Lithium secondary battery

Abstract

[Problem] To provide a lithium deposition-type lithium secondary battery, wherein there is a means for more reliably suppressing dendrite growth. [Solution] Provided is a lithium secondary battery provided with a lithium deposition-type power generation element, wherein: an ionic conductivity reaction-suppressing layer, which has lithium ion conductivity and suppresses a reaction between lithium metal and a solid electrolyte, is provided in a region which is on a negative electrode current collector side of a face where a solid electrolyte layer faces a negative electrode current collector and in which a positive electrode active material layer faces the negative electrode current collector; and an ion permeation-suppressing layer for suppressing the permeation of lithium ions is provided so as to be adjacent to at least part of the outer periphery of the solid electrolyte layer.

Description

[Technical Field]

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

[0002] In recent years, there has been active research and development into all-solid-state lithium secondary batteries that use oxide- or sulfide-based solid electrolytes. Solid electrolytes are materials primarily composed of ionic conductors that allow ionic conduction in a solid state. Therefore, all-solid-state lithium secondary batteries do not, in principle, encounter the various problems associated with flammable organic electrolytes that occur in conventional liquid-based lithium-ion secondary batteries. In addition, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally leads to significant improvements in the output density and energy density of the battery.

[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 (see, for example, U.S. Patent Application Publication No. 2019 / 0157723). During charging of such a lithium deposition type all-solid-state lithium secondary battery, lithium metal is deposited between the solid electrolyte layer and the negative electrode current collector. U.S. Patent Application Publication No. 2019 / 0157723 discloses a technology in which a fine particle layer made of fine particles of amorphous carbon, silicon, silver, tin, aluminum, bismuth, or the like is disposed between the negative electrode current collector and the solid electrolyte layer, which constitute the power generating element of the lithium secondary battery. According to U.S. Patent Application Publication No. 2019 / 0157723, this configuration serves as a protective layer for the lithium metal layer when lithium metal is deposited between the fine particle layer and the negative electrode current collector during charging, and also suppresses the growth of dendrites from the lithium metal layer, thereby preventing short-circuiting of the lithium secondary battery and resulting capacity reduction. Summary of the Invention [Problem to be solved by the invention]

[0004] However, the inventors' investigations have revealed that even when the technology described in U.S. Patent Application Publication No. 2019 / 0157723 is used, it may still not be possible to prevent dendrite growth.

[0005] Therefore, an object of the present invention is to provide a means for more reliably suppressing the growth of dendrites in a lithium deposition type lithium secondary battery. [Means for solving the problem]

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that in a lithium secondary battery including a lithium deposition-type power generating element, the above-mentioned problems can be solved by providing an ion-conductive reaction-suppressing layer that has lithium ion conductivity and suppresses a reaction between lithium metal and the solid electrolyte on the negative electrode current collector side of the surface of the solid electrolyte layer that faces the negative electrode current collector, and in a region where the positive electrode active material layer faces the negative electrode current collector, and by providing an ion-permeation-suppressing layer that suppresses the permeation of lithium ions so as to be adjacent to at least a portion of the outer periphery of the solid electrolyte layer, thereby completing the present invention.

[0007] That is, one aspect of the present invention relates to a lithium secondary battery including a power generating element including: a positive electrode including a positive electrode active material layer containing a positive electrode active material capable of absorbing and releasing lithium ions, disposed on the surface of a positive electrode current collector; a negative electrode including a negative electrode current collector and in which lithium metal is deposited on the negative electrode current collector during charging; and a solid electrolyte layer containing a solid electrolyte interposed between the positive electrode and the negative electrode. The lithium secondary battery is characterized in that an ion-conductive reaction-suppressing layer having lithium ion conductivity and suppressing reaction between the lithium metal and the solid electrolyte is provided on at least a portion of a region of the main surface of the solid electrolyte layer facing the negative electrode current collector, the region being in a region where the positive electrode active material layer faces the negative electrode current collector, and an ion-permeation-suppressing layer that suppresses permeation of lithium ions is provided adjacent to at least a portion of the outer periphery of the solid electrolyte layer. [Effects of the Invention]

[0008] According to the present invention, it is possible to more reliably suppress the growth of dendrites in a lithium deposition type lithium secondary battery. [Brief explanation of the drawings]

[0009] [Figure 1] 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. [Figure 2] 2 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to one embodiment of the present invention, and corresponds to the configuration of an evaluation cell produced in Example 1, which will be described later. [Figure 3] 1 is a perspective view of a stacked secondary battery according to one embodiment of the present invention; [Figure 4] FIG. 4 is a side view seen from a direction A shown in FIG. 3. [Figure 5] 1 is a perspective view showing the appearance of a stacked secondary battery according to one embodiment of the present invention; [Figure 6] 6 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to another embodiment of the present invention, which corresponds to the configuration of an evaluation cell fabricated in Example 2 described below. [Figure 7] 7 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to still another embodiment of the present invention, which corresponds to the configuration of an evaluation cell fabricated in Example 3 described below. [Figure 8] 8 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to still another embodiment of the present invention, which corresponds to the configuration of an evaluation cell fabricated in Example 4 described below. [Figure 9] FIG. 10 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to still another embodiment of the present invention. [Figure 10] FIG. 10 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to still another embodiment of the present invention. [Figure 11] FIG. 10 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to still another embodiment of the present invention. [Figure 12]FIG. 10 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to still another embodiment of the present invention. [Figure 13] FIG. 10 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to still another embodiment of the present invention. [Figure 14] FIG. 10 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to still another embodiment of the present invention. [Figure 15] FIG. 10 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to still another embodiment of the present invention. [Figure 16] FIG. 10 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to still another embodiment of the present invention. [Figure 17] FIG. 10 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery according to still another embodiment of the present invention. [Figure 18] 10 is an enlarged cross-sectional view of a unit cell layer of a stacked secondary battery corresponding to the configuration of the evaluation cell produced in Comparative Example 3. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present embodiment will be described below with reference to the drawings, but the technical scope of the present invention should be determined based on the description of the claims and is not limited to the following embodiment. Note that 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 simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. 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 occur, is sealed inside a laminate film 29, which is a battery exterior. Note that FIG. 1 shows a cross section of the stacked-type secondary battery during charging, and thus, a negative electrode active material layer 13 made of lithium metal is present between a negative electrode current collector 11' and a solid electrolyte layer 17. Furthermore, a pressure member (not shown) applies a restraining pressure to the stacked-type secondary battery 10a in the stacking direction of the power generating element 21. Therefore, the volume of the power generating element 21 is maintained constant.

