Lithium secondary battery

Abstract

[Problem] To provide a means that makes it possible to improve initial charge / discharge efficiency by suppressing any decrease in capacity during initial discharge in a lithium-deposition-type lithium secondary battery that has a negative electrode intermediate layer. [Solution] This lithium secondary battery comprises an electric power generation element including: a positive electrode that has a positive electrode active material layer containing a positive electrode active material; a negative electrode that has a negative electrode current collector, lithium metal being deposited in the negative electrode during charging; a solid electrolyte layer that is interposed between the positive electrode and the negative electrode and contains a solid electrolyte; and a negative electrode intermediate layer that is present adjacent to the negative-current-collector-side surface of the solid electrolyte layer, the average pore diameter of the negative electrode intermediate layer being 15 nm or greater, and the average volume of pores in the negative electrode intermediate layer being 0.5 cm3 / g or less.

Description

Lithium secondary battery

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

[0002] In recent years, research and development on lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively conducted. Solid electrolytes are materials composed primarily 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 caused by flammable organic electrolytes that occur in conventional liquid-based batteries using nonaqueous electrolytes.

[0003] Conventionally, one type of lithium secondary battery using a solid electrolyte 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 2020-167146 A). In such lithium deposition type lithium secondary batteries, 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. To prevent this, a technology has been reported in which a negative electrode active material layer (negative electrode intermediate layer) is provided between the negative electrode current collector and the solid electrolyte layer. However, when such a layer is included, there is a problem that battery characteristics such as cycle characteristics and discharge rate characteristics may not be sufficiently improved.

[0004] This document discloses a technique for incorporating a predetermined amount of amorphous carbon into a negative electrode active material layer (negative electrode intermediate layer) between a negative electrode current collector and a solid electrolyte layer, and further controlling the specific surface area of ​​the amorphous carbon to a certain level or less, or limiting the DBP oil absorption to a certain level or more. This configuration is said to reduce the number of interfaces between carbon particles or carbon particle aggregates (primary agglomerates), facilitating lithium diffusion within the negative electrode active material layer, thereby improving cycle characteristics and discharge rate characteristics.

[0005] However, the inventors have found through their investigations that when a negative electrode intermediate layer as described in the above document is present, the capacity at the time of initial discharge decreases, and the initial charge / discharge efficiency may be reduced.

[0006] Therefore, an object of the present invention is to provide a means for suppressing the decrease in capacity during initial discharge and improving the initial charge-discharge efficiency 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 problems. As a result, in a lithium secondary battery equipped with a lithium deposition type power generating element, it has been discovered that the average pore diameter of the negative electrode intermediate layer is 15 nm or more, and the pore volume of the negative electrode intermediate layer is 0.5 cm ３ The present inventors have found that the above problems can be solved by controlling the porosity of the polymer to 0.1g or less, and have completed the present invention.

[0008] That is, one aspect of the present invention relates to 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 and on which lithium metal is deposited during charging; a solid electrolyte layer containing a solid electrolyte interposed between the positive electrode and the negative electrode; and a negative electrode intermediate layer present adjacent to a surface of the solid electrolyte layer facing the negative electrode current collector. In this lithium secondary battery, the negative electrode intermediate layer has an average pore diameter of 15 nm or more, and an average pore volume of 0.5 cm ３ / g or less.

[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 provides 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 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 a surface of the solid electrolyte layer facing the negative electrode current collector, wherein the negative electrode intermediate layer has an average pore diameter of 15 nm or more and an average pore volume of 0.5 cm ３ / g or less. According to this embodiment, in a lithium deposition type lithium secondary battery having a negative electrode intermediate layer, it is possible to suppress a decrease in capacity during initial discharge and improve the initial charge / discharge efficiency. The lithium secondary battery according to this embodiment 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 all-solid-state 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 that is adjacent to the surface of the solid electrolyte layer that faces the negative electrode current collector. The negative electrode intermediate layer preferably has electrical conductivity as a whole. The volume resistivity of the negative electrode intermediate layer is not particularly limited, but is preferably 10 ２ The volume resistivity of the negative electrode intermediate layer is preferably 10 Ω·cm or less, and more preferably 10 Ω·cm or less. 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). Furthermore, lithium metal may be precipitated inside the negative electrode intermediate layer during charging. Since the negative electrode intermediate layer has a structure suitable for precipitation and elution, the effects of the present invention can be further enhanced by precipitation and elution of lithium inside the layer.

[0017] The negative electrode intermediate layer may contain a lithium reactive material. The lithium reactive material is not particularly limited, but may be, for example, a carbon material. When the carbon material is contained in the negative electrode intermediate layer, the generation and growth of dendrites can be particularly effectively suppressed.

