Lithium secondary battery

Abstract

A lithium secondary battery (10) according to the present disclosure includes a positive electrode (11), a negative electrode (12), a separator (13), and a non-aqueous electrolyte. The negative electrode (12) includes a current collector and a lithium alloy layer formed on the current collector. The lithium alloy layer contains magnesium. The negative electrode (12) is a negative electrode in which a metal layer containing lithium and magnesium is formed on a lithium alloy layer by depositing lithium metal during charging, and the lithium metal is dissolved during discharging. The ratio of the number of moles of magnesium (RAM) to the number of moles of lithium (RAL) in the lithium alloy layer (RAM / RAL) is greater than the ratio of the number of moles of magnesium (RMM) to the number of moles of lithium (RML) in the metal layer (RMM / RML) in a state where the state of charge is 80% or more.

Description

Technical Field

[0001] The present disclosure relates to a lithium secondary battery. Background Art

[0002] A lithium secondary battery (also referred to as a lithium metal secondary battery) using lithium metal as a negative electrode active material has a high theoretical capacity density. In a lithium secondary battery, lithium is deposited on a negative electrode current collector during a charging process, and the deposited lithium is dissolved into a non-aqueous electrolyte during a discharging process. In a lithium secondary battery, a negative electrode containing a lithium alloy is sometimes used. Regarding the negative electrode of a lithium secondary battery, various proposals have been made so far.

[0003] Claim 1 of Patent Document 1 (International Publication No. 2022 / 138490) describes a lithium secondary battery including a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains a magnesium-containing lithium alloy at least in a charged state, lithium metal is deposited during charging at the negative electrode, and the lithium metal is dissolved during discharging, and the non-aqueous electrolyte contains a hydrofluoroether and a lithium ion.

[0004] Claim 1 of Patent Document 2 (Japanese Patent Publication No. 05-050815) describes a high-density anode capable of maintaining good performance and capable of withstanding at least 20 charge cycles, which includes a high-density metal foil alloyed with an alkali metal having a thickness between 5 and 100 μm.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: International Publication No. 2022 / 138490

[0008] Patent Document 2: Japanese Patent Publication No. 05-050815 Summary of the Invention

[0009] Problems to be Solved by the Invention

[0010] A lithium secondary battery is required to improve cycle characteristics, and suppressing the swelling of the negative electrode is important for improving the cycle characteristics. An object of the present disclosure is to provide a lithium secondary battery having good cycle characteristics.

[0011] Means for Solving the Problems

[0012] One aspect of the present disclosure relates to a lithium secondary battery. The lithium secondary battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The negative electrode includes a current collector and a lithium alloy layer formed on the current collector. The lithium alloy layer contains magnesium. The negative electrode is a negative electrode in which a metal layer containing lithium and magnesium is formed on the lithium alloy layer by depositing lithium metal during charging, and the lithium metal dissolves during discharging. In a state where the charging state is 80% or more, the number of moles of magnesium R AM in the lithium alloy layer relative to the number of moles of lithium R AL The ratio R AM / R AL is greater than the number of moles of magnesium R MM in the metal layer relative to the number of moles of lithium R ML The ratio R MM / R ML .

[0013] Advantages of the Invention

[0014] According to the present disclosure, a lithium secondary battery with good cycle characteristics can be obtained.

[0015] The novel features of the present invention are set forth in the appended claims, but the configuration and content of the present invention, as well as other objects and features of the present invention, can be further understood from the following detailed description with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Figure 1 is a cross-sectional view schematically showing the lithium secondary battery of Embodiment 1.

[0017] Figure 2 Figure 2 is a graph showing an example of the evaluation results in the examples.

[0018] Figure 3 Figure 3 is a graph showing another example of the evaluation results in the examples.

[0019] Figure 4 Figure 4 is an image showing an example of the manufacturing process of the negative electrode.

[0020] Figure 5 Figure 5 is a diagram showing Figure 4 an example of the process after the manufacturing process shown.

[0021] Figure 6 Figure 6 is an image showing an example of the state of the negative electrode.

[0022] ​​​​​​​​​​​​​Figure 7 Figure 7 shows an image obtained by binarizing a part of the image shown Figure 6 . DETAILED DESCRIPTION

[0023] Hereinafter, embodiments of the present disclosure will be described by way of examples. However, the present disclosure is not limited to the examples described below. In the following description, specific numerical values and materials are sometimes exemplified. However, as long as the effects of the present disclosure can be obtained, other numerical values and other materials can also be used. In this specification, the description of "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be written as "not less than numerical value A and not more than numerical value B". In the following description, when the lower limit and the upper limit of a numerical value regarding a specific physical property, condition, etc. are exemplified, as long as the lower limit is not more than the upper limit, any lower limit exemplified can be arbitrarily combined with any upper limit exemplified. In the following description, when examples of components and examples of methods are listed, as long as there is no particular description, only one of the listed examples can be used, or a plurality of the listed examples can be used in combination.

[0024] (Lithium secondary battery)

[0025] The lithium secondary battery of the present embodiment is sometimes hereinafter referred to as "lithium secondary battery (B)". The lithium secondary battery (B) includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The negative electrode includes a current collector (negative current collector) and a lithium alloy layer formed on the current collector. The lithium alloy layer contains magnesium. The negative electrode is a negative electrode that forms a metal layer containing lithium and magnesium on the lithium alloy layer by depositing lithium metal during charging and in which lithium metal dissolves during discharging. In a state where the state of charge (SOC) is 80% or more, the molar number R of magnesium AM in the lithium alloy layer relative to the molar number R of lithium AL The ratio R AM / R AL is greater than the molar number R of magnesium MM in the metal layer relative to the molar number R of lithium ML The ratio R MM / R ML .