[0012] As shown in FIG. 1, a power generating element 21 of a stacked secondary battery 10a of this embodiment has a configuration in which a negative electrode in which negative electrode active material layers 13 containing lithium metal are arranged on both sides of a negative electrode current collector 11′, a solid electrolyte layer 17, and a positive electrode in which positive electrode active material layers 15 containing a lithium transition metal composite oxide are arranged on both sides of a positive electrode current collector 11″ are stacked. Specifically, the negative electrode, solid electrolyte layer, and positive electrode are stacked in this order such that one negative electrode active material layer 13 and an adjacent positive electrode active material layer 15 face each other with the solid electrolyte layer 17 interposed therebetween. As a result, adjacent negative electrode, solid electrolyte layer, and positive electrode constitute one unit cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can also be said to have a configuration in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel.

[0013] A negative electrode current collector 25 and a positive electrode current collector 27 that are electrically connected to the respective electrodes (negative and positive electrodes) are attached to the negative electrode current collector 11′ and the positive electrode current collector 11″, respectively, and are configured to be sandwiched between the ends of the laminate film 29 and extended to the outside of the laminate film 29. The negative electrode current collector 25 and the positive electrode current collector 27 may be attached to the negative electrode current collector 11′ and the positive electrode current collector 11″ of the respective electrodes by ultrasonic welding, resistance welding, or the like, via a negative electrode terminal lead and a positive electrode terminal lead (not shown), respectively, as necessary.

[0014] FIG. 2 is an enlarged cross-sectional view of a cell layer 19 of a stacked secondary battery according to one embodiment of the present invention. As shown in FIG. 2, the cell layer 19 constituting the stacked secondary battery 10a according to this embodiment has a positive electrode constituted by a positive electrode current collector 11" and a positive electrode active material layer 15 disposed on the surface thereof. A solid electrolyte layer 17 containing a solid electrolyte is disposed on the surface of the positive electrode active material layer 15 opposite the positive electrode current collector 11". In the embodiment shown in FIG. 2, the solid electrolyte layer 17 extends to reach the positive electrode current collector 11" so as to cover the entire outer periphery of the positive electrode active material layer 15. In the embodiment shown in FIG. 2, a carbon black layer 18a containing nanoparticles of carbon black is provided on the main surface of the solid electrolyte layer 17 facing the negative electrode current collector 11' in a region including the entire region where the positive electrode active material layer 15 faces the negative electrode current collector 11' (in other words, one size larger than the positive electrode active material layer 15 when the power generating element 21 is viewed from above). This carbon black layer 18a The carbon black constituting the negative electrode current collector 11' has lithium ion conductivity, and therefore the carbon black layer 18a can conduct lithium ions. Therefore, the provision of the carbon black layer 18a does not impede the progress of the battery reaction. The carbon black layer 18a also has the function of suppressing the reaction between the lithium metal (negative electrode active material layer 13) deposited on the negative electrode current collector 11' and the solid electrolyte contained in the solid electrolyte layer 17 during charging. Therefore, the carbon black layer 18a can be said to function as an ion-conductive reaction suppression layer.

[0015] As shown in FIG. 2, in the cell layer 19 constituting the stacked secondary battery 10a according to this embodiment, an alumina layer 18b containing nanoparticles of alumina (aluminum oxide) is provided around the entire periphery of the solid electrolyte layer 17. The alumina constituting the alumina layer 18b is a material that does not have lithium ion conductivity. Therefore, the alumina layer 18b functions as an ion permeation suppression layer that suppresses the permeation of lithium ions. As shown in FIG. 2, the solid electrolyte layer 17 extends to cover the periphery of the positive electrode active material layer 15, and an alumina layer (ion permeation suppression layer) is provided adjacent to the periphery of the extended solid electrolyte layer 17. This can further improve the QC performance of the resulting battery. This is because the solid electrolyte layer 17 extends to the periphery of the positive electrode active material layer 15, thereby providing a physical barrier. In addition, the alumina layer (ion permeation suppression layer) has stronger adhesion to the solid electrolyte layer 17 than to the positive electrode active material layer 15, thereby more effectively preventing dendrite growth and resulting short circuits.

