Lithium secondary battery

Abstract

[Problem] To provide a means capable of suppressing occurrence of a short circuit in a lithium-precipitation-type lithium secondary battery having a negative electrode intermediate layer. [Solution] A lithium secondary battery comprising a power-generating element comprising: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode which has a negative electrode current collector and in which lithium metal precipitates during charging; a solid electrolyte layer which is interposed between the positive electrode and the negative electrode and contains a solid electrolyte; and a negative electrode intermediate layer which is present adjacent to a surface of the solid electrolyte layer on the negative electrode current collector side and contains a carbon material and metal particles which are not solid-solved with lithium, wherein the thickness of the negative electrode intermediate layer is between 0.1 μm and 5 μm (non-inclusive).

Description

Lithium secondary battery

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

[0002] In recent years, research and development on all-solid-state batteries using oxide- or sulfide-based solid electrolytes has been actively conducted. Solid electrolytes are materials primarily composed of ionic conductors that can conduct ions in a solid state. Therefore, all-solid-state batteries have the advantage that, in principle, they do not encounter the various problems associated with flammable organic electrolytes, as occurs in conventional liquid-based batteries that use nonaqueous electrolytes.

[0003] Conventionally, one type of all-solid-state lithium secondary battery is known as a so-called lithium deposition type, in which lithium metal is deposited on a negative electrode current collector during charging (for example, JP 2020-191202 A). During the charging process of such a lithium deposition type all-solid-state lithium secondary battery, lithium metal is deposited between the solid electrolyte layer and the negative electrode current collector. At this time, dendrites grow from the lithium metal layer and penetrate the solid electrolyte layer, which can cause a short circuit.

[0004] This document discloses a technique for providing a negative electrode active material layer (negative electrode intermediate layer) containing silver (Ag) between a negative electrode current collector and a solid electrolyte layer. With this configuration, lithium (Li) precipitates as a Li(Ag) alloy containing Ag in a solid solution during charging. Then, during discharging, only Li dissolves from the Li(Ag) alloy, leaving the dissolved Ag remaining. This technique is believed to be able to suppress the generation of voids and the resulting degradation of battery performance.

[0005] However, the inventors have found through their investigations that in lithium deposition-type secondary batteries having a negative electrode intermediate layer containing metal particles such as Ag, a short circuit may occur if the thickness of the negative electrode intermediate layer is thin.

[0006] Therefore, an object of the present invention is to provide a means for suppressing the occurrence of short circuits in a lithium deposition type lithium secondary battery having a negative electrode intermediate layer.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that in a lithium secondary battery equipped with a lithium deposition-type power generating element, the above-mentioned problems can be solved by incorporating metal particles that do not form a solid solution with lithium into a negative electrode intermediate layer together with a carbon material, and by controlling the thickness of the negative electrode intermediate layer to be in the range of more than 0.1 μm and less than 5 μm, thereby completing the present invention.

[0008] That is, one aspect of the present invention relates to a lithium secondary battery including a power generating element having a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode current collector and on which lithium metal is deposited during charging, a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a negative electrode intermediate layer present adjacent to the surface of the solid electrolyte layer facing the negative electrode current collector. The lithium secondary battery is characterized in that the negative electrode intermediate layer contains metal particles and a carbon material that do not form a solid solution with lithium, and the thickness of the negative electrode intermediate layer is more than 0.1 μm and less than 5 μm.

[0009] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (stacked-type secondary battery) according to one embodiment of the present invention.

[0010] One aspect of the present invention is a lithium secondary battery including a power generating element including: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a negative electrode intermediate layer adjacent to the surface of the solid electrolyte layer facing the negative electrode current collector and containing metal particles that do not form a solid solution with lithium and a carbon material, wherein the thickness of the negative electrode intermediate layer is greater than 0.1 μm and less than 5 μm. According to this aspect, the occurrence of a short circuit can be suppressed in a lithium deposition-type lithium secondary battery having a negative electrode intermediate layer. The lithium secondary battery according to this aspect may be an all-solid-state battery.

