Lithium secondary battery

Abstract

To provide a lithium deposition type lithium secondary battery with further improved charging and discharging efficiency.SOLUTION: A lithium secondary battery includes: a power generation element including a positive electrode including a positive electrode active material layer 15, a negative electrode in which lithium metal is deposited on a negative electrode current collector 11' during charging, and a solid electrolyte layer 17 interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a pressurizing member for pressurizing the power generating element in a stacking direction with predetermined pressure. At least part of a main surface of the solid electrolyte layer facing the negative electrode current collector and at least part of a side surface of the solid electrolyte layer have electronic insulation and lithium ion conductivity. A protective layer 18 that is more stable than the solid electrolyte with respect to reductive decomposition due to contact with the lithium metal is provided. The shear elastic modulus of the protective layer is set to 2 [GPa] or more, and a difference between the shear elastic modulus of the protective layer and the shear elastic modulus of the solid electrolyte layer is set to 50 [GPa] or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] In recent years, there has been a strong desire to reduce carbon dioxide emissions in order to combat global warming. The automotive industry is hoping that the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs) will help reduce carbon dioxide emissions, and there has been active development of non-aqueous electrolyte secondary batteries, such as secondary batteries for driving motors, which hold the key to putting these vehicles into practical use.

[0003] Secondary batteries for driving motors are required to have extremely high output characteristics and high energy compared to consumer lithium-ion secondary batteries used in mobile phones, laptops, etc. Therefore, lithium-ion secondary batteries, which have the highest theoretical energy of all practical batteries, have attracted attention and are currently being rapidly developed.

[0004] Currently widely used lithium-ion secondary batteries use flammable organic electrolytes, and these liquid-based lithium-ion secondary batteries require stricter safety measures against leakage, short circuits, overcharging, and other issues than other batteries.

[0005] Therefore, 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. For this reason, 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. Furthermore, 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.

[0006] Conventionally, one type of all-solid-state lithium secondary battery is known as a so-called lithium deposition type, in which lithium metal is deposited on a negative electrode current collector during charging (see, for example, Patent Document 1). 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. In the lithium secondary battery described in Patent Document 1, an electrolyte layer interposed between a lithium-containing positive electrode and a lithium-containing negative electrode is composed of an electrolyte layer made of a first electrolyte and a second electrolyte containing iodine provided between the electrolyte layer and the negative electrode, and the ionic conductivity of the second electrolyte is lower than that of the first electrolyte. Patent Document 1 claims that this configuration eliminates non-uniform lithium deposition and suppresses dendrite formation, even if the interface between the electrolyte layer and the negative electrode is uneven. As a result, it is claimed that variations in internal resistance and reductions in discharge capacity due to lithium dendrite growth are improved, thereby providing a lithium battery with excellent charge / discharge characteristics. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2019-61867 Summary of the Invention [Problem to be solved by the invention]

[0008] However, according to the investigations of the present inventors, it has been found that even if the technique described in Patent Document 1 is used, there are still cases where sufficient charge / discharge efficiency cannot be achieved.

[0009] Therefore, an object of the present invention is to provide a means for further improving the charge / discharge efficiency of a lithium deposition type lithium secondary battery. [Means for solving the problem]

[0010] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that in a secondary battery including a lithium deposition-type power generating element and a pressing member that presses the power generating element in the stacking direction, the above-mentioned problems can be solved by providing a predetermined protective layer on at least a part of a main surface of the solid electrolyte layer that faces the negative electrode current collector and on at least a part of a side surface of the solid electrolyte layer, and by controlling the profile of the shear modulus of the protective layer, which led to the completion of the present invention.

[0011] Specifically, one aspect of the present invention relates to a lithium secondary battery comprising: a power generating element including a positive electrode having a positive electrode active material layer disposed on the surface of a positive electrode current collector, the negative electrode having a negative electrode current collector and in which lithium metal is deposited on the negative electrode current collector during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a pressure member that applies a predetermined pressure to the power generating element in the stacking direction. The lithium secondary battery further comprises a protective layer that is electronically insulating and lithium-ion conductive, and is more stable than the solid electrolyte with respect to reductive decomposition due to contact with the lithium metal, on at least a portion of the main surface of the solid electrolyte layer facing the negative electrode current collector and on at least a portion of the side surface of the solid electrolyte layer. The protective layer has a shear modulus of 2 GPa or more, and the difference between the shear modulus of the protective layer and that of the solid electrolyte layer is 50 GPa or less. [Effects of the Invention]

[0012] According to the present invention, the charge / discharge efficiency of a lithium deposition type lithium secondary battery can be further improved. [Brief explanation of the drawings]

[0013] [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 cell layer of a stacked secondary battery according to one embodiment of the present invention. FIG. [Figure 3] FIG. 10 is an enlarged cross-sectional view of a unit cell layer showing a modified example of a stacked secondary battery according to the present invention. [Figure 4] 1 is a perspective view of a stacked secondary battery according to one embodiment of the present invention; [Figure 5] FIG. 5 is a side view seen from a direction A shown in FIG. [Figure 6] FIG. 10 is an enlarged cross-sectional view of a unit cell layer showing a modified example of a stacked secondary battery according to the present invention. [Figure 7] FIG. 10 is an enlarged cross-sectional view of a unit cell layer showing a modified example of a stacked secondary battery according to the present invention. [Figure 8] FIG. 10 is an enlarged cross-sectional view of a unit cell layer showing a modified example of a stacked secondary battery according to the present invention. [Figure 9] 1 is a perspective view showing the appearance of a stacked secondary battery according to one embodiment of the present invention; [Figure 10] 1 is a graph in which the charge / discharge efficiency after 100 cycles is plotted against the difference between the shear modulus of elasticity of the protective layer and the shear modulus of elasticity of the solid electrolyte layer. DETAILED DESCRIPTION OF THE INVENTION