[0016] FIG. 3 is a perspective view of a stacked secondary battery according to one embodiment of the present invention. FIG. 4 is a side view seen from direction A shown in FIG. 3. As shown in FIGS. 3 and 4, the stacked secondary battery 100 according to this embodiment includes the stacked secondary battery 10a shown in FIG. 1, two metal plates 200 that sandwich the stacked secondary battery 10a, and bolts 300 and nuts 400 as fastening members. These fastening members (bolts 300 and nuts 400) function to secure the stacked secondary battery 10a in a sandwiched state between the metal plates 200. As a result, the metal plates 200 and the fastening members (bolts 300 and nuts 400) function as pressure members that pressurize (restrain) the stacked secondary battery 10a in the stacking direction. Note that the pressure members are not particularly limited as long as they are members that can pressurize the stacked secondary battery 10a in the stacking direction. A combination of a plate made of a rigid material, such as the metal plate 200, and the above-mentioned fastening members is typically used as the pressure member. Furthermore, the fastening members are not limited to the bolts 300 and nuts 400, and may include tension plates or the like that fix the ends of the metal plates 200 so as to restrain the stacked secondary batteries 10a in the stacking direction.

[0017] The lower limit of the load applied to the stacked secondary battery 10a (confining pressure in the stacking direction of the stacked secondary battery) is, for example, 0.1 MPa or more, preferably 1 MPa or more, more preferably 3 MPa or more, and even more preferably 5 MPa or more. The upper limit of the confining pressure in the stacking direction of the stacked secondary battery is, for example, 100 MPa or less, preferably 70 MPa or less, more preferably 40 MPa or less, and even more preferably 10 MPa or less.

[0018] The main components of the above-described stacked secondary battery 10a will be described below.

[0019] [Positive electrode current collector] There are no particular limitations on the material that constitutes the positive electrode current collector, and examples of materials that can be used for the positive electrode current collector include metals and conductive resins (such as resins in which a conductive filler is added to a non-conductive polymer material).

[0020] The current collector may have a single layer structure made of a single material, or may have a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector contains at least a conductive resin layer made of a resin having conductivity. Furthermore, from the viewpoint of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on a part of the current collector. There are no particular restrictions on the thickness of the positive electrode current collector, but an example is 10 to 100 μm.

[0021] [Cathode active material layer] The positive electrode constituting the lithium secondary battery according to this embodiment has a positive electrode active material layer containing a positive electrode active material capable of absorbing and releasing lithium ions. The positive electrode active material layer 15 is disposed on the surface of a positive electrode current collector 11″ as shown in FIG. 1.

[0022] The positive electrode active material is not particularly limited as long as it can release lithium ions during the charging process of the secondary battery and absorb lithium ions during the discharging process. An example of such a positive electrode active material is one containing an M1 element and an O element, where the M1 element contains at least one element selected from the group consisting of Li, Mn, Ni, Co, Cr, Fe, and P. Examples of such a positive electrode active material include layered rock salt active materials such as LiCoO2, LiMnO2, LiNiO2, and Li(Ni-Mn-Co)O2; LiMn2O4; and LiNi 0.5 Mn 1.5 Examples of oxide active materials include spinel-type active materials such as LiFePO4 and LiMnPO4, olivine-type active materials such as LiFeSiO4 and LiMnSiO4, and Si-containing active materials such as LiFeSiO4 and LiMnSiO4. 12 , LiVO2. In some cases, two or more types of positive electrode active materials may be used in combination. Of course, positive electrode active materials other than those mentioned above may also be used. In a preferred embodiment, the positive electrode active material layer 15 constituting the lithium secondary battery according to this embodiment contains, from the viewpoint of output characteristics, a layered rock salt type active material containing lithium and cobalt (for example, Li(Ni-Mn-Co)O2) as the positive electrode active material.

[0023] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 30 to 99 mass %, more preferably in the range of 40 to 90 mass %, and even more preferably in the range of 45 to 80 mass %.

[0024] In the lithium secondary battery according to this embodiment, the positive electrode active material layer 15 preferably further includes a solid electrolyte. Examples of the solid electrolyte include a sulfide solid electrolyte and an oxide solid electrolyte. In a preferred embodiment of the lithium secondary battery according to this embodiment, the solid electrolyte is preferably a sulfide solid electrolyte containing an S element, from the viewpoint of exhibiting excellent lithium ion conductivity and being able to better follow volume changes of the electrode active material that occur during charge and discharge. More preferably, the solid electrolyte is 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 a S element, a Li element, and a P element.

[0025] The content of the solid electrolyte in the positive electrode active material layer is not particularly limited, but is preferably within the range of 1 to 70 mass %, more preferably within the range of 10 to 60 mass %, and even more preferably within the range of 20 to 55 mass %.

[0026] (Conductive additives and binders) The positive electrode active material layer may further contain at least one of a conductive additive and a binder in addition to the positive electrode active material and the solid electrolyte.

[0027] The thickness of the positive electrode active material layer varies depending on the intended configuration of the lithium secondary battery, but is preferably within the range of 0.1 to 1000 μm, and more preferably 40 to 100 μm, for example.

[0028] [Solid electrolyte layer] The solid electrolyte layer is a layer that is usually interposed between the positive electrode active material layer and the negative electrode current collector during full discharge, and contains a solid electrolyte (usually as a main component). The specific form of the solid electrolyte contained in the solid electrolyte layer is the same as that described above, so a detailed description will be omitted here. The content of the solid electrolyte in the solid electrolyte layer is, for example, preferably in the range of 10 to 100 mass%, more preferably in the range of 50 to 100 mass%, and even more preferably in the range of 90 to 100 mass%, relative to the total mass of the solid electrolyte layer.