[0011] Hereinafter, a secondary battery according to the present embodiment will be described with reference to the accompanying drawings. The technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. In the description of the drawings, the same elements are given the same reference numerals, and redundant description will be omitted. In addition, the dimensional proportions in the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0012] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. FIG. 1 shows a cross section of the stacked-type secondary battery during charging. The stacked-type secondary battery 10a shown in FIG. 1 has a structure in which a substantially rectangular power-generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior. The power-generating element 21 has a structure in which a negative electrode, a solid electrolyte layer 17, and a positive electrode are stacked. The negative electrode has a structure in which a negative electrode current collector 11' and a negative electrode active material layer 13 made of lithium metal deposited on the surface of the negative electrode current collector 11' are stacked. A negative electrode intermediate layer 14 is disposed adjacent to the surface of the negative electrode active material layer 13 facing the solid electrolyte layer 17. The positive electrode has a structure in which a positive electrode active material layer 15 is disposed on the surface of a positive electrode current collector 11". The negative electrode, solid electrolyte layer, and positive electrode are laminated in this order, with the negative electrode intermediate layer 14 and the positive electrode active material layer 15 facing each other with the solid electrolyte layer 17 interposed therebetween. As a result, adjacent negative electrodes, solid electrolyte layers, and positive electrodes constitute one unit cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can be said to have a structure in which a plurality of unit cell layers 19 are laminated and electrically connected in parallel. A negative electrode current collector 25 and a positive electrode current collector 27 that are electrically connected to the respective electrodes (negative electrode and positive electrode) are attached to the negative electrode current collector 11' and the positive electrode current collector 11", respectively, and are structured so as to be sandwiched between the ends of the laminate film 29 and extended to the outside of the laminate film 29. A constraining pressure is applied to the stacked secondary battery 10a in the stacking direction of the power generating element 21 by a pressure member (not shown). Therefore, the volume of the power generating element 21 is kept constant.

[0013] The main components of the lithium secondary battery according to this embodiment will be described below.

[0014] [Current Collector] The current collector (negative electrode current collector, positive electrode current collector) has the function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There are no particular restrictions on the material that constitutes the current collector. Examples of materials that can be used for the current collector include metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins. There are also no particular restrictions on the thickness of the current collector, but an example is 10 to 100 μm.

[0015] [Negative Electrode Active Material Layer] The lithium secondary battery according to this embodiment is a so-called lithium deposition type in which lithium metal is deposited as the negative electrode active material during the charging process. The layer composed of lithium metal deposited during this charging process is the negative electrode active material layer of the lithium secondary battery according to this embodiment. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and the thickness of the negative electrode active material layer decreases as the discharging process progresses. Although the negative electrode active material layer does not need to be present during full discharge, in some cases, a negative electrode active material layer composed of a certain amount of lithium metal may be present during full discharge. Furthermore, the thickness of the negative electrode active material layer (lithium metal layer) during full charge is not particularly limited, but is typically 0.1 to 1000 μm.

[0016] [Negative electrode intermediate layer] The negative electrode intermediate layer is a layer adjacent to the surface of the solid electrolyte layer facing the negative electrode current collector, and contains metal particles that do not form a solid solution with lithium and a carbon material. The negative electrode intermediate layer is preferably conductive as a whole. The volume resistivity of the negative electrode intermediate layer is not particularly limited, but is preferably 10 ２ In this specification, the volume resistivity of the negative electrode intermediate layer is measured using an electrode resistance measurement system (manufactured by Hioki E.E. Corporation, product name: RM2610).

[0017] The metal species of the metal particles that do not form a solid solution with lithium are not particularly limited as long as they do not form a solid solution with lithium, and may be a metal that can be alloyed with lithium or a metal that does not form an alloy with lithium. In this specification, a metal that forms a solid solution with lithium refers to a metal that can form an intermetallic compound with lithium at any composition ratio. In addition, in this specification, being alloyable with lithium refers to a metal that can form an intermetallic compound with lithium by selecting an appropriate composition ratio. Among metals that do not form a solid solution with lithium, examples of metals that can be alloyed with lithium include tin (Sn) and silicon (Si), and examples of metals that do not form an alloy include nickel (Ni) and copper (Cu). Among these, from the viewpoint of improving the film strength of the negative electrode intermediate layer, it is more preferable that the metal material contains at least one of tin, nickel, and copper, and even more preferable that it contains at least one of tin and nickel. In addition, from the viewpoint of excellent cycle characteristics, it is most preferable that it contains nickel.