[0018] 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. Here, the carbon material is preferably amorphous carbon. Specific examples of amorphous carbon include, but are not limited to, carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). When the negative electrode intermediate layer contains amorphous carbon, it can have a structure more suitable for lithium precipitation and elution reactions during charge and discharge.

[0019] The carbon material may be contained in the negative electrode intermediate layer, for example, in the form of carbon particles. This can further suppress 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. In this specification, the average primary particle diameter of particles refers to the 50% cumulative diameter (D50) of particle diameters of the particles observed within several to several tens of fields of view when a cross section of a layer containing the particles is observed with a scanning electron microscope (SEM) (the maximum distance between any two points on the outline of the observed particles).

[0020] The content of the carbon material is, for example, 99% by mass or less, preferably 95% by mass or less, and more preferably 90% by mass or less, relative to the total mass of the negative electrode intermediate layer. The lower limit of the content of the carbon material is not particularly limited, but is, for example, 50% by mass or more, preferably 70% by mass or more, and more preferably 80% by mass or more, relative to the total mass of the negative electrode intermediate layer. When the content of the carbon material is within the above range, the generation and growth of dendrites can be suppressed.

[0021] The negative electrode intermediate layer may contain a binder material as needed. The type of binder material is not particularly limited, and binder materials known in the art can be appropriately used. The binder material preferably contains a polymer. Examples of polymers 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. In particular, the binder material preferably contains a fluorine-based resin.

[0022] The content of the binder material in the negative electrode intermediate layer is not particularly limited, but is preferably in the range of 1 to 20 mass %, and more preferably in the range of 5 to 15 mass %, relative to the total mass of the negative electrode intermediate layer. When the content of the binder material is 1 mass % or more, a negative electrode intermediate layer having sufficient strength can be formed. When the content of the binder material is 15 mass % or less, a negative electrode intermediate layer having sufficient lithium ion conductivity can be formed. From the same viewpoint, the content of the binder material in the negative electrode intermediate layer is preferably 1 mass % or more and 15 mass % or less, relative to the total mass of the carbon particles and the binder material (100 mass %).

[0023] The negative electrode intermediate layer may additionally contain a metal material. Examples of metal materials include, but are not limited to, In, Al, Si, Sn, Mg, Au, Ag, and Zn. The content of the metal material is, for example, 0 to 35 mass%, preferably 0 to 10 mass%, more preferably 0 to 1 mass%, and even more preferably 0 to 0.01 mass%, relative to the total mass of the negative electrode intermediate layer. In one embodiment, the negative electrode intermediate layer preferably does not contain a metal material. In this specification, the term "free of" a material or the like refers not only to "substantially free of" but also to a material or the like being below the detection limit of a measuring instrument (e.g., an absorptiometer or mass spectrometer). "Substantially free of" means that a certain material or the like is not intentionally added, and does not exclude, for example, unintentional inclusion due to unintentional mixing or insufficient removal. In one embodiment, when a material is described as being "free" or "substantially free," the content of the material is preferably less than 10% by mass, more preferably 5% by mass or less, even more preferably 1% by mass or less, and even more preferably 0.1% by mass or less. In one embodiment, when a material is described as being "free" or "substantially free," the content of the material may be 1,000 ppm by mass or less, 100 ppm by mass or less, 10 ppm by mass or less, 1 ppm by mass or less, or 0.1 ppm by mass or less.

[0024] In one embodiment, the negative electrode intermediate layer preferably contains at least one of a carbon material and a binder material, and more preferably contains a carbon material and a binder material. By including a carbon material and a binder material in the negative electrode intermediate layer, a structure more suitable for the negative electrode reaction during charge and discharge can be obtained. The negative electrode intermediate layer may also be composed only of a carbon material and a binder material. In this specification, "consisting only of" includes not only an embodiment containing only a specific compound, but also an embodiment substantially free of compounds other than the specific compound.

[0025] In the lithium secondary battery according to the present embodiment, the average pore diameter of the negative electrode intermediate layer is 15 nm or more, and the average pore volume is 0.5 cm ３ When the average pore size of the negative electrode intermediate layer is less than 15 nm, the structure becomes unfavorable for the lithium deposition and elution reaction, and the reaction efficiency may decrease. ３ If it is greater than 1 / g, the electronic conductivity may decrease, resulting in a decrease in charge / discharge efficiency.

[0026] In this specification, the average pore size and average pore volume of the negative electrode intermediate layer are evaluated by the BET method in the molded negative electrode intermediate layer, and the values ​​obtained by measurement according to the BJH method described in E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 73, 373 (1951) are used. In other words, "pore" means a space having a shape that can be measured by this method. Therefore, the pores of the negative electrode intermediate layer may include pores in the surface of the material (e.g., carbon material) contained in the negative electrode intermediate layer. For example, the carbon material may take the form of primary particles, a form in which the primary particles are aggregated (primary aggregates), or a form in which these particles are further aggregated (secondary aggregates). When a carbon material is contained in the negative electrode intermediate layer, pores that can form these forms may be included in the pores of the negative electrode intermediate layer.