[0029] The solid electrolyte layer may further contain a binder in addition to the above-mentioned solid electrolyte. The thickness of the solid electrolyte layer varies depending on the configuration of the intended lithium secondary battery, but is preferably within a range of, for example, 0.1 to 1000 μm, and more preferably 10 to 40 μm.

[0030] [Negative electrode current collector] The negative electrode current collector is a conductive member that functions as a flow path for electrons that are released from the negative electrode toward the power source as the battery reaction (charge / discharge reaction) progresses, or that flow from an external load toward the negative electrode. There are no particular limitations on the material that constitutes the negative electrode current collector. For example, metals and conductive resins can be used as materials for the negative electrode current collector. There are no particular limitations on the thickness of the negative electrode current collector, but an example is 10 to 100 μm.

[0031] [Negative electrode active material layer] The lithium secondary battery according to the present embodiment is a so-called lithium deposition type in which lithium metal is deposited on the negative electrode current collector during charging. The layer of lithium metal deposited on the negative electrode current collector during charging is the negative electrode active material layer of the lithium secondary battery according to the present embodiment. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharging process progresses. The negative electrode active material layer does not need to be present during full discharge, but in some cases, a negative electrode active material layer consisting of a certain amount of lithium metal may be present during full discharge. 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.

[0032] [Ion-conductive reaction suppression layer] One feature of the lithium secondary battery according to this embodiment is that an ion-conductive reaction-suppressing layer is provided on at least a portion of the main surface of the solid electrolyte layer facing the negative electrode current collector, in a region where the positive electrode active material layer faces the negative electrode current collector. This ion-conductive reaction-suppressing layer has lithium ion conductivity and suppresses the reaction between lithium metal (negative electrode active material layer) and the solid electrolyte. Therefore, by providing the ion-conductive reaction-suppressing layer, it is possible to prevent degradation of the solid electrolyte and a decrease in battery capacity caused by the reaction between lithium metal (negative electrode active material layer) and the solid electrolyte without impeding the progress of the battery reaction.

[0033] Here, a material "has lithium ion conductivity" means that the lithium ion conductivity of the material at 25°C is 1×10 -4 On the other hand, a material "does not have lithium ion conductivity" means that the lithium ion conductivity of the material at 25°C is 1 x 10 -4 In the lithium secondary battery according to the present embodiment, the lithium ion conductivity of the constituent material of the ion-conductive reaction suppression layer at 25°C is less than 1×10 -4 [S / cm] or more, preferably 1.5 × 10 -4 [S / cm] or more, more preferably 2.0 × 10 -4[S / cm] or more, and more preferably 2.5 × 10 -4 [S / cm] or more, and particularly preferably 3.0 × 10 -4 [S / cm] or more.

[0034] The constituent material of the ion-conductive reaction suppression layer is not particularly limited, and various materials capable of exhibiting the above-mentioned functions can be used. One example of a constituent material of the ion-conductive reaction suppression layer is nanoparticles with lithium ion conductivity (herein, nanoparticles as a constituent material of the ion-conductive reaction suppression layer are also simply referred to as "first nanoparticles"). By including the first nanoparticles in the ion-conductive reaction suppression layer, a lithium secondary battery with particularly excellent ion-conductive reaction suppression layer functionality can be provided. Here, "nanoparticles" refers to particles with an average particle diameter on the nanometer (nm) scale. The "average particle diameter" of nanoparticles refers to the 50% cumulative diameter (D50) of the particle diameter (the maximum distance between any two points on the outline of the observed particle) measured by observing the cross section of a layer containing the nanoparticles with a scanning electron microscope (SEM). The average particle diameter of the first nanoparticles is preferably 500 nm or less, more preferably 300 nm or less, even more preferably 150 nm or less, particularly preferably 100 nm or less, and most preferably 60 nm or less. In particular, when the average particle size of the first nanoparticles is 60 nm or less, a lithium secondary battery having a particularly excellent dendrite growth suppression effect can be provided. Although there is no particular lower limit for the average particle size of the first nanoparticles, it is usually 10 nm or more, and preferably 20 nm or more.

[0035] From the viewpoint of achieving particularly excellent functionality as an ion-conductive reaction suppression layer, the first nanoparticles preferably contain one or more elements selected from the group consisting of carbon, gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc, and more preferably comprise one or more of these elements as simple substances or alloys. The first nanoparticles preferably contain carbon, and more preferably comprise simple carbon. Examples of such materials comprised of simple carbon include acetylene black, Vulcan (registered trademark), Black Pearl (registered trademark), carbon nanofiber, Ketjen Black (registered trademark), carbon nanotubes, carbon nanohorns, carbon nanoballoons, and fullerene. When the ion-conductive reaction suppression layer contains such nanoparticles, the layer may further contain a binder.

[0036] There are no particular limitations on the method for forming the ion-conductive reaction suppression layer containing the first nanoparticles described above on the surface of the solid electrolyte layer facing the negative electrode current collector. For example, a method can be used in which a slurry containing the nanoparticles and, if necessary, a binder dispersed in an appropriate solvent is applied to the surface of the solid electrolyte layer facing the negative electrode current collector, and the solvent is then dried. Alternatively, the slurry can be applied to the surface of a support such as stainless steel foil, the solvent is dried, and the resulting coating is bonded to the surface of the solid electrolyte layer facing the negative electrode current collector using a method such as hydrostatic pressing, and the support is then peeled off to form the ion-conductive reaction suppression layer. In some cases, the ion-conductive reaction suppression layer can be formed by forming a continuous layer containing any of the above-mentioned materials by a method such as sputtering, rather than in the form of nanoparticles.