[0014] One aspect of the present invention is a lithium secondary battery comprising: a power generating element including a positive electrode having 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, where lithium metal is deposited on the negative electrode current collector during charging; and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a pressure member that applies a predetermined pressure to the power generating element in a stacking direction, wherein a protective layer that is electronically insulating and lithium ion conductive and is more stable than the solid electrolyte with respect to reductive decomposition due to contact with the lithium metal is provided on at least a portion of a main surface of the solid electrolyte layer facing the negative electrode current collector and at least a portion of a side surface of the solid electrolyte layer, the protective layer having a shear modulus of 2 GPa or more and a difference in the shear modulus of the protective layer and the solid electrolyte layer being 50 GPa or less. The lithium secondary battery of this aspect can further improve the charge / discharge efficiency of lithium-deposition-type lithium secondary batteries.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] In the above description, an embodiment of the lithium secondary battery according to the present invention has been described using a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery as an example. However, the type of lithium secondary battery to which the present invention is applicable is not particularly limited, and the present invention can also be applied to a bipolar-type lithium secondary battery.

[0020] 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 to the positive electrode current collector 11". Here, in the embodiment shown in FIG. 2, the outer peripheral edge portion of the solid electrolyte layer 17 extends over its entire periphery to the side surface of the positive electrode active material layer 15. As a result, the positive electrode active material layer 15 is configured to be slightly smaller than the solid electrolyte layer 17. In other words, when the power generating element 21 is viewed from above, the entire periphery of the outer peripheral edge of the positive electrode active material layer 15 is configured to be located inside the outer peripheral edge of the solid electrolyte layer 17. With this configuration, even if the lithium metal constituting the negative electrode active material layer 13 is pushed from the outer circumferential edge of the solid electrolyte layer 17 toward the positive electrode active material layer 15 due to the restraining pressure of the pressure member, the lithium metal is less likely to come into contact with the side surface of the positive electrode active material layer 15. As a result, the effect of preventing short circuits is further enhanced. Note that the "side surface of the positive electrode active material layer" refers to the surface of the positive electrode active material layer that is not in contact with the positive electrode current collector and does not face the negative electrode current collector. Here, as shown in FIG. 3 , the outer circumferential edge of the solid electrolyte layer 17 does not have to extend to the side surface of the positive electrode active material layer 15. However, even in this case, it is preferable that at least a portion of the outer circumferential edge of the positive electrode active material layer 15 is positioned inside the outer circumferential edge of the solid electrolyte layer 17 when the power generating element 21 is viewed in plan, as shown in FIG. 3 . This is because even if the lithium metal constituting the negative electrode active material layer 13 is pushed out from the outer peripheral edge of the solid electrolyte layer 17 toward the positive electrode active material layer 15 due to the restraining pressure of the pressure member, the lithium metal is less likely to come into contact with the side surface of the positive electrode active material layer 15, and short circuits can be more effectively prevented.

[0021] In the embodiment shown in FIG. 2 , a protective layer 18 is provided on the entire main surface of the solid electrolyte layer 17 facing the negative electrode current collector 11′ and on the entire side surface of the solid electrolyte layer 17. The “side surface of the solid electrolyte layer” refers to the surface of the solid electrolyte layer that faces neither the positive electrode active material layer nor the negative electrode current collector during discharge when the negative electrode active material layer 13 made of lithium metal is not present. This protective layer 18 is a layer that has electronic insulation and lithium ion conductivity. The protective layer 18 is also characterized by being more stable than the solid electrolyte constituting the solid electrolyte layer 17 with respect to reductive decomposition due to contact with lithium metal. In the embodiment shown in FIG. 2 , the protective layer 18 is made of lithium bromide (LiBr). By providing such a protective layer on the side surface of the solid electrolyte layer, even when lithium metal deposited on the surface of the negative electrode current collector during charge is pushed out from the outer periphery of the solid electrolyte layer by the restraining pressure of the pressing member, contact between the solid electrolyte layer and the negative electrode active material layer is prevented, thereby suppressing deterioration of the solid electrolyte layer due to reductive decomposition. Furthermore, since the effective area of ​​the lithium metal facing the positive electrode active material layer via the protective layer and the solid electrolyte layer becomes larger, there is an advantage in that the charge / discharge efficiency can be further improved.

[0022] Furthermore, in the embodiment shown in FIG. 2 , the negative electrode current collector 11′ is configured to be slightly smaller than the solid electrolyte layer 17. The negative electrode current collector 11′ is also configured to be slightly larger than the positive electrode active material layer 15. That is, when the power generating element 21 is viewed from above, the entire periphery of the negative electrode current collector 11′ is positioned inside the outer periphery of the solid electrolyte layer 17 and outside the outer periphery of the positive electrode active material layer 15. This configuration suppresses the generation of dendrites made of lithium metal at the end of the negative electrode, while preventing the negative electrode active material layer 13 made of lithium metal from moving beyond the outer periphery of the solid electrolyte layer 17 and wrapping around to the positive electrode active material layer 15, resulting in a short circuit. However, depending on the case, the negative electrode current collector 11′ may be configured to be the same size as or slightly larger than the solid electrolyte layer 17, or the same size as or slightly smaller than the positive electrode active material layer 15.

