder may be, for example, 0.5% by mass and 30% by mass or less relative to the entire positive electrode 140. The total content of the solid polymer electrolyte and the inorganic solid electrolyte may be, for example, 0.5% by mass and 30% by mass or less relative to the entire positive electrode 140.

[0070] (electrolyte) The secondary battery 100 preferably further comprises an electrolyte.

[0071] The electrolyte is a liquid containing a solvent and an electrolyte, and it is ionic conductive. The electrolyte can also be called a liquid electrolyte, and it acts as a conductive pathway for lithium ions. Therefore, when the secondary battery 100 contains an electrolyte, its internal resistance tends to decrease further, and its energy density, capacity, and cycle characteristics tend to improve further.

[0072] The electrolyte may be impregnated into the separator 110, or the electrolyte may be sealed together with the laminate of the negative electrode 120, separator 110, positive electrode 140, and positive electrode current collector to form the finished secondary battery 100.

[0073] The electrolyte contained in the electrolyte solution can be an electrolyte that can be found in polymer electrolytes and gel electrolytes, in particular the lithium salts mentioned above, either alone or in combination of two or more. Preferred lithium salts are the same as those used in polymer electrolytes and gel electrolytes.

[0074] Examples of solvents included in the electrolyte are non-aqueous solvents containing fluorine atoms (hereinafter referred to as "fluorinated solvents") and non-aqueous solvents that do not contain fluorine atoms (hereinafter referred to as "non-fluorinated solvents").

[0075] The fluorinating solvent is not particularly limited, but examples include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether.

[0076] Examples of non-fluorinated solvents include, but are not limited to, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,2-dimethoxyethane, dimethoxyethane, dimethoxypropane, dimethoxybutane, diethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, ethylene carbonate, propylene carbonate, chloroethylene carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, trimethyl phosphate, triethyl phosphate, and 12-crown-4.

[0077] The above-mentioned fluorinated solvent and / or non-fluorinated solvent can be used individually or in any combination of two or more in any proportion. The content of the fluorinated solvent and non-fluorinated solvent is not particularly limited, and the proportion of the fluorinated solvent to the total solvent may be 0 to 100% by volume, and the proportion of the non-fluorinated solvent to the total solvent may be 0 to 100% by volume.

[0078] (Positive terminal and negative terminal) As shown in Figure 3(e), which illustrates the wound secondary battery 100, the secondary battery 100 has at least one negative electrode terminal 310 on the flat portion 170 of the laminate 150. The secondary battery 100 also has a positive electrode terminal 320 on each positive electrode 140. The negative electrode terminal 310 and the positive electrode terminal 320 are connected to an external circuit. The material of the negative electrode terminal 310 and the positive electrode terminal 320 is not particularly limited as long as it is conductive, but examples include Al and Ni.

[0079] (Use of rechargeable batteries) The secondary battery 100 is charged and discharged by connecting the negative terminal 310 to one end of the external circuit and the positive terminal 320 to the other end of the external circuit. If there are multiple negative terminals 310, they are all connected to the external circuit so that they are at the same potential. Similarly, the positive terminals 320 are all connected to the external circuit so that they are at the same potential.

[0080] The secondary battery 100 is charged by applying a voltage between the positive terminal 320 and the negative terminal 310 such that current flows from the negative terminal 310 through the external circuit to the positive terminal 320. Charging the secondary battery 100 causes the deposition of carrier metal at the interface between the negative electrode 120 and the first separator 110a, and at the interface between the negative electrode 120 and the second separator 110b. The deposited carrier metal is typically lithium metal.

[0081] The secondary battery 100 may have a solid electrolyte interface layer (SEI layer) formed at the interface between the negative electrode 120 and the first separator 110a, and / or at the interface between the negative electrode 120 and the second separator 110b, upon initial charging. The formed SEI layer is not particularly limited, but may contain, for example, an inorganic carrier metal and an organic carrier metal. Typically, it may contain an inorganic compound containing lithium and an organic compound containing lithium. The typical average thickness of the SEI layer is 1 nm to 10 μm.

[0082] If an SEI layer is formed on the secondary battery 100, the carrier metal deposited during charging of the secondary battery 100 may be deposited at the interface between the negative electrode 120 and the SEI layer, at the interface between the SEI layer and the first separator 110a, or at the interface between the SEI layer and the second separator 110b.