[0018] In one embodiment, the metal material contained in the negative electrode intermediate layer may be a metal that does not form a solid solution with lithium, or may be used together with a metal that forms a solid solution with lithium. That is, the negative electrode intermediate layer may contain or not contain a metal that forms a solid solution with lithium. Furthermore, the metal that does not form a solid solution with lithium may contain only a metal that can be alloyed with lithium, only a metal that does not form an alloy with lithium, or a metal that can be alloyed with lithium and a metal that does not form an alloy with lithium. In this case, the metal that does not form an alloy with lithium may be used alone or in combination of two or more. The metal that can be alloyed with lithium may be used alone or in combination of two or more. The metal that forms a solid solution with lithium may be used alone or in combination of two or more. Examples of metals that form a solid solution with lithium include magnesium (Mg), gold (Au), silver (Ag), and zinc (Zn). In one embodiment, the metal material contained in the negative electrode intermediate layer may be tin and / or nickel alone, or nickel alone. The metal material contained in the negative electrode intermediate layer does not necessarily need to contain silver.

[0019] The metal particles contained in the negative electrode intermediate layer may also contain oxygen atoms on their surfaces. In one embodiment, the content (or molar percentage) of oxygen atoms present on the surface of the metal particles may be greater than 80 mol%, for example, 82 mol% or more, 85 mol% or more, 88 mol% or more, or 90 mol% or more, based on 100 mol% of all atoms present on the particle surface. In one embodiment, the content (or molar percentage) of metal oxide present on the surface of the metal particles may be greater than 80 mol%, for example, 82 mol% or more, 85 mol% or more, 88 mol% or more, or 90 mol% or more, based on 100 mol% of all metal atoms present on the particle surface. In this specification, the content (or molar percentage) of atoms present on the surface of the metal particles is measured by analyzing the elemental composition of the surface (region 0 to 5 nm deep) using X-ray photoelectron spectroscopy.

[0020] The average primary particle size of metal particles that do not form a solid solution with lithium is, for example, less than 1000 nm, preferably 500 nm or less, more preferably 300 nm or less, even more preferably 200 nm or less, and particularly preferably 150 nm or less. The lower limit of the average primary particle size of the metal particles is not particularly limited, but is, for example, 10 nm or more, preferably 20 nm or more, even more preferably 30 nm or more, and even more preferably 40 nm or more. When the metal particles are made of a metal that does not alloy with lithium, the average primary particle size of the metal particles is preferably 10 to 1000 nm, more preferably 20 to 200 nm, and even more preferably 40 to 100 nm. When the metal particles are made of a metal that can alloy with lithium, the average primary particle size of the metal particles is preferably 10 to 1000 nm, more preferably 50 to 300 nm, and even more preferably 100 to 200 nm. In this specification, the average particle diameter of particles refers to the 50% cumulative diameter (D50) of particle diameters (the maximum distance between any two points on the contour lines of the observed particles) of 50 particles measured by observation with a scanning electron microscope (SEM).

[0021] The negative electrode intermediate layer further contains a carbon material in addition to metal particles that do not form a solid solution with lithium. The carbon material can contribute to suppressing the generation and growth of dendrites. Specific examples of carbon materials include carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.), carbon nanotubes (CNT), graphite, hard carbon, etc. Among these, carbon black is preferred, and at least one selected from the group consisting of acetylene black, Ketjen Black (registered trademark), furnace black, channel black, and thermal lamp black is more preferred.

[0022] The carbon material may be contained in the negative electrode intermediate layer in the form of carbon particles. This may further enhance the effect of suppressing the generation and growth of dendrites. The average primary particle diameter of the carbon particles is, for example, 200 nm or less, preferably 100 nm or less, more preferably 50 nm or less, and even more preferably 45 nm or less. The lower limit of the average primary particle diameter of the carbon particles is not particularly limited, but is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 25 nm or more. In one embodiment, the carbon material contained in the negative electrode intermediate layer contains particulate carbon and fibrous carbon. In this case, the content of particulate carbon is preferably greater than the content of fibrous carbon. In another embodiment, the carbon material contained in the negative electrode intermediate layer preferably does not contain fibrous carbon, and more preferably contains only particulate carbon.

