Lithium secondary battery

Abstract

A purpose of the present invention is to provide a means for making an internal short circuit of a battery less likely to occur in a lithium deposition type lithium secondary battery.　The present invention provides a lithium secondary battery comprising an electric power generation element that includes: a positive electrode that has a positive electrode active material layer that contains a positive electrode active material; a negative electrode that has a negative electrode collector and on which lithium metal is deposited during charging; a solid electrolyte layer that is interposed between the positive electrode and the negative electrode, and contains a solid electrolyte; and a negative electrode intermediate layer that is interposed between the solid electrolyte layer and the negative electrode collector, and contains a lithium-reactive material that is selected from the group consisting of a carbon material that can occlude lithium during charging and a metal material that can be alloyed with lithium, wherein a lithium high-occlusion portion that can occlude more lithium than the central portion of the negative electrode intermediate layer is provided to at least a part of the outer peripheral edge portion of the negative electrode intermediate layer.

Description

Lithium-ion battery

[0001] This invention relates to a lithium secondary battery.

[0002] In recent years, research and development on lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials mainly composed of ion conductors capable of ion conduction in a solid state. Generally, using high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials can significantly improve the power density and energy density of the battery. As one type of lithium secondary battery using a solid electrolyte, for example, Japanese Patent Publication No. 2022-98487 proposes a technique in which a negative electrode active material layer (negative electrode intermediate layer) made of a metal-carbon composite is placed between the solid electrolyte layer and the negative electrode current collector. According to the above document, this configuration can prevent the aggregation phenomenon of metal particles and make the current distribution within the negative electrode layer uniform.

[0003] However, our investigations have revealed that in lithium secondary batteries described in the above-mentioned literature, it may not be possible to prevent internal short circuits. Therefore, the present invention aims to provide a means to make internal short circuits less likely to occur in lithium deposition type lithium secondary batteries.

[0004] The inventors of the present invention conducted diligent studies to solve the above problems. As a result, they found that the above problems can be solved by providing a lithium-high storage region at the outer edge of the negative electrode intermediate layer containing a lithium-reactive material, which can absorb more lithium than the central portion, in a lithium-deposit type lithium secondary battery, and thus completed the present invention.

[0005] That is, one embodiment of the present invention includes a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode current collector, and lithium metal being deposited during charging, a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a negative electrode intermediate layer interposed between the solid electrolyte layer and the negative electrode current collector and containing a lithium reactive material selected from the group consisting of a carbon material capable of occluding lithium and a metal material capable of alloying with lithium during charging. A power generation element is provided with a lithium high-occlusion portion capable of occluding more lithium than the central portion of the negative electrode intermediate layer at at least a part of the outer peripheral edge portion of the negative electrode intermediate layer. It is a lithium secondary battery.

[0006] FIG. 1 is a cross-sectional view schematically showing the overall structure of a laminated (internally parallel-connected type) lithium secondary battery (laminated secondary battery) according to an embodiment of the present invention. FIG. 2 is an enlarged cross-sectional view of a single battery layer constituting the laminated secondary battery of the embodiment shown in FIG. 1. FIG. 3 is an enlarged cross-sectional view of a single battery layer constituting the cells manufactured in Examples 1 to 3. FIG. 4 is an enlarged cross-sectional view of a single battery layer constituting the cells manufactured in Example 4, Example 5, and Comparative Example 1.

[0007] One embodiment of the present invention includes a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode current collector, and lithium metal being deposited during charging, a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a negative electrode intermediate layer interposed between the solid electrolyte layer and the negative electrode current collector and containing a lithium reactive material selected from the group consisting of a carbon material capable of occluding lithium and a metal material capable of alloying with lithium during charging. A power generation element is provided with a lithium high-occlusion portion capable of occluding more lithium than the central portion of the negative electrode intermediate layer at at least a part of the outer peripheral edge portion of the negative electrode intermediate layer. It is a lithium secondary battery. According to the lithium secondary battery of this embodiment, in a lithium precipitation type lithium secondary battery, it is possible to make it difficult for an internal short circuit of the battery to occur.

[0008] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted. Also, the dimensional ratios in the drawings are exaggerated for convenience of explanation and may be different from the actual ratios.

[0009] FIG. 1 is a cross-sectional view schematically showing the overall structure of a laminated (internally parallel-connected type) all-solid-state lithium secondary battery (hereinafter, also simply referred to as “laminated secondary battery”) according to an embodiment of the present invention. Note that FIG. 1 shows a cross-section of the laminated secondary battery during charging. The laminated secondary battery 10a shown in FIG. 1 has a structure in which a substantially rectangular power generation element 21 in which a charge-discharge reaction actually proceeds is sealed inside a laminate film 29 which is a battery exterior body. Here, the power generation element 21 has a structure in which a negative electrode, a solid electrolyte layer 17, and a positive electrode are laminated. The negative electrode has a structure in which a negative electrode current collector 11′ and a negative electrode active material layer 13 made of lithium metal deposited on the surface of the negative electrode current collector 11′ are laminated. And a negative electrode intermediate layer 14 is disposed so as to contact the negative electrode active material layer 13 and the solid electrolyte layer 17, respectively. In the present embodiment, as shown in FIG. 1 and FIGS. 2(a) to 2(c) described later, the negative electrode intermediate layer 14 is provided with a high lithium occlusion portion 14a over the entire outer peripheral edge thereof. This high lithium occlusion portion 14a is a portion that can occlude more lithium than the central portion 14b of the negative electrode intermediate layer 14.

[0010] The positive electrode has a structure in which a positive electrode active material layer 15 is arranged on the surface of the positive electrode current collector 11''. As a result, the negative electrode current collector 11'', the negative electrode active material layer 13, the negative electrode intermediate layer 14, the solid electrolyte layer 17, the positive electrode active material layer 15, and the positive electrode current collector 11'' constitute a single cell layer 19. Therefore, the stacked secondary battery 10a shown in Figure 1 can also be said to have a configuration in which multiple single cell layers 19 are stacked and electrically connected in parallel. The negative electrode current collector 11' and the positive electrode current collector 11'' are each fitted with a negative electrode current collector plate 25 and a positive electrode current collector plate 27, which are electrically connected to the respective electrodes (negative and positive electrodes), and are structured to be sandwiched between the edges of the laminate film 29 and led out to the outside of the laminate film 29. The stacked secondary battery 10a is subjected to restraining pressure in the stacking direction of the power generation elements 21 by a pressurizing member (not shown). As a result, the volume of the power generation elements 21 is kept constant.

[0011] Figure 2(a) is an enlarged cross-sectional view of a single cell layer constituting a stacked secondary battery according to one embodiment of the present invention shown in Figure 1. As shown in Figure 2(a), the single cell layer 19 constituting the stacked secondary battery according to this embodiment has a positive electrode composed of a positive electrode current collector 11" and a positive electrode active material layer 15 disposed on its surface. A solid electrolyte layer 17 containing a solid electrolyte is disposed on the side of the positive electrode active material layer 15 opposite to the positive electrode current collector 11". In the embodiment shown in Figure 2(a), the outer peripheral edge of the solid electrolyte layer 17 extends to at least a portion of the side surface of the positive electrode active material layer 15 along its entire circumference. As a result, the positive electrode active material layer 15 is configured to be slightly smaller than the solid electrolyte layer 17. With this configuration, 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 pressurizing 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. Furthermore, "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 is not facing the negative electrode current collector. Also, the outer peripheral edge of the solid electrolyte layer 17 does not necessarily have to extend to at least a portion of the side surface of the positive electrode active material layer 15.