[0037] Although the first nanoparticles of the constituent material of the ion-conductive reaction suppression layer have been described above, the ion-conductive reaction suppression layer may be composed of other constituent materials. Examples of other constituent materials include one or more lithium-containing compounds selected from the group consisting of lithium halides (lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), and lithium iodide (LiI)), composite metal oxides represented by Li-MO (where M is one or more metal elements selected from the group consisting of Mg, Au, Al, Sn, and Zn), and Li-Ba-TiO composite oxides. All of these materials are more stable than solid electrolytes with respect to reductive decomposition upon contact with lithium metal. That is, when comparing the tendency of the solid electrolyte constituting the solid electrolyte layer to undergo reductive decomposition upon contact with lithium metal with the tendency of the lithium-containing compound constituting the ion-conductive reaction suppression layer to undergo reductive decomposition upon contact with lithium metal, the latter tendency is smaller. Therefore, the lithium-containing compound can also function as the ion-conductive reaction suppression layer. There are no particular limitations on the method for forming such an ion-conductive reaction suppression layer containing a lithium-containing compound. For example, the ion-conductive reaction suppression layer can be formed by forming a continuous layer containing the above-mentioned lithium-containing compound by a method such as sputtering.

[0038] The average thickness of the ion-conductive reaction-suppressing layer is not particularly limited, as long as it is disposed at a thickness that allows the above-mentioned functions to be exhibited. However, if the average thickness of the ion-conductive reaction-suppressing layer is too large, it increases internal resistance, thereby reducing charge / discharge efficiency. For this reason, the average thickness of the ion-conductive reaction-suppressing layer is preferably smaller than the average thickness of the solid electrolyte layer. Furthermore, if the average thickness of the ion-conductive reaction-suppressing layer is too small, the reaction-suppressing effect of providing the ion-conductive reaction-suppressing layer may not be fully achieved. From these perspectives, when the ion-conductive reaction-suppressing layer is a layer containing first nanoparticles, the average thickness is preferably 300 nm to 20 μm, more preferably 500 nm to 15 μm, and even more preferably 1 to 10 μm. Furthermore, when the layer is a continuous layer made of a lithium-containing compound formed by a technique such as sputtering, the average thickness is preferably 0.5 to 20 nm. The "average thickness" of the protective layer refers to the value calculated as the arithmetic mean value of thickness measurements taken at several to several dozen different locations on the ion-conductive reaction-suppressing layer constituting a lithium secondary battery.

[0039] [Ion permeation suppression layer] The lithium secondary battery according to this embodiment is also characterized in that an ion permeation suppression layer is provided on at least a portion of the outer periphery of the solid electrolyte layer, as shown in Fig. 2. This ion permeation suppression layer is a layer that suppresses the permeation of lithium ions. Therefore, by providing the ion permeation suppression layer, even if dendrites are generated from the lithium metal in the negative electrode active material layer 13, it is possible to effectively prevent the dendrites from growing around the outer periphery of the solid electrolyte layer 17, thereby preventing a short circuit from occurring.

[0040] There are no particular limitations on the material constituting the ion permeation suppression layer, and various materials capable of exhibiting the above-mentioned functions can be used. The material constituting the ion permeation suppression layer is preferably a material that does not have lithium ion conductivity. In the lithium secondary battery according to this embodiment, the material constituting the ion permeation suppression layer has a lithium ion conductivity of 1×10 at 25° C. -4[S / cm], but preferably less than 1 × 10 -5 [S / cm] or less, and more preferably 1×10 -6 [S / cm] or less, and more preferably 1×10 -7 [S / cm] or less, and particularly preferably 1×10 -8 [S / cm] or less. When the lithium ion conductivity of the constituent material of the ion permeation-suppressing layer is within these ranges, the effect of suppressing lithium ion permeation is particularly high. Furthermore, in terms of the relative relationship between the lithium ion conductivity of the constituent material of the ion-conductive reaction-suppressing layer and the lithium ion conductivity of the constituent material of the ion-conductive reaction-suppressing layer, the lithium ion conductivity (25°C) of the constituent material of the ion-conductive reaction-suppressing layer is preferably 10 times or more, more preferably 100 times or more, even more preferably 300 times or more, and particularly preferably 500 times or more than the lithium ion conductivity (25°C) of the constituent material of the ion permeation-suppressing layer. When the lithium ion conductivities of the respective materials differ by this degree, it can be said that the constituent materials of each layer exhibit lithium ion conductivities that are preferable.

[0041] Examples of materials for the ion permeation suppression layer include nanoparticles that do not have lithium ion conductivity (in this specification, nanoparticles serving as a material for the ion permeation suppression layer are also simply referred to as "second nanoparticles"). By including second nanoparticles in the ion permeation suppression layer, a lithium secondary battery with particularly excellent ion permeation suppression layer functionality can be provided. The average particle diameter of the second nanoparticles is preferably 500 nm or less, more preferably 300 nm or less, even more preferably 150 nm or less, even more preferably 100 nm or less, particularly preferably 70 nm or less, and most preferably 40 nm or less. In particular, when the average particle diameter of the second nanoparticles is 40 nm or less, a lithium secondary battery with particularly excellent dendrite growth suppression effect can be provided. There is no particular lower limit for the average particle diameter of the second nanoparticles, but it is usually 10 nm or more, preferably 20 nm or more.