[0027] The average pore diameter of the negative electrode intermediate layer is preferably 20 nm or more, more preferably 25 nm or more, and even more preferably 30 nm or more. The upper limit of the average pore diameter of the negative electrode intermediate layer is not particularly limited, but is preferably 100 nm or less, more preferably 80 nm or less, and even more preferably 60 nm or less. That is, the average pore diameter of the negative electrode intermediate layer is preferably 20 nm or more and 100 nm or less, more preferably 25 nm or more and 80 nm or less, and even more preferably 30 nm or more and 60 nm or less. When the average pore diameter is within the above range, the negative electrode intermediate layer can have an advantageous structure for lithium deposition and elution reactions.

[0028] The average volume of the pores in the negative electrode intermediate layer is preferably 0.5 cm ３ / g, more preferably less than 0.48 cm ３ / g or less, and more preferably 0.35 cm ３ The lower limit of the average pore volume of the negative electrode intermediate layer is not particularly limited, but is, for example, 0.05 cm ３ / g or more, preferably 0.1 cm ３ / g or more, more preferably 0.2 cm ３ / g or more, and more preferably 0.25 cm ３ That is, the average volume of the pores in the negative electrode intermediate layer is preferably 0.1 cm ３ / g or more 0.5cm ３ / g, more preferably less than 0.2 cm ３ / g or more 0.48cm ３ / g or less, and more preferably 0.25 cm ３ / g or more 0.35cm ３ When the average pore volume is within the above range, the electron conductivity of the negative electrode intermediate layer can be improved.

[0029] The mechanism by which the initial charge-discharge efficiency is improved by controlling the average pore diameter and average pore volume of the negative electrode intermediate layer within the above ranges is not fully understood, but the following mechanism is presumed. During charge and discharge, lithium deposition and elution reactions proceed in the negative electrode. If the average pore diameter is too small, there may not be enough space for lithium deposition during the initial charge, making the deposition reaction difficult to proceed. Furthermore, the lack of sufficient space during lithium deposition can cause deterioration of the lithium secondary battery due to repeated deposition and elution. On the other hand, if the pore volume is too large, the electron curvature tends to increase when electrons move between the particles and structures constituting the carbon material, which can reduce electronic conductivity. Furthermore, if the pore volume is too large, the contact area between the constituent material of the negative electrode intermediate layer and the deposited lithium may decrease, preventing the elution reaction from proceeding sufficiently during discharge, resulting in a deterioration of charge-discharge efficiency. Note that the above mechanism is based on speculation, and its accuracy does not affect the technical scope of the present invention.

[0030] The DPB absorption capacity of the negative electrode intermediate layer is not particularly limited as long as it is within a range exhibiting an appropriate average pore diameter and average pore volume. For example, it is 10 mL / 100 g or more and 1,000 mL / 100 g or less, preferably 30 mL / 100 g or more and 200 mL / 100 g or less, more preferably 50 mL / 100 g or more and 150 mL / 100 g or less, and even more preferably 70 mL / 100 g or more and 100 mL / 100 g or less. Within the above ranges, the effects of the present invention can be more significantly achieved. In this specification, the DPB absorption capacity of the negative electrode intermediate layer is the value calculated for a molded negative electrode intermediate layer in accordance with JIS K 6217-4:2017 "Carbon black for rubber - Fundamental properties - Part 4: Determination of oil absorption (including compressed samples)."

[0031] The BET specific surface area of ​​the negative electrode intermediate layer is not particularly limited, but is preferably 500 m ２ / g or less, and ２ / g or less is more preferable, and 130m ２ / g or less is more preferable, and 50m２ / g or less is even more preferable. There is no particular restriction on the lower limit, but it is preferably 10 m ２ / g or more, and ２ / g or more is more preferable, and 20m ２ / g or more is more preferable, and 30m ２ / g or more. ２ / g or more 500m ２ / g or less, and ２ / g or more 200m ２ / g or less is more preferable, and 20m ２ / g or more 130m ２ / g or less is more preferable, and 30m ２ / g or more 50m ２ / g or less. Within this range, the effects of the present invention can be obtained more significantly. The BET specific surface area of ​​the negative electrode intermediate layer is a value measured according to the BJH method described in JIS Z 8830:2013 (ISO 9277:2010).