[0083] When the positive terminal 320 and negative terminal 310 of the charged secondary battery 100 are connected, the secondary battery 100 is discharged. The carrier metal precipitates formed at at least one of the interfaces between the negative electrode 120 and the SEI layer, the interface between the SEI layer and the first separator 110a, and the interface between the SEI layer and the second separator 110b dissolve.

[0084] (Method of manufacturing secondary batteries) Figures 2 and 3 are diagrams showing the manufacturing process of the secondary battery 100 according to this embodiment. Figure 4 is a flowchart showing the manufacturing process of the secondary battery 100 according to this embodiment. The manufacturing method of the secondary battery 100 will be described in detail below with reference to these figures.

[0085] (Sheet preparation process) First, a sheet 130 is prepared, which has a negative electrode 120 without negative electrode active material and separators 110 arranged on both sides of the negative electrode 120 (S1 in Figure 4). The sheet preparation process is not particularly limited as long as it involves arranging separators 110 on both sides of the negative electrode 120, but for example, a roll-to-roll method can be used.

[0086] The roll-to-roll method is performed, for example, as follows: A roll is prepared on which a sheet containing the material constituting the negative electrode 120 (hereinafter referred to as "negative electrode sheet 121") is wound, a roll is prepared on which a sheet containing the material constituting the first separator 110a (hereinafter referred to as "first separator sheet 111a") is wound, and a roll is prepared on which a sheet containing the material constituting the second separator 110b (hereinafter referred to as "second separator sheet 111b") is wound. Figure 2(a) shows the negative electrode sheet 121, the first separator sheet 111a, and the second separator sheet 111b. These rolls are placed in a predetermined apparatus, and while returning each roll to a sheet shape, the negative electrode sheet 121 is sandwiched between the first separator sheet 111a and the second separator sheet 111b and pressed in the thickness direction of the sheets to form a sheet 130 on which the first separator sheet and the second separator sheet are arranged on both sides of the negative electrode sheet. Figure 2(b) shows the sheet 130 produced in this manner. The obtained sheet 130 can be wound into a roll and used for the next process.

[0087] By using the roll-to-roll method in the sheet preparation process, a sheet 130 can be formed in which a first separator sheet and a second separator sheet are arranged on both sides of the negative electrode sheet while pulling the negative electrode sheet in the planar direction, thus reducing the likelihood of wrinkles forming on the negative electrode sheet. Furthermore, since the resulting sheet 130 is wound into a roll, it is easy to use in subsequent processes, resulting in even greater productivity.

[0088] The negative electrode sheet may be the same thickness as the negative electrode 120, or it may be thicker than the negative electrode 120. If the negative electrode sheet is thicker than the negative electrode 120, the negative electrode sheet can be thinned by rolling it before the step of sandwiching it between the first separator sheet and the second separator sheet.

[0089] The sheet preparation step may include a washing step and a drying step before and / or after forming the sheet 130 having a negative electrode 120 and separators 110 arranged on both sides of the negative electrode 120. Examples of the washing step include washing the negative electrode sheet with a solvent containing sulfamic acid, followed by ultrasonic cleaning with ethanol.

[0090] (Positive electrode preparation process) Next, the positive electrode 140 is prepared (S2 in Figure 4). The method for manufacturing the positive electrode 140 is not particularly limited as long as it is a method that can obtain the positive electrode 140 as described above. For example, it may be obtained by mixing a positive electrode active material, a known conductive additive, and a known binder to obtain a positive electrode mixture, coating one side of a metal foil (e.g., Al foil) that is, for example, 5 μm or more and 1 mm or less in thickness, and then press molding it. Alternatively, a commercially available positive electrode for secondary batteries may be used.

[0091] (Positive electrode placement process) Next, the positive electrodes 140 are placed on the sheet 130 at predetermined intervals (S3 in Figure 4). As shown in Figure 2(c), no positive electrodes 140 are placed at the right end of the diagram, which is the side where winding begins. As the sheet 130 is wound, the laminate 150 is formed, so the spacing between the positive electrodes 140 widens as it moves away from the side where winding begins.