[0023] Fig. 4 is a perspective view of a stacked secondary battery according to one embodiment of the present invention, and Fig. 5 is a side view seen from direction A shown in Fig. 4.

[0024] As shown in FIGS. 4 and 5 , the stacked secondary battery 100 according to this embodiment includes the power generating element 21 sealed in the laminate film 29 shown in FIG. 1 , two metal plates 200 sandwiching the power generating element 21 sealed in the laminate film 29, and bolts 300 and nuts 400 as fastening members. The fastening members (bolts 300 and nuts 400) function to secure the power generating element 21 sealed in the laminate film 29 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 power generating element 21 in the stacking direction. Note that the pressure members are not particularly limited as long as they are capable of pressing the power generating element 21 in the stacking direction. A combination of a plate made of a rigid material, such as the metal plate 200, and the above-described 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 power generating element 21 in the stacking direction.

[0025] The lower limit of the load applied to the power generating element 21 (restraint pressure in the stacking direction of the power generating element) 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 restraint pressure in the stacking direction of the power generating element 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.

[0026] The main components of the above-described stacked secondary battery 10a will be described below.

[0027] [Positive electrode current collector] The positive electrode current collector is a conductive member that functions as a flow path for electrons that are released from the positive electrode toward an external load or flow from a power source toward the positive electrode as the battery reaction (charge / discharge reaction) progresses. There are no particular limitations on the material that constitutes the positive electrode current collector. Examples of materials that can be used for the positive electrode current collector include metals and conductive resins.

[0028] Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. Other examples include clad materials of nickel and aluminum, and clad materials of copper and aluminum. Foils in which aluminum is coated on a metal surface may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and the like. Examples of the latter conductive resin include resins in which a conductive filler is added to a non-conductive polymer material.

[0029] 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 include at least a conductive resin layer made of a resin having electrical 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.

[0030] There is no particular limitation on the thickness of the positive electrode current collector, but an example is 10 to 100 μm.

[0031] [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.

[0032] 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.

[0033] In some cases, two or more positive electrode active materials may be used in combination. Of course, positive electrode active materials other than those mentioned above may also be used.

[0034] In a preferred embodiment, the positive electrode active material layer 15 constituting the lithium secondary battery according to this embodiment contains a layered rock salt active material (e.g., Li(Ni-Mn-Co)O2) containing lithium and cobalt as the positive electrode active material from the viewpoint of output characteristics.

[0035] The shape of the positive electrode active material may be, for example, particulate (spherical, fibrous), thin film, etc. When the positive electrode active material is particulate, its average particle size (D 50 ) is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. In this specification, the average particle size (D 50 The value of can be measured by a laser diffraction scattering method.

[0036] 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 %.

[0037] In the lithium secondary battery according to this embodiment, the positive electrode active material layer 15 preferably further contains a solid electrolyte. Examples of the solid electrolyte include a sulfide solid electrolyte, a resin solid electrolyte, and an oxide solid electrolyte. Note that, as the solid electrolyte, a material having a desired bulk modulus can be appropriately selected depending on the degree of volume expansion accompanying charge and discharge of the electrode active material used.

[0038] In a preferred embodiment of the secondary battery according to the present invention, the solid electrolyte preferably includes a resin solid electrolyte, which can better accommodate changes in the volume of the electrode active material during charging and discharging. Examples of such a resin solid electrolyte include fluororesin, polyethylene oxide, polyacrylonitrile, polyacrylate, and derivatives and copolymers thereof.

[0039] Examples of fluororesins include those containing, as structural units, vinylidene fluoride (VdF), hexafluoropropylene (HFP), tetrafluoroethylene (TFE), and derivatives thereof. Specific examples include homopolymers such as polyvinylidene fluoride (PVdF), polyhexafluoropropylene (PHFP), and polytetrafluoroethylene (PTFE), and binary copolymers such as a copolymer of VdF and HFP.

[0040] In another preferred embodiment of the secondary battery according to this embodiment, the solid electrolyte preferably is a sulfide solid electrolyte containing S element from the viewpoint that it exhibits excellent lithium ion conductivity and can better follow the volume change of the electrode active material accompanying charge and discharge. More preferably, it is a sulfide solid electrolyte containing Li element, M element and S element, and 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, it is a sulfide solid electrolyte containing S element, Li element and P element.

[0041] The sulfide solid electrolyte may have a Li3PS4 skeleton, a Li4P2S7 skeleton, or a Li4P2S6 skeleton. Examples of the sulfide solid electrolyte having a Li3PS4 skeleton include LiI-Li3PS4, LiI-LiBr-Li3PS4, and Li3PS4. Examples of the sulfide solid electrolyte having a Li4P2S7 skeleton include, for example, a Li-P-S based solid electrolyte called LPS. Also, as the sulfide solid electrolyte, for example, LGPS represented by Li (4-x) Ge (1-x) P x S4 (where x satisfies 0 < x < 1) such as LGPS may be used. More specifically, for example, LPS (Li2S-P2S5), Li7P3S 11 , Li 3.2 P 0.96 S, Li 3.25 Ge 0.25 P 0.75 S4, Li 10 GeP2S 12 , or Li6PS5X (where X is Cl, Br or I), etc. are included. The description of "Li2S-P2S5" means a sulfide solid electrolyte formed using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions. Among them, the sulfide solid electrolyte preferably is LPS (Li2S-P2S5), Li6PS5X (where X is Cl, Br or I), Li7P3S 11 , Li3.2 P 0.96 S and Li3PS4.