[0032] The porosity of the negative electrode intermediate layer is not particularly limited, but is, for example, 10% to 70%, preferably 20% to 60%, and particularly preferably 30% to 58%. Within the above range, the effects of the present invention can be more significantly achieved. The porosity of the negative electrode intermediate layer can be estimated using the volume of the voids in the negative electrode intermediate layer measured by mercury intrusion porosimetry using a mercury porosimeter, as described below, and the size and thickness of the negative electrode intermediate layer. The thickness of the negative electrode intermediate layer can be determined by SEM observation of the cross section.

[0033] The thickness of the negative electrode intermediate layer is not particularly limited, but may be 0.1 μm or more and less than 100 μm. When the thickness of the negative electrode intermediate layer is 0.1 μm or more, the occurrence of a short circuit during charging can be suppressed. Furthermore, when the thickness of the negative electrode intermediate layer is 100 μm or less, the increase in lithium ion conduction resistance can be suppressed, and sufficient charge capacity of the secondary battery can be obtained. In one embodiment, the thickness of the negative electrode intermediate layer is preferably 1 μm or more and 20 μm or less, more preferably 1 μm or more and 15 μm or less, and even more preferably 1 μm or more and 10 μm or less.

[0034] [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 , Li ３．２ 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.

[0035] The ionic conductivity (e.g., Li ion conductivity) of the sulfide solid electrolyte at room temperature (25°C) is, for example, 1 × 10 －５S / cm or more, and preferably 1×10 －４ The ionic conductivity of the solid electrolyte can be measured by an AC impedance method.

[0036] 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.

[0037] The content of the solid electrolyte in the solid electrolyte layer is preferably 50 to 100 mass %, more preferably 90 to 100 mass %.

[0038] The solid electrolyte layer may further contain a binder material in addition to the solid electrolyte. The binder material is not particularly limited, and known binder materials can be used as appropriate. For example, the binder material described above for the negative electrode intermediate layer can be similarly employed. The content of the binder material in the solid electrolyte layer is not particularly limited, and is, for example, 1 to 10 mass %.

[0039] 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.

[0040] The ratio of the thickness of the negative electrode intermediate layer to the thickness of the solid electrolyte layer (thickness of the negative electrode intermediate layer:thickness of the solid electrolyte layer) is, for example, 1:100 to 1:1, preferably 1:50 to 1:2, and more preferably 1:10 to 1:3.

[0041] 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.

[0042] [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 material, and a conductive additive as needed.

[0043] 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 also simply referred to as "NMC composite oxides") are preferably used as the positive electrode active material.

[0044] 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.

[0045] 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 %.

[0046] 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.

[0047] The binder material used in the positive electrode active material layer is not particularly limited, and known binder materials can be used as appropriate. For example, the binder materials described above for the negative electrode intermediate layer can be used. The content of the binder material in the positive electrode active material layer is not particularly limited, and is, for example, 1 to 10 mass %.

[0048] 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%.

[0049] 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.

[0050] The ratio of the thickness of the negative electrode intermediate layer to the thickness of the positive electrode active material layer (thickness of the negative electrode intermediate layer:thickness of the positive electrode active material layer) is, for example, 1:100 to 1:3, preferably 1:50 to 1:5, and more preferably 1:30 to 1:10.

[0051] [Method for Manufacturing Lithium Secondary Battery] Next, a method for manufacturing the lithium secondary battery according to the present embodiment will be described. The lithium secondary battery according to the present embodiment can be manufactured by a manufacturing method including: mixing a carbon material, a binder material, a first solvent selected from the group consisting of linear or branched dialkyl ketones having 5 to 7 carbon atoms and saturated alicyclic ketones having 5 to 6 carbon atoms, and a second solvent selected from the group consisting of N-methyl-2-pyrrolidone (NMP), N,N-dimethylpropionamide, and dimethyl sulfoxide (DMSO) at a temperature of 80°C or higher to prepare a negative electrode intermediate layer slurry (slurry preparation step); applying the negative electrode intermediate layer slurry to the surface of a negative electrode current collector to form a coating film (coating film formation step); heating the coating film to 70°C or lower to remove the first solvent from the coating film, and then heating the coating film to 100°C or higher to remove the second solvent from the coating film (drying step). That is, according to another aspect of the present invention, there is provided a method for manufacturing a lithium secondary battery including the steps described above. The lithium secondary battery manufactured by the method described above can have an improved initial charge / discharge efficiency because the average pore size and average pore volume in the negative electrode intermediate layer are appropriately controlled.

[0052] The method for producing the positive electrode and the solid electrolyte layer of the lithium secondary battery is not particularly limited, and any conventionally known method can be appropriately adopted. For example, the positive electrode and the solid electrolyte layer of the lithium secondary battery can be produced by the method described in the Examples section.