[0026] ​In a typical lithium secondary battery, lithium metal is deposited on the negative electrode during charging, and the deposited lithium metal dissolves during discharging. Generally, in a lithium secondary battery, a film is formed on the negative electrode by decomposition and / or reaction of components contained in the electrolyte during charging. This film is called an SEI (Solid Electrolyte Interphase) film. The SEI film can also be formed on the surface of the deposited lithium metal. During charging, the SEI film is formed while lithium metal is deposited on the negative electrode. Therefore, compared with a lithium ion secondary battery, the charging reaction of a lithium secondary battery is likely to become non-uniform, and as a result, the capacity and cycle characteristics may sometimes decrease. In addition, if the charging reaction occurs locally due to the SEI film, the negative electrode is likely to expand because lithium metal is deposited dendritically. If the negative electrode expands, the electrolyte contained in the positive electrode and the separator is extruded, the lithium ion concentration decreases, and the cycle characteristics decrease. Further, if lithium metal is deposited dendritically, the side reaction between the lithium metal and the electrolyte increases due to the increased specific surface area of the lithium metal, and thus the cycle characteristics decrease significantly.

[0027] On the other hand, in the lithium secondary battery (B) of the present embodiment, the negative electrode has the above-described configuration. Through research, the inventors of the present invention found that by using a negative electrode having the above-described configuration, the expansion of the negative electrode can be reduced and the cycle characteristics can be improved. The present disclosure is made based on this new finding.

[0028] It is considered that the negative electrode of the lithium secondary battery (B) of the present embodiment contains magnesium having a standard electrode potential higher than that of lithium, so that the reductive decomposition reaction of the electrolyte can be suppressed, and thus the growth of the SEI film can be suppressed. In addition, the negative electrode of the lithium secondary battery (B) not only simply contains magnesium, but the distribution of magnesium is unevenly present. Therefore, as shown in the examples, the expansion of the negative electrode can be particularly suppressed, and the cycle characteristics can be significantly improved.

[0029] In this specification, a state where the state of charge (SOC) is 80% means a state where the battery is charged to 80% of the rated capacity. When the SOC is 100%, it means that the battery is in a fully charged state. Regarding SOC 100%, for example, a state where the battery is charged to the rated voltage with a current value of 0.2C and then continuously charged at the rated voltage until the current value becomes 0.02C or less can be defined as SOC 100%. Here, 1C (A) is a current value obtained by 1C (A) = rated capacity (Ah) / 1 (h).

[0030] When a lithium metal layer is formed by depositing lithium metal, magnesium in the lithium alloy layer diffuses into the metal layer. At least a part of the diffused magnesium can form an alloy with lithium in the metal layer.

[0031] The metal layer may include a high-magnesium portion having a higher ratio of magnesium than other portions of the metal layer. At least a part of the high-magnesium portion may be aggregated in a particulate form to form a plurality of particulate portions. With this configuration, a high effect can be obtained.

[0032] The average particle size of the plurality of particulate portions may be 5 μm or less. With this configuration, the specific surface area of the aggregated magnesium can be increased, and the decomposition reaction of the electrolytic solution can be suppressed. The average particle size of the particulate portions can be measured by the following steps. First, the negative electrode is taken out from a lithium secondary battery with an SOC of 80% or more. The cross-section of the portion of the taken-out negative electrode where the metal layer is deposited is analyzed by SEM-EDX (scanning electron microscope - energy dispersive X-ray analysis). Specifically, an image showing the distribution of magnesium is obtained by SEM-EDX. The average particle size R of the particulate portions of magnesium can be calculated from the obtained image. Specifically, N particulate portions are arbitrarily selected from the particulate portions in the image, and the area Sn of each particulate portion is obtained from the image. Then, the particle size (equivalent circle diameter) Rn = 2×(Sn / π) of each particulate portion is obtained from the area Sn of each particulate portion. 0.5 . Here, n is n = 1 to N. The average particle size R can be obtained by R = (R1 + ··· + Rn) / N. In addition, the obtained image can be binarized as needed. The number N of the selected particulate portions is 5 or more.

[0033] At least a part of the plurality of particulate portions may be connected together by the above-mentioned high-magnesium portion. For example, at least a part of the plurality of particulate portions may be connected together by a linear high-magnesium portion.

[0034] The magnesium content rate in the lithium alloy layer may be 0.1 mass% or more and 30 mass% or less. By making the content rate 0.1 mass% or more, the effect brought about by adding magnesium can be high. If the content rate exceeds 30 mass%, the lithium alloy layer becomes hard, and the adhesion to the current collector is reduced. By making the content rate 30 mass% or less, the adhesion between the lithium alloy layer and the current collector can be improved, and the manufacture of the negative electrode becomes easy. In addition, by making the content rate 30 mass% or less, a decrease in battery performance due to an excessive amount of magnesium can be suppressed. The content rate may be 0.1 mass% or more, 0.5 mass% or more, or 1.0 mass% or more. The content rate may be 30 mass% or less, 20 mass% or less, 10 mass% or less, or 5.0 mass% or less.

[0035] The average thickness Ta of the lithium alloy layer can be 5 μm or more and 100 μm or less. By making the average thickness Ta 5 μm or more, the volume when forming a lithium secondary battery can be reduced, and the volumetric energy density of the lithium secondary battery can be increased. By making the average thickness Ta 100 μm or less, the lithium ions consumed by charge-discharge cycles can be replenished, and the cycle retention rate can be increased. The average thickness Ta can be 5 μm or more, 10 μm or more, 20 μm or more, or 50 μm or more. The average thickness Ta can be 100 μm or less, 80 μm or less, 60 μm or less, or 40 μm or less. In addition, the average thickness Ta of the lithium alloy layer gradually decreases due to repeated charge-discharge cycles. The average thickness Ta can be the average thickness of the lithium alloy layer in the lithium secondary battery (B) after battery manufacturing and with 20 charge-discharge cycles or less.