[0012] Furthermore, in the embodiment shown in Figure 2(a), a negative electrode intermediate layer 14 is arranged on the side of the solid electrolyte layer 17 opposite to the positive electrode active material layer 15. In this negative electrode intermediate layer 14, a lithium high-storage portion 14a is provided around the entire circumference of its outer edge. Here, when the power generation element is viewed from above (i.e., when the single cell layer 19 is viewed from above), the inner edge of the lithium high-storage portion is located at or outside the position of the outer edge of the positive electrode active material layer 15, and the lithium high-storage portion 14a includes a region that is thicker than the central portion 14b. Also, the lithium high-storage portion 14a is arranged to cover at least a part of the side surface of the solid electrolyte layer 17. Note that in the embodiment shown in Figure 2(a), the central portion 14b is either the portion facing the positive electrode active material layer 15 or includes the portion facing the positive electrode active material layer 15, and the lithium high-storage portion 14a is the portion not facing the positive electrode active material layer 15.

[0013] Figure 2(b) is an enlarged cross-sectional view of a single cell layer 19 constituting a stacked secondary battery in another embodiment of the present invention. In the embodiment shown in Figure 2(b), the single cell layer 19 is arranged such that the lithium high storage portion 14a of the negative electrode intermediate layer 14 covers at least a portion of the side surface of the negative electrode active material layer 13. Otherwise, it is the same as the single cell layer 19 shown in Figure 2(a).

[0014] Figure 2(c) is an enlarged cross-sectional view of a single cell layer constituting a stacked secondary battery in another embodiment of the present invention. In the embodiment shown in Figure 2(c), the negative electrode intermediate layer 14 has a lithium high storage portion 14a around its entire outer edge, and the content ratio of lithium reactive material in this lithium high storage portion 14a is greater than the content ratio of lithium reactive material in the central portion 14b. Except as described above, it is the same as the single cell layer 19 shown in Figure 2(a).

[0015] The main components of the lithium secondary battery according to this embodiment will be described below.

[0016] [Current Collector] The current collector (negative electrode current collector, positive electrode current collector) has the function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There are no particular restrictions on the material that constitutes the current collector, but for example, metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, or conductive resins may be used. There are also no particular restrictions on the thickness of the current collector, but for example, it is 10 to 100 μm.

[0017] [Negative Electrode Active Material Layer] The lithium secondary battery according to this embodiment is a so-called lithium deposition type, in which lithium metal is deposited during the charging process. The layer made of lithium metal deposited during this charging process is the negative electrode active material layer of the lithium secondary battery according to this embodiment. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharging process progresses. The negative electrode active material layer does not need to be present during complete discharge, but in some cases, a negative electrode active material layer made of a certain amount of lithium metal may be present during complete discharge. Furthermore, the thickness of the negative electrode active material layer (lithium metal layer) during complete charge is not particularly limited, but it is usually 0.1 to 1000 μm.

[0018] [Negative Electrode Interlayer] The negative electrode intermediate layer is a layer interposed between the negative electrode current collector and the solid electrolyte layer, and contains a lithium-reactive material selected from the group consisting of carbon materials capable of adsorbing lithium during charging and metallic materials capable of alloying with lithium. By containing such a lithium-reactive material, the negative electrode intermediate layer exhibits the function of allowing lithium metal to be deposited more uniformly on the negative electrode (for example, on the negative electrode current collector).

[0019] The volume resistivity of the negative electrode intermediate layer is not particularly limited, but is preferably 10. ２ The resistivity is Ω·cm or less, and more preferably 10Ω·cm or less. In this specification, the volume resistivity of the negative electrode intermediate layer is the value measured using an electrode resistance measurement system (manufactured by HIOKI E.E. CORPORATION, product name: RM2610).

[0020] Lithium-reactive materials can be carbon materials capable of absorbing lithium during charging. Specific examples of such carbon materials include carbon black (specifically, acetylene black, Ketjenblack®, furnace black, channel black, thermal black, thermal lamp black, etc.), graphitized carbon black (derived from these), carbon nanotubes (CNTs), graphite, graphene, and hard carbon. Among these, the carbon material is preferably one or more selected from the group consisting of carbon black, graphite, and graphene, and more preferably carbon black, as this allows for even more pronounced effects of the present invention. The carbon black is more preferably at least one selected from the group consisting of acetylene black, Ketjenblack®, furnace black, channel black, thermal black, and thermal lamp black, and particularly preferably acetylene black.

[0021] When the carbon material is in the form of particles, its average particle diameter (average primary particle diameter) is, for example, 10 nm to 300 nm, for example, 10 nm to 200 nm, preferably 15 nm to 150 nm, more preferably 20 nm to 100 nm, and even more preferably 20 nm to 70 μm. In this specification, the average particle diameter of the carbon material is the 50% cumulative diameter (D) of the particle diameter of the particles observed in several to tens of fields of view when a cross-section of the layer containing the particles is observed with a scanning electron microscope (SEM) (the maximum distance between any two points on the contour line of the observed particles). ５０ ) refers to.

[0022] Furthermore, the lithium-reactive material may include, in addition to or instead of, the carbon material described above, a metallic material that can alloy with lithium during charging. The metallic material that can alloy with lithium is preferably one or more selected from the group consisting of Ag, Al, Mg, Si, Zn, In, and Sn, and more preferably Ag, as this further enhances the effects of the present invention. Note that the lithium high-storage portion and the central portion may each contain only either the carbon material or the metallic material. However, it is preferable that the lithium high-storage portion and the central portion each contain both the carbon material and the metallic material. This ensures sufficient mechanical strength and lithium ion conductivity of the negative electrode intermediate layer. It also further suppresses the occurrence of short circuits.

[0023] When the above metal material is in the form of particles, its average particle diameter (average primary particle diameter) is preferably 500 nm or less, more preferably 300 nm or less, even more preferably 200 nm or less, and particularly preferably 100 nm or less. The lower limit of the average particle diameter of the metal particles is not particularly limited, but is preferably 20 nm or more. In this specification, the average particle diameter of the metal particles is the 50% cumulative diameter (D) of the particle diameter of the particles observed in several to tens of fields of view when a cross-section of the layer containing the particles is observed with a scanning electron microscope (SEM) (the maximum distance between any two points on the contour line of the observed particles). ５０ ) refers to.

[0024] The negative electrode intermediate layer may consist solely of lithium-reactive material if a self-supporting film can be fabricated using only lithium-reactive material, but may also contain a binder as needed. Here, only the lithium-high storage portion and the central portion may contain a binder, but it is preferable that both contain a binder. The type of binder is not particularly limited, and any known in the art can be used as appropriate. Examples include fluororesins such as polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements) and polytetrafluoroethylene (PTFE), as well as styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC). Among these, the binder of the negative electrode intermediate layer preferably contains a fluororesin, more preferably contains PVDF, and even more preferably consists solely of PVDF.