[0023] The ratio of the average particle size of the metal particles that do not form a solid solution with lithium to the carbon particles is preferably 10:1 to 1.1:1, more preferably 8:1 to 1.2:1, even more preferably 5:1 to 1.3:1, and particularly preferably 3:1 to 1.5:1.

[0024] The blending ratio (mass ratio) of the metal particles that do not form a solid solution with lithium to the carbon particles is not particularly limited, but the metal particles:carbon particles ratio is preferably 10:1 to 1:1, and more preferably 5:1 to 2:1. The blending ratio (volume ratio) on a volume basis of the metal particles that do not form a solid solution with lithium to the carbon particles is also not particularly limited, but the metal particles:carbon particles ratio is preferably 1:99 to 30:70, and more preferably 5:95 to 25:75.

[0025] The content of metal particles that do not form a solid solution with lithium in the negative electrode intermediate layer (when two or more materials are used in combination, this refers to the total content of those materials; the same applies hereinafter) is not particularly limited, but is preferably in the range of 50 to 100 mass %, more preferably in the range of 70 to 100 mass %, even more preferably in the range of 85 to 100 mass %, and particularly preferably in the range of 90 to 99 mass %.

[0026] The negative electrode intermediate layer may be composed solely of a mixture of metal particles and carbon particles that do not form a solid solution with lithium, as long as a free-standing film can be produced using only the mixture of metal particles and carbon particles that do not form a solid solution with lithium. However, if necessary, a binder may be included. The type of binder is not particularly limited, and binders known in the art can be appropriately used. Examples include fluorine-based resins such as polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements) and polytetrafluoroethylene (PTFE), as well as styrene-butadiene rubber (SBR) and carboxymethyl cellulose. Among these, it is preferable that the binder contain a fluorine-based resin.

[0027] The content of the binder in the negative electrode intermediate layer is not particularly limited, but is preferably in the range of 1 to 15 mass%, and more preferably in the range of 5 to 10 mass%, relative to 100 mass% of the total amount of the negative electrode intermediate layer. A binder content of 1 mass% or more can form a negative electrode intermediate layer with sufficient strength. A binder content of 15 mass% or less can form a negative electrode intermediate layer with sufficient lithium ion conductivity. In one embodiment, from the viewpoint of improving lithium ion conductivity, the binder content in the negative electrode intermediate layer is preferably 1 mass% to 15 mass%, more preferably 2 mass% to 10 mass%, even more preferably 3 mass% to less than 8 mass%, and even more preferably 3 mass% to 7 mass%, relative to 100 mass% of the total amount of the negative electrode intermediate layer. From the same viewpoint, the content of the binder in the negative electrode intermediate layer is preferably 1% by mass or more and 15% by mass or less, more preferably 2% by mass or more and 10% by mass or less, even more preferably 3% by mass or more and less than 8% by mass, and even more preferably 3% by mass or more and 7% by mass or less, relative to 100% by mass, which is the total of the masses of the metal particles, the carbon particles, and the binder.

[0028] The negative electrode intermediate layer essentially contains metal particles that do not form a solid solution with lithium and a carbon material, and may additionally contain at least one of a binder, a solid electrolyte, and metal particles that form a solid solution with lithium. In this case, from the viewpoint of improving the film strength of the negative electrode intermediate layer, the content of the metal particles that do not form a solid solution with lithium is preferably greater on a mass and / or volume basis than the content of the metal particles that form a solid solution with lithium. In one embodiment, the negative electrode intermediate layer preferably does not contain metal particles that form a solid solution with lithium. In one embodiment, the negative electrode intermediate layer preferably does not contain a solid electrolyte. In one embodiment, the negative electrode intermediate layer preferably consists only of metal particles that do not form a solid solution with lithium, a carbon material, and a binder.