[0036] The average thickness Ta of the lithium alloy layer can be obtained by the following method. First, remove the negative electrode from the lithium secondary battery. Analyze the cross-section of the removed negative electrode by SEM-EDX (scanning electron microscope - energy dispersive X-ray analysis). Specifically, obtain an image showing the distribution of lithium and magnesium by SEM-EDX, and identify the part of the lithium alloy layer. Next, select any 5 positions of the lithium alloy layer, and find the thickness of each position. The average thickness Ta is obtained by taking the arithmetic mean of the thicknesses of the 5 measured positions.

[0037] Except for using the above negative electrode, there are no particular limitations on the components of the lithium secondary battery (B). As components other than the negative electrode, components used in known lithium secondary batteries can be used. Examples of the components of the lithium secondary battery (B) are described below. However, the components of the lithium secondary battery (B) are not limited to the examples described below.

[0038] (Negative electrode)

[0039] As described above, the negative electrode includes a negative electrode current collector and a lithium alloy layer formed on the negative electrode current collector. In addition, in the negative electrode during charging, magnesium diffuses into the precipitated lithium, thereby forming a metal layer containing lithium and magnesium on the lithium alloy layer. Lithium in the metal layer dissolves in the non-aqueous electrolyte during discharge. The lithium alloy layer is formed on one or both sides of the negative electrode current collector. The lithium alloy layer is formed at least on the side facing the positive electrode mixture layer across the separator. When both sides of the negative electrode face the positive electrode mixture layer across the separator, the lithium alloy layer is formed on both sides of the negative electrode current collector.

[0040] In a state where the SOC is 80% or more, the number of moles R of magnesium AM relative to the number of moles R of lithium AL ratio R AM / R AL in the lithium alloy layer, and the number of moles R of magnesium MM relative to the number of moles R of lithiumML Ratio R MM / R ML The magnitude relationship between them can be determined by the following method.

[0041] In Step 1, first, the negative electrode is taken out from the lithium secondary battery in a charged state of 80% SOC or more. By immersing the taken-out negative electrode in the solvent used in the electrolytic solution, the lithium salt attached to the surface of the negative electrode is removed. Next, the negative electrode is cut into a specified area S1, and the negative electrode taken out from the solvent is dried and the mass M10 after drying is measured. Next, the dried negative electrode is immersed in pure water to completely dissolve the metal layer and the lithium alloy layer contained in the negative electrode, thereby obtaining an aqueous solution. Next, the concentrations of lithium ions and magnesium ions in the aqueous solution are measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). In addition, the remaining part (specifically, the negative electrode current collector) after removing the metal layer and the lithium alloy layer from the negative electrode is taken out from the aqueous solution and dried. Then, the mass M11 of the obtained dried product is measured. The mass M12 of the lithium alloy layer and the metal layer can be calculated by subtracting the mass M11 from the mass M10. In addition, the lithium content rate Li1 and the magnesium content rate Mg1 in the lithium alloy layer and the metal layer are calculated from the concentrations of lithium ions and magnesium ions in the aqueous solution. The mass MLi1 of lithium in the lithium alloy layer and the metal layer can be calculated by mass M12 × Li1. The mass MMg1 of magnesium in the lithium alloy layer and the metal layer can be calculated by mass M12 × Mg1.

[0042] In step 2, first, the negative electrode near the negative electrode cut out in step 1 is cut to an area larger than S1, and a lead is installed. Then, the negative electrode with the lead installed is stacked with the lithium metal (or copper foil, etc.) with the lead installed through a separator to form a stack. Then, the stack and the electrolyte are sealed in an outer packaging body to make a single cell. The single cell is discharged until the negative electrode is in a fully discharged state. Specifically, the discharge is performed with the same amount of electricity as when charging in step 1. The metal layer in the negative electrode is dissolved by this discharge. Then, the negative electrode with the dissolved metal layer is taken out from the single cell. The lithium salt attached to the surface of the negative electrode is removed by immersing the removed negative electrode in a solvent. The negative electrode thus obtained is cut into the same area S1 as in step 1. The cut negative electrode is dried, and the mass M20 after drying is measured. Then, the dried negative electrode is immersed in pure water to completely dissolve the lithium alloy layer contained in the negative electrode, thereby obtaining an aqueous solution. Next, the concentrations of lithium ions and magnesium ions in the aqueous solution are measured by ICP-AES. In addition, the remaining portion after removing the lithium alloy layer from the negative electrode (specifically, the negative electrode collector) is taken out from the aqueous solution and dried. Then, the mass M21 of the obtained dry product is measured. The mass M22 of the lithium alloy layer is calculated by subtracting the mass M20 from the mass M21. In addition, the lithium content Li2 and the magnesium content Mg2 in the lithium alloy layer are calculated from the concentrations of lithium ions and magnesium ions in the aqueous solution. The mass MLi2 of lithium in the lithium alloy layer can be calculated by mass M22×Li2. The mass MLi3 of lithium in the metal layer can be calculated by subtracting MLi2 calculated in step 2 from MLi1 calculated in step 1.

