Lithium secondary battery

Abstract

The present disclosure addresses the problem of providing a means that enables the discharge rate characteristics to be improved in a lithium precipitation-type lithium secondary battery that includes two negative electrode intermediate layers. In the present invention, the foregoing problem can be solved by: causing a predetermined carbon material to be contained in each of two negative electrode intermediate layers; and controlling the value of the peel strength at the interface between the respective layers of a solid electrolyte layer, the two negative electrode intermediate layers, and a negative electrode current collector to be a value within a predetermined range.

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 known is a so-called lithium deposition type, in which lithium metal is deposited on a negative electrode current collector during charging (for example, JP-A 2020-534663 (corresponding to the specification of U.S. Patent Application Publication No. 2020 / 235386)). This document discloses a technology that prevents the growth of lithium dendrites while suppressing side reactions between the electrolyte and lithium polysulfide and the lithium metal negative electrode by providing two protective layers with different ionic conductivities or electrolyte impregnation rates between the solid electrolyte layer and the negative electrode current collector.

[0004] The present inventors further investigated the technology of providing a protective layer (negative electrode intermediate layer) having a different composition between the solid electrolyte layer and the negative electrode current collector, as described in the above-mentioned literature, and found that even if the configuration described in the above-mentioned literature is adopted, sufficient discharge rate characteristics may not be obtained.

[0005] Therefore, an object of the present invention is to provide a means for improving the discharge rate characteristics of a lithium deposition type lithium secondary battery having two negative electrode intermediate layers.

[0006] The present inventors conducted extensive research to solve the above-mentioned problems, and as a result, they found that the above-mentioned problems can be solved by including a predetermined carbon material in each of the two negative electrode intermediate layers and controlling the peel strength at the interface between each of the solid electrolyte layer, the two negative electrode intermediate layers, and the negative electrode current collector to a value within a predetermined range, thereby completing the present invention.

[0007] That is, one embodiment 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 from which lithium metal is deposited during charging, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte. The negative electrode further includes a first negative electrode intermediate layer adjacent to the negative electrode current collector side of the solid electrolyte layer and containing a first carbon material, and a second negative electrode intermediate layer adjacent to both the first negative electrode intermediate layer and the negative electrode current collector and containing a second carbon material having a smaller capacity as a negative electrode active material than the first carbon material, wherein the peel strength (at full discharge) between the solid electrolyte layer and the first negative electrode intermediate layer is 0.015 [N / mm] or more, the peel strength (at full discharge) between the first negative electrode intermediate layer and the second negative electrode intermediate layer is less than 0.015 [N / mm], and the peel strength (at full discharge) between the second negative electrode intermediate layer and the negative electrode current collector is 0.8 [N / mm] or more.

[0008] 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.

[0009] 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, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, wherein the negative electrode is adjacent to a surface of the solid electrolyte layer facing the negative electrode current collector and includes a first negative electrode intermediate layer containing a first carbon material, and a second negative electrode intermediate layer present between the first negative electrode intermediate layer and the negative electrode current collector so as to be adjacent to both the first negative electrode intermediate layer and the negative electrode current collector, and the first carbon material and and a second anode intermediate layer containing a second carbon material having a smaller capacity as a negative electrode active material than the first anode intermediate layer, wherein the peel strength (when fully discharged) between the solid electrolyte layer and the first anode intermediate layer is 0.015 [N / mm] or more, the peel strength (when fully discharged) between the first anode intermediate layer and the second anode intermediate layer is less than 0.015 [N / mm], and the peel strength (when fully discharged) between the second anode intermediate layer and the anode current collector is 0.8 [N / mm] or more. The lithium secondary battery according to this embodiment can improve the discharge rate characteristics of a lithium deposition type lithium secondary battery having two anode intermediate layers.

[0010] 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.

[0011] 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 negative electrode current collector 11'. The negative electrode has a first negative electrode intermediate layer 14 adjacent to the surface of the solid electrolyte layer 17 facing the negative electrode current collector 11'. The first negative electrode intermediate layer 14 contains a first carbon material (e.g., acetylene black). The negative electrode has a second negative electrode intermediate layer 14' adjacent to the surface of the negative electrode current collector 11' on the side of the first negative electrode intermediate layer 14. The second negative electrode intermediate layer 14' contains a second carbon material (e.g., nanoparticles of acetylene black). The second carbon material has a smaller capacity as a negative electrode active material than the first carbon material.