[0029] The thickness of the negative electrode intermediate layer is more than 0.1 μm and less than 5 μm. If the thickness of the negative electrode intermediate layer is 0.1 μm or less, there is a risk of a short circuit occurring during charging. Furthermore, if the thickness of the negative electrode intermediate layer is 5 μm or less, the lithium ion conduction resistance increases, and there is a risk that the charge capacity of the secondary battery cannot be sufficiently obtained. In one embodiment, the thickness of the negative electrode intermediate layer is preferably 1 μm or more and 4 μm or less, more preferably 1 μm or more and 2 μm or less, and even more preferably 1 μm or more and 1.5 μm or less. If the thickness of the negative electrode intermediate layer is 1 μm or more, the function of the negative electrode intermediate layer can be fully exhibited. If the thickness of the negative electrode intermediate layer is 4 μm or less, a decrease in energy density can be suppressed.

[0030] In another embodiment, the thickness of the negative electrode intermediate layer may be more than 0.1 μm and less than 5 μm, and is preferably 0.5 μm or more and 4 μm or less, more preferably 0.5 μm or more and 2 μm or less, even more preferably 0.5 μm or more and 1.5 μm or less, and even more preferably 0.5 μm or more and 1.2 μm or less.

[0031] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as a main component). The solid electrolyte contained in the solid electrolyte layer is not particularly limited, and any solid electrolyte known in the art can be appropriately adopted, for example, a sulfide solid electrolyte and an oxide solid electrolyte. This solid electrolyte exhibits excellent lithium ion conductivity, and is therefore preferably a sulfide solid electrolyte containing an S element, more preferably a sulfide solid electrolyte containing an Li element, an M element, and an S element, wherein the M element contains at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I, and even more preferably a sulfide solid electrolyte containing an S element, an Li element, and an P element. One example is LPS (Li ２ S-P ２ S ５ ), Li ６ P.S. ５ X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li３．２ P ０．９６ S ４ and Li ３ P.S. ４ These sulfide solid electrolytes are preferably used because they have excellent lithium ion conductivity.

[0032] The ionic conductivity (e.g., Li ion conductivity) of the sulfide solid electrolyte at room temperature (25°C) is, for example, 1 × 10 －５ S / cm or more, and preferably 1×10 －４ The ionic conductivity of the solid electrolyte can be measured by an AC impedance method.

[0033] Examples of the shape of the solid electrolyte include particulate shapes such as spherical shapes and oval spherical shapes, thin films, etc. When the solid electrolyte is particulate, its average particle size (D50) is not particularly limited, but is preferably 0.01 μm or more and 40 μm or less, more preferably 0.1 μm or more and 20 μm or less, and even more preferably 0.5 μm or more and 10 μm or less.

[0034] The content of the solid electrolyte in the solid electrolyte layer is preferably 50 to 100 mass %, more preferably 90 to 100 mass %.

[0035] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. The binder is not particularly limited, and known binders can be used as appropriate. For example, the binders described above for the negative electrode intermediate layer can be similarly employed. The content of the binder in the solid electrolyte layer is not particularly limited, and is, for example, 1 to 10 mass %.

[0036] The thickness of the solid electrolyte layer varies depending on the intended configuration of the lithium secondary battery, but is usually 0.1 to 1000 μm, preferably 10 to 100 μm.

[0037] The thickness ratio between the negative electrode intermediate layer and the solid electrolyte layer is, for example, 1:100 to 1:10, preferably 1:80 to 1:15, and more preferably 1:40 to 1:20.

[0038] Although one embodiment of the lithium secondary battery of the present invention has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0039] [Positive Electrode Active Material Layer] The positive electrode active material layer essentially contains a positive electrode active material, and may contain a solid electrolyte, a binder, and a conductive additive as needed.

[0040] The type of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, but may be LiCoO ２ , LiMnO ２ , LiNiO ２ , LiVO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt active materials such as LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ Spinel-type active materials such as LiFePO ４ , LiMnPO ４ Olivine type active materials such as Li ２ FeSiO ４ , Li ２ MnSiO ４ Examples of oxide active materials other than those mentioned above include Si-containing active materials such as Li ４ Ti ５ O １２ Among them, Li(Ni-Mn-Co)O ２ Also, those in which part of these transition metals has been replaced with other elements (hereinafter simply referred to as "NMC composite oxides") are preferably used as the positive electrode active material.