[0043] In step 3, the negative electrode near the negative electrode cut out in step 1 is cut, and the cross-section of the part where the metal layer is precipitated is analyzed by SEM-EDX (scanning electron microscope-energy dispersive X-ray analysis). Specifically, first, an image showing the distribution of magnesium is obtained by SEM-EDX. The area of ​​the lithium alloy layer and the area of ​​the metal layer are determined from the obtained image. Next, the ratio of magnesium in the lithium alloy layer Mg31 and the ratio of magnesium in the metal layer Mg32 are calculated from the area of ​​the part showing magnesium in each area. Using the MMg1 obtained in step 1, the mass MMg31 of magnesium in the lithium alloy layer is calculated by Mg31×MMg1. In addition, the mass MMg32 of magnesium in the metal layer can be calculated by Mg32×MMg1. If the atomic weight of lithium is set to RLi and the atomic weight of magnesium is set to RMg, it can be calculated by R AL =MLi2 / RLi calculate the number of moles of lithium in the lithium alloy layer R AL , through R AM =MMg31 / RMg Calculate the molar number of magnesium in the lithium alloy layer through R ML =MLi3 / RLi Calculate the molar number of lithium in the metal layer, and use R MM= MMg32 / RMg calculates the number of moles of magnesium in the metal layer. The ratio R can be determined through the above steps 1 to 3. AM / R AL With the ratio R MM / R ML The magnitude relationship between them.

[0044] The lithium alloy layer may contain a third element other than lithium (the first element) and magnesium (the second element). Examples of the third element include aluminum, indium, calcium, lead, hydrogen, sodium, bismuth, copper, and zinc. The lithium alloy may contain one third element or two or more third elements. The content rate of the third element in the lithium alloy layer is lower than the content rate of magnesium, and may be 10% by mass or less, 1% by mass or less, or may be less than 0.1% by mass. The content rate of the third element in the lithium alloy layer is determined according to the content rate of magnesium. The third element contained in the lithium alloy layer may diffuse into the metal layer in the subsequent step (iii). The metal layer deposited during charging in the charge-discharge cycle after step (iii) may contain the third element.

[0045] In a battery such as a lithium secondary battery where lithium metal is deposited on the negative electrode during charging, the open circuit voltage (OCV: Open Circuit Voltage) of the negative electrode at full charge with respect to lithium metal (the dissolution and deposition potential of lithium) can be, for example, 70 mV or less. Regarding the OCV of the negative electrode at full charge, the battery in the fully charged state is disassembled in an argon atmosphere to take out the negative electrode, and a single cell is assembled with lithium metal as the counter electrode for measurement. The composition of the non-aqueous electrolyte of the single cell can be the same as the non-aqueous electrolyte in the disassembled battery.

[0046] (Negative electrode current collector)

[0047] The negative electrode current collector can use a conductive sheet. As the conductive sheet, foil, film, etc. can be used. Examples of the material of the negative electrode current collector include metal materials (metals composed of a single element, alloys). The negative electrode current collector is preferably made of a material that does not react with lithium. Examples of metal materials include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metal elements. Examples of alloys include copper alloys, stainless steel (SUS), etc. Among these metal materials, copper and copper alloys are preferred from the aspect of high conductivity. Examples of elements other than copper contained in the copper alloy include magnesium, aluminum, zinc, etc. The thickness of the negative electrode current collector can be in the range of 5 μm to 300 μm (for example, in the range of 50 μm to 200 μm).

[0048] (Non-aqueous electrolyte)

[0049] The non-aqueous electrolyte (non-aqueous electrolyte solution) contains a solvent (non-aqueous solvent) and a solute dissolved in the solvent. Examples of the solute include lithium salts. Various additives can also be added to the electrolyte.

[0050] As the solvent, known materials can be used. As the solvent, for example, cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, chain ethers, fluorinated chain ethers, cyclic ethers, fluorinated cyclic ethers, etc. can be used. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), vinylene carbonate (VC), etc. Chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), etc. Examples of cyclic carboxylates include γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. Examples of chain carboxylates include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP), etc. Examples of chain ethers include dimethyl ether, methyl ethyl ether, diethyl ether, ethyl propyl ether, di-n-propyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,1-dimethoxymethane, 1,1-diethoxyethane, etc. The chain ether preferably contains at least a chain ether having two or more ether bonds. Examples of such chain ethers include 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, etc. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-di ane, 1,4-di ane, 1,3,5-tri ane, furan, 2-methylfuran, 1,8-cineole, and crown ethers, etc. The fluorinated chain ether has a structure in which one or more hydrogens of the above chain ether are replaced by fluorine. Examples include bis(2,2,2-trifluoroethyl) ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, etc. The fluorinated cyclic ether has a structure in which one or more hydrogens of the above cyclic ether are replaced by fluorine. Examples include 3,3,4,4-tetrafluorotetrahydrofuran. The non-aqueous solvent can be used alone or in combination of two or more.

[0051] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10etc.), lithium salts of fluorinated acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorinated acid imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(FSO2), LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), lithium salts containing oxalate complexes (LiB(C2O4)2, LiBF2(C2O4), LiPF4(C2O4), LiPF2(C2O4)2, etc.), etc. The lithium salt can be used alone or in combination of two or more kinds.

[0052] The concentration of the lithium salt in the electrolyte can be 1 mol / L or more and 5 mol / L or less, or can be 1 mol / L or more and 3 mol / L or less. By making the concentration of the lithium salt in the above range, an electrolyte with excellent ionic conductivity and moderate viscosity can be obtained.

[0053] The electrolyte can contain additives (e.g., well-known additives). Examples of the additives include 1,3-propane sultone, methyl benzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene, etc.

[0054] (Positive electrode)

[0055] The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode current collector can be sheet-shaped (e.g., foil, film) or porous. The positive electrode mixture layer can be formed on both main surfaces of the positive electrode current collector or only on one main surface. The positive electrode mixture layer can be formed in a state of being filled in a mesh-shaped positive electrode current collector.

[0056] Examples of the material of the positive electrode current collector include metal materials such as Al, Al alloy, Ti, Ti alloy, and Fe alloy. The Fe alloy can be stainless steel.

[0057] The positive electrode mixture layer contains a positive electrode active material. The positive electrode mixture layer can also contain at least one selected from binders, conductive materials, tackifiers, and additives in addition to the positive electrode active material. A conductive carbonaceous material can be disposed between the positive electrode current collector and the positive electrode mixture layer as needed.