[0025] When the negative electrode intermediate layer contains a binder, the binder content is preferably more than 10 parts by mass, and more preferably 15 parts by mass or more, relative to 100 parts by mass of the total of the carbon material and the metallic material that can be alloyed with lithium. When the binder content is within the above range, the strength of the negative electrode intermediate layer is sufficiently ensured, and cracks can be suppressed. There is no particular upper limit to the binder content, but from the viewpoint of suppressing an increase in resistance, it is preferably 35 parts by mass or less, relative to 100 parts by mass of the total of the carbon material and the metallic material.

[0026] The ratio of the total mass of the carbon material, the metal material, and the binder to the total mass of the negative electrode intermediate layer is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 98% by mass or more, particularly preferably 99% by mass or more, and most preferably 100% by mass.

[0027] As described above, the lithium secondary battery according to this embodiment is provided with a lithium high-storage portion in at least a part of the outer edge of the negative electrode intermediate layer, which is capable of absorbing more lithium than the central part of the negative electrode intermediate layer. The lithium high-storage portion is a part of the negative electrode intermediate layer that can absorb more lithium per unit area than the central part, and it is sufficient that it is provided in at least a part of the outer edge of the negative electrode intermediate layer.

[0028] In lithium-deposited lithium secondary batteries, lithium ions are supplied from the positive electrode side via a solid electrolyte layer during the charging process, and lithium metal is deposited between the negative electrode current collector and the negative electrode intermediate layer. It is known that the presence of a negative electrode intermediate layer can reduce cell short circuits during charging and discharging. However, our studies have revealed that even with a negative electrode intermediate layer, lithium metal can still be deposited on the outer edge of the negative electrode intermediate layer, which can cause cell short circuits. Normally, lithium secondary batteries are subjected to restraining pressure in the stacking direction of the power generation elements by a pressurizing member. While sufficient restraining pressure is applied to the center of the negative electrode intermediate layer, it may not be reliably applied to the outer edge. In this case, the lithium metal deposited on the outer edge does not return to the positive electrode during discharge, and the deposited lithium metal grows to form lithium dendrites. It is believed that this deposited / grown lithium metal comes into contact with the positive electrode active material layer or the positive electrode current collector, leading to a cell short circuit.

[0029] In contrast, in this embodiment of lithium secondary battery, a high lithium storage region is provided on at least a portion of the outer edge of the negative electrode intermediate layer, which is capable of absorbing more lithium than the central part of the negative electrode intermediate layer. Since the high lithium storage region can absorb more lithium during charging, the growth of lithium dendrites on the outer edge of the negative electrode intermediate layer is suppressed. This is thought to reduce cell short circuits.

[0030] In addition, if a lithium-high storage area is not provided at the outer edge of the negative electrode intermediate layer, the outer edge of the negative electrode intermediate layer will not be sufficiently pressurized. As a result, the deposited lithium metal will not return to the positive electrode during discharge and will not contribute to the subsequent charge-discharge reaction, which may lead to a decrease in the battery's charge-discharge capacity. In contrast, in this embodiment of lithium secondary battery, the deposition and growth of lithium metal at the outer edge of the negative electrode intermediate layer are suppressed by providing a lithium-high storage area. Therefore, it is believed that the battery's charge-discharge capacity will be improved.

[0031] Here, in a lithium deposition type lithium secondary battery, before the first charge or in a completely discharged state, the lithium-reactive material contained in the negative electrode intermediate layer does not contain lithium (for example, lithium is not adsorbed into a carbon material capable of adsorbing lithium, and / or lithium is not alloyed into a metallic material capable of alloying with lithium). When a lithium secondary battery is charged, lithium ions move from the positive electrode to the negative electrode, and at this time, lithium is first stored in the negative electrode intermediate layer. The storage of lithium in the negative electrode intermediate layer includes the following forms: (i) If the lithium-reactive material contained in the negative electrode intermediate layer includes a carbon material, lithium is adsorbed into the carbon material. (ii) If the lithium-reactive material contained in the negative electrode intermediate layer includes a metallic material capable of alloying with lithium, the metallic material and lithium are alloyed. (iii) Lithium metal is deposited in the voids of the negative electrode intermediate layer. Then, after lithium equivalent to the capacity of the lithium-reactive material and voids in the negative electrode intermediate layer is stored, lithium metal (negative electrode active material layer) is deposited between the negative electrode intermediate layer and the negative electrode current collector.

[0032] Therefore, the amount of lithium that can be absorbed per unit area of ​​the negative electrode intermediate layer corresponds to the amount of lithium that can be stored according to (i) to (iii) above, and can be estimated from the sum of the capacity of the lithium-reactive material per unit area of ​​the negative electrode intermediate layer and the capacity based on the voids per unit area of ​​the negative electrode intermediate layer (the capacity of lithium when lithium is filled into the voids of the negative electrode intermediate layer).

[0033] <Capacity of lithium-reactive material per unit area of ​​the negative electrode intermediate layer> In this specification, the capacity C of the lithium-reactive material per unit area of ​​the negative electrode intermediate layer ｘ [mAh / cm ２shall adopt the value calculated by multiplying the capacity per unit mass of the lithium reactive material contained per unit area of the negative electrode intermediate layer by the mass of the lithium reactive material contained per unit area of the negative electrode intermediate layer. The capacity per unit mass of each material of the lithium reactive material is determined by the following method. Weigh 0.1 g of sample A, which is the lithium reactive material to be measured, put it into an SLD sleeve (Φ10), sandwich both ends with SLD pins plated with hard Cr, and press for 1 minute at a pressure of 390 MPa at room temperature (25°C) to produce a pellet made of sample A. Also, weigh 0.1 g of Li ６ PS ５ Cl, and produce a solid electrolyte pellet by the same method as above. Stack a SUS foil as a current collector, a pellet made of sample A, a solid electrolyte pellet, a lithium metal as a counter electrode, and a SUS foil as a current collector in sequence to produce a half cell for capacity measurement. Apply a restraint pressure of 3 MPa using a pressure member in the stacking direction of the half cell for capacity measurement, and charge at a constant current of 1.5 [mA / cm ２ at a temperature of 60°C to move lithium ions from the lithium metal to the pellet made of sample A. Measure the behavior of the cell voltage at this time, and determine the current capacity [mAh] of the lithium reactive material from the behavior. The cut-off voltage varies depending on the type of lithium reactive material, but the part where the cell voltage rapidly decreases is taken as the cut-off voltage. The value obtained by dividing the product of the time T (h) from the start of charging to the cut-off and the constant charging current of 1.5 [mA / cm ２ by the mass (0.1 g) of sample A used in the measurement is the capacity per unit mass [mA / (g·cm ２ )] of sample A. Then, the product of the mass M (g) of the lithium reactive material contained per unit area of the negative electrode intermediate layer and the capacity per unit mass [mA / (g·cm ２ )] of each material obtained above is C ｘ [mAh / cm ２ . When two or more kinds of lithium reactive materials are contained in the negative electrode intermediate layer, for each material, the capacity per unit mass is determined by the above method, and the product with the mass of each material contained per unit area of the negative electrode intermediate layer is calculated. Then, the sum of the products calculated for all materials is used to obtain the capacity Cｘ [mAh / cm ２ ] can be calculated.