[0041] In addition, a sulfur-based positive electrode active material is also one of the preferred embodiments. Examples of the sulfur-based positive electrode active material include particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and any material can be used as long as it is capable of releasing lithium ions during charging and absorbing lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur.

[0042] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably, for example, 30 to 99 mass %, more preferably 40 to 95 mass %, and even more preferably 45 to 90 mass %.

[0043] The positive electrode active material layer preferably further contains a solid electrolyte. The specific form of the solid electrolyte contained in the positive electrode active material layer may be the same as that described in the solid electrolyte layer section. A sulfide solid electrolyte is preferably used because it has excellent lithium ion conductivity and a low bulk modulus, allowing it to follow the volumetric changes of the positive electrode active material that occur during charging and discharging. The content of the solid electrolyte in the positive electrode active material layer is not particularly limited, but is, for example, 1 to 70% by mass, preferably 3 to 60% by mass, and more preferably 5 to 55% by mass.

[0044] The binder used in the positive electrode active material layer is not particularly limited, and any known binder can be used as appropriate. For example, the binder described above for the negative electrode intermediate layer can be used. The content of the binder in the positive electrode active material layer is not particularly limited, and is, for example, 1 to 10 mass %.

[0045] The conductive additive used in the positive electrode active material layer is not particularly limited, and may be, for example, carbon such as carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). The content of the conductive additive in the positive electrode active material layer is not particularly limited, and is, for example, 1 to 30 mass%.

[0046] The thickness of the positive electrode active material layer varies depending on the intended configuration of the lithium secondary battery, but is usually 0.1 to 1000 μm, preferably 10 to 300 μm.

[0047] The thickness ratio between the negative electrode intermediate layer and the positive electrode active material layer is, for example, 1:300 to 1:10, preferably 1:200 to 1:50, and more preferably 1:150 to 1:100.

[0048] Although one embodiment of the lithium secondary battery of the present invention has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0049] The following embodiments are also included within the scope of the present invention: a lithium secondary battery according to claim 1 having the features of claim 2; a lithium secondary battery according to claim 1 or 2 having the features of claim 3; a lithium secondary battery according to any one of claims 1 to 3 having the features of claim 4; a lithium secondary battery according to any one of claims 1 to 4 having the features of claim 5; and a lithium secondary battery according to any one of claims 1 to 5 having the features of claim 6.

[0050] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. Note that the following operations were carried out in a glove box with a dew point of -68°C or less. Furthermore, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0051] <Examples of Preparation of Evaluation Cell> [Example 1] (Preparation of Positive Electrode) LiNi as Positive Electrode Active Material ０．８ Mn ０．１ Co ０．１ O ２ , acetylene black as a conductive additive, and Li as a solid electrolyte. ６ P.S. ５ Cl was weighed out to give a mass ratio of 90:1:9. These were mixed using an agate mortar and then further stirred and mixed using a planetary ball mill. 2 parts by mass of styrene-butadiene rubber (SBR) as a binder was added to 100 parts by mass of the obtained mixed powder, and mesitylene was added as a solvent and mixed to prepare a positive electrode active material slurry. The positive electrode active material slurry was applied to the surface of an aluminum foil as a positive electrode current collector, dried, and pressed to obtain a positive electrode having a positive electrode active material layer (thickness 120 μm) on the surface of the positive electrode current collector.

[0052] (Preparation of solid electrolyte layer) Li as solid electrolyte ６ P.S. ５A solid electrolyte slurry was prepared by adding 2 parts by mass of SBR as a binder to 100 parts by mass of Cl, and adding mesitylene as a solvent and mixing them. The solid electrolyte slurry was applied to the surface of a stainless steel foil as a support and dried to obtain a solid electrolyte layer (thickness: 30 μm).