[0053] The negative electrode of a lithium secondary battery is produced by a method including the above-mentioned three steps of slurry preparation, coating film formation, and drying. Each step will be described below.

[0054] (Slurry Preparation Step) In the slurry preparation step, a carbon material, a binder material, a first solvent, and a second solvent are mixed. The carbon material and binder material may be the same as those described in the above section [Negative Electrode Intermediate Layer].

[0055] The first solvent is selected from the group consisting of linear or branched dialkyl ketones having 5 to 7 carbon atoms and saturated alicyclic ketones having 5 to 6 carbon atoms. By including such a solvent in the negative electrode intermediate layer slurry, a well-dispersed slurry can be obtained, and the solvent can be slowly converted to a gas at a temperature of 70°C or less in the subsequent drying step and removed from the coating. The first solvent can be, for example, a solvent selected from the group consisting of linear or branched dialkyl ketones having 5 to 6 carbon atoms and saturated alicyclic ketones having 5 to 6 carbon atoms, and preferably a saturated alicyclic ketone having 5 or 6 carbon atoms. Specific examples of the first solvent include 2-pentanone, 3-pentanone, 2-hexanone, 3-hexanone, 2-heptanone, 3-heptanone, 4-heptanone, 3-methyl-2-butanone, 3-methyl-2-pentanone, 4-methyl-2-pentanone, 2-methyl-3-pentanone, 3-methyl-2-hexanone, 4-methyl-2-hexanone, 5-methyl-2-hexanone, 2-methyl-3-hexanone, 4-methyl-3-hexanone, 5-methyl-3-hexanone, cyclopentanone, and cyclohexanone. Among these, from the viewpoints of ease of drying and dispersibility, 2-pentanone, 2-hexanone, 3-hexanone, 3-methyl-2-butanone, 3-methyl-2-pentanone, 4-methyl-2-pentanone, 2-methyl-3-pentanone, cyclopentanone, or cyclohexanone is preferred, cyclopentanone or cyclohexanone is more preferred, and cyclohexanone is particularly preferred.

[0056] The boiling point of the first solvent is preferably 100° C. or higher and lower than 165° C., and more preferably 120° C. or higher and 160° C. or lower. When the boiling point is within the above range, evaporation and concentration during preparation of the slurry can be suppressed, and the first solvent can be easily removed in the drying step.

[0057] The second solvent is selected from the group consisting of N-methyl-2-pyrrolidone (NMP), N,N-dimethylpropionamide, and dimethyl sulfoxide (DMSO). The inclusion of such a solvent in the negative electrode intermediate layer slurry results in a well-dispersed slurry and facilitates two-stage drying in the subsequent drying step. Among these, N-methyl-2-pyrrolidone (NMP) is particularly preferred from the viewpoint of dispersibility (or solubility).

[0058] The slurry is prepared at a temperature of 80° C. or higher. If the temperature is lower than 80° C., the mixed solution may not disperse sufficiently, resulting in a non-uniform negative electrode intermediate layer. The upper limit of the temperature is not particularly limited as long as it is lower than the boiling points of the first and second solvents to be mixed, but is, for example, 130° C. or lower, and preferably 120° C. or lower.

[0059] (Coating Film Formation Step) In the coating film formation step, the slurry prepared above is applied to the surface of the negative electrode current collector. The negative electrode current collector may be the same as that described in the [Current Collector] section above. The method for applying the slurry is not particularly limited, and a conventionally known method may be appropriately adopted. For example, the method described in the Examples may be adopted.

[0060] (Drying Step) The drying step includes a first drying step for removing the first solvent and a second drying step for removing the second solvent. By performing the drying step in two steps, the resulting negative electrode intermediate layer can have an appropriate shape, thereby achieving the effects of the present invention.

[0061] In the first drying stage, the first solvent is removed by heating the coating film to 70°C or less. If the heating temperature exceeds 70°C to remove the first solvent, the removal of the second solvent may not be suppressed, and the binder may melt, preventing the negative electrode intermediate layer from assuming an appropriate shape. The lower limit of the heating temperature is not particularly limited as long as the first solvent can be appropriately removed, but is, for example, 50°C or higher, preferably 55°C or higher. The pressure during heating may be, but is not limited to, 1 atmosphere. The heating time is not particularly limited, but is, for example, 5 minutes to 3 hours, preferably 10 minutes to 1 hour, and more preferably 20 minutes to 40 minutes.