[0034] <Capacity based on voids per unit area of ​​the negative electrode intermediate layer> In this specification, the capacity C based on voids per unit area of ​​the negative electrode intermediate layer ｙ [mAh / cm ２ The value used for [V] is calculated using the following method. First, the volume of the voids (pores) in the negative electrode intermediate layer (total pore volume) is measured using the mercury intrusion method with a mercury porosimeter. Specifically, mercury is injected under pressure into the voids of the negative electrode intermediate layer to be measured, and the volume of the voids V (total pore volume) [cm] is calculated from the pressure applied and the volume of mercury injected. ３ The volume of the voids in the negative electrode intermediate layer (total pore volume) V [cm³] was calculated. A Micromeritics Autopore IV 9510 was used as the measuring device, with a pore diameter range of 0.003 to 500 μm, a mercury contact angle of 130°, and a mercury surface tension of 485 dynes / cm³. ３ ] The theoretical capacity of lithium per unit volume (2.062 × 10⁻¹⁰ －３ [mAh / cm ３ Multiply by ]) and determine the unit area of ​​the negative electrode intermediate layer [cm² ２ The value obtained by dividing by ] is the capacity C ｙ [mAh / cm ２ ]

[0035] The lithium high-storage portion may be provided on at least a part of the outer edge of the negative electrode intermediate layer, but it is preferable that it be provided around the entire circumference of the outer edge of the negative electrode intermediate layer. This further enhances the effect of making it difficult for the battery to short-circuit. Furthermore, when the power generation element is viewed in plan, it is preferable that at least a part of the inner edge of the lithium high-storage portion is located at or outside the outer edge of the positive electrode active material layer, and it is more preferable that the inner edge of the lithium high-storage portion is located at or outside the outer edge of the positive electrode active material layer along its entire circumference. In other words, it is preferable that the lithium high-storage portion is provided on a part that is not facing the positive electrode active material layer. This further enhances the effect of making it difficult for the battery to short-circuit.

[0036] The specific form of the negative electrode intermediate layer equipped with a lithium high storage portion and the lithium secondary battery having said negative electrode intermediate layer are not particularly limited. In one embodiment, as shown in Figures 2(a) and 2(b), when the power generation element is viewed from above, at least a portion of the inner circumferential edge of the lithium high storage portion is positioned at or outside the outer circumferential edge of the positive electrode active material layer and includes a region that is thicker than the central portion. This allows more lithium to be absorbed in the lithium high storage portion. As a result, the deposition and growth of lithium metal at the outer circumferential edge can be suppressed, and cell short circuits can be suppressed. Furthermore, the battery capacity can be further improved.

[0037] In the embodiments shown in Figures 2(a) and 2(b), the thickness of the lithium high-absorption portion 14a is not particularly limited as long as it is greater than the thickness of the central portion 14b. The thickness of the central portion 14b is not particularly limited, but is preferably 2 to 10 μm, and more preferably 3 to 8 μm. If the thickness of the central portion 14b is 2 μm or more, the strength of the negative electrode intermediate layer is excellent. If it is 10 μm or less, the energy density of the battery is excellent. Furthermore, although the thickness of the central portion 14b is not particularly limited, it is preferably smaller than the thickness of the solid electrolyte layer. This can make the effects of the present invention even more pronounced.

[0038] Furthermore, the thickness of the lithium high-absorption portion 14a is not particularly limited, but is, for example, 4 μm or more, preferably more than 4 μm, more preferably 8 μm or more, even more preferably 10 μm or more, and particularly preferably 12 μm or more. The upper limit of the thickness of the lithium high-absorption portion 14a is also not particularly limited, but is, for example, 15 μm or less. That is, the thickness of the lithium high-absorption portion 14a is preferably 4 to 15 μm. Furthermore, the difference between the thickness of the lithium high-absorption portion 14a and the thickness of the central portion 14b is, for example, 0.5 μm or more, preferably 1 μm or more, and more preferably 2 μm or more. The difference in thickness is, for example, 5 μm or less, and preferably 4 μm or less.

[0039] In the embodiments shown in Figures 2(a) and 2(b), the porosity in the lithium high-storage portion of the negative electrode intermediate layer is larger than the porosity in the central portion, and it is preferable that the difference between the porosity in the lithium high-storage portion and the porosity in the central portion (the difference between the porosity values) is within 20%. As a result, sufficient lithium is absorbed into the voids of the lithium high-storage portion, and the effects of the present invention can be obtained even more significantly. The value of the porosity in the lithium high-storage portion of the negative electrode intermediate layer is not particularly limited, but is, for example, 50% or less, and preferably 45% or less. Also, the porosity of the lithium high-storage portion is, for example, 15% or more, preferably more than 15%, and more preferably 20% or more. That is, the porosity of the lithium high-storage portion 14a is preferably 15 to 50%. Also, the value of the porosity in the central portion is not particularly limited, but is, for example, 40% or less, and preferably 30% or less. If the value of the porosity in the central portion is within these ranges, the effect of making it difficult for the battery to short-circuit will be even greater. Furthermore, the void ratio of the central portion is, for example, 5% or more, and more preferably 10% or more. That is, the void ratio of the central portion 14b is preferably 5 to 40%.

[0040] Furthermore, in the embodiments shown in Figures 2(a) and 2(b), it is preferable that, when the power generation element 21 is viewed from above (i.e., when the single cell layer 19 is viewed from above), each layer is arranged such that the inner circumferential end of the lithium high storage portion 14a is located at or outside the position of the outer circumferential end of the positive electrode active material layer 15 along its entire circumference. This allows lithium to be more easily absorbed into the lithium high storage portion 14a located at the outer circumferential edge of the negative electrode intermediate layer 14, and the occurrence of short circuits at the ends is more effectively prevented.

[0041] In the negative electrode intermediate layer 14, there are no particular limitations on the means for forming a lithium high-absorption portion 14a that is thicker than the central portion 14b in the portion not facing the positive electrode active material layer 15, and various methods can be employed. As an example, in the manufacture of a lithium secondary battery, a two-stage pressing process can be employed, in which the solid electrolyte layer is pressed, and then the solid electrolyte layer and the negative electrode intermediate layer are laminated and pressed. More specifically, a solid electrolyte slurry containing a solid electrolyte is coated onto the surface of a support, and the coating film is dried to obtain a solid electrolyte layer formed on the surface of the support. Subsequently, the exposed surface of the positive electrode active material layer, which has been separately manufactured on a positive electrode current collector, is placed on the exposed surface of the solid electrolyte layer, and the solid electrolyte layer and the positive electrode active material layer are pressed together (first pressing step). At this time, by making the size of the solid electrolyte layer sufficiently larger than the size of the positive electrode active material layer, the outer edge of the solid electrolyte layer can be made to extend to at least a part of the side surface of the positive electrode active material layer (so that the positive electrode active material layer is enveloped by the solid electrolyte layer).