[0042] 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 %.

[0043] 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.

[0044] 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.

[0045] [Solid electrolyte layer] The solid electrolyte layer is a layer interposed between the positive electrode active material layer and the negative electrode current collector during 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, and therefore a detailed description thereof will be omitted here.

[0046] 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.

[0047] The solid electrolyte layer may further contain a binder in addition to the above-mentioned solid electrolyte.

[0048] The thickness of the solid electrolyte layer varies depending on the configuration of the intended lithium secondary battery, but is preferably within the range of 0.1 to 1000 μm, and more preferably 10 to 40 μm, for example.

[0049] As described above with reference to FIG. 2, from the viewpoint of preventing a short circuit due to contact between the negative electrode active material layer and the positive electrode active material layer, it is preferable that at least a portion (preferably the entire periphery) of the outer periphery of the solid electrolyte layer extends to the side surface of the positive electrode active material layer. In a more preferred embodiment, the outer periphery of the solid electrolyte layer extends to the positive electrode current collector, thereby covering the entire side surface of the positive electrode active material layer, thereby achieving a particularly high short circuit prevention effect. In this case, the outer periphery of the solid electrolyte layer covering the side surface of the positive electrode active material layer may be arranged so as to be approximately parallel to the side surface of the positive electrode active material layer, or may be arranged in a tapered shape so as to be inclined at a certain angle thereto (see, for example, FIG. 6).

[0050] [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.

[0051] [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.

[0052] [Protective layer] In the lithium secondary battery according to this embodiment, a protective layer is provided on at least a portion (preferably the entirety of) the main surface of the solid electrolyte layer facing the negative electrode current collector and on at least a portion (preferably the entirety of the side surface) of the solid electrolyte layer. This protective layer is a layer having electronic insulation and lithium ion conductivity. Furthermore, the protective layer must be more stable than the solid electrolyte with respect to reductive decomposition upon contact with lithium metal. Here, "more stable than the solid electrolyte with respect to reductive decomposition upon contact with lithium metal" means that the tendency of the solid electrolyte constituting the solid electrolyte layer to undergo reductive decomposition upon contact with lithium metal is smaller than the tendency of the constituent material of the protective layer to undergo reductive decomposition upon contact with lithium metal. Whether the constituent material of the protective layer satisfies this condition can be determined by determining whether the current flowing through the protective layer is smaller than the current flowing through the solid electrolyte layer when a voltage is swept around 0 V [vs. Li / Li+] by cyclic voltammetry using the solid electrolyte layer and the protective layer as working electrodes and lithium metal as a counter electrode.

[0053] By providing such a protective layer on the side surfaces of the solid electrolyte layer, even when lithium metal deposited on the surface of the negative electrode current collector is pushed out from the outer periphery of the solid electrolyte layer by the restraining pressure of the pressing member during charging, contact between the solid electrolyte layer and the negative electrode active material layer is prevented, thereby suppressing degradation of the solid electrolyte layer due to reductive decomposition. Furthermore, since the effective area of ​​the lithium metal facing the positive electrode active material layer via the protective layer and the solid electrolyte layer is increased, there is also the advantage of further improving charge / discharge efficiency. Whether or not a protective layer is provided in the lithium secondary battery according to this embodiment can be determined, for example, by observing the cross section of the lithium secondary battery using SEM-EDX to confirm whether or not a layer corresponding to the protective layer is present on the main surface and side surfaces of the solid electrolyte layer, and then analyzing its composition by elemental analysis, etc. Furthermore, if the above-mentioned method is difficult to determine due to reasons such as the thinness of the protective layer, it can also be determined by analyzing the layer corresponding to the protective layer while etching it using XPS.

[0054] Furthermore, in the lithium secondary battery according to the present embodiment, the shear modulus of the protective layer is also controlled. Specifically, the shear modulus of the protective layer must be 2 GPa or greater, and the difference between the shear modulus of the protective layer and that of the solid electrolyte layer must be 50 GPa or less. Here, "shear modulus" is also referred to as the shear modulus, rigidity modulus, or transverse modulus. It is a material-specific physical property defined as the ratio of shear stress to shear strain, and indicates the resistance to deformation due to shear force. Here, if the shear modulus of the protective layer is less than 2 GPa, when lithium metal dendrites form on the surface of the negative electrode current collector during charging, the dendrites may penetrate the protective layer, causing an internal short circuit. Furthermore, if the difference between the shear modulus of the protective layer and that of the solid electrolyte layer exceeds 50 GPa, the protective layer may not be able to adequately respond to the stresses that occur during the deposition / dissolution of lithium metal in the negative electrode during repeated charge / discharge, and cracks may occur in the protective layer. As a result, the deposited lithium metal comes into contact with the solid electrolyte layer, resulting in reduced cycle durability due to reductive decomposition of the solid electrolyte. It has also been found that this decrease in cycle durability is particularly pronounced during rapid charging, which tends to cause lithium metal to preferentially deposit unevenly from active sites. In contrast, if the shear modulus of the protective layer satisfies the above-mentioned requirements, it can prevent penetration of lithium metal dendrites while sufficiently adapting to stresses caused by repeated charge-discharge cycles, thereby contributing to improved cycle durability. The difference between the shear modulus of the protective layer and that of the solid electrolyte layer is 50 GPa or less, as described above, but is preferably 43 GPa or less, more preferably 20 GPa or less, particularly preferably 13 GPa or less, and most preferably 5 GPa or less. While the shear modulus of the protective layer is typically greater than that of the solid electrolyte layer, this is not a limitation.The shear modulus of the protective layer is not particularly limited, but is preferably 51 GPa or less, more preferably 30 GPa or less, even more preferably 21 GPa or less, and most preferably 10 GPa or less. As described above, the lower limit is 2 GPa or more. The shear modulus of the solid electrolyte layer is also not particularly limited, but is preferably 20 GPa or less, more preferably 15 GPa or less, and even more preferably 10 GPa or less. The lower limit may be approximately 1 GPa or more. The shear modulus of the solid electrolyte layer and the protective layer is measured by the nanoinden- tation method described in the literature (Kunkun Fu et al., Vacuum, Volume 112, February 2015, Pages 29-32) at the time of fabrication of these layers during the manufacture of a lithium secondary battery.