[0012] The stacked secondary battery 10a according to the embodiment shown in FIG. 1 is in a charging state. The negative electrode further includes a negative electrode active material layer 13 made of lithium metal deposited between a first negative electrode intermediate layer 14 and a second negative electrode intermediate layer 14'. During discharge, this negative electrode active material layer 13 (lithium metal) gradually disappears, and upon full discharge, the negative electrode active material layer 13 may completely disappear. That is, upon full discharge, the first negative electrode intermediate layer 14 and the second negative electrode intermediate layer 14' are adjacent to each other.

[0013] The positive electrode constituting the stacked secondary battery 10a 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 stacked in this order, with the first negative electrode intermediate layer 14 and the positive electrode active material layer 15 constituting the negative electrode facing each other with the solid electrolyte layer 17 interposed therebetween. Adjacent negative electrodes, solid electrolyte layers, and positive electrodes thereby constitute one unit cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can also be said to have a structure in which a plurality of unit cell layers 19 are stacked 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 to be sandwiched between the ends of the laminate film 29 and led out of the laminate film 29. The stacked secondary battery 10a is subjected to a restraining pressure in the stacking direction of the power generating element 21 by a pressure member (not shown), so that the volume of the power generating element 21 is kept constant.

[0014] The main components of the lithium secondary battery according to this embodiment will be described below.

[0015] [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.

[0016] [Negative Electrode] The negative electrode of the lithium secondary battery according to this embodiment includes a negative electrode current collector, a first negative electrode intermediate layer, and a second negative electrode intermediate layer. The lithium secondary battery according to this embodiment is a so-called lithium deposition type in which lithium metal is deposited on the negative electrode during charging. Therefore, at times other than full discharge, the negative electrode also includes a negative electrode active material layer made of lithium metal, as shown in FIG.

[0017] (Negative Electrode Active Material Layer) The negative electrode active material layer of the lithium secondary battery according to this embodiment is a layer made of lithium metal that deposits on the negative electrode during the charging process of the lithium secondary battery. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharging process progresses. The thickness of the negative electrode active material layer (lithium metal layer) at full charge is not particularly limited, but is typically 0.1 to 1000 μm. It is preferable that the negative electrode active material layer (layer made of lithium metal) that deposits during the charging process as described above be located between the first negative electrode intermediate layer and the second negative electrode intermediate layer (lithium metal deposits between the first negative electrode intermediate layer and the second negative electrode intermediate layer). This configuration can prevent the deposition and growth of dendrites in the solid electrolyte layer, which is more effective in preventing short circuits and improving performance by reducing inactive lithium.

[0018] [Negative Electrode Intermediate Layers (First Negative Electrode Intermediate Layer and Second Negative Electrode Intermediate Layer)] The negative electrode intermediate layer is a layer that is present between the solid electrolyte layer and the negative electrode current collector and contains a carbon material. The negative electrode of the lithium secondary battery according to this embodiment has two negative electrode intermediate layers: a first negative electrode intermediate layer and a second negative electrode intermediate layer. Of these, as shown in FIG. 1 , the first negative electrode intermediate layer 14 contains a carbon material and is a layer that is present adjacent to the surface of the solid electrolyte layer 17 facing the negative electrode current collector 11′. In this specification, the carbon material contained in the first negative electrode intermediate layer 14 is also referred to as the “first carbon material.” On the other hand, the second negative electrode intermediate layer 14′ is a layer that contains a carbon material and is present between the first negative electrode intermediate layer 14 and the negative electrode current collector 11′ so as to be adjacent to both of them during full discharge. In this specification, the carbon material contained in the second negative electrode intermediate layer 14′ is also referred to as the “second carbon material.” Here, the second carbon material has a smaller capacity as a negative electrode active material than the first carbon material.