[0042] From the viewpoint of low lithium ion conductivity and effective suppression of dendrite growth, the second nanoparticles preferably contain a metal oxide or nitride. Examples of such metal oxides or nitrides include oxides or nitrides of metals such as aluminum, silicon, magnesium, calcium, potassium, tin, sodium, boron, titanium, lead, zirconium, and yttrium. Among these, the second nanoparticles preferably contain an oxide of these metals, more preferably an oxide of aluminum (aluminum oxide; alumina) or an oxide of silicon (silicon oxide; silica), and even more preferably alumina. When the ion permeation suppression layer contains such nanoparticles, the layer may further contain a binder.

[0043] There are no particular limitations on the method for forming the ion permeation suppression layer containing the second nanoparticles described above adjacent to the periphery of the solid electrolyte layer. For example, a method can be used in which a slurry prepared by dispersing the nanoparticles and, if necessary, a binder in an appropriate solvent is applied to the periphery of the solid electrolyte layer and the solvent is then dried. Alternatively, the slurry can be applied to the surface of a support such as stainless steel foil, the solvent is dried, and the resulting coating is bonded to the periphery of the solid electrolyte layer using a method such as a hydrostatic press, and the support is then peeled off to form the ion permeation suppression layer. In some cases, the ion permeation suppression layer can be formed by forming a continuous layer containing one of the above-mentioned materials by a method such as sputtering, rather than in the form of nanoparticles.

[0044] In some cases, the ion permeation suppression layer may be made of an inorganic powder such as SB-Na-based glass frit, a resin material, or a rubber material. Resin materials and rubber materials, in particular, have elasticity, so even if internal stress occurs in the region where the ion permeation suppression layer is formed, the ion permeation suppression layer can stretch without breaking, effectively preventing the occurrence of a short circuit.

[0045] Fig. 5 is a perspective view showing the appearance of a stacked secondary battery according to one embodiment of the present invention. As shown in Fig. 5, a flat stacked secondary battery 50 has a flat, rectangular shape, with a positive electrode tab 58 and a negative electrode tab 59 for extracting power extending from both sides. A power generating element 57 is wrapped in a battery exterior body (laminate film 52) of the stacked secondary battery 50, and the periphery is heat-sealed, with the power generating element 57 sealed in a state in which the positive electrode tab 58 and the negative electrode tab 59 extend to the outside.

[0046] The above description has been given using the stacked structure and layer arrangement of the stacked secondary battery 10a (lithium secondary battery) according to the embodiment shown in FIG. 2 as an example. However, the lithium secondary battery according to this embodiment can have various other stacked structures and layer arrangements. Examples of such other embodiments include those shown in FIGS. 6 to 8. In the embodiment shown in FIG. 6, the solid electrolyte layer 17 does not extend to the outer periphery of the positive electrode active material layer 15, but is disposed only between the positive electrode active material layer 15 and the carbon black layer 18a (ion-conductive reaction-suppressing layer). An alumina layer 18b (ion permeation suppressing layer) is disposed on the outer periphery of the solid electrolyte layer 17. In the embodiment shown in FIG. 7, the solid electrolyte layer 17 and the positive electrode active material layer 15 are fabricated to the same size, and the alumina layer 18b (ion permeation suppressing layer) is disposed adjacent to the outer periphery of both layers. Having the ion permeation suppressing layer adjacent to at least a portion of the outer periphery of the positive electrode active material layer in addition to the outer periphery of the solid electrolyte layer is also a preferred embodiment for effectively preventing dendrite growth and resulting short circuits. Furthermore, in the embodiment shown in Fig. 8, the carbon black layer 18a (ion-conductive reaction-suppressing layer) extends so as to cover at least a portion of the outer periphery of the solid electrolyte layer 17. This configuration is also one of the preferable configurations for effectively preventing dendrite growth and resulting short circuits. In addition to the above-described embodiment, the lithium secondary battery according to this embodiment may also employ stacked structures and arrangements of layers such as those shown in Figs. 10 to 17.

[0047] Although the above description has been given taking the case where the secondary battery according to the present embodiment is an all-solid-state lithium secondary battery as an example, the lithium secondary battery according to the present embodiment does not have to be an all-solid-state type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolytic solution).

[0048] The following embodiments are also within the scope of the present invention: the lithium secondary battery of claim 1 including the features of claim 2; the lithium secondary battery of claim 1 or 2 including the features of claim 3; the lithium secondary battery of any of claims 1 to 3 including the features of claim 4; the lithium secondary battery of any of claims 1 to 4 including the features of claim 5; the lithium secondary battery of any of claims 1 to 5 including the features of claim 6; the lithium secondary battery of any of claims 1 to 6 including the features of claim 7; the lithium secondary battery of claim 7 including the features of claim 8; the lithium secondary battery of claim 7 or 8 including the features of claim 9; the lithium secondary battery of any of claims 7 to 9 including the features of claim 10; and the lithium secondary battery of any of claims 1 to 10 including the features of claim 11. [Example]

[0049] 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 performed in a glove box. Furthermore, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0050] Example 1 [Preparation of evaluation cell] First, LiNi as the positive electrode active material 0.8 Mn 0.1 Co 0.1O2, acetylene black as a conductive additive, and sulfide solid electrolyte (LPS (Li2S-P2S5)) were weighed in a mass ratio of 70:5:25 and mixed in an agate mortar in a glove box. The mixture was then further mixed and stirred in a planetary ball mill. Two parts by mass of styrene-butadiene rubber (SBR) were 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. The positive electrode active material slurry was then applied to the surface of stainless steel (SUS) foil as a positive electrode current collector and dried to form a positive electrode active material layer (50 μm thick). This produced a positive electrode.