[0042] On the other hand, a negative electrode intermediate layer slurry containing the materials included in the negative electrode intermediate layer (such as lithium-reactive material and binder) is coated onto the surface of the negative electrode current collector and dried to obtain a coating film for the negative electrode intermediate layer formed on the surface of the negative electrode current collector. Then, the support is peeled off to expose the solid electrolyte layer, and the exposed surface of the solid electrolyte layer and the exposed surface of the coating film for the negative electrode intermediate layer are stacked and pressed together (second pressing step). At this time, by making the size of the negative electrode intermediate layer and the negative electrode current collector sufficiently larger than the size of the positive electrode active material layer and the solid electrolyte layer, the pressing pressure applied to the non-facing portion of the negative electrode intermediate layer against the positive electrode active material layer becomes relatively lower than the pressing pressure applied to the central portion (the portion facing the positive electrode active material layer), and the thickness of the non-facing portion becomes relatively larger. And, because the size of the solid electrolyte layer is relatively small, the non-facing portion is positioned to cover at least a part of the side surface of the solid electrolyte layer. At this time, for example, the thickness and porosity of the central portion and the lithium high storage portion of the negative electrode intermediate layer can be adjusted by adjusting the pressing conditions.

[0043] Furthermore, by adjusting the size of the negative electrode current collector to be equal to or less than the size of the positive electrode active material layer and performing the second pressing step, a lithium secondary battery can be obtained having a negative electrode intermediate layer having a lithium high-storage portion that includes a region thicker than the central portion in the non-facing portion of the positive electrode active material layer and is arranged to cover at least a part of the side surface of the negative electrode active material layer, as shown in Figure 2(b).

[0044] The pressing pressure in the first pressing step varies depending on the materials contained in the solid electrolyte layer and the positive electrode active material layer, and can be appropriately set by those skilled in the art. For example, the pressing pressure in the first pressing step is preferably 300 MPa to 1000 MPa, more preferably 300 MPa to 800 MPa, and even more preferably 500 MPa to 700 MPa. The pressing temperature in the first pressing step is not particularly limited, but for example, it is 20 to 80°C, and preferably 20 to 40°C. The pressing time in the first pressing step is not particularly limited, but for example, it is 10 seconds to 30 minutes, and preferably 10 seconds to 10 minutes.

[0045] The press pressure in the second pressing step varies depending on the material contained in the negative electrode intermediate layer and can be appropriately set by those skilled in the art. For example, the press pressure in the second pressing step is, for example, 50 MPa to 700 MPa, preferably 100 MPa to 700 MPa. The press temperature in the second pressing step is also not particularly limited, but for example, 20 to 80°C, preferably 20 to 40°C. The press time in the second pressing step is also not particularly limited, but for example, 10 seconds to 30 minutes, preferably 10 seconds to 10 minutes.

[0046] Furthermore, in another embodiment of the present invention, as shown in Figure 2(c), the negative electrode intermediate layer 14 has a higher proportion (by mass) of lithium-reactive material in the lithium-high storage portion 14a than in the central portion 14b. This allows for the storage of more lithium in the lithium-high storage portion 14a. As a result, the deposition and growth of lithium metal at the outer edge can be suppressed, and cell short circuits can be suppressed. In addition, the battery capacity can be further improved.

[0047] In the embodiment shown in Figure 2(c), the content of the lithium-reactive material in the lithium-high storage portion is preferably more than 80% by mass, more preferably 85% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more, based on 100% by mass of the total amount of material constituting the lithium-high storage portion. The upper limit of the content of the lithium-reactive material in the lithium-high storage portion is not particularly limited, but is for example 100% by mass or less, preferably 99% by mass or less, and more preferably 98% by mass or less. On the other hand, the content of the lithium-reactive material in the central portion is preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less, based on 100% by mass of the total amount of material constituting the central portion. The lower limit of the content of the lithium-reactive material in the central portion is not particularly limited, but is for example 65% by mass or more. In the lithium-high storage portion and the central portion, if the lithium-reactive material includes both a carbon material capable of absorbing lithium during charging and a metal material capable of alloying with lithium during charging, the mixing ratio (mass ratio) of these materials is not particularly limited. However, in the central portion, the carbon material:metal material (mass ratio) is preferably 10:1 to 1:1, and more preferably 5:1 to 2:1. Within this range, battery performance is superior. In the lithium-high storage portion, the carbon material:metal material (mass ratio) is preferably 1:1 to 1:5, and more preferably 1:2 to 1:3. Within this range, the effects of the present invention can be obtained even more significantly. In this case, the mass of the carbon material is the total mass of multiple types of carbon materials. Furthermore, if multiple types of metals capable of alloying with lithium are included, the mass is the total amount of the multiple types of metals capable of alloying with lithium.

[0048] Furthermore, in the embodiment shown in Figure 2(c), when the power generation element 21 is viewed from above (i.e., when the single cell layer 19 is viewed from above), it is preferable that each layer is arranged such that at least a portion of the inner circumference of the lithium high storage portion 14a is located at or outside the outer circumference of the positive electrode active material layer 15. This allows lithium to be more easily absorbed into the lithium high storage portion 14a located at the outer circumference of the negative electrode intermediate layer 14, thus making the effects of the present invention even more pronounced. Moreover, it is preferable that the inner circumference of the lithium high storage portion 14a is located at or outside the outer circumference of the positive electrode active material layer 15 along its entire circumference, as this is even more effective.

[0049] In the embodiment shown in Figure 2(c), the thickness of the lithium high-absorption portion 14a and the central portion 14b of the negative electrode intermediate layer is not particularly limited. However, from the same viewpoint as the embodiments shown in Figures 2(a) and (b), the thickness of the central portion 14b is preferably 2 to 10 μm, and more preferably 3 to 8 μm. Furthermore, it is preferable that the thickness of the central portion 14b is smaller than the thickness of the solid electrolyte layer.

[0050] The method for manufacturing the negative electrode intermediate layer in the lithium secondary battery of the embodiment shown in Figure 2(c) is not particularly limited. For example, the negative electrode intermediate layer constituting the lithium secondary battery according to this embodiment can be manufactured by separately manufacturing a component that constitutes the central part of the negative electrode intermediate layer (for example, a square or rectangular shape) and a component that constitutes the lithium high-absorption part of the negative electrode intermediate layer (for example, a frame shape), and combining these. In this case, each part can be manufactured by preparing a slurry according to the composition of each part, and then coating and drying the slurry. Alternatively, the negative electrode intermediate layer constituting the lithium secondary battery according to this embodiment can also be manufactured by sequentially coating and drying the same slurry on the surface of a substrate (for example, a negative electrode current collector).

[0051] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as the main component). The solid electrolyte contained in the solid electrolyte layer is not particularly limited, and any known in the art can be used as appropriate. One example is LPS (Li ２ S-P２ S ５ ), Li ６ PS ５ X (where X is Cl, Br, or I), Li ７ P ３ S １１ Li ３．２ P ０．９６ S and Li ３ PS ４ Examples of sulfide solid electrolytes include the following. These sulfide solid electrolytes are preferred because they have excellent lithium-ion conductivity and a low bulk modulus, allowing them to follow the volume changes of the electrode active material during charging and discharging. These solid electrolytes may be used individually or in combination of two or more. Of course, other solid electrolytes may also be used.

[0052] The ionic conductivity of a solid electrolyte at room temperature (25°C) (for example, the Li ionic conductivity) is, for example, 1 × 10⁻⁶. －５ It is preferable that the S / cm is greater than or equal to 1 × 10 －４ A value of S / cm or higher is more preferable. The ionic conductivity of the solid electrolyte can be measured by the AC impedance method.