[0019] (Carbon Material) The carbon material contained in the first negative electrode intermediate layer and the second negative electrode intermediate layer is not particularly limited, but is preferably at least one selected from the group consisting of carbon materials capable of absorbing and desorbing lithium ions. When the negative electrode intermediate layer (particularly the first negative electrode intermediate layer) contains a carbon material capable of absorbing lithium ions, the precipitation and growth of lithium dendrites can be suppressed. Specific examples of carbon materials capable of absorbing and desorbing lithium ions 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, the carbon material preferably contains at least one selected from the group consisting of carbon black, and more preferably contains at least one selected from the group consisting of acetylene black, Ketjen Black (registered trademark), furnace black, channel black, and thermal lamp black.

[0020] The carbon material may be contained in the negative electrode intermediate layer 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 made of the first carbon material is, for example, 200 nm or less, preferably 100 nm or less, and more preferably 60 nm or less. The lower limit of the average primary particle diameter of the carbon particles made of the first carbon material 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. The average primary particle diameter of the carbon particles made of the second carbon material 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 made of the second carbon material 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 this specification, the average particle diameter of the particles is defined as the 50% cumulative diameter (D50) of the particle diameters of the particles observed in several to several tens of fields of view when the 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).

[0021] There are no particular limitations on the absolute values ​​of the capacities (per unit mass) of the first and second carbon materials as negative electrode active materials, and the materials and average particle sizes may be selected and adjusted so as to satisfy the above-mentioned predetermined relationship. The capacities of these carbon materials as negative electrode active materials can be compared using the method described in the Examples section below.

[0022] The content of the carbon material in each negative electrode intermediate layer is not particularly limited, but is, for example, 60 to 98 mass%, preferably 70 to 95 mass%, and more preferably 80 to 90 mass%, relative to 100 mass% of the total amount of the negative electrode intermediate layers. In particular, when the content of the carbon material in the second negative electrode intermediate layer is a value within these ranges, the cycle durability of the battery is improved.

[0023] (Metallic Material Alloyable with Lithium) The negative electrode intermediate layer (particularly the first negative electrode intermediate layer) preferably contains a metallic material alloyable with lithium. By containing a metallic material alloyable with lithium in the negative electrode intermediate layer, lithium metal can be more uniformly deposited on the current collector surface. Specific examples of metallic materials alloyable with lithium include indium (In), aluminum (Al), silicon (Si), tin (Sn), magnesium (Mg), gold (Au), silver (Ag), zinc (Zn), and alloys containing at least one of these. Among these, the metallic material alloyable with lithium preferably contains at least one selected from the group consisting of In, Al, Si, Sn, Mg, Au, Ag, and Zn, more preferably contains at least one selected from the group consisting of Ag, Mg, Zn, and Al, even more preferably contains at least one selected from the group consisting of Ag, Mg, and Zn, and particularly preferably contains Ag.

[0024] The content of the metal material capable of being alloyed with lithium in each negative electrode intermediate layer is not particularly limited, but is, for example, 0 to 30 mass %, preferably 5 to 25 mass %, and more preferably 10 to 20 mass %, relative to 100 mass % of the total amount of each negative electrode intermediate layer.

[0025] (Metallic Material Not Alloyable with Lithium) In addition to the above-described carbon material and, if necessary, a metallic material capable of being alloyed with lithium, the negative electrode intermediate layer may further contain a metallic material other than the metallic material capable of being alloyed with lithium (a metallic material not alloyable with lithium). Examples of metallic materials that are not alloyable with lithium include nickel (Ni) and copper (Cu). The content of the metallic material that is not alloyable with lithium in each negative electrode intermediate layer is not particularly limited, but is, for example, 0 to 20% by mass, preferably 2 to 15% by mass, and more preferably 5 to 10% by mass, relative to 100% by mass of the total amount of each negative electrode intermediate layer.

[0026] (Binder) Furthermore, the first negative electrode intermediate layer and the second negative electrode intermediate layer each preferably contain a binder. The type of binder is not particularly limited, and binders known in the art can be appropriately used. Examples of binders include polyvinylidene fluoride (PVDF), compounds in which hydrogen atoms of PVDF are substituted with other halogen elements, polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC). Among these, the binder preferably contains polyvinylidene fluoride (PVDF), and more preferably is polyvinylidene fluoride (PVDF).