[0092] (molding process) Next, the sheet 130 on which the positive electrodes 140 are placed is wound up from one end to the other, as shown in Figure 3 (S4 in Figure 4). At this time, the sheet 130 is wound up by folding it back so that the center of the fold is approximately midway between the adjacent positive electrodes 140. As a result, a molded body as shown in Figure 1 is obtained, which consists of a laminate 150 formed by winding the sheet 130 in a way that folds it back multiple times, and a positive electrode 140 placed in each gap formed between the separators 110 facing each other in the laminate 150.

[0093] Thus, in the molding process, the negative electrode 120, separator 110, and positive electrode 140 necessary for forming the molded body are prepared and simply wound together, eliminating the need to use an arm or other device to place the positive electrode 140 or negative electrode 120 along the way. Therefore, compared to conventional manufacturing methods that require the use of an arm or other device to place the positive electrode 140 or negative electrode 120, the time required to assemble the molded body can be significantly reduced.

[0094] (Sealing process) Next, a sealed body is obtained by sealing the molded body shown in Figure 1 into a sealed container, which becomes the secondary battery 100 (S5 in Figure 4). In the sealing process, an electrolyte may also be sealed into the sealed container. By sealing the electrolyte in this way, the internal resistance of the secondary battery 100 is further reduced, and the energy density, capacity, and cycle characteristics of the secondary battery 100 become even better.

[0095] The sealed container used in the sealing process is not particularly limited, but an example is a laminate film.

[0096] [Embodiment 2] The secondary battery 101 of Embodiment 2 differs from the secondary battery 100 of Embodiment 1 in that sheet 131, corresponding to sheet 130, has a buffering layer 190. That is, in the laminate 151, corresponding to the laminate 150 of Embodiment 1, the buffering layer 190 is placed between the separator 110 and the negative electrode 120. The following description will focus on the differences between this embodiment and Embodiment 1, and will omit the explanation of parts common to both Embodiment 1 and Embodiment 1.

[0097] (Configuration of a secondary battery) Figure 5 is a schematic cross-sectional view of an anode-free secondary battery 101 according to Embodiment 2. As shown in Figure 5, the secondary battery 101 comprises a laminate 151 and a plurality of positive electrodes 140. The laminate 151 is formed by winding a sheet 131 so that it is folded multiple times. The sheet 131 comprises a negative electrode 120 without negative electrode active material, a first separator 110a and a second separator 110b, and a buffering layer 190. As shown in Figure 5, the buffering layer 190 is placed between the negative electrode 120 and the first separator 110a and the second separator 110b.

[0098] (buffer layer) The buffering layer 190 of Embodiment 2 is porous and has ionic and electrical conductivity. In this embodiment, porous includes fibrous material. Here, because the buffering layer 190 is porous, it has solid portions that have ionic and electrical conductivity, and void portions (synonymous with "gap portions"; the same applies hereinafter in this specification) formed by the gaps in the solid portions. In this specification, the "solid portion" of the buffering layer 190 includes gel-like portions.

[0099] As the buffering layer 190 is configured as described above, when the secondary battery is charged, electrons from the negative electrode and lithium ions from the separator and / or electrolyte are supplied to the solid portion. As a result, the electrons and lithium ions supplied as described above react on the surface of the solid portion, and lithium metal is deposited in the vacant portions (surface of the solid portion). Therefore, the buffering layer 190 has the effect of suppressing the volume expansion of the battery caused by lithium metal deposition when the battery is charged. In other words, the secondary battery 101 can suppress volume expansion or volume change caused by charging and discharging even more effectively than the secondary battery 100 of Embodiment 1.

[0100] The inventors have found that, in the secondary battery of this embodiment, the cycle characteristics of the battery do not deteriorate even when a buffering layer is introduced, or may even improve. The inventors surmise that this is because the introduction of the buffering layer increases the surface area of ​​the site where lithium metal can be deposited, thereby slowly controlling the reaction rate of the lithium metal deposition reaction and more reliably suppressing the formation of dendrite-like lithium metal.

[0101] In this embodiment, "lithium metal is deposited on the negative electrode" means, unless otherwise specified, that lithium metal is deposited on at least one location on the surface of the negative electrode and on the surface of the solid portion of the buffering layer. Therefore, in the lithium secondary battery of this embodiment, lithium metal may be deposited, for example, on the surface of the negative electrode (the interface between the negative electrode and the buffering layer) or inside the buffering layer (the surface of the solid portion of the buffering layer).