[0051] A solid electrolyte slurry was prepared by adding 2 parts by mass of styrene-butadiene rubber (SBR) to 100 parts by mass of a sulfide solid electrolyte (LPS (Li2S-P2S5)) and mesitylene as a solvent. The solid electrolyte slurry was then applied to the surface of a stainless steel foil support and dried to form a solid electrolyte layer (30 μm thick) on the surface of the stainless steel foil. The outer periphery of the solid electrolyte layer was slightly larger than that of the positive electrode active material layer. The positive electrode active material layer and the solid electrolyte layer were then stacked facing each other. The solid electrolyte layer was then bonded together using a hydrostatic press (700 MPa, 25°C, 1 minute) so that the outer periphery of the solid electrolyte layer covered the entire periphery of the positive electrode active material layer. The stainless steel foil on the solid electrolyte layer side was then peeled off to obtain a positive electrode current collector / positive electrode active material layer / solid electrolyte layer laminate.

[0052] Meanwhile, alumina nanoparticles were prepared as a constituent material for the ion permeation suppression layer. 10 parts by mass of styrene-butadiene rubber (SBR) was added to 100 parts by mass of these alumina nanoparticles, and mesitylene was added as a solvent to prepare an alumina nanoparticle slurry. The alumina nanoparticle slurry prepared above was then applied to the surface of a stainless steel foil support and dried to prepare an alumina layer (5 μm thick) as an ion permeation suppression layer on the surface of the stainless steel foil. The average particle diameter (D50) of the alumina nanoparticles contained in the alumina layer thus prepared was measured by SEM observation of the cross section of the alumina layer and found to be 40 nm.

[0053] Next, the outer periphery of the solid electrolyte layer in the laminate of positive electrode current collector / positive electrode active material layer / solid electrolyte layer prepared above was superimposed on the alumina layer (ion permeation suppressing layer) prepared above so that they faced each other, and then they were bonded together using a hydrostatic press (700 MPa, 25°C, 1 minute), and the stainless steel foil on the alumina layer side was peeled off, forming an alumina layer on the entire outer periphery of the solid electrolyte layer.

[0054] Furthermore, carbon black nanoparticles were prepared as a constituent material for the ion-conductive reaction suppression layer. A carbon black nanoparticle slurry was prepared by adding 10 parts by mass of styrene-butadiene rubber (SBR) to 100 parts by mass of the carbon black nanoparticles and adding mesitylene as a solvent. The carbon black nanoparticle slurry was then applied to the surface of a stainless steel foil support and dried to form a carbon black layer (5 μm thick) as an ion-conductive reaction suppression layer on the surface of the stainless steel foil. The outer periphery of the carbon black layer (ion-conductive reaction suppression layer) was slightly larger than the area where the positive electrode active material layer faces the negative electrode current collector, as shown in Figure 2. The average particle diameter (D50) of the carbon black nanoparticles contained in the carbon black layer thus prepared was measured by SEM observation of the cross section of the carbon black layer and found to be 60 nm.

[0055] The alumina layer covering the exposed surface of the solid electrolyte layer and the entire periphery of the solid electrolyte layer in the positive electrode current collector / positive electrode active material layer / solid electrolyte layer laminate was then laminated so that the end face of the alumina layer faced the carbon black layer (ion-conductive reaction-suppressing layer) prepared above. The resulting laminate was then bonded using a hydrostatic press (700 MPa, 25°C, 1 minute). The stainless steel foil on the carbon black layer side was peeled off, forming a carbon black layer on the exposed surface of the solid electrolyte layer. Finally, a stainless steel foil anode current collector was placed to cover the carbon black layer, producing an evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) with the configuration shown in Figure 2, except that no anode active material layer was present.

[0056] <Example 2> 6 except that no negative electrode active material layer was present, a test cell (lithium deposition-type all-solid-state lithium secondary battery) was fabricated in the same manner as in Example 1. When fabricating the test cell, the positive electrode current collector, positive electrode active material layer, solid electrolyte layer, alumina layer, carbon black layer, and negative electrode current collector were formed (laminated) in this order.

[0057] Example 3 7 except that no negative electrode active material layer was present, a test cell (lithium deposition-type all-solid-state lithium secondary battery) was fabricated in the same manner as in Example 1. When fabricating the test cell, the positive electrode current collector, positive electrode active material layer, solid electrolyte layer, alumina layer, carbon black layer, and negative electrode current collector were formed (laminated) in this order.

[0058] Example 4 An evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) having the configuration shown in Fig. 8 was fabricated using the same method as in Example 1 described above, except that no negative electrode active material layer was present. When fabricating the evaluation cell, the positive electrode current collector, positive electrode active material layer, solid electrolyte layer, alumina layer, carbon black layer, and negative electrode current collector were formed (laminated) in this order.

[0059] <Comparative Example 1> An evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) of this comparative example was produced in the same manner as in Example 1 described above, except that neither the ion-conductive reaction-suppressing layer (carbon black layer) nor the ion-permeation-suppressing layer (alumina layer) was formed.