[0027] The binder content in each anode intermediate layer is not particularly limited, but is preferably within the range of 1 to 15 mass%, and more preferably within the range of 5 to 10 mass%, relative to 100 mass% of the total amount of each anode intermediate layer. Here, when both the first anode intermediate layer and the second anode intermediate layer contain a binder, the binder content in the first anode intermediate layer (content concentration in the first anode intermediate layer) is preferably greater than the binder content in the second anode intermediate layer (content concentration in the second anode intermediate layer). This configuration not only ensures sufficient discharge rate characteristics of the battery, but also improves the cycle durability of the battery. This is thought to be because lithium metal deposited between the first anode intermediate layer and the second anode intermediate layer during the charging process is less likely to penetrate the first anode intermediate layer and more likely to penetrate into the second anode intermediate layer, thereby sufficiently maintaining the conduction path of lithium ions during the charging and discharging process.

[0028] 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).

[0029] The thickness of the first negative electrode intermediate layer is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 40 μm, and even more preferably 10 to 30 μm. When the thickness of the first negative electrode intermediate layer is 1 μm or more, the functions of the negative electrode intermediate layer can be fully exhibited. When the thickness of the first negative electrode intermediate layer is 50 μm or less, a decrease in energy density can be suppressed. Furthermore, from the viewpoint of facilitating the deposition of lithium metal between the first negative electrode intermediate layer and the second negative electrode intermediate layer during the charging process, the thickness of the second negative electrode intermediate layer is preferably smaller than the thickness of the first negative electrode intermediate layer. The thickness of the second negative electrode intermediate layer is also not particularly limited, but is preferably 50 nm to 50 μm, more preferably 200 nm to 20 μm, and even more preferably 400 nm to 4 μm.

[0030] Although not shown in FIG. 1 , it is preferable that these negative electrode intermediate layers are arranged so that, when the power generating element 21 is viewed from above, at least a portion (preferably the entire circumference) of the outer periphery of the second negative electrode intermediate layer 14′ is located outside the outer periphery of the first negative electrode intermediate layer 14 (i.e., so that the second negative electrode intermediate layer 14′ is slightly larger than the first negative electrode intermediate layer 14). This configuration ensures that the second negative electrode intermediate layer 14′ is always present between the lithium metal and the negative electrode current collector 11′ during the charging process. As a result, the conduction path of lithium ions is sufficiently maintained during the charging and discharging process, which not only ensures the discharge rate characteristics of the battery but also improves the cycle durability of the battery.

[0031] [Peel Strength Between Layers] One of the features of the lithium secondary battery according to this embodiment is that, during full discharge, the peel strength between the solid electrolyte layer and each layer constituting the negative electrode (first negative electrode intermediate layer, second negative electrode intermediate layer, and negative electrode current collector) is controlled to a predetermined value. Specifically, the lithium secondary battery according to this embodiment is configured to satisfy the following requirements (a) to (c) during full discharge (a state in which a negative electrode active material layer made of lithium metal is not present): (a) The peel strength between the solid electrolyte layer and the first negative electrode intermediate layer is 0.015 [N / mm] or more, preferably 0.016 [N / mm] or more, more preferably 0.017 [N / mm] or more, and even more preferably 0.018 [N / mm] or more; (b) The peel strength between the first negative electrode intermediate layer and the second negative electrode intermediate layer is less than 0.015 [N / mm], preferably 0.014 [N / mm] or less, more preferably 0.013 [N / mm] or less, and even more preferably 0.012 [N / mm] or less; (c) The peel strength between the second negative electrode intermediate layer and the negative electrode current collector is 0.8 [N / mm] or more, preferably 1.5 [N / mm] or more, more preferably 2.0 [N / mm] or more, and even more preferably 2.3 [N / mm] or more.

[0032] The lithium secondary battery according to the present embodiment that satisfies the above requirements (a) to (c) has the advantage of being excellent in discharge rate characteristics. Furthermore, the lithium secondary battery that satisfies the requirements according to the preferred embodiment described above is preferable because it not only has excellent discharge rate characteristics but also excellent cycle durability.

[0033] In this specification, the peel strength values ​​relating to the above provisions (a) to (c) are values ​​measured using the method described in the Examples section below.