[0102] One embodiment of the buffering layer 190 is a fibrous buffering layer. Figure 6(A) shows a schematic cross-sectional view of the fibrous buffering layer. The buffering layer 190 shown in Figure 6(A) consists of ion-electrical conductive fibers 410, which are fibers having ion conductivity and electrical conductivity. In other words, in this embodiment, "the buffering layer is fibrous" means that the buffering layer contains fibers or is composed of fibers, and has solid portions and vacant portions composed of gaps between the solid portions.

[0103] When a lithium secondary battery having the buffering layer 190 shown in Figure 6(A) is charged, lithium metal is deposited on the surface of the solid portion of the buffering layer, that is, on the surface of the ion-conducting fiber 410. Therefore, in such an embodiment, as shown in the schematic cross-sectional view in Figure 6(B), lithium metal 420 is deposited on the surface of the ion-conducting fiber 410, which is the solid portion of the buffering layer, filling the voids in the buffering layer. In other words, the lithium metal 420 is deposited inside the buffering layer 190.

[0104] One embodiment of the ion-conducting fiber 410 is shown as a schematic cross-sectional view in Figure 6(C). As shown in Figure 6(C), in one embodiment, the ion-conducting fiber 410 comprises a fibrous ion-conducting layer 430 and an electrical conductive layer 440 covering the surface of the ion-conducting layer 430. The ion-conducting layer 430 may have the configuration described above as an ion-conducting layer, for example, and the electrical conductive layer 440 may have the configuration described above as an electrical conductive layer.

[0105] The average fiber diameter of the fibrous ion-conducting layer 430 is preferably 30 nm to 5000 nm, more preferably 50 nm to 2000 nm, even more preferably 70 nm to 1000 nm, and even more preferably 80 nm to 500 nm. Having the average fiber diameter of the ion-conducting layer within the above range allows for a more appropriate surface area of ​​the reaction field where lithium metal can be deposited, thus tending to further improve the cycle characteristics.

[0106] The average thickness of the electrical conductive layer 440 is preferably 1 nm to 300 nm, more preferably 5 nm to 200 nm, and even more preferably 10 nm to 150 nm. By having the average thickness of the electrical conductive layer within the above range, the electrical conductivity of the ion electrical conductive fiber 410 can be maintained more appropriately, and thus the cycle characteristics tend to improve further.

[0107] The buffering layer 190 of this embodiment is not limited to the configuration described above, and is not particularly limited as long as it is porous (including fibrous) and has a porosity of 50% to 97%. Non-limiting examples of the buffering layer 190 include, for example, nonwoven fabrics or woven fabrics made of fibers. The material of the buffering layer 190 may be inorganic, organic, metallic, or a combination of these. Furthermore, a material that has been plated to impart conductivity to a framework that does not have electronic conductivity may be used.

[0108] The porosity (also called void ratio) of the buffer functional layer 190 is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more, in volume percent. When the porosity of the buffer functional layer is within the above range, the surface area of ​​the reaction field where lithium metal can be deposited increases further, thus tending to further improve the cycle characteristics. Furthermore, according to such an embodiment, the effect of suppressing cell volume expansion tends to be more effective and reliable. The porosity of the buffer functional layer may be 97% or less, 95% or less, or 90% or less, in volume percent.

[0109] The average thickness of the buffering layer 190 is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 30 μm or less. By having the average thickness of the buffering layer within the above range, the volume occupied by the buffering layer 190 in the secondary battery 101 is reduced, thus further improving the energy density of the battery. Alternatively, the average thickness of the buffering layer is preferably 1 μm or more, more preferably 4 μm or more, and even more preferably 7 μm or more. By having the average thickness of the buffering layer within the above range, the surface area of ​​the reaction field where lithium metal can be deposited is further increased, thus tending to further improve the cycle characteristics. Furthermore, according to such an embodiment, the effect of suppressing cell volume expansion tends to be more effective and reliable.

[0110] The fiber diameter of the fibrous ion-conducting layer, the thickness of the electrical-conducting layer, the porosity of the buffering layer, and the thickness of the buffering layer can be measured by known measurement methods. For example, the thickness of the buffering layer can be measured by etching the surface of the buffering layer with a focused ion beam (FIB) to expose its cross-section, and then observing the thickness of the buffering layer at the exposed cross-section with an electron microscope.