[0053] (Preparation of Negative Electrode Intermediate Layer) Nickel (Ni) nanoparticles (average particle diameter 70 nm) and carbon black nanoparticles (average particle diameter 35 nm) were weighed and mixed in a volume ratio of 7:93. To 93 parts by mass of the resulting mixture, 7 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added, and N-methyl-2-pyrrolidone (NMP) was added as a solvent and mixed to prepare a negative electrode intermediate layer slurry. The negative electrode intermediate layer slurry was applied to the surface of a stainless steel foil as a negative electrode current collector and dried to obtain a negative electrode intermediate layer (thickness 1 μm) formed on the negative electrode current collector.

[0054] (Preparation of Evaluation Cell) A positive electrode active material layer formed on the surface of an aluminum foil (positive electrode current collector) and a solid electrolyte layer formed on the surface of a stainless steel foil were stacked so that the exposed surface of the positive electrode active material layer and the exposed surface of the solid electrolyte layer faced each other, and then bonded together by cold isostatic pressing (CIP; 700 MPa, 25 ° C, 1 minute). After peeling off the stainless steel foil adjacent to the solid electrolyte layer, the solid electrolyte layer and the negative electrode intermediate layer formed on the surface of the stainless steel foil (negative electrode current collector) were stacked so that the exposed surface of the solid electrolyte layer and the exposed surface of the negative electrode intermediate layer faced each other, and then bonded together by cold isostatic pressing (CIP; 80 ° C, 500 MPa, 1 minute). Finally, an aluminum positive electrode tab and a nickel negative electrode tab were bonded to each of the aluminum foil (positive electrode current collector) and the stainless steel foil (negative electrode current collector) using an ultrasonic welding machine, and the resulting laminate was placed inside an aluminum laminate film and vacuum sealed to obtain an evaluation cell, which is a lithium deposition-type all-solid-state lithium secondary battery.

[0055] Example 2 An evaluation cell for this example was produced in the same manner as in Example 1, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 2 μm.

[0056] Example 3 An evaluation cell for this example was produced in the same manner as in Example 1, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 4 μm.

[0057] [Example 4] An evaluation cell for this example was produced in the same manner as in Example 1, except that in the above (production of the negative electrode intermediate layer), nickel (Ni) nanoparticles (average particle diameter 70 nm) were replaced with tin (Sn) nanoparticles (average particle diameter 150 nm).

[0058] Example 5 An evaluation cell for this example was produced in the same manner as in Example 4, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 2 μm.

[0059] Example 6 An evaluation cell for this example was produced in the same manner as in Example 4, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 4 μm.

[0060] Comparative Example 1 An evaluation cell for this comparative example was fabricated in the same manner as in Example 1, except that in the above (fabrication of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 0.1 μm.

[0061] Comparative Example 2 An evaluation cell for this comparative example was produced in the same manner as in Example 1, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 5 μm.

[0062] Comparative Example 3 An evaluation cell for this comparative example was produced in the same manner as in Example 1, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 10 μm.

[0063] Comparative Example 4 An evaluation cell for this comparative example was fabricated in the same manner as in Example 4, except that in the above (fabrication of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 0.1 μm.

[0064] Comparative Example 5 An evaluation cell for this comparative example was produced in the same manner as in Example 4, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 5 μm.

[0065] Comparative Example 6 An evaluation cell for this comparative example was produced in the same manner as in Example 4, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 10 μm.

[0066] Comparative Example 7 An evaluation cell for this comparative example was produced in the same manner as in Comparative Example 1, except that in the above (production of the negative electrode intermediate layer), nickel (Ni) nanoparticles (average particle diameter 70 nm) were replaced with silver (Ag) nanoparticles (average particle diameter 60 nm).

[0067] Comparative Example 8 An evaluation cell for this comparative example was produced in the same manner as in Comparative Example 7, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 1 μm.

[0068] Comparative Example 9 An evaluation cell for this comparative example was produced in the same manner as in Comparative Example 7, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 2 μm.

[0069] Comparative Example 10 An evaluation cell for this comparative example was produced in the same manner as in Comparative Example 7, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 4 μm.

[0070] Comparative Example 11 An evaluation cell for this comparative example was produced in the same manner as in Comparative Example 7, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 5 μm.