[0034] Furthermore, there are no particular limitations on the method for controlling the peel strength between each layer so as to satisfy the above requirements (a) to (c), and a person skilled in the art can appropriately implement this method in consideration of the common general technical knowledge at the time of filing of this application. As an example, a lithium secondary battery satisfying the above requirements can be fabricated by adjusting the composition of the components of the solid electrolyte layer and the first and second negative electrode intermediate layers when forming these layers, or by adjusting the conditions of the pressing process when performing a pressing process when stacking these layers to fabricate a power generating element. To give a more specific example, the peel strength of the above requirement (a) tends to increase when the pressing pressure of the cold isostatic pressing (CIP) used when stacking and bonding the solid electrolyte layer and the first negative electrode intermediate layer is increased. Furthermore, the peel strength of the above requirement (b) tends to increase when the restraining pressure applied in the stacking direction of the power generating element when fabricating a lithium secondary battery is increased. Furthermore, the peel strength of the above requirement (c) tends to increase when the binder concentration in the second negative electrode intermediate layer is increased. However, these control means are merely examples, and other means can be used to control the value to the above values, and lithium secondary batteries obtained in this way are also within the scope of the present invention.

[0035] [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. Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes. 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 a 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, a Li element, and a P element. One example is LPS (Li ２ S-P ２ S ５ ), Li ６ P.S. ５X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S and Li ３ P.S. ４ These sulfide solid electrolytes are preferably used because they have excellent lithium ion conductivity.

[0036] 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.

[0037] 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.

[0038] The content of the solid electrolyte in the solid electrolyte layer is preferably 50 to 100 mass %, more preferably 90 to 100 mass %.

[0039] 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 %.

[0040] The thickness of the solid electrolyte layer varies depending on the intended configuration of the all-solid-state battery, but is usually 0.1 to 1000 μm, and preferably 10 to 100 μm.

[0041] [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.

[0042] 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.

[0043] 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.

[0044] 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 %.

[0045] 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.

[0046] 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 %.

[0047] 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%.

[0048] 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.

[0049] Although one embodiment of a lithium secondary battery according to one aspect 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.

[0050] 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 claim 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; a lithium secondary battery according to any one of claims 1 to 5 having the features of claim 6; and a lithium secondary battery according to any one of claims 1 to 6 having the features of claim 7.

[0051] 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 an argon atmosphere at a dew point of -68°C or less. Furthermore, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0052] Example 1 (Fabrication of Positive Electrode) First, LiNi was used as a positive electrode active material. ０．８ Mn ０．１ Co ０．１ O ２ , acetylene black as a conductive additive, and a sulfide solid electrolyte (LPS(Li ２ S-P ２ S ５ )) were weighed out to a mass ratio of 50:30:20, mixed in an agate mortar in a glove box, and then further mixed and stirred in a planetary ball mill. 2 parts by mass of styrene-butadiene rubber (SBR) was added to 100 parts by mass of the resulting mixed powder, and mesitylene was added as a solvent to prepare a positive electrode active material slurry. Next, the positive electrode active material slurry prepared above was applied to the surface of aluminum foil as a positive electrode current collector, dried, and pressed to form a positive electrode active material layer (thickness 50 μm), thereby producing a positive electrode.

[0053] (Preparation of solid electrolyte layer) Sulfide solid electrolyte (LPS(Li ２ S-P ２ S ５To 100 parts by mass of the cellulose acetate solution, 2 parts by mass of styrene-butadiene rubber (SBR) was added, and mesitylene was added as a solvent to prepare a solid electrolyte slurry. The solid electrolyte slurry prepared above was then applied to the surface of a stainless steel foil as a support, and dried to obtain a solid electrolyte layer (thickness: 30 μm) as a free-standing membrane.

[0054] The solid electrolyte layer prepared above was transferred to the positive electrode active material layer side of the positive electrode prepared above by cold isostatic pressing (CIP) so that the exposed surface of the solid electrolyte layer faced the positive electrode active material layer, thereby producing a solid electrolyte layer / positive electrode active material layer / positive electrode current collector laminate (I). At this time, the pressing pressure during the CIP treatment was controlled so that the outer peripheral edge of the solid electrolyte layer completely covered the side surface of the positive electrode active material layer.