[0111] The fiber diameter of the fibrous ion-conducting layer, the thickness of the electrical-conducting layer, and the porosity of the buffering layer can be measured by observing the surface of the buffering layer with an electron microscope. The porosity of the buffering layer can be calculated by performing a binary analysis of the observed image of the buffering layer's surface using image analysis software and determining the proportion of the buffering layer that occupies the total area of ​​the image. Each of the above measurements is calculated by taking the average of three or more measurements, preferably ten or more.

[0112] Furthermore, if the buffering layer 190 contains a metal that can react with lithium, the sum of the capacities of the negative electrode 120 and the buffering layer 190 is sufficiently small compared to the capacity of the positive electrode 140, for example, 20% or less, 15% or less, 10% or less, or 5% or less. The capacities of the positive electrode 140, the negative electrode 120, and the buffering layer 190 can be measured by conventionally known methods.

[0113] The weight per surface area of ​​the cushioning layer 190 is preferably 3 g / m². 2 More than 20g / m 2 The following is more preferable: 4 g / m 2 More than 15g / m 2 The following is more preferably 5 g / m 2 More than 10g / m 2 The following applies:

[0114] (Method of manufacturing secondary batteries) The secondary battery 101 of this embodiment differs from that of Embodiment 1 in that, as described above, sheet 130 is replaced with sheet 131. Here, we will explain the differences between this embodiment and Embodiment 1 in the sheet preparation step for preparing sheet 131. The manufacturing method other than the sheet preparation step in this embodiment is the same as that of Embodiment 1, and therefore its explanation will be omitted.

[0115] (Sheet preparation process) In the sheet preparation step of this embodiment, a sheet 131 is prepared having a negative electrode 120 without negative electrode active material, a buffering functional layer 190, and a separator 110. The sheet preparation step is not particularly limited as long as it involves arranging separators 110 on both sides of the negative electrode 120, but for example, the roll-to-roll method can be used.

[0116] In this embodiment, as shown in Figure 7(a), in addition to the negative electrode sheet 121, the first separator sheet 111a, and the second separator sheet 111b, a buffer function layer sheet 191 having the material constituting the buffer function layer 190 is prepared, and these are joined together to form the sheet 131 shown in Figure 7(c). The process for forming the sheet 131 is the same as in Embodiment 1.

[0117] Next, as shown in Figure 7(d), the positive electrodes 140 are placed on the sheet 131 at predetermined intervals. The steps from this point onward are the same as in Embodiment 1. [Examples]

[0118] The present invention will be described more specifically below using examples and comparative examples. The present invention is not limited in any way by the following examples.

[0119] [Example 1] A Cu substrate with a thickness of 8 μm was prepared as the negative electrode sheet. The negative electrode terminals were attached to the negative electrode sheet in advance by ultrasonic welding to Ni terminals. Separators (thickness: 15 μm) with a surface coated with a mixture of polyvinylidene fluoride (PVDF) and Al2O3 were prepared as the first and second separator sheets. The negative electrode sheet was sandwiched between the first and second separator sheets and pressed in the thickness direction of the sheets to obtain a sheet in which separators were arranged on both sides of the negative electrode.

[0120] N-methylpyrrolidone (NMP) is used as the solvent, and LiNi is used as the positive electrode active material. 0.8 Co 0.15 Al 0.05A mixture of 96 parts by mass of O2, 2 parts by mass of carbon black as a conductive additive, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder was applied to both sides of a 12 μm aluminum foil and press-molded. The resulting molded body was punched out to a predetermined size to obtain a positive electrode. The positive electrode terminal was pre-attached to the aluminum foil by joining aluminum terminals using ultrasonic welding. The obtained positive electrode was charged with a current equivalent to 0.1C until it reached 4.2V (vs. lithium metal counter electrode), and then discharged until it reached 3.0V (vs. lithium metal counter electrode), resulting in a discharge capacity of 4.8 mAh / cm². 2 They demanded that it be so.

[0121] Next, the positive electrode prepared above was placed on the sheet on which the first and second separator sheets were arranged on both sides of the negative electrode sheet, resulting in the state shown in Figure 2(c).

[0122] Next, the sheet with the positive electrodes was placed in an automatic winding device and wound sequentially from one end, resulting in the structure shown in Figure 1, where positive electrodes are placed in each gap of the laminate. This was then inserted into a laminated outer casing. The number of laminate layers was adjusted so that the initial capacity of the resulting secondary battery would be 10 Ah.