[0058] As the positive electrode active material, a material capable of electrochemically occluding and releasing lithium ions can be used. As such a material, for example, at least one selected from lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides can be used. From the viewpoints of high average discharge voltage and cost advantage, the positive electrode active material can contain a lithium-containing transition metal oxide.

[0059] Examples of transition metal elements contained in lithium-containing transition metal oxides include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, W, etc. The lithium-containing transition metal oxide may contain only one transition metal element or may contain two or more. The transition metal element may be at least one selected from Co, Ni, and Mn. The lithium-containing transition metal oxide may contain one or two or more typical metal elements as needed. Examples of typical metal elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, Bi, etc. The typical metal element may be Al, etc.

[0060] Examples of binders include fluororesins, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, rubber-like polymers, etc. Examples of fluororesins include polytetrafluoroethylene, polyvinylidene fluoride, etc.

[0061] As the conductive material, for example, a conductive carbonaceous material, etc. can be used. Examples of the conductive carbonaceous material include carbon black, carbon nanotubes, graphite, etc. Examples of carbon black include acetylene black, Ketjen black, etc.

[0062] As the tackifier, for example, cellulose derivatives such as cellulose ether, etc. can be used. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modified products, methyl cellulose, etc. Examples of modified products of CMC may also include salts of CMC. As the salt, alkali metal salts (for example, sodium salts), ammonium salts, etc. can be cited.

[0063] As the carbonaceous material disposed between the positive electrode current collector and the positive electrode mixture layer, for example, at least one selected from the conductive carbonaceous materials exemplified as the conductive material can be used.

[0064] The positive electrode can be obtained, for example, by coating or filling a slurry containing the constituent components of the positive electrode mixture layer and a dispersion medium on the positive electrode current collector and drying and compressing the coating film. As needed, a conductive carbonaceous material can also be coated on the surface of the positive electrode current collector. As the dispersion medium, at least one selected from water and organic solvents (N-methyl-2-pyrrolidone, etc.) can be used.

[0065] (Separator)

[0066] A separator that can be used is a porous sheet having ion permeability and insulation. Examples of the form of the porous sheet include microporous membranes, woven fabrics, non-woven fabrics, etc. The material of the separator can be a polymer material. Examples of the polymer material include olefin resins, polyamide resins, and cellulose, etc. Examples of olefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene, etc. The separator may contain additives as needed. Examples of the additives include inorganic fillers, etc.

[0067] The separator may include a plurality of layers that differ in at least one of morphology and composition. Such a separator may be, for example, a laminate of a polyethylene microporous membrane and a polypropylene microporous membrane, or a laminate of a nonwoven fabric containing cellulose fibers and a nonwoven fabric containing thermoplastic resin fibers.

[0068] (Shape, etc.)

[0069] The positive electrode, negative electrode, and separator constitute an electrode assembly. Examples of the electrode assembly include an electrode assembly (wound-type electrode assembly) in which the positive electrode and the negative electrode are wound with a separator interposed therebetween. The electrode assembly may also have a morphology other than the wound type. For example, the electrode assembly may be an electrode assembly (laminated-type electrode assembly) in which the positive electrode and the negative electrode are laminated with a separator interposed therebetween. The morphology of the lithium secondary battery is not limited and may be cylindrical, square, coin-shaped, button-shaped, laminated, etc. The lithium secondary battery (B) includes an outer package corresponding to these morphologies. The electrode assembly and the non-aqueous electrolyte are accommodated in the outer package. The outer package is not particularly limited, and the same outer package as that of a known lithium secondary battery may be used.

[0070] (An example of the manufacturing method of the lithium secondary battery (B))

[0071] As an example of the manufacturing method of the lithium secondary battery (B), the manufacturing method (M) will be described below. However, the lithium secondary battery (B) may also be manufactured by a manufacturing method other than the manufacturing method (M) described below. Matters described for the lithium secondary battery (B) can be used for the manufacturing method (M), so repeated descriptions may sometimes be omitted. Matters described for the manufacturing method (M) can also be used for the lithium secondary battery (B).

[0072] The manufacturing method (M) is a manufacturing method using a negative electrode substrate including a negative electrode current collector and a lithium alloy layer containing magnesium and formed on the negative electrode current collector. The manufacturing method (M) includes the following steps: a step (i) of assembling a battery including the negative electrode substrate, a step (ii) of depositing a lithium metal layer on the lithium alloy layer by charging the battery, and a step (iii) of diffusing magnesium in the lithium alloy layer into the lithium metal layer by holding the battery at a prescribed temperature for a prescribed time.

[0073] The negative electrode substrate can be obtained by forming a lithium alloy layer containing magnesium on the surface of the negative electrode current collector. For example, the negative electrode substrate can be formed by pressing a foil of a lithium alloy containing magnesium onto the surface of the negative electrode current collector. The lithium alloy layer may have the above-described magnesium content rate and the above-described thickness.

[0074] (Step (i)) (Battery assembly step)

[0075] In step (i), the battery can be assembled according to the form of the lithium secondary battery (B). The method of assembling the battery is not limited, and it can be assembled in the same manner as the well-known method of assembling a lithium secondary battery. When the lithium secondary battery (B) includes a wound electrode assembly, first, the positive electrode, the negative electrode, and the separator are wound in such a way that the separator is disposed between the positive electrode and the negative electrode to produce a wound electrode assembly. Then, the electrode assembly and the non-aqueous electrolyte are housed in an outer package. Thus, the battery is assembled. The components other than the negative electrode can be commercially available products or can be produced by a well-known method.