[0055] The material for the protective layer is not particularly limited, and any material that satisfies the above-mentioned conditions can be suitably used. Examples of materials for the protective layer include one or more materials 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. These materials are particularly stable to reductive decomposition upon contact with lithium metal, making them suitable for use as protective layer materials.

[0056] There are no particular limitations on the average thickness of the protective layer, 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 protective layer is too large, it will increase the internal resistance, which will cause a decrease in charge / discharge efficiency. For this reason, the average thickness of the protective layer is preferably smaller than the average thickness of the solid electrolyte layer. Furthermore, if the average thickness of the protective layer is too small, the protective effect of providing the protective layer may not be sufficiently obtained. From these perspectives, the average thickness of the protective layer is preferably 0.5 nm to 20 μm, and more preferably 5 nm to 10 μm. The "average thickness" of the protective layer refers to the value calculated as the arithmetic average of thicknesses measured at several to several dozen different locations on the protective layer constituting the lithium secondary battery.

[0057] Furthermore, the smaller the surface roughness of the protective layer, the better. Specifically, the arithmetic mean roughness (Ra) of the protective layer measured in accordance with JIS B 0601:2013 is preferably 1 μm or less, more preferably 500 nm or less, even more preferably 100 nm or less, even more preferably 50 nm or less, particularly preferably 10 nm or less, and most preferably 5 nm or less. If the surface roughness (Ra) of the protective layer is 1 μm or less, the interface between the lithium metal deposited on the negative electrode and the solid electrolyte layer (protective layer) can maintain good contact for a long period of time, which can effectively contribute to maintaining cycle durability.

[0058] [Insulating layer] In the lithium secondary battery according to this embodiment, as shown in FIG. 7 , when the power generating element 21 is viewed from above, it is preferable that at least a part of the outer circumferential edge of the positive electrode active material layer 15 is located inside the outer circumferential edge of the positive electrode current collector 11″, and that an insulating layer 20 made of an electronically insulating material is disposed on the surface of the positive electrode current collector 11″ on the side of the solid electrolyte layer 17 where the positive electrode active material layer 15 is not disposed. This configuration has the advantage that, even when lithium metal deposited on the surface of the negative electrode current collector during charging is extruded from the outer circumferential edge of the solid electrolyte layer by the restraining pressure of the pressing member, contact between the positive electrode active material layer 15 and the negative electrode active material layer 13 is prevented, thereby suppressing the occurrence of a short circuit. In the embodiment shown in FIG. 7 , the insulating layer 20 is disposed so that the entire side surface of the positive electrode active material layer 15 is covered by the insulating layer 20. However, as in the embodiment shown in FIG. 8 , the insulating layer 20 may be disposed so that a part of the side surface of the positive electrode active material layer 15 is exposed.

[0059] The constituent material of the insulating layer described above is not particularly limited, and any material that satisfies the above conditions can be suitably used. Examples of constituent materials for the insulating layer include materials in which inorganic powders such as aluminum oxide, zirconium oxide, silicon oxide, and SB-Na-based glass frit are dispersed in a solid electrolyte that constitutes the solid electrolyte layer. However, the constituent material for the insulating layer is preferably a resin material or a rubber material. These materials are highly durable and elastic, so even if internal stress occurs in the region where the insulating layer is formed, the insulating layer can stretch without breaking, thereby effectively preventing the occurrence of a short circuit. Examples of such resin materials include polyolefin resins such as polyethylene (e.g., low-density polyethylene, high-density polyethylene, etc.), polypropylene, polyester resins such as polyethylene terephthalate (PET), thermoplastic resins such as polyvinyl chloride, acrylic resin, methacrylic resin, acrylonitrile-butadiene-styrene resin, vinyl acetate resin, ethylene-vinyl acetate resin, and styrene-butadiene resin; and thermosetting resins such as silicone resin, urethane resin, melamine resin, thermosetting acrylic resin, urea resin, phenolic resin, resorcinol resin, alkylresorcinol resin, epoxy resin, and thermosetting polyester. Examples of rubber materials include latex rubber, chloroprene rubber (CR), styrene-butadiene rubber (SBR), ethylene-propylene-diene rubber (EPDM), and acrylonitrile-butadiene rubber (NBR).

[0060] [Positive and negative current collector plates] The material constituting the current collector plates (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metallic materials such as aluminum, carbon-coated aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as the constituent material of the current collector plates. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The negative electrode current collector plate 25 and the positive electrode current collector plate 27 may be made of the same material or different materials.