[0123] Furthermore, a 4M LiN(SO2F)2 (hereinafter also referred to as "LFSI") dimethoxyethane (hereinafter also referred to as "DME") solution was injected into the above-mentioned casing as the electrolyte. A secondary battery was obtained by sealing the casing.

[0124] [Example 2] A negative electrode sheet, a first separator sheet, and a second separator sheet were prepared in the same manner as in Example 1, and a buffering layer was prepared on the surfaces of the first separator sheet and the second separator sheet that were in contact with the negative electrode sheet. The buffering layer was formed on one side of the separator as follows.

[0125] A resin solution, obtained by dissolving PVDF resin in N-methylpyrrolidone (NMP), was applied to a separator using a bar coater. Next, the separator coated with the resin solution was immersed in a water bath and then thoroughly dried at room temperature to form a fibrous ion-conducting layer on the separator. (Note that the ion-conducting layer exhibits its ion-conducting function when the electrolyte solution (4M LiN(SO2F)2(LFSI) in dimethoxyethane (DME) solution), described later, is injected during battery assembly.)

[0126] The average fiber diameter of the fibrous ion-conducting layer formed on the separator was observed and measured using a scanning electron microscope (SEM) and found to be 100 nm.

[0127] Next, Ni was deposited onto a separator with a fibrous ion-conducting layer under vacuum conditions. Observation of the ion-conducting layer after Ni deposition using an energy-dispersive X-ray spectrometer (EDX) equipped SEM confirmed that the Ni was distributed to cover the fibrous ion-conducting layer, and that a fibrous buffer layer was obtained in which the surface of the fibrous ion-conducting layer was covered by an electrical conductive layer.

[0128] Furthermore, when a cross-section of the buffering layer was prepared using FIB and observed with SEM, the average thickness of the buffering layer was found to be 10 μm. When the buffering layer was observed with a transmission electron microscope, the average thickness of the Ni thin film, which is the electrical conductive layer, and the porosity of the buffering layer were found to be 20 nm and 90%, respectively.

[0129] A negative electrode sheet and a first separator sheet and a second separator sheet having a buffering layer were laminated together to obtain the laminate shown in Figure 7(c). Subsequently, the structure shown in Figure 5 was obtained using the same method as in Example 1 and inserted into a laminated outer casing. The number of laminates was adjusted so that the initial capacity of the resulting secondary battery would be 10 Ah.

[0130] [Comparative Example 1] A negative electrode sheet and first and second separator sheets with the same configuration as those used in Example 1 were prepared in a size appropriate for lamination with the positive electrode. These positive electrode, negative electrode, and separator sheets were placed in an automatic lamination device and laminated sequentially to form a laminate as shown in Figure 8. This laminate was then inserted into a laminated outer casing to obtain the secondary battery 102. The number of layers in the laminate was adjusted so that the initial capacity of the resulting secondary battery would be 10 Ah.

[0131] [Comparative Example 2] Separator sheets identical to the first and second separator sheets used in Example 1, along with a positive electrode, were prepared. Furthermore, similar to Comparative Example 1, a negative electrode with the same configuration as the negative electrode sheet used in Example 1 was prepared in an appropriate size.

[0132] Next, the separator sheets were placed in an automatic lamination device, and the separator sheets were automatically folded alternately at multiple acute angles, thereby alternately arranging the positive and negative electrodes between opposing separator sheets, resulting in a laminate with a zigzag structure (also called a "cross-fold structure") as shown in Figure 9.

[0133] In this process, the automatic lamination device pressed a first flat plate against the sheet from a first direction perpendicular to the lamination direction of the laminate, pressed a second flat plate against the separator sheet from a second direction opposite to the first direction, and pressed the separator sheet from the opposite direction to the lamination direction of the laminate, thereby bending the separator sheet. The process of removing the first and second flat plates was then repeated, thereby automatically bending the separator sheet multiple times at sharp angles alternately. In this process, tension was applied to the separator sheet in the direction of its long axis while bending. The number of layers in the laminate was adjusted so that the initial capacity of the resulting secondary battery would be 10 Ah.