[0055] (Preparation of Negative Electrode Intermediate Layer) Acetylene black (average primary particle diameter: 200 nm) was prepared as the first carbon material, and acetylene black nanoparticles (average primary particle diameter: 35 nm) were prepared as the second carbon material. Coin cells (half cells) were prepared using these carbon materials as the working electrode and lithium metal as the counter electrode, and the capacity [mAh / g] of each carbon material as a negative electrode active material was measured from the amount of current until the cell voltage reached 0 V. As a result, the capacity of the second carbon material as a negative electrode active material was smaller than the capacity of the first carbon material as a negative electrode active material.

[0056] Next, silver nanoparticles (average primary particle diameter: 60 nm), which are metal materials that can be alloyed with lithium during charging, and acetylene black (average primary particle diameter: 200 nm) (first carbon material) were weighed (Ag:C = 1:3 (mass ratio)) and mixed. 15 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added to 85 parts by mass of the resulting mixture, and N-methyl-2-pyrrolidone (NMP) was added as a solvent and mixed to prepare a negative electrode intermediate layer slurry (1). Furthermore, 10 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added to 90 parts by mass of acetylene black nanoparticles (average primary particle diameter: 35 nm) (second carbon material), and N-methyl-2-pyrrolidone (NMP) was added as a solvent and mixed to prepare a negative electrode intermediate layer slurry (2). The negative electrode intermediate layer slurry (2) obtained above was then coated on the surface of a stainless steel foil serving as a negative electrode current collector and dried to obtain a second negative electrode intermediate layer (thickness 500 nm). Furthermore, the negative electrode intermediate layer slurry (1) obtained above was coated on the exposed surface of the second negative electrode intermediate layer and dried to produce a first negative electrode intermediate layer (thickness 10 μm) / second negative electrode intermediate layer / negative electrode current collector laminate (II). The negative electrode intermediate layer slurry (1) was applied so that, when the power generating element was viewed from above, the entire periphery of the outer periphery of the second negative electrode intermediate layer was positioned outside the outer periphery of the first negative electrode intermediate layer (i.e., so that the second negative electrode intermediate layer was slightly larger than the first negative electrode intermediate layer 14).

[0057] (Preparation of Evaluation Cell) The laminate (I) and laminate (II) prepared above were stacked so that the exposed surface of the solid electrolyte layer and the exposed surface of the first negative electrode intermediate layer faced each other, and pressed using a hot isostatic press (WIP) at 700 MPa and 80 ° C. for 1 minute. This transferred the first negative electrode intermediate layer to the exposed surface of the solid electrolyte layer, and adjusted the porosity of the first and second negative electrode intermediate layers. Finally, an aluminum positive electrode current collector and a nickel negative electrode current collector were bonded to an aluminum foil (positive electrode current collector) and a stainless steel foil (negative electrode current collector), respectively, using an ultrasonic welder. The resulting laminate was placed inside an aluminum laminate film, vacuum sealed, and restrained at a restraining pressure of 3 MPa using a pressure member, thereby preparing an evaluation cell (lithium deposition type lithium secondary battery) for this example. Note that this evaluation cell is in a fully discharged state without a negative electrode active material layer.

[0058] Example 2 An evaluation cell of this example (lithium deposition-type lithium secondary battery) was produced using the same method as in Example 1 described above, except that the solid content composition of the negative electrode intermediate layer slurry (2) was changed to 95 parts by mass of acetylene black nanoparticles (average particle diameter: 35 nm) and 5 parts by mass of polyvinylidene fluoride (PVDF) as a binder.

[0059] Example 3 An evaluation cell (lithium deposition-type lithium secondary battery) of this comparative example was produced using the same method as in Example 1 described above, except that the solid content composition of the negative electrode intermediate layer slurry (2) was changed to 97.5 parts by mass of acetylene black nanoparticles (average particle diameter: 35 nm) and 2.5 parts by mass of polyvinylidene fluoride (PVDF) as a binder.

[0060] Comparative Example 1 An evaluation cell (lithium deposition-type lithium secondary battery) of this comparative example was produced using the same method as in Example 1 described above, except that the pressing pressure when bonding the laminate (I) and the laminate (II) together by cold isostatic pressing (CIP) was changed to 500 MPa and the restraining pressure when producing the evaluation cell was changed to 2 MPa.