[0053] Examples of solid electrolyte shapes include spherical, ellipsoidal, or other particulate forms, as well as thin films. When the solid electrolyte is particulate, its average particle size (D50) is not particularly limited, but is preferably 0.01 μm or more and 40 μm or less, more preferably 0.1 μm or more and 20 μm or less, and even more preferably 0.5 μm or more and 10 μm or less.

[0054] The solid electrolyte content in the solid electrolyte layer is preferably 50% by mass or more and 100% by mass or less, and more preferably 90% by mass or more and 99% by mass or less. In addition to the solid electrolyte, the solid electrolyte layer may further contain a binder. The binder that can be used in the solid electrolyte layer is the same as that described above for the negative electrode intermediate layer. The thickness of the solid electrolyte layer is, for example, 0.1 μm or more and 1000 μm or less, and preferably 10 μm or more and 40 μm or less.

[0055] [Positive Electrode Active Material Layer] The positive electrode active material layer essentially contains positive electrode active material and may optionally contain a solid electrolyte, binder, and / or conductive additive. Typically, the positive electrode active material layer is placed on the surface of the positive electrode current collector as shown in Figure 1, but if the positive electrode active material layer 15 itself has a certain degree of conductivity, it is also possible for the positive electrode active material layer itself to constitute the positive electrode without using a positive electrode current collector.

[0056] The type of positive electrode active material included in the positive electrode active material layer is not particularly limited, but lithium-containing metal oxides are preferred. A specific example of a lithium-containing metal oxide is LiCoO ２ LiMnO ２ LiNiO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt type active materials such as LiMn ２ O ４ LiNi ０．５ Mn １．５ O ４ spinel-type active materials such as LiFePO ４ LiMnPO ４ Olivine-type active materials such as Li ２ FeSiO ４ Li ２ MnSiO ４ Examples of Si-containing active materials include the above. Other oxide active materials include, for example, Li ４ Ti ５ O １２ LiVO ２ These include Li(Ni-Mn-Co)O ２ Furthermore, those in which some of these transition metals are substituted with other elements (NMC composite oxides) are preferably used as positive electrode active materials. These positive electrode active materials may be used individually or in combination of two or more types.

[0057] The shape of the positive electrode active material can be, for example, particulate (spherical, fibrous), thin film, etc. If the positive electrode active material is in particulate form, its average particle diameter (D ５０The average particle diameter (D) of the positive electrode active material is 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 diameter (D) of the positive electrode active material is used. ５０ The value of ) can be measured by laser diffraction scattering.

[0058] The content of the positive electrode active material is not particularly limited, but from the viewpoint of energy density, it is preferably, for example, 50% to 99% by mass, 70% to 99% by mass, more preferably 80% to 99% by mass, even more preferably 83% to 97% by mass, and particularly preferably 83% to 95% by mass, relative to the total mass of the positive electrode active material layer.

[0059] The positive electrode active material layer may further contain a solid electrolyte, a binder, and / or a conductive additive in addition to the positive electrode active material. Here, the solid electrolyte that can be used in the positive electrode active material layer is the same as that described for the solid electrolyte layer. The binder that can be used in the positive electrode active material layer is the same as that described for the negative electrode intermediate layer. Examples of conductive additives include, but are not limited to, carbon fibers (vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.), carbon nanotubes (CNTs), and carbon black (acetylene black, Ketjenblack®, furnace black, channel black, thermal lamp black, etc.).

[0060] The thickness of the positive electrode active material layer is, for example, 0.1 μm or more and 1000 μm or less, preferably 30 μm or more and 300 μm or less, more preferably 50 μm or more and 200 μm or less, and even more preferably 70 μm or more and 150 μm or less.

[0061] The lithium secondary battery according to this embodiment has a configuration in which multiple single cell layers are connected in parallel, resulting in high capacity and excellent cycle durability. Therefore, the lithium secondary battery according to this embodiment is suitable for use as a power source for EVs and HEVs.

[0062] Although one embodiment of the lithium secondary battery of the present invention has been described above, the present invention is not limited to the configuration described in the above-mentioned embodiment, and can be modified as appropriate based on the description of the claims.

[0063] For example, one type of battery to which the lithium secondary battery according to the present invention is applied is a bipolar battery that includes a bipolar electrode having a positive electrode active material layer electrically coupled to one side of a current collector and a negative electrode active material layer electrically coupled to the opposite side of the current collector.

[0064] Furthermore, the lithium secondary battery according to this embodiment does not have to be all-solid type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolyte). There are no particular restrictions on the amount of liquid electrolyte (electrolyte) 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 (electrolyte) does not occur.

[0065] The following embodiments are also included in the scope of the present invention: a lithium secondary battery according to claim 1 having the features of claim 2; a lithium secondary battery according to claim 2 having the features of claim 3; a lithium secondary battery according to claim 1 having the features of claim 4; a lithium secondary battery according to claim 4 having the features of claim 5; a lithium secondary battery according to any one of claims 1 to 5 having the features of claim 6; a lithium secondary battery according to any one of claims 1 to 6 having the features of claim 7; a lithium secondary battery according to any one of claims 1 to 7 having the features of claim 8; a lithium secondary battery according to any one of claims 1 to 8 having the features of claim 9.

[0066] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following, the operations were carried out in a glove box with a dew point of -68°C or lower. Furthermore, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0067] <Examples of evaluation cell fabrication> [Example 1] (Fabrication of positive electrode) NMC composite oxide (LiNi) as positive electrode active material ０．８ Mn ０．１ Co ０．１ O ２ ), carbon fiber as a conductive additive, and argyrodite-type sulfide solid electrolyte (Li ６ PS ５ Cl) was weighed in a mass ratio of 85:15:5. These were mixed using an agate mortar and then further stirred and mixed using a planetary ball mill. 2 parts by mass of polytetrafluoroethylene (PTFE) as a binder were added to 100 parts by mass of the resulting mixed powder and mixed. The resulting mixture was placed on top of aluminum foil as a positive electrode current collector and subjected to a pressing process to obtain a positive electrode having a positive electrode active material layer (20 mm x 20 mm square, 100 μm thick) on the surface of the positive electrode current collector.

[0068] (Preparation of the solid electrolyte layer) Algyrodite-type sulfide solid electrolyte (Li ６ PS ５ Cl, average particle diameter (D ５０ A solid electrolyte slurry was prepared by adding 2 parts by mass of styrene-butadiene rubber (SBR) as a binder to 100 parts by mass of (0.8 μm) material, and then adding and mixing with mesitylene as a solvent. A solid electrolyte layer (25 mm x 25 mm square, 40 μm thick) was obtained by coating the surface of a polyethylene terephthalate (PET) sheet, which served as a support, with the solid electrolyte slurry and drying it. In this case, the solid electrolyte layer was made to be larger than the positive electrode active material layer.