[0071] Comparative Example 12 An evaluation cell for this comparative example was produced in the same manner as in Comparative Example 7, except that in the above (production of the negative electrode intermediate layer), the thickness of the negative electrode intermediate layer was changed to 10 μm.

[0072] <Evaluation of Capacity Characteristics and Charge Rate Characteristics> A charge-discharge test was performed on the evaluation cells prepared in the above Examples and Comparative Examples, while applying a confining pressure of 3 MPa in the stacking direction of the evaluation cells using a pressure member. First, as cell conditioning, the cells were charged at a rate of 0.01 C for 25 hours in a thermostatic chamber set at 60°C, and then constant-current constant-voltage (CCCV) charging at 0.05 C and 4.3 V was performed with a cutoff current value of 0.01 C. Subsequently, constant-current (CC) discharging was performed at 0.1 C to 2.5 V.

[0073] Next, constant-current constant-voltage (CCCV) charging at 0.1 C and 4.3 V was performed with a cutoff current of 0.1 C, and the charge capacity value at 0.1 C was measured. Similarly, constant-current constant-voltage (CCCV) charging at 1.0 C and 4.3 V was performed with a cutoff current of 0.1 C, and the charge capacity value at 1.0 C was also measured. The percentage of the charge capacity value obtained at 1.0 C charging relative to the charge capacity value obtained at 0.1 C charging (charge capacity retention rate) was then calculated. The results are shown in Table 1 below, along with the charge capacity values ​​at each rate. In Table 1, when a short circuit occurred, the charge capacity value was marked as "short circuit." In Table 1, "-" indicates that no calculation was performed.

[0074]

[0075] As shown in Table 1, according to the present invention, in a lithium deposition-type lithium secondary battery equipped with a negative electrode intermediate layer, by including a metal that does not form a solid solution with lithium in the negative electrode intermediate layer together with a carbon material, it is possible to further improve the charge rate characteristics without causing a short circuit during charging even when the thickness of the negative electrode intermediate layer is less than 5 μm.

[0076] <Cycle Test> A cycle charge-discharge test was performed on the evaluation cell prepared in Example 1 above, while applying a restraining pressure of 3 MPa in the stacking direction of the evaluation cell using a pressure member. First, the cell was charged for 25 hours at a rate of 0.01 C in a thermostatic chamber set at 60°C, followed by a constant current, constant voltage (CCCV) charge at 0.05 C and 4.3 V with a cutoff current value of 0.01 C. Subsequently, a constant current (CC) discharge was performed at 0.1 C to 2.5 V. This charge-discharge was designated the first cycle. Next, as the second and subsequent cycles, constant current (CC) charge-discharge was repeated at 0.5 C in the cell voltage range of 2.5 V to 4.3 V for up to 100 cycles. No short circuits occurred even after 100 cycles.

[0077] REFERENCE SIGNS LIST 10a laminated secondary battery, 11' negative electrode current collector, 11" positive electrode current collector, 13 negative electrode active material layer, 14 negative electrode intermediate layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 19 single cell layer, 21 power generating element, 25 negative electrode current collector, 27 positive electrode current collector, 29 laminate film.

Claims

1. A lithium secondary battery comprising: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector, on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a negative electrode intermediate layer present adjacent to the surface of the solid electrolyte layer on the negative electrode current collector side and containing metal particles that do not form a solid solution with lithium and a carbon material, wherein the thickness of the negative electrode intermediate layer is more than 0.1 μm and less than 5 μm.

2. The lithium secondary battery according to claim 1, wherein the thickness of the negative electrode intermediate layer is 1 μm or more and 4 μm or less.

3. The lithium secondary battery according to claim 1, wherein the thickness of the negative electrode intermediate layer is 1 μm or more and 2 μm or less.

4. The lithium secondary battery according to claim 1, wherein the metal particles that do not form a solid solution with lithium include at least one of Sn particles and Ni particles.

5. The lithium secondary battery according to claim 4, wherein the metal particles that do not form a solid solution with lithium include Ni particles.

6. The lithium secondary battery according to claim 1, wherein the negative electrode intermediate layer does not contain a metal that forms a solid solution with lithium.

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