[0061] Comparative Example 2 An evaluation cell of this comparative example (lithium deposition type lithium secondary battery) was produced using the same method as in Example 1 described above, except that the confining pressure during production of the evaluation cell was changed to 10 MPa.

[0062] Comparative Example 3 An evaluation cell (lithium deposition-type lithium secondary battery) of this comparative example was produced using the same method as in Example 1 described above, except that the pressing pressure when bonding the laminate (I) and the laminate (II) together by cold isostatic pressing (CIP) was changed to 500 MPa and the restraining pressure when producing the evaluation cell was changed to 10 MPa.

[0063] Comparative Example 4 An evaluation cell (lithium deposition-type lithium secondary battery) of this comparative example was produced using the same method as in Example 1 described above, except that in the "production of negative electrode intermediate layer", the steps of applying and drying the negative electrode intermediate layer slurry (2) were omitted and a laminate (II) consisting of a first negative electrode intermediate layer / negative electrode current collector was produced.

[0064] [Measurement of Peel Strength Between Layers in Evaluation Cell] The evaluation cells produced in the above Examples and Comparative Examples in the fully discharged state (no lithium metal deposited on the negative electrode) were disassembled, and the power generating elements inside the cells were removed. The following (a) to (c) peel strengths [N / mm] were measured for each intact power generating element (for Comparative Example 4, only (a) and "peel strength between the negative electrode intermediate layer (1) and the negative electrode current collector" were measured): (a) peel strength between the solid electrolyte layer and the first negative electrode intermediate layer (b) peel strength between the first negative electrode intermediate layer and the second negative electrode intermediate layer (c) peel strength between the second negative electrode intermediate layer and the negative electrode current collector Specifically, a test piece was cut out from the power generating element, and the test piece was measured according to "JIS Z The adhesive strength was measured by peeling using a tensile tester at a peel angle of 90° and a peel speed of 200 mm / min in accordance with the "Peel Adhesion Test Method Using the Backside of a Tape as a Test Plate" described in "Test Methods for Adhesive Tapes and Adhesive Sheets, JIS No. 0237:2009." At this time, the positive electrode current collector side of the power generating element was placed downward, and the layer located on the negative electrode current collector side was peeled off. Next, the peel strength [N / mm] was calculated by dividing this adhesive strength (N) by the width (mm) of the test piece. The results are shown in Table 1 below.

[0065] [Evaluation of Evaluation Cells] A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector plate and the negative electrode current collector plate, respectively, of the evaluation cells (restrained at a restraining pressure of 3 MPa) produced in the above Examples and Comparative Examples, and the following evaluations were performed at 25°C.

[0066] First, aging treatment was performed at charge rates of 0.01C and 0.5C (CC mode) (upper limit voltage 4.3V). Next, the voltage range was set to 3.0 to 4.3V, and the initial charge / discharge capacity was measured at a charge / discharge rate of 0.1C (CC mode). A 30-minute rest period was provided between the charge and discharge treatments (the same applies below). The initial discharge capacity measured in this manner (relative values ​​when the value of Example 1 is set to 100) is shown in Table 1 below.

[0067] Next, the charge / discharge capacity under high-rate conditions was measured under the same charge / discharge conditions as above, except that the charge / discharge rate was 2.0 C (CC mode). The discharge capacity results under high-rate conditions measured in this manner (relative values ​​when the value of Example 1 is set to 100) are shown in Table 1 below.

[0068] Finally, a 100-cycle charge-discharge cycle test was conducted under the same charge-discharge conditions as above, except that the charge-discharge rate was 0.5 C (CC mode). The discharge capacities at the 50th and 100th cycles were measured. The results (all relative values ​​when the value of Example 1 was set to 100) are shown in Table 1 below.

[0069] After the above charge-discharge cycle test, the evaluation cells were further charged and disassembled, and the vertical cross sections of the power-generating elements were observed. This confirmed that lithium metal had precipitated between the first and second negative electrode intermediate layers in the evaluation cells of Examples 1 to 3. In contrast, no lithium metal had precipitated between the first and second negative electrode intermediate layers in the evaluation cells of Comparative Examples 1 to 4.