[0076] (Step (ii)) (Precipitation step of the lithium metal layer)

[0077] In step (ii), the lithium secondary battery (B) is charged. By charging, lithium in the positive electrode active material precipitates onto the lithium alloy layer of the negative electrode substrate to form a lithium metal layer. In order to form the lithium metal layer, the positive electrode binder layer contains the positive electrode active material just required to form the lithium metal layer. In step (ii), charging is performed until the SOC reaches 80% or more (in the range of 80 to 100%). Step (ii) can be carried out at room temperature (for example, at a temperature in the range of 10 to 30 °C).

[0078] (Step (iii)) (Diffusion step of magnesium)

[0079] In step (iii), the battery is maintained at a temperature of 40 °C or higher. The maintained temperature T is preferably 45 °C or higher and can also be 50 °C or higher. The maintained temperature can be 60 °C or lower, or 55 °C or lower. The time D for maintaining the battery at the above temperature T varies according to the temperature and can be 1 day or more (24 hours or more), 5 days or more, 7 days or more, or 14 days or more. The time D can be 21 days or less, 7 days or less, or 5 days or less. The higher the temperature T, the shorter the time D can be. In an example of step (iii), the battery is maintained at a temperature of 40 °C or higher for 4 days or more.

[0080] Step (iii) can be carried out with the battery open or in a state of being charged once or regularly. For example, step (iii) can be carried out while charging the battery with a very small current value. By charging the battery in step (iii), the diffusion of magnesium can be promoted. The current value during charging varies according to the length of the charging time Dc, but can be 0.005C or more, or 0.01C or more, and can be 0.2C or less or 0.1C or less. The charging time Dc can be 0.1 times or more, 0.3 times or more, 0.5 times or more, 0.7 times or more, or 0.9 times or more of the time D maintained at the temperature T, and can be 1 time or less, 0.9 times or less, 0.7 times or less, 0.5 times or less, or 0.3 times or less. Here, 1C(A) is the current value obtained by 1C(A) = rated capacity (Ah) / 1(h). In one example, during the execution of step (iii), regular charging is carried out with a current value in the range of 0.01C to 0.05C.

[0081] The negative electrode after step (iii) includes a negative electrode current collector, a lithium alloy layer disposed on the negative electrode current collector, and a metal layer formed on the lithium alloy layer. The metal layer is formed by diffusing magnesium in the lithium alloy layer into the lithium metal layer. When step (iii) is carried out under the above conditions, at least a part of the diffused magnesium can form the above-mentioned particulate portion in the metal layer.

[0082] In the above manner, the lithium secondary battery (B) can be obtained. If discharging is carried out after the above steps, the lithium metal in the metal layer dissolves. That is, lithium ions dissolve into the electrolyte. If charging is carried out after discharging, on the negative electrode, a metal layer containing lithium and magnesium is formed on the lithium alloy layer.

[0083] Hereinafter, an example of the lithium secondary battery (B) will be specifically described with reference to the drawings. However, the lithium secondary battery (B) is not limited to the configuration shown in the drawings. The examples described below can be changed based on the above description. In addition, the matters described below can also be applied to the above-described embodiments.

[0084] (Embodiment 1)

[0085] Figure 1It is a longitudinal sectional view schematically showing the lithium secondary battery 10 of Embodiment 1. The lithium secondary battery 10 is a cylindrical battery. The lithium secondary battery 10 includes a cylindrical battery case, a wound electrode group 14 housed in the battery case, and a non-aqueous electrolyte (not shown). The battery case includes a bottomed cylindrical case body 15 and a sealing body 16 that seals the opening of the case body 15. The case body 15 is made of metal. A gasket 27 is disposed between the case body 15 and the sealing body 16. The airtightness of the battery case can be ensured by the gasket 27. The case body 15, the sealing body 16, and the gasket 27 constitute an outer package. Inside the case body 15, insulating plates 17 and 18 are disposed at both ends in the winding axis direction of the electrode group 14.

[0086] The case body 15 has a stepped portion 21. The sealing body 16 is supported by the stepped portion 21. The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a lid 26. The above-described components constituting the sealing body 16 are electrically connected to each other except for the insulating member 24. The lid 26 has the function of a positive electrode terminal. The case body 15 has the function of a negative electrode terminal.

[0087] The electrode group 14 is a wound type electrode group composed of a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all strip-shaped. The negative electrode 12 is the above-described negative electrode.

[0088] The positive electrode 11 is electrically connected to the lid 26 via a positive electrode lead 19. One end of the positive electrode lead 19 is connected to the positive electrode 11. The other end of the positive electrode lead 19 is connected to the sealing body 16 (filter 22). The negative electrode 12 is electrically connected to the case body 15 via a negative electrode lead 20. One end of the negative electrode lead 20 is connected to the negative electrode 12. The other end of the negative electrode lead 20 is connected to the case body 15.

[0089] (Additional description)

[0090] The following technology is disclosed by the above description.

[0091] (Technology 1)

[0092] A lithium secondary battery, which includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte,

[0093] The negative electrode includes a current collector and a lithium alloy layer formed on the current collector,

[0094] The lithium alloy layer contains magnesium,

[0095] The negative electrode is a negative electrode that forms a metal layer containing lithium and magnesium on the lithium alloy layer by depositing lithium metal during charging and dissolves the lithium metal during discharging,

[0096] In a state where the charging state is above 80%, the molar number R of magnesium in the lithium alloy layer AM relative to the molar number R of lithium AL The ratio R AM / R AL is greater than the molar number R of magnesium in the metal layer MM relative to the molar number R of lithium ML The ratio R MM / R ML .

[0097] (Technology 2)

[0098] In the lithium secondary battery according to Technology 1, the metal layer includes a high-magnesium portion, and the ratio of magnesium in the high-magnesium portion is higher than that of other portions of the metal layer.

[0099] At least a part of the high-magnesium portion aggregates in a particulate form to form a plurality of particulate portions.