[0060] <Comparative Example 2> An evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) of this comparative example was produced in the same manner as in Example 2 above, except that neither the ion-conductive reaction-suppressing layer (carbon black layer) nor the ion-permeation-suppressing layer (alumina layer) was formed.

[0061] <Comparative Example 3> An evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) having the configuration shown in FIG. 18 was fabricated using the same method as in Example 1 described above, except that the negative electrode active material layer was not present. When fabricating the evaluation cell, first, a positive electrode current collector, a positive electrode active material layer, and a solid electrolyte layer were formed in this order. Next, an alumina layer was formed with the exposed surface of the solid electrolyte layer masked, and then the mask was removed, and a carbon black layer and a negative electrode current collector were formed (laminated) in this order.

[0062] [Evaluation of the evaluation cell (evaluation of the presence or absence of short circuits caused by dendrites)] A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector of the evaluation cell prepared above, respectively, and the cell was placed in a thermostatic chamber at 60°C and subjected to a current of 3 [mA / cm 2 ], 3.5 [mA / cm 2 ] or 4 [mA / cm 2 The test cells were charged from 0% SOC at a current density of 0.05% and the presence or absence of a short circuit within 30 minutes was checked. A short circuit was deemed to have occurred when the test cell's voltage dropped during the charging process. Test cells that were deemed to have short-circuited were disassembled and the interior of the cells was inspected, and the formation of lithium metal dendrites was confirmed in all cases. The results are shown in Table 1 below. Among the evaluation results shown in Table 1, those that did not have a short circuit are indicated with an "O" and those that did have a short circuit are indicated with an "X."

[0063] [Table 1]

[0064] The results shown in Table 1 reveal that, according to one embodiment of the present invention, in a lithium secondary battery including a lithium deposition-type power generating element, by providing an ion-conductive reaction-suppressing layer on the negative electrode current collector side of the solid electrolyte layer and by arranging an ion permeation-suppressing layer adjacent to the outer periphery of the solid electrolyte layer, dendrite growth can be more reliably suppressed compared to a case in which these layers are not provided. In particular, in Example 1, in which the ion-conductive reaction-suppressing layer and ion permeation-suppressing layer were provided and the solid electrolyte layer extended to the outer periphery of the positive electrode active material layer, it was also found that dendrite formation was suppressed even by current treatment at a higher current density.

[0065] This application is based on Japanese Patent Application No. 2021-202389, filed on December 14, 2021, the disclosure of which is incorporated by reference in its entirety.

Claims

1. a positive electrode including a positive electrode active material layer disposed on a surface of a positive electrode current collector, the positive electrode active material layer containing a positive electrode active material capable of absorbing and releasing lithium ions; a negative electrode having a negative electrode current collector on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; a power generating element having an ion-conductive reaction-suppressing layer having lithium ion conductivity and suppressing a reaction between the lithium metal and the solid electrolyte is provided on at least a part of a region of the main surface of the solid electrolyte layer facing the negative electrode current collector, where the positive electrode active material layer faces the negative electrode current collector; an ion permeation suppression layer that suppresses permeation of lithium ions is provided adjacent to at least a portion of the outer periphery of the solid electrolyte layer; The solid electrolyte layer extends so as to cover at least a portion of the outer periphery of the positive electrode active material layer.

2. 2. The lithium secondary battery according to claim 1, wherein the solid electrolyte layer extends to reach the positive electrode current collector so as to cover the entire outer periphery of the positive electrode active material layer.

3. The lithium secondary battery according to claim 1 , wherein the ion permeation suppression layer is further adjacent to at least a part of the outer periphery of the positive electrode active material layer.

4. 3. The lithium secondary battery according to claim 1, wherein the ion-conductive reaction suppression layer extends so as to cover at least a portion of the outer periphery of the solid electrolyte layer.

5. 3. The lithium secondary battery according to claim 1, wherein the lithium ion conductivity of the constituent material of the ion-conductive reaction-suppressing layer is at least 100 times that of the constituent material of the ion permeation-suppressing layer.

6. The lithium ion conductivity of the constituent material of the ion permeation suppression layer at 25°C is 1×10 －５ 3. The lithium secondary battery according to claim 1, wherein the resistance is 0.1 [S / cm] or less.

7. 3. The lithium secondary battery according to claim 1, wherein the ion-conductive reaction-suppressing layer contains first nanoparticles having lithium ion conductivity, and the ion-permeation-suppressing layer contains second nanoparticles not having lithium ion conductivity.

8. 8. The lithium secondary battery according to claim 7, wherein the average particle diameter of the first nanoparticles (the 50% cumulative diameter (D50) of particle diameters measured by observing a cross-section of a layer containing nanoparticles with a scanning electron microscope (SEM) (the maximum distance between any two points on the outlines of the observed particles)) is 60 nm or less, and the average particle diameter of the second nanoparticles (the 50% cumulative diameter (D50) of particle diameters measured by observing a cross-section of a layer containing nanoparticles with a scanning electron microscope (SEM) (the maximum distance between any two points on the outlines of the observed particles)) is 40 nm or less.

9. 9. The lithium secondary battery according to claim 8, wherein the first nanoparticles contain one or more elements selected from the group consisting of carbon, gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc.

10. The lithium secondary battery according to claim 8 , wherein the second nanoparticles comprise a metal oxide or nitride.

11. 3. The lithium secondary battery according to claim 1, wherein the ion-conductive reaction-suppressing layer is provided in an area including the entire region, and the ion permeation-suppressing layer is provided around the entire outer periphery of the solid electrolyte layer.

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