[0069] (Preparation of the negative electrode intermediate layer) Silver nanoparticles (average primary particle size: 60 nm), a metallic material that can alloy with lithium during charging, and acetylene black (average primary particle size: 200 nm), a carbon material that can absorb lithium during charging, were weighed (Ag:C = 25:50 (mass ratio)) and mixed. To 80 parts by mass of the resulting mixture, 20 parts by mass of polyvinylidene fluoride (PVDF) as a binder were added, and N-methyl-2-pyrrolidone (NMP) was added as a solvent and mixed to prepare a negative electrode intermediate layer slurry. Subsequently, the negative electrode intermediate layer slurry obtained above was coated in a square shape onto the surface of a stainless steel foil (28 mm x 28 mm square shape) used as a negative electrode current collector, and dried to produce a negative electrode intermediate layer (28 mm x 28 mm square shape). At this time, the negative electrode current collector and the negative electrode intermediate layer were made to be larger than the solid electrolyte layer and the positive electrode active material layer.

[0070] (Preparation of evaluation cell) A positive electrode active material layer formed on the surface of an aluminum foil (positive electrode current collector) and a solid electrolyte layer formed on the surface of a PET sheet were placed on top of each other so that the exposed surfaces of the positive electrode active material layer and the exposed surfaces of the solid electrolyte layer faced each other, and pressed at 700 MPa for 1 minute using a cold isostatic press (CIP). This transferred the solid electrolyte layer to the exposed surface of the positive electrode active material layer, and at the same time, the outer edge of the solid electrolyte layer extended all the way around to the entire side surface of the positive electrode active material layer. After peeling off the PET sheet adjacent to the solid electrolyte layer, the solid electrolyte layer and a negative electrode intermediate layer formed on the surface of a stainless steel foil (negative electrode current collector) were placed on top of each other so that the exposed surfaces of the solid electrolyte layer and the exposed surfaces of the negative electrode intermediate layer faced each other, and pressed at 500 MPa for 1 minute using a cold isostatic press (CIP). This transferred the negative electrode intermediate layer to the exposed surface of the solid electrolyte layer. Furthermore, the outer edge of the negative electrode intermediate layer covered a portion of the side surface of the solid electrolyte layer around its entire circumference, resulting in a region of the negative electrode intermediate layer that was thicker than the portion facing the positive electrode active material layer (a high lithium storage region). Finally, an aluminum positive electrode tab and a nickel negative electrode tab were joined to the aluminum foil (positive electrode current collector) and stainless steel foil (negative electrode current collector) respectively using an ultrasonic welding machine, and the resulting laminate was placed inside an aluminum laminate film and vacuum sealed to create an evaluation cell for this embodiment, which is a lithium deposition type lithium secondary battery.

[0071] In Example 2 (fabrication of the solid electrolyte layer), the evaluation cell for this example was fabricated using the same method as in Example 1 described above, except that the size of the solid electrolyte layer was the same as the size of the positive electrode active material layer (20 mm x 20 mm square). In the evaluation cell for this example, the outer edge of the solid electrolyte layer does not extend to the side surface of the positive electrode active material layer. Furthermore, the outer edge of the negative electrode intermediate layer covers a portion of the side surface of the solid electrolyte layer around its entire circumference, and a region (high lithium storage region) thicker than the thickness of the portion facing the positive electrode active material layer was obtained in the entire portion of the negative electrode intermediate layer that does not face the positive electrode active material layer.

[0072] In Example 3 (fabrication of the negative electrode intermediate layer), the evaluation cell for this example was fabricated using the same method as in Example 2 described above, except that the negative electrode current collector was made to be the same size as the positive electrode active material layer (20 mm x 20 mm square). In the evaluation cell for this example, the outer edge of the solid electrolyte layer does not extend to the side surface of the positive electrode active material layer. Furthermore, the outer edge of the negative electrode intermediate layer covers the entire side surface of the negative electrode current collector in the pre-charge state, and a region (high lithium storage region) thicker than the thickness of the portion facing the positive electrode active material layer was obtained in the entire portion of the negative electrode intermediate layer that does not face the positive electrode active material layer.

[0073] [Example 4] (Fabrication of the positive electrode) NMC composite oxide (LiNi) as the positive electrode active material ０．８ Mn ０．１ Co ０．１ O ２ ), carbon fiber as a conductive additive, and argyrodite-type sulfide solid electrolyte (Li ６ PS ５ Cl) was weighed in a mass ratio of 85:15:5. These were mixed using an agate mortar and then further stirred and mixed using a planetary ball mill. 2 parts by mass of PTFE as a binder were added to 100 parts by mass of the resulting mixed powder and mixed. The resulting mixture was placed on top of aluminum foil as a positive electrode current collector and subjected to a pressing process to obtain a positive electrode having a positive electrode active material layer (18 mm x 18 mm square, 100 μm thick) on the surface of the positive electrode current collector.

[0074] (Preparation of the solid electrolyte layer) Algyrodite-type sulfide solid electrolyte (Li ６ PS ５ Cl, average particle diameter (D ５０ A solid electrolyte slurry was prepared by adding 2 parts by mass of SBR as a binder to 100 parts by mass of (0.8 μm) material, and then adding and mixing with mesitylene as a solvent. A solid electrolyte layer (23 mm x 23 mm square, 40 μm thick) was obtained by coating the surface of a PET sheet, which served as a support, with the solid electrolyte slurry and drying it. In this case, the solid electrolyte layer was made to be larger than the positive electrode active material layer.

[0075] (Preparation of the negative electrode intermediate layer) Silver nanoparticles (average primary particle size: 60 nm), a metallic material that can alloy with lithium during charging, and acetylene black (average primary particle size: 200 nm), a carbon material that can absorb lithium during charging, were weighed (Ag:C = 60:30 (mass ratio)) and mixed. To 95 parts by mass of the resulting mixture, 5 parts by mass of PVDF as a binder and NMP as a solvent were added and mixed to prepare the negative electrode intermediate layer slurry (1) (for forming a high lithium absorption region). On the other hand, silver nanoparticles (average primary particle size: 60 nm) and acetylene black (average primary particle size: 200 nm) were weighed (Ag:C = 25:50 (mass ratio)) and mixed. To 80 parts by mass of the resulting mixture, 20 parts by mass of PVDF as a binder and NMP as a solvent were added and mixed to prepare the negative electrode intermediate layer slurry (2) (for forming the central region).

[0076] Subsequently, the negative electrode intermediate layer slurry (2) obtained above was coated in a square shape onto the surface of the stainless steel foil used as the negative electrode current collector and dried to create the central part of the negative electrode intermediate layer (a 20 mm x 20 mm square). Furthermore, the negative electrode intermediate layer slurry (1) obtained above was coated in a 1.5 mm width around the outer circumference of the central part and dried to create the lithium high-storage portion of the negative electrode intermediate layer (a frame-shaped portion with outer dimensions of 23 mm x 23 mm and inner dimensions of 20 mm x 20 mm). At this time, the size of the negative electrode intermediate layer was adjusted so that, when the power generation element is viewed in plan, the inner edge of the lithium high-storage portion is located outside the outer edge of the positive electrode active material layer along its entire circumference.