[0062] In the second drying step, the coating film from which the first solvent has been removed in the first drying step is heated to 100°C or higher to remove the second solvent from the coating film. If the heating temperature is lower than 100°C, the second solvent may not be properly removed. The upper limit of the heating temperature is not particularly limited, but is, for example, 180°C or lower, preferably 150°C or lower. In one embodiment, heating may be performed under reduced pressure, i.e., at a pressure lower than 1 atmosphere. The pressure during heating may be, for example, 0.01 mmHg or higher but lower than 5 mmHg, 0.01 mmHg or higher but 2 mmHg or lower, 0.01 mmHg or higher but 1 mmHg or lower, 0.01 mmHg or higher but 0.5 mmHg or lower, or 0.01 mmHg or higher but 0.2 mmHg or lower. In still other embodiments, the pressure during heating may be, for example, 0.01 mmHg or more and 200 mmHg or less, 5 mmHg or more and 100 mmHg or less, 5 mmHg or more and 50 mmHg or less, 5 mmHg or more and 30 mmHg or less, or 5 mmHg or more and 10 mmHg or less. In still other embodiments, the pressure during heating may be 1 atmosphere. The heating time is not particularly limited, but is, for example, 5 minutes to 48 hours, preferably 1 hour to 24 hours, and more preferably 3 hours to 18 hours.

[0063] The method for producing a lithium secondary battery using each power generating element is not particularly limited, and any conventionally known method can be appropriately adopted. For example, the battery can be produced by the method described in the Examples section.

[0064] The above describes one embodiment of the present invention, but the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0065] The following items are also included within the scope of the present invention: Item 1: 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 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 a surface of the solid electrolyte layer facing the negative electrode current collector, wherein the average pore diameter of the negative electrode intermediate layer is 15 nm or more, and the average volume of the pores in the negative electrode intermediate layer is 0.5 cm ３ Item 2: The lithium secondary battery according to Item 1, in which lithium metal precipitates inside the negative electrode intermediate layer during charging; Item 3: The lithium secondary battery according to Item 1 or 2, in which the negative electrode intermediate layer has an average pore size of 30 nm or more (preferably, 30 nm or more and 60 nm or less); Item 4: The lithium secondary battery according to Item 1, in which the average volume of the pores in the negative electrode intermediate layer is 0.35 cm ３ / g or less (preferably 0.25 cm ３ / g or more 0.35cm ３ Item 5: The lithium secondary battery according to any one of Items 1 to 3, wherein the porosity of the negative electrode intermediate layer is 20% or more and 60% or less (preferably 30% or more and 58% or less); Item 6: The lithium secondary battery according to any one of Items 1 to 4, wherein the specific surface area of ​​the negative electrode intermediate layer measured by the BET method is 130 m or less. ２ / g or less (preferably 20m ２ / g or more 130m ２ / g or less, more preferably 30m ２ / g or more 50m ２Item 7: The lithium secondary battery according to any one of Items 1 to 5, wherein the thickness of the negative electrode intermediate layer is 1 μm or more and 20 μm or less (preferably 1 μm or more and 15 μm or less, more preferably 1 μm or more and 10 μm or less); Item 8: The lithium secondary battery according to any one of Items 1 to 7, wherein the negative electrode intermediate layer contains a carbon material and a binder material (preferably does not contain a metal material, more preferably consists of only a carbon material and a binder material); Item 9: The lithium secondary battery according to Item 8, wherein the carbon material contains amorphous carbon (preferably carbon black, more preferably acetylene black); Item 10: The lithium secondary battery according to Item 8 or 9, wherein the binder material contains a polymer (preferably a fluorine-based resin, more preferably polyvinylidene fluoride); Item 11: A lithium secondary battery comprising: a positive electrode having a positive electrode active material layer containing a positive electrode active material; and a negative electrode having a negative electrode current collector, from which lithium metal precipitates during charging, a negative electrode intermediate layer adjacent to a surface of the solid electrolyte layer facing the negative electrode current collector, the negative electrode intermediate layer being formed on the surface of the solid electrolyte layer facing the negative electrode current collector; the negative electrode intermediate layer being formed on the surface of the solid electrolyte layer facing the negative electrode current collector; the negative electrode intermediate layer being formed on the surface of the solid electrolyte layer facing the negative electrode current collector; Item 12: The manufacturing method according to Item 11, wherein the carbon material contains amorphous carbon (preferably carbon black, more preferably acetylene black); Item 13: The manufacturing method according to Item 11 or 12, wherein the binder material contains a fluorine-based resin (preferably polyvinylidene fluoride).

[0066] 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. In the following, unless otherwise specified, operations were performed in a glove box with a dew point of -68°C or less. In addition, the instruments and devices used in the glove box and dry room were thoroughly dried beforehand. In addition, in this specification, "room temperature" means a temperature of 20°C or higher and 25°C or lower.