[0061] [Positive and negative leads] Although not shown, the current collectors (11", 11') and the current collector plates (27, 25) may be electrically connected via positive and negative electrode leads. Materials used in known lithium secondary batteries may be used as the constituent materials of the positive and negative electrode leads. The parts removed from the exterior are preferably covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, resulting in electrical leakage and affecting products (e.g., automobile parts, particularly electronic devices).

[0062] [Battery exterior] As the battery exterior, a known metal can case can be used, or a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power-generating element as shown in FIG. 1 can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited to these. A laminate film is desirable from the viewpoint of achieving high output and excellent cooling performance, making it suitable for use in batteries for large devices such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable for the exterior because it allows for easy adjustment of the collective pressure applied to the power-generating element from the outside.

[0063] FIG. 9 is a perspective view showing the appearance of a stacked secondary battery according to one embodiment of the present invention. As shown in FIG. 9, a flat stacked secondary battery 50 has a rectangular, flat shape, and a positive electrode tab 58 and a negative electrode tab 59 for extracting power are pulled out from both sides thereof. A power generating element 57 is wrapped in a battery exterior body (laminate film 52) of the stacked secondary battery 50, and the periphery thereof is heat-sealed, and the power generating element 57 is sealed with the positive electrode tab 58 and the negative electrode tab 59 pulled out to the outside. Here, the power generating element 57 corresponds to the power generating element 21 of the stacked secondary battery 10a shown in FIG. 1 described above. The power generating element 57 is formed by stacking a plurality of unit cell layers (unit cells) 19, each of which is composed of a positive electrode (positive electrode current collector 11″ and positive electrode active material layer 15), a solid electrolyte layer 17, and a negative electrode (negative electrode current collector 11′).

[0064] The lithium secondary battery according to this embodiment is not limited to a flat shape. A wound-type lithium secondary battery may be cylindrical, or may be a cylindrical battery modified to have a flat rectangular shape, and is not particularly limited. The cylindrical battery may be casing a laminate film or a conventional cylindrical can (metal can), and is not particularly limited. Preferably, the power generating element is casing a laminate film. This configuration can achieve weight reduction. The removal of the tabs 58 and 59 shown in FIG. 9 is also not particularly limited. The positive electrode tab 58 and the negative electrode tab 59 may be pulled out from the same side, or the positive electrode tab 58 and the negative electrode tab 59 may be divided into multiple tabs and pulled out from each side, and are not limited to the configuration shown in FIG. 9. In addition, a wound-type lithium secondary battery may use, for example, a cylindrical can (metal can) instead of tabs to form terminals.

[0065] 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). There is no particular limitation on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferable that the amount is such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and leakage of the liquid electrolyte (electrolytic solution) does not occur. Note that, as the liquid electrolyte (electrolytic solution), a solution in the form of a conventionally known lithium salt dissolved in a conventionally known organic solvent is used. The liquid electrolyte (electrolytic solution) may further contain additives other than the organic solvent and the lithium salt. These additives may be used alone or in combination of two or more. Furthermore, when an additive is used in the electrolyte solution, the amount used can be appropriately adjusted.

[0066] [Battery pack] A battery pack is made up of multiple batteries connected together. Specifically, it is made up of at least two batteries connected in series, parallel, or both. By connecting them in series or parallel, it is possible to freely adjust the capacity and voltage.

[0067] A small, detachable battery pack can be formed by connecting multiple batteries in series or in parallel. Furthermore, a large-capacity, high-output battery pack suitable for vehicle drive power sources and auxiliary power sources, which require high volumetric energy density and high volumetric power density, can be formed by further connecting multiple batteries in series or in parallel. The number of batteries to be connected to form a battery pack and the number of stacked small batteries to form a large-capacity battery pack can be determined based on the battery capacity and output of the vehicle (electric vehicle) in which it will be installed.

[0068] [vehicle] The lithium secondary battery according to the present embodiment has a high energy density per volume. In vehicle applications such as electric vehicles, hybrid electric vehicles, fuel cell vehicles, and hybrid fuel cell vehicles, higher capacity and larger size are required compared to applications in electrical and portable electronic devices. Therefore, the lithium secondary battery according to the present embodiment can be suitably used as a power source for vehicles, for example, as a power source for driving a vehicle or an auxiliary power source.

[0069] Specifically, a battery or a battery pack formed by combining a plurality of such batteries can be mounted on a vehicle. Since the present invention can provide a high-capacity battery with excellent output characteristics, installing such a battery can enable the construction of plug-in hybrid electric vehicles with long EV driving distances and electric vehicles with long driving distances per charge. For example, a battery or a battery pack formed by combining a plurality of such batteries can be used in hybrid vehicles, fuel cell vehicles, and electric vehicles (all of which include four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, and buses, and light vehicles), as well as two-wheeled vehicles (motorcycles) and three-wheeled vehicles) to provide vehicles with long driving distances. However, the applications are not limited to automobiles, and the battery pack can also be used as a power source for other vehicles, such as trains, and can also be used as an on-board power source for uninterruptible power supplies and the like. [Example]

[0070] 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.

[0071] Example 1 [Preparation of evaluation cell] (Preparation of positive electrode) First, LiNi as the positive electrode active material 0.8 Mn 0.1 Co 0.1O2, acetylene black as a conductive additive, and a sulfide solid electrolyte (LPS (Li2S-P2S5)) were weighed out in a mass ratio of 50:30:20 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 an aluminum foil positive electrode current collector, dried, and pressed to form a positive electrode active material layer (50 μm thick). This produced a positive electrode.