[0077] (Preparation of evaluation cell) A positive electrode active material layer formed on the surface of an aluminum foil (positive electrode current collector) and a solid electrolyte layer formed on the surface of a PET sheet were placed on top of each other so that the exposed surfaces of the positive electrode active material layer and the exposed surfaces of the solid electrolyte layer faced each other, and pressed at 700 MPa for 1 minute using a cold isostatic press (CIP). This transferred the solid electrolyte layer to the exposed surface of the positive electrode active material layer, and at the same time, the outer edge of the solid electrolyte layer extended all the way around to the entire side surface of the positive electrode active material layer. After peeling off the PET sheet adjacent to the solid electrolyte layer, the solid electrolyte layer and a negative electrode intermediate layer formed on the surface of a stainless steel foil (negative electrode current collector) were placed on top of each other so that the exposed surfaces of the solid electrolyte layer and the exposed surfaces of the negative electrode intermediate layer faced each other, and pressed at 500 MPa for 1 minute using a cold isostatic press (CIP). This transferred the negative electrode intermediate layer to the exposed surface of the solid electrolyte layer. Finally, an aluminum positive electrode tab and a nickel negative electrode tab were joined to the aluminum foil (positive electrode current collector) and stainless steel foil (negative electrode current collector) respectively using an ultrasonic welding machine. The resulting laminate was then placed inside an aluminum laminate film and vacuum-sealed to create an evaluation cell for this embodiment, which is a lithium deposition type lithium secondary battery.

[0078] [Example 5] An evaluation cell for this example was manufactured in the same manner as in Example 4, except that the lithium high storage portion of the negative electrode intermediate layer was made into a frame shape with outer dimensions of 26 mm x 26 mm and inner dimensions of 20 mm x 20 mm.

[0079] [Comparative Example 1] An evaluation cell for this comparative example was prepared using the same method as in Example 1 described above, except that the size of the negative electrode current collector and the negative electrode intermediate layer were made equivalent to the size of the solid electrolyte layer in the preparation of the negative electrode intermediate layer in Example 1.

[0080] Figure 3 shows an enlarged cross-sectional view of the single cell layers constituting the cells fabricated in Examples 1 to 3, and Figure 4 shows an enlarged cross-sectional view of the single cell layers constituting the cells fabricated in Example 4, Example 5, and Comparative Example 1.

[0081] <Measurement of Thickness and Porosity of the Negative Electrode Interlayer> For each of the evaluation cells prepared in the above-described examples and comparative examples, the thickness of the lithium high-storage portion and the central portion constituting the negative electrode interlayer was measured. Here, the thickness of the negative electrode interlayer was calculated from the negative electrode interlayer in the discharged state after the cell constraints were released and the outer casing was removed after the lithium secondary battery was completely discharged. Specifically, the power generation element was removed from the evaluation cell after complete discharge, and a cross section perpendicular to the plane direction (stacked cross section) was exposed by ion milling. The above cross section was then observed by SEM, and the thickness was measured at several to several dozen different locations in both the lithium high-storage portion and the central portion of the negative electrode interlayer, and the arithmetic mean of these measurements was taken as the thickness of the negative electrode interlayer. The results are shown in Table 1 below. Note that the term "SE layer" in Table 1 refers to the solid electrolyte layer.

[0082] Regarding the porosity, the volume of voids in the high lithium storage region and the central region of the negative electrode intermediate layer was measured using the mercury intrusion method described above in <Capacity based on voids per unit area of ​​the negative electrode intermediate layer>, and the porosity was determined. The porosity values ​​for the high lithium storage region and the central region are shown in Table 1 below.

[0083] <Charge / Discharge Test> The evaluation cells prepared above (before initial charge) were charged and discharged according to the following charge / discharge test conditions. During this test, a constraining pressure of 3 MPa was applied to the stacking direction of the evaluation cells using a pressurizing member, and the following charge / discharge test was performed: (Charge / Discharge Test Conditions) Evaluation temperature: 333K (60℃) Voltage range: 2.5 to 4.25V Charging process: CCCV (cutoff at 0.1mA) Charging rate: 0.1C Discharging process: CC Discharge rate: 0.1C After charging and discharging, the cells were left to rest for 30 minutes.

[0084] The evaluation cells were tested using a charge / discharge tester in a constant temperature chamber set to the evaluation temperature described above. During the charging process (deposition of lithium metal on the negative electrode current collector), a constant current / constant voltage (CCCV) mode was used. The cell was charged to 4.25V with a constant current of 0.1C, and then charged at a constant voltage until the current dropped to 0.1mA. Subsequently, during the discharging process (dissolution of lithium metal on the negative electrode current collector), a constant current (CC) mode was used, and the cell was discharged to 2.5V at 0.1C. Here, 1C refers to the current value at which charging for one hour results in the battery being fully charged (100% charged). The discharge capacity of the first cycle was defined as the discharge capacity of the evaluation cell. Furthermore, when the evaluation cell was charged with a current of 2C, if the voltage was unstable during charging or did not reach the upper voltage limit, it was determined that the cell had short-circuited. The results are shown in Table 1 below.

[0085]

[0086] The results shown in Table 1 indicate that lithium-deposited lithium secondary batteries, such as the evaluation cells in Examples 1 to 5, which have a high lithium storage area at the outer edge of the negative electrode intermediate layer, are less prone to internal short circuits. Furthermore, it was found that they can achieve high discharge capacity.

[0087] 10a Stacked secondary battery, 11' Negative electrode current collector, 11'' Positive electrode current collector, 13 Negative electrode active material layer, 14 Negative electrode intermediate layer, 14a High lithium storage section, 14b Central section, 15 Positive electrode active material layer, 17 Solid electrolyte layer, 19 Single cell layer, 21 Power generation element, 25 Negative electrode current collector plate, 27 Positive electrode current collector plate, 29 Laminate film.

Claims

1. A lithium secondary battery comprising a power generation element having: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a negative electrode intermediate layer interposed between the solid electrolyte layer and the negative electrode current collector and containing a lithium-reactive material selected from the group consisting of a carbon material capable of absorbing lithium during charging and a metallic material capable of alloying with lithium, wherein at least a portion of the outer peripheral edge of the negative electrode intermediate layer is provided with a lithium high-absorption portion capable of absorbing more lithium than the central portion of the negative electrode intermediate layer.

2. The lithium secondary battery according to claim 1, wherein, when the power generation element is viewed from above, at least a portion of the inner circumferential end of the lithium high storage portion is located at or outside the position of the outer circumferential end of the positive electrode active material layer and includes a region that is thicker than the central portion.

3. The lithium secondary battery according to claim 2, wherein the porosity in the lithium high storage portion is greater than the porosity in the central portion, and the difference between the porosity in the lithium high storage portion and the porosity in the central portion is within 20%.

4. The lithium secondary battery according to claim 1, wherein the content ratio of the lithium-reactive material in the lithium high storage portion is greater than the content ratio of the lithium-reactive material in the central portion.

5. The lithium secondary battery according to claim 4, wherein, when the power generation element is viewed in plan, at least a portion of the inner circumferential end of the lithium high storage portion is located at or outside the position of the outer circumferential end of the positive electrode active material layer.

6. The lithium secondary battery according to any one of claims 1 to 5, wherein the lithium-reactive material includes one or more carbon materials selected from the group consisting of carbon black, graphite, and graphene.

7. The lithium-reactive material comprises one or more metals selected from the group consisting of Ag, Al, Mg, Si, Zn, In, and Sn as the metallic material, the lithium secondary battery according to any one of claims 1 to 5.

8. The lithium secondary battery according to any one of claims 1 to 5, wherein the thickness of the central portion is less than the thickness of the solid electrolyte layer.

9. The lithium secondary battery according to any one of claims 1 to 5, wherein the thickness of the central portion is 2 to 10 μm.

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