[0067] <Example of Preparation of Evaluation Cell> [Example 1] (Preparation of Positive Electrode) LiNi was used as the positive electrode active material. ０．８ Co ０．１ Mn ０．１ O ２ (NCM811), sulfide solid electrolyte (manufactured by Ampcera, Li ６ P.S. ５ Cl), a binder material (Daikin, PTFE), and a conductive additive (carbon nanotubes) were mixed in a mass ratio of 70:20:5:5. The resulting mixture was stretched into a sheet to prepare a positive electrode active material sheet (positive electrode active material layer). This positive electrode active material sheet was then molded and pressure-bonded to a positive electrode current collector made of 20 μm thick aluminum foil, thereby preparing a positive electrode.

[0068] (Preparation of Solid Electrolyte Layer) A sulfide solid electrolyte (manufactured by Ampcera, Li ６ P.S. ５ 1.1 g of PVdF (Aldrich, 182702) was added to a 10 μm thick SUS304 foil support with stirring to prepare a slurry. The prepared slurry was applied to a 10 μm thick SUS304 foil support using a blade coater and dried at 80° C. for 2 hours in a glove box. The laminate was punched out, and the support was peeled off to obtain a solid electrolyte layer.

[0069] (Preparation of Negative Electrode) The preparation of the negative electrode was carried out in a dry room with a dew point temperature of -50 ° C or less. 1.4 g of cyclohexanone (c-HEXO in Table 1) and 7.6 g of N-methylpyrrolidone (NMP) were added to 0.86 g of amorphous carbon (Denka Black, Li100) and 0.14 g of binder material (#9307 manufactured by Kureha Corporation) and stirred at 100 ° C to prepare a slurry. The prepared slurry was applied to a negative electrode current collector made of 10 μm thick SUS304 foil using a blade coater. Then, it was dried in air at 60 ° C for 30 minutes and then vacuum dried at 120 ° C for 12 hours. This laminate was punched out to prepare a negative electrode.

[0070] (Preparation of Evaluation Cell) The positive electrode prepared above was placed at the center of the solid electrolyte layer prepared above and pressed at 25°C for 1 minute at 700 MPa to prepare a laminate of the positive electrode and solid electrolyte layer. The negative electrode prepared above was placed at the center of the exposed surface of the solid electrolyte layer and pressed at 60°C at 500 MPa, and sealed in a laminate film to prepare an evaluation cell. The thicknesses of the positive electrode active material layer, solid electrolyte layer, and negative electrode intermediate layer were 85 μm, 40 μm, and 7 μm, respectively. A pressure of 3 MPa was applied in the stacking direction to the prepared evaluation cell, and the following battery evaluation was performed.

[0071] [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 negative electrode), the stirring temperature when preparing the slurry was changed from 100°C to 120°C, and in the above (production of evaluation cell), the temperature when pressing after disposing the negative electrode was changed from 60°C to 80°C.

[0072] Comparative Example 1 The above (fabrication of the positive electrode) and (fabrication of the solid electrolyte layer) were carried out in the same manner as in Example 1.

[0073] (Preparation of Negative Electrode) 0.86 g of amorphous carbon (Denka Black, Li100) and 0.14 g of binder material (#9307 manufactured by Kureha Corporation) were mixed with 6.74 g of N-methylpyrrolidone (NMP) and stirred to prepare a slurry. The prepared slurry was applied to a negative electrode current collector made of 10 μm thick SUS304 foil using a blade coater. The resulting mixture was then dried in air at 80° C. for 30 minutes and then vacuum dried at 120° C. for 12 hours. This laminate was punched out to prepare a negative electrode.

[0074] An evaluation cell for this comparative example was produced in the same manner as in Example 1, except that in the above (production of evaluation cell), the pressure used in pressing after disposing the negative electrode was changed from 500 MPa to 700 MPa.

[0075] Comparative Example 2 The above (fabrication of the positive electrode) and (fabrication of the solid electrolyte layer) were carried out in the same manner as in Comparative Example 1.

[0076] (Preparation of Negative Electrode) 1.4 g of cyclohexanone and 7.6 g of N-methylpyrrolidone (NMP) were added to 0.86 g of amorphous carbon (Denka Black, Li100) and 0.14 g of binder material (Kureha #9307) and stirred at room temperature to prepare a slurry. The prepared slurry was applied to a negative electrode current collector made of 10 μm thick SUS304 foil using a blade coater. The resulting mixture was then dried in air at 80°C for 30 minutes and then vacuum dried at 120°C for 12 hours. This laminate was punched out to form a negative electrode.

[0077] An evaluation cell for this comparative example was prepared in the same manner as in Comparative Example 1, except that in the above (preparation of evaluation cell), the temperature during pressing after disposing the negative electrode was changed from 60°C to 25°C.

[0078] Comparative Example 3 An evaluation cell for this comparative example was prepared in the same manner as in Comparative Example 2, except that in the above (preparation of negative electrode), the stirring temperature during preparation of the slurry was changed from room temperature to 60°C, and in the above (preparation of evaluation cell), the temperature during pressing after disposing the negative electrode was changed from 25°C to 80°C.