[0070]

[0071] From the results shown in Table 1, it can be seen that the evaluation cells (lithium deposition-type lithium secondary batteries) of Examples 1 to 3 have excellent discharge rate characteristics because the first negative electrode intermediate layer and the second negative electrode intermediate layer each contain carbon materials that satisfy a predetermined relationship, and the peel strength between each layer in the negative electrode is controlled to a predetermined value. Furthermore, it can be seen that the evaluation cells of Examples 1 and 2 have particularly excellent cycle durability compared to the evaluation cell of Example 3, and the evaluation cell of Example 1 has particularly excellent cycle durability compared to the evaluation cell of Example 2. These results are thought to reflect the difference in binder content in the second negative electrode intermediate layer and the resulting difference in peel strength between the second negative electrode intermediate layer and the negative electrode current collector.

[0072] On the other hand, in the evaluation cell of Comparative Example 1, the peel strength between the solid electrolyte layer and the first anode intermediate layer was insufficient, resulting in increased interlayer resistance at the interface between them, and it is believed that this resulted in insufficient discharge rate characteristics and cycle durability. In the evaluation cell of Comparative Example 2, the peel strength between the first anode intermediate layer and the second anode intermediate layer was too high, resulting in lithium metal being less likely to deposit at the interface during charging, and it is believed that this resulted in insufficient discharge rate characteristics and cycle durability. Furthermore, in Comparative Example 3, in addition to this, the peel strength between the solid electrolyte layer and the first anode intermediate layer was also low, resulting in almost all lithium metal being deposited between the solid electrolyte layer and the first anode intermediate layer during charging, which is likely to have caused short circuits and resulted in insufficient battery performance. Finally, in Comparative Example 4, the absence of a second anode intermediate layer and the lack of the function of maintaining a lithium conduction path precluded smooth migration of lithium metal deposited on the surface of the anode current collector during charging, and it is believed that this resulted in insufficient discharge rate characteristics.

[0073] REFERENCE SIGNS LIST 10a laminated secondary battery, 11' negative electrode current collector, 11" positive electrode current collector, 13 negative electrode active material layer, 14 first negative electrode intermediate layer, 14' second 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 generation 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 in which lithium metal is deposited during charging, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, wherein the negative electrode is present adjacent to the surface of the solid electrolyte layer on the negative electrode current collector side, and includes a first negative electrode intermediate layer containing a first carbon material, and a second negative electrode intermediate layer that is present between the first negative electrode intermediate layer and the negative electrode current collector so as to be adjacent to both of them and contains a second carbon material having a smaller capacity as a negative electrode active material than the first carbon material, and the peeling strength (at full discharge) between the solid electrolyte layer and the first negative electrode intermediate layer is 0.015 [N / mm] or more, the peeling strength (at full discharge) between the first negative electrode intermediate layer and the second negative electrode intermediate layer is less than 0.015 [N / mm], and the peeling strength (at full discharge) between the second negative electrode intermediate layer and the negative electrode current collector is 0.8 [N / mm] or more, a lithium secondary battery.

2. The lithium secondary battery according to claim 1, wherein lithium metal is deposited between the first negative electrode intermediate layer and the second negative electrode intermediate layer during the charging process.

3. The lithium secondary battery according to claim 1 or 2, wherein the peeling strength (at full discharge) between the second negative electrode intermediate layer and the negative electrode current collector is 1.5 [N / mm] or more.

4. The lithium secondary battery according to claim 3, wherein the peeling strength (at full discharge) between the second negative electrode intermediate layer and the negative electrode current collector is 2.0 [N / mm] or more.

5. The lithium secondary battery according to claim 1 or 2, wherein the thickness of the second negative electrode intermediate layer is smaller than the thickness of the first negative electrode intermediate layer.

6. The lithium secondary battery according to claim 1 or 2, wherein the first negative electrode intermediate layer and the second negative electrode intermediate layer each further contain a binder, and the content of the binder in the first negative electrode intermediate layer is larger than the content of the binder in the second negative electrode intermediate layer.

7. The lithium secondary battery according to claim 1 or 2, wherein when the power generation element is viewed in plan, at least a part of the outer peripheral end of the second negative electrode intermediate layer is located outside the outer peripheral end of the first negative electrode intermediate layer.

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