[0100] (Technology 3)

[0101] In the lithium secondary battery according to Technology 2, the average particle size of the plurality of particulate portions is 5 μm or less.

[0102] (Technology 4)

[0103] In the lithium secondary battery according to Technology 2 or 3, at least a part of the plurality of particulate portions are connected together through the high-magnesium portion.

[0104] (Technology 5)

[0105] In the lithium secondary battery according to any one of Technologies 1 to 4, the magnesium content in the lithium alloy layer is 0.1% by mass or more and 30% by mass or less.

[0106] (Technology 6)

[0107] In the lithium secondary battery according to any one of Technologies 1 to 5, the average thickness of the lithium alloy layer is 5 μm or more and 100 μm or less.

[0108] Examples

[0109] The present disclosure will be described in detail below by way of examples. In this example, a plurality of lithium secondary batteries were fabricated and evaluated by the following method.

[0110] (Fabrication of Battery A1)

[0111] (1) Fabrication of the positive electrode

[0112] A mixture is obtained by mixing a positive electrode active material, acetylene black (conductive material), and polyvinylidene fluoride (binder) in a mass ratio of 95:2.5:2.5. An appropriate amount of N-methyl-2-pyrrolidone (dispersion medium) is added to the mixture and stirred, thereby preparing a positive electrode paste. A lithium-containing transition metal oxide containing Ni, Co, and Al is used as the positive electrode active material.

[0113] The positive electrode paste is coated on both sides of an aluminum foil (positive electrode current collector) and dried, thereby forming a laminate. The laminate is compressed in the thickness direction using a roller. The compressed laminate is cut into a specified size. Thus, a positive electrode including a positive electrode current collector and positive electrode binder layers formed on both sides of the positive electrode current collector is manufactured.

[0114] (2) Fabrication of negative electrode substrate

[0115] In an inert atmosphere, a lithium-magnesium alloy foil (thickness: 35 μm) is pressed onto the surface of a copper foil (negative electrode current collector, thickness: 14 μm), thereby fabricating a negative electrode substrate. The magnesium content rate in the lithium-magnesium alloy foil is 5% by mass.

[0116] (3) Preparation of non-aqueous electrolyte

[0117] LiPF6 and LiBF2(C2O4) are dissolved in a non-aqueous solvent, thereby preparing a liquid non-aqueous electrolyte. The concentrations of LiPF6 and LiBF2(C2O4) in the non-aqueous electrolyte are 1 mol / L and 0.1 mol / L, respectively. As the non-aqueous solvent, a solvent obtained by mixing propylene carbonate (PC) and 1,2-dimethoxyethane (DME) in a volume ratio of 1:2 is used.

[0118] (4) Assembly of battery

[0119] One end of an aluminum positive electrode lead is attached to the positive electrode current collector by welding. One end of a nickel negative electrode lead is attached to the negative electrode current collector by welding. In an inert atmosphere, the positive electrode and the negative electrode are laminated with a separator therebetween, thereby fabricating a laminated electrode assembly. A polyethylene microporous membrane is used as the separator.

[0120] Next, the electrode assembly is housed in an outer package. A bag-shaped outer package formed of a laminate sheet having an Al layer is used as the outer package. The non-aqueous electrolyte is injected into the outer package and then the outer package is sealed. Thus, a lithium secondary battery is assembled. In addition, when the electrode assembly is housed in the outer package, the other ends of the positive electrode lead and the negative electrode lead are exposed outside the outer package.

[0121] (5) Process (ii)

[0122] Next, perform the above-mentioned step (ii) on the assembled battery. Specifically, charge the battery at a current value of 0.2C until the SOC reaches 80%. Through this step, a lithium metal layer is deposited on the lithium alloy layer.

[0123] (6) Step (iii)

[0124] Next, perform the above-mentioned step (iii). Specifically, keep the battery in an environment of 40°C for 120 hours (5 days). Thereby, magnesium in the lithium magnesium alloy layer diffuses into the lithium metal layer. Thus, Battery A1 (lithium secondary battery (B)) was fabricated.

[0125] (Battery A2)

[0126] Except for performing step (iii) while charging, Battery A2 (lithium secondary battery (B)) was fabricated under the same conditions and methods as those for fabricating Battery A1. In step (iii), similar to the fabrication of Battery A1, the battery was kept in an environment of 40°C for 120 hours (5 days). Among them, step (iii) was performed while continuously charging the battery at a current value of 0.01C.

[0127] (Battery C1)

[0128] Except for not performing step (iii), Battery C1 was fabricated under the same conditions and methods as those for fabricating Battery A1. Battery C1 is a comparative example.

[0129] (7) Evaluation

[0130] Perform a charge-discharge cycle test on the fabricated lithium secondary battery. Specifically, in a constant temperature bath at 25°C, charge the lithium secondary battery under the following charging conditions, stop for 20 minutes, and then discharge it under the following discharge conditions. Consider these charging, stopping, and discharging as one cycle, and perform a charge-discharge test for 50 cycles. At this time, install a film thickness gauge on the battery to observe the expansion and contraction of the negative electrode.

[0131] (Charging)

[0132] Perform constant current charging at a current of 0.2C until the battery voltage reaches 4.1V, and then perform constant voltage charging at a voltage of 4.1V until the charging current becomes 0.02C.

[0133] (Discharging)

[0134] Perform constant current discharging at a current of 0.2C until the battery voltage reaches 3.0V.

[0135] Through the above cycle test, evaluate how the maximum thickness and capacity retention rate of the negative electrode change. The maximum thickness of the negative electrode is obtained from the measured value of the film thickness gauge. The capacity retention rate is obtained by measuring the battery capacity E0 before the cycle test and the battery capacity E1 during the cycle test using the following formula.