[0079] Comparative Example 4 An evaluation cell for this comparative example was prepared in the same manner as in Comparative Example 3, except that in the above (preparation of negative electrode), the stirring temperature during preparation of the slurry was changed from 60°C to 100°C.

[0080] <Measurement of DBP absorption> The DPB absorption value of the negative electrode intermediate layer was calculated after press-molding only the negative electrode intermediate layer under the same conditions as in each of the above examples and comparative examples, according to JIS K 6217-4:2017 "Carbon black for rubber - Fundamental properties - Part 4: Determination of oil absorption (including compressed samples)". The results are shown in Table 1 below.

[0081] <Measurement of Average Pore Diameter and Average Pore Volume> The average pore diameter and average pore volume of the negative electrode intermediate layer were evaluated by the BET method after press-molding only the negative electrode intermediate layer under the same conditions as in each of the above examples and comparative examples, and the values ​​obtained by measurement according to the BJH method described in E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 73, 373 (1951) were used. The results are shown in Table 1 below.

[0082] <Measurement of BET specific surface area> The BET specific surface area of ​​the negative electrode intermediate layer was measured according to the BJH method described in JIS Z 8830:2013 (ISO 9277:2010) after press-molding only the negative electrode intermediate layer under the same conditions as in each of the above examples and comparative examples. The results are shown in Table 1 below.

[0083] <Evaluation of Initial Charge-Discharge Efficiency> The evaluation cells prepared in the above Examples and Comparative Examples were evaluated for initial charge-discharge efficiency using a charge-discharge tester (HJ-SD8, manufactured by Hokuto Denko Corporation) in a thermostatic chamber set at 60°C. The evaluation cells were placed in the thermostatic chamber, and after the cell temperature became constant, a current of 0.6 mA / cm was applied. ２ The battery was charged at a constant current density of 0.6 mA / cm until the battery voltage reached 4.25 V, and then at a constant voltage of 4.25 V until the current reached 0.2 mA. Thereafter, the battery was charged at a constant current density of 0.6 mA / cm until the battery voltage reached 2.5 V. ２ The initial charge-discharge efficiency (%) was calculated using the following formula 1. The results are shown in Table 1 below.

[0084]

[0085]

[0086] As shown in Table 1, according to the present invention, in a lithium deposition type lithium secondary battery provided with a negative electrode intermediate layer, it is possible to further improve the initial charge / discharge efficiency by appropriately controlling the average pore diameter and the average pore volume of the negative electrode intermediate layer.

[0087] 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 power generating element 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 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, wherein the average pore diameter of the negative electrode intermediate layer is 15 nm or more, and the average volume of the pores in the negative electrode intermediate layer is 0.5 cm ３ / g or less.

2. The lithium secondary battery according to claim 1, wherein lithium metal is deposited inside the negative electrode intermediate layer during charging.

3. The lithium secondary battery according to claim 1 or 2, wherein the average pore size of the negative electrode intermediate layer is 30 nm or more.

4. The average volume of the pores in the negative electrode intermediate layer is 0.35 cm ３ 3. The lithium secondary battery according to claim 1, wherein the SiO2 content is 0.1 / g or less.

5. The lithium secondary battery according to claim 1 or 2, wherein the porosity of the negative electrode intermediate layer is 30% or more and 58% or less.

6. The specific surface area of ​​the negative electrode intermediate layer measured by the BET method is 130 m ２ 3. The lithium secondary battery according to claim 1, wherein the SiO2 content is 0.1 / g or less.

7. The lithium secondary battery according to claim 1 or 2, wherein the negative electrode intermediate layer contains a carbon material and a binder material.

8. The lithium secondary battery according to claim 7, wherein the carbon material comprises amorphous carbon.

9. The lithium secondary battery according to claim 7, wherein the binder material includes a polymer.

10. A method for producing a lithium secondary battery equipped with 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 method comprising: mixing a carbon material, a binder material, a first solvent selected from the group consisting of linear or branched dialkyl ketones having 5 to 7 carbon atoms and saturated alicyclic ketones having 5 to 6 carbon atoms, and a second solvent selected from the group consisting of N-methyl-2-pyrrolidone (NMP), N,N-dimethylpropionamide, and dimethyl sulfoxide (DMSO) at a temperature of 80°C or higher to prepare a negative electrode intermediate layer slurry; and applying the negative electrode intermediate layer slurry to the surface of the negative electrode current collector to form a coating film. heating the coating film to 70°C or less to remove the first solvent from the coating film, and then heating the coating film to 100°C or more to remove the second solvent from the coating film.

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