[0072] (Preparation of solid electrolyte layer and protective layer) A solid electrolyte slurry was prepared by adding 2 parts by mass of styrene-butadiene rubber (SBR) to 100 parts by mass of 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 obtain a free-standing solid electrolyte layer (30 μm thick). The shear modulus of the resulting solid electrolyte layer was measured using the nanoinden- tation method described in the literature (Kunkun Fu et al., Vacuum, Volume 112, February 2015, Pages 29-32), and was found to be 8 [GPa].

[0073] A protective layer (20 nm thick) made of lithium bromide (LiBr) was then formed by sputtering over one main surface and the entire side surface of the resulting solid electrolyte layer. The shear modulus of the resulting protective layer was measured using the same method as above and found to be 8 GPa. In other words, in this example, the difference between the shear modulus of the protective layer and that of the solid electrolyte layer was 0 GPa. Furthermore, the arithmetic mean roughness (Ra) of the resulting protective layer was measured in accordance with JIS B 0601:2013 and found to be less than 5 nm.

[0074] (Preparation of evaluation cells) A solid electrolyte layer with the protective layer formed similarly to the 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. The pressure during the CIP treatment was controlled so that the outer peripheral edge of the solid electrolyte layer extended halfway along the side of the positive electrode active material layer. Finally, a stainless steel foil was laminated on the exposed surface of the protective layer as a negative electrode current collector to prepare an evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) with the configuration shown in Figure 2, except that there was no negative electrode active material layer.

[0075] [Evaluation of the evaluation cell (measurement of charge / discharge efficiency)] 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 test was carried out for 100 cycles under the following conditions: A pressure of 5 MPa was applied to the evaluation cell in the stacking direction using a pressure member.

[0076] (Charge / discharge test conditions) 1) Charge / discharge conditions [Voltage range] 3.0~4.3V [Charging process] CCCV [Discharge process]CC [Charge / discharge rate] 0.2C (After charging and discharging, rest for 30 minutes each time) 2) Evaluation temperature: 298K (25℃).

[0077] The evaluation cell was charged in a constant current / constant voltage (CCCV) mode at 0.2 C from 3.0 V to 4.3 V during the charging process (lithium metal deposition on the negative electrode current collector) in a thermostatic chamber set at the evaluation temperature using a charge / discharge tester. The cell was then discharged in constant current (CC) mode at 0.2 C from 4.3 V to 3.0 V during the discharging process (lithium metal dissolution on the negative electrode current collector). Note that 1 C refers to the current value at which the battery reaches a full charge (100% charge) after one hour of charging. During the charge / discharge process, the charge capacity (battery capacity at charge) and discharge capacity (battery capacity at discharge) of the evaluation cell were measured. The charge / discharge efficiency (Coulomb efficiency) was calculated as the ratio of the battery capacity at discharge to the battery capacity at charge at the 100th cycle. The charge / discharge efficiency at 100 cycles in this example was 99%.

[0078] <Example 2> An evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) for this example was fabricated using the same method as in Example 1 described above, except that the material for forming the protective layer was changed to lithium chloride (LiCl) and the thickness of the protective layer was changed to 5 nm. The shear modulus of the protective layer was measured using the same method as above and found to be 21 [GPa]. That is, in this example, the difference between the shear modulus of the protective layer and that of the solid electrolyte layer was 13 [GPa]. Furthermore, the arithmetic mean roughness (Ra) of the resulting protective layer was measured in accordance with JIS B 0601:2013 and found to be below the measurement limit. The charge-discharge efficiency (Coulomb efficiency) was calculated using the same method as above and found to be 99% after 100 cycles in this example.

[0079] Example 3 An evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) for this example was fabricated using the same method as in Example 1 described above, except that the material for forming the protective layer was changed to lithium fluoride (LiF). The shear modulus of the protective layer was measured using the same method as above and found to be 51 [GPa]. In other words, in this example, the difference between the shear modulus of the protective layer and that of the solid electrolyte layer was 43 [GPa]. Furthermore, the arithmetic mean roughness (Ra) of the resulting protective layer was measured in accordance with JIS B 0601:2013 and found to be less than 5 nm. The charge-discharge efficiency (Coulomb efficiency) was calculated using the same method as above and found to be 94% after 100 cycles in this example.

[0080] <Comparative Example 1> The protective layer is made of a lithium-containing oxide solid electrolyte, Li7La3Zr2O 12 An evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) for this comparative example was fabricated using the same method as in Example 1 described above, except that the ion-exchange membrane (ELT) was changed to (LLZO). The shear modulus of the protective layer was measured using the same method as above and found to be 64 [GPa]. That is, in this comparative example, the difference between the shear modulus of the protective layer and that of the solid electrolyte layer was 56 [GPa]. Furthermore, the arithmetic mean roughness (Ra) of the resulting protective layer was measured in accordance with JIS B 0601:2013 and found to be less than 5 nm. The charge-discharge efficiency (Coulomb efficiency) was calculated using the same method as above and found to be 76% after 100 cycles in this example.