[0136] Capacity retention rate (%) = 100 × E1 / E0

[0137] The relationship between the number of cycles of the cycle test and the maximum thickness of the negative electrode is shown in Figure 2 . The relationship between the number of cycles of the cycle test and the capacity retention rate is shown in Figure 3 .

[0138] As shown in Figure 2 and Figure 3 , compared with the battery C1 of the comparative example, the swelling of the negative electrode of the battery A1 and the battery A2 of the present disclosure is suppressed, showing good cycle characteristics. If the battery A1 and the battery A2 are compared, the battery A2 further suppresses the swelling of the negative electrode, showing better cycle characteristics.

[0139] The negative electrode of the wound lithium secondary battery (B) of the present disclosure was analyzed by SEM-EDX (scanning electron microscope-energy dispersive X-ray spectroscopy). The image obtained by observing the negative electrode after the process (iii) by SEM-EDX is shown in Figure 4 . The process (iii) is carried out by holding the battery charged in the process (ii) in an open state at an environment temperature of 45 °C for 168 hours (7 days) (the conditions of the process (iii) of the battery of the negative electrode shown in Figure 5 and Figure 6 are the same). Figure 4 The upper image of Figure 4 shows the dispersion state of lithium, and the lower image of Figure 5 and Figure 6 shows the dispersion state of magnesium (the same applies to Figure 4 ). In Figure 5 and Figure 6 , the upper image and the lower image are images of the same position (the same applies to

[0140] ). As shown in Figure 4 , a lithium alloy layer 102 is disposed on the current collector 101 of the negative electrode, and a metal layer 103 is formed on the lithium alloy layer 102. The lithium alloy layer 102 is a lithium-magnesium alloy layer containing magnesium. The metal layer 103 is a layer obtained by diffusing magnesium into the lithium metal layer deposited during charging.

[0141] Discharge the battery that has undergone the above process (iii) at 25 °C. The image obtained by observing the discharged battery by SEM-EDX is shown in Figure 5 . AsFigure 5 As shown, most of the metal layer 103 has dissolved.

[0142] After discharging the battery that has undergone the above process (iii) at 25°C, it is charged at 25°C until the SOC reaches 80%. The image obtained by observing the charged battery through SEM-EDX is shown in Figure 6 . As Figure 6 shown, a metal layer 103 containing lithium and magnesium is formed on the lithium alloy layer 102. At this time, the molar number R of magnesium in the lithium alloy layer 102 AM relative to the molar number R of lithium AL ratio R AM / R AL is greater than the molar number R of magnesium in the metal layer 103 MM relative to the molar number R of lithium ML ratio R MM / R ML . In addition, the ratio R AM / R AL and the ratio R MM / R ML are substantially the same throughout the negative electrode. Therefore, by comparing the ratio R AM / R AL and the ratio R MM / R ML for a part of the negative electrode, it can be speculated which one of the ratio R AM / R AL and the ratio R MM / R ML is larger for the entire negative electrode.

[0143] There is a high-magnesium part 103a in the metal layer 103 where the ratio of magnesium is higher than that of other parts of the metal layer 103. At least a part of the high-magnesium part 103a aggregates in a particulate form to form a plurality of particulate parts. The average particle size of the plurality of particulate parts is 5 μm or less. The plurality of particulate parts are connected together by a linear high-magnesium part 103a.

[0144] The image obtained by binarizing a part of Figure 6 is shown in Figure 7 . By binarizing the image, Figure 6 the particulate part of the high-magnesium part 103a surrounded by the circle on the right side of

[0145] Industrial Applicability

[0146] The present disclosure can be used in lithium secondary batteries.

[0147] Although the preferred embodiments of the present invention have been described, the present invention should not be construed as being limited to such disclosure. Various modifications and changes will no doubt be apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Therefore, it should be construed that the appended claims cover all modifications and changes without departing from the spirit and scope of the present invention.

[0148] Description of Reference Numerals

[0149] 10: Lithium secondary battery

[0150] 11: Positive electrode

[0151] 12: Negative electrode

[0152] 13: Separator

[0153] 14: Electrode assembly

[0154] 15: Outer shell body

[0155] 16: Sealing body

[0156] 27: Gasket

[0157] 101: Current collector

[0158] 102: Lithium alloy layer

[0159] 103: Metal layer

[0160] 103a: High magnesium part

Claims

1. A lithium secondary battery, comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the negative electrode includes a current collector and a lithium alloy layer formed on the current collector, the lithium alloy layer contains magnesium, the negative electrode is a negative electrode that forms a metal layer containing lithium and magnesium on the lithium alloy layer by depositing lithium metal during charging and dissolves the lithium metal during discharging, In a state where the charging state is above 80%, the molar number R of magnesium in the lithium alloy layer AM relative to the molar number R of lithium AL The ratio R AM / R AL is greater than the molar number R of magnesium in the metal layer MM relative to the molar number R of lithium ML The ratio R MM / R ML .

2. The lithium secondary battery according to claim 1, wherein the metal layer includes a high-magnesium portion, and the ratio of magnesium in the high-magnesium portion is higher than that of other portions of the metal layer, at least a part of the high-magnesium portion aggregates in a particulate form to form a plurality of particulate portions.

3. The lithium secondary battery according to claim 2, wherein the average particle size of the plurality of particulate portions is 5 μm or less.

4. The lithium secondary battery according to claim 2 or 3, wherein at least a part of the plurality of particulate portions are connected together through the high-magnesium portion.

5. The lithium secondary battery according to claim 1 or 2, wherein the magnesium content in the lithium alloy layer is 0.1 mass% or more and 30 mass% or less.

6. The lithium secondary battery according to claim 1 or 2, wherein the average thickness of the lithium alloy layer is 5 μm or more and 100 μm or less.

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