[0134] Next, the positive and negative electrodes prepared above were alternately inserted into each gap of the laminate obtained as described above. In this way, a structure was obtained in which the positive electrode 140 and negative electrode 120 are alternately arranged in each gap of the separator 110 in the separator sheet, as shown in Figure 9. This was then inserted into a laminate outer casing to obtain the secondary battery 103.

[0135] [Comparative Example 3] A sheet was prepared by laminating the negative electrode sheet used in Example 1 with the first and second separator sheets, and a positive electrode was also prepared.

[0136] Next, this sheet was placed in an automatic lamination apparatus, and the sheet was automatically folded multiple times at sharp angles alternately, thereby positioning the positive electrode between opposing separator sheets, and a zigzag-structured laminate as shown in Figure 10 was obtained. The process of forming the laminate using the automatic lamination apparatus was the same as in Comparative Example 2, except that the separator sheet was replaced with the sheet of this comparative example.

[0137] Next, the positive electrodes prepared above were inserted into each gap in the laminate obtained as described above. In this way, a structure in which positive electrodes 140 are arranged in each gap of the separator 110 was obtained, as shown in Figure 10. This was then inserted into a laminate outer casing to obtain a secondary battery 104.

[0138] [evaluation] Table 1 shows the time required to assemble the secondary batteries of the above examples and comparative examples, and the volume expansion rate of the cells after 20 charge-discharge cycles in which these secondary batteries are charged and discharged.

[0139] The assembly time for the secondary battery was measured in Examples 1 and 2 from the point where the positive electrode was placed on the sheet. In Comparative Examples 2 and 3, it was measured from the point where the sheet was prepared. In each example and modification, the measurement was stopped when the formation of the laminate was completed.

[0140] [Table 1]

[0141] As shown in Table 1 above, in Example 1 and Example 2, the volume expansion rates in the discharged state after 20 charge-discharge cycles were 18% and 11%, respectively, indicating favorable results. Furthermore, the time required for manufacturing was 20 seconds in both cases, confirming that laminates can be formed in a shorter time compared to the examples. [Industrial applicability]

[0142] The secondary battery of the present invention has high energy density and capacity, and excellent cycle characteristics, making it industrially applicable as an energy storage device for various uses. [Explanation of symbols]

[0143] 100, 101…Secondary battery 110... Separator 120...Negative electrode 130, 131…sheets 140...Positive electrode 150, 151... Laminate 160...Folded section 170...Plane part 180...winding end 190...Buffer function layer 310...Negative terminal 320... Positive terminal

Claims

1. A laminate formed by winding a sheet having a negative electrode without a negative electrode active material and separators arranged on both sides of the negative electrode, in a manner that it is folded multiple times, The laminate comprises a plurality of positive electrodes arranged in a gap formed between the separators facing each other, Secondary battery.

2. The sheet comprises a buffering layer disposed between the negative electrode and the separators arranged on both sides of the negative electrode. The secondary battery according to claim 1.

3. The buffering layer has a higher porosity than the separator. The secondary battery according to claim 2.

4. The porosity of the buffering layer is 50% or more and 97% or less. The secondary battery according to claim 2.

5. The electrolyte in which the separator is immersed is further comprising The secondary battery according to claim 2.

6. The negative electrode is formed from at least one material selected from the group consisting of metals that do not react with Cu, Ni, Ti, Fe, and Li, alloys thereof, and stainless steel. The secondary battery according to claim 2.

7. The positive electrode is positioned at a distance of 0.01 mm to 5 mm from the folded portion of the sheet in the laminate. The secondary battery according to claim 2.

8. The average thickness of the negative electrode is 1 μm or more and 30 μm or less. The secondary battery according to claim 2.

9. The buffering layer has an average thickness of 100 μm or less, a porosity of 50% to 97%, and a weight of 10 g / m² per unit area. 2 The following is: The secondary battery according to claim 3.

10. A step of preparing a sheet having a negative electrode without a negative electrode active material and separators arranged on both sides of the negative electrode, The steps include: arranging a plurality of positive electrodes on the sheet along the longitudinal direction of the sheet; The step includes winding the sheet on which the positive electrode is placed so that the positive electrode is positioned in a gap formed between the separators facing each other, A method for manufacturing secondary batteries.

11. The sheet comprises a buffering layer disposed between the negative electrode and the separators arranged on both sides of the negative electrode. A method for manufacturing a secondary battery according to claim 10.

Images