[0081] <Comparative Example 2> An evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) for this comparative example was fabricated using the same method as in Example 1 described above, except that the material forming the protective layer was changed to lithium oxide (LiO). The shear modulus of the protective layer was measured using the same method as above and found to be 70 [GPa]. That is, in this comparative example, the difference between the shear modulus of the protective layer and that of the solid electrolyte layer was 62 [GPa]. Furthermore, the arithmetic mean roughness (Ra) of the resulting protective layer was measured in accordance with JIS B 0601:2013 and found to be less than 5 nm. The charge-discharge efficiency (Coulomb efficiency) was calculated using the same method as above and found to be 72% after 100 cycles in this example.

[0082] <Comparative Example 3> An evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) for this comparative example was fabricated in the same manner as in Example 1 described above, except that the material for the protective layer was changed to polyethylene glycol (PEG (polyethylene oxide; PEO); number-average molecular weight 200,000) and the thickness of the protective layer was changed to 500 nm. The shear modulus of the protective layer was measured using the same method as above and found to be 0.1 [GPa]. That is, in this comparative example, the difference between the shear modulus of the protective layer and that of the solid electrolyte layer was 50 [GPa] or less, but the shear modulus of the protective layer itself was less than 2 [GPa]. Furthermore, the arithmetic mean roughness (Ra) of the resulting protective layer was measured in accordance with JIS B 0601:2013 and found to be 100 nm. A charge-discharge test was then performed using the same method as above, but an internal short circuit occurred in the cell due to lithium metal dendrites penetrating the protective layer and the solid electrolyte layer, making it impossible to perform 100 charge-discharge cycles.

[0083] FIG. 10 shows a graph in which the charge / discharge efficiency after 100 cycles is plotted against the difference between the shear modulus of elasticity of the protective layer and the shear modulus of elasticity of the solid electrolyte layer. [Explanation of symbols]

[0084] 10a, 50, 100 stacked secondary battery, 11' negative electrode current collector, 11” positive electrode current collector, 13 negative electrode active material layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 18 protective layer, 19 cell layer, 20 insulating layer, 21, 57 Power generation elements, 25 negative electrode current collector plate (negative electrode tab), 27 Positive current collector plate (positive tab), 29, 52 Laminated film, 58 positive tab, 59 negative electrode tab, 200 metal plate, 300 volts, 400 Nuts.

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; and a pressure member that applies a predetermined pressure to the power generating element in the stacking direction, a protective layer having electronic insulation and lithium ion conductivity and being more stable than the solid electrolyte with respect to reductive decomposition due to contact with the lithium metal is provided on at least a portion of a main surface of the solid electrolyte layer facing the negative electrode current collector and on at least a portion of a side surface of the solid electrolyte layer; the protective layer has a shear modulus of 2 [GPa] or more, and the difference between the shear modulus of the protective layer and the shear modulus of the solid electrolyte layer is 50 [GPa] or less; a lithium secondary battery in which, when the power generating element is viewed from above, an outer peripheral edge of the negative electrode current collector is located inside an outer peripheral edge of the solid electrolyte layer and outside an outer peripheral edge of the positive electrode active material layer.

2. A positive electrode comprising a positive electrode active material layer disposed on the 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; and a pressure member that applies a predetermined pressure to the power generating element in the stacking direction, a protective layer having electronic insulation and lithium ion conductivity and being more stable than the solid electrolyte with respect to reductive decomposition due to contact with the lithium metal is provided on at least a portion of a main surface of the solid electrolyte layer facing the negative electrode current collector and on at least a portion of a side surface of the solid electrolyte layer; the protective layer has a shear modulus of 2 [GPa] or more, and the difference between the shear modulus of the protective layer and the shear modulus of the solid electrolyte layer is 50 [GPa] or less; when the power generating element is viewed from above, at least a part of the outer circumferential edge of the positive electrode active material layer is located inside the outer circumferential edge of the positive electrode current collector, a lithium secondary battery, wherein an insulating layer made of an electronically insulating material is disposed on a surface of the positive electrode current collector on the side of the solid electrolyte layer where the positive electrode active material layer is not disposed;

3. 3. The lithium secondary battery according to claim 1, wherein, in a plan view of the power generating element, at least a part of the outer circumferential edge of the positive electrode active material layer is located inside the outer circumferential edge of the solid electrolyte layer.

4. 4. The lithium secondary battery according to claim 1, wherein at least a portion of the outer periphery of the solid electrolyte layer extends to at least a portion of the side surface of the positive electrode active material layer.

5. 5. The lithium secondary battery according to claim 1, wherein the protective layer has an average thickness smaller than that of the solid electrolyte layer.

6. 6. The lithium secondary battery according to claim 1, wherein the protective layer has an average thickness of 5 nm to 10 μm.

7. 7. The lithium secondary battery according to claim 1, wherein the surface roughness (Ra) of the protective layer is 1 μm or less.

8. The protective layer may be formed of a lithium halide, a composite metal oxide represented by Li-MO (wherein M is one or more metal elements selected from the group consisting of Mg, Au, Al, Sn, and Zn), or Li-Ba-TiO ３ 8. The lithium secondary battery according to claim 1, which is composed of one or more materials selected from the group consisting of composite oxides.

9. 9. The lithium secondary battery according to claim 1, wherein the solid electrolyte contained in the solid electrolyte layer contains an S element.

10. The solid electrolyte contained in the solid electrolyte layer is Li ２ S-P ２ S ５ , Li ６ P.S. ５ X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S and Li ３ P.S. ４ The lithium secondary battery according to any one of claims 1 to 9, selected from the group consisting of:

11. The lithium secondary battery according to any one of claims 1 to 10, which is an all-solid-state lithium secondary battery.

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