Lithium secondary battery

Abstract

The objective of the present invention is to provide a means whereby cycle characteristics can be further improved in a lithium secondary battery of the lithium precipitation type provided with a negative electrode interlayer. This objective is achieved by means of a lithium secondary battery comprising a power generation element including: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector, in which lithium metal precipitates on the negative electrode current collector during charging; a solid electrolyte layer which is interposed between the positive electrode and the negative electrode and which contains a solid electrolyte; and a negative electrode interlayer which is interposed between the negative electrode current collector and the solid electrolyte layer and which includes a lithium-reactive material. Provided that A [mAh / cm2] denotes the irreversible capacity, which is the difference between the initial charge capacity and initial discharge capacity per unit area of the positive electrode active material layer, and B [mAh / cm2] denotes the charging capacity, which is the sum of a capacity b1 of the lithium-reactive material per unit area of the negative electrode interlayer and a capacity b2 based on voids, the relation A ≥ B holds.

Description

Lithium secondary battery

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

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

[0003] One type of all-solid-state lithium secondary battery is known as a lithium deposition type, in which lithium metal is deposited on a negative electrode current collector during charging. While lithium deposition type all-solid-state lithium secondary batteries have excellent energy density and output characteristics, they suffer from the problem of susceptibility to short circuits due to dendrites from the lithium metal layer. As a means of suppressing dendrite growth, a technique has been proposed in which a negative electrode active material layer (negative electrode intermediate layer) that forms an alloy or compound with lithium is provided between the solid electrolyte layer and the negative electrode current collector.

[0004] For example, Japanese Patent Laid-Open Publication No. 2020-167146 discloses a technology relating to an all-solid-state secondary battery having, in this order, a positive electrode active material layer, a solid electrolyte layer, and the above-mentioned negative electrode active material layer (negative electrode intermediate layer). According to this document, an all-solid-state secondary battery having excellent battery characteristics (particularly, cycle characteristics and discharge rate characteristics) is provided by including 33 mass% or more of amorphous carbon having at least one of a predetermined nitrogen adsorption specific surface area and a predetermined DBP oil absorption in the negative electrode active material layer (negative electrode intermediate layer), and controlling the initial charge capacity of the positive electrode active material layer to be 0.01<b / a<0.5, where a (mAh) is the initial charge capacity of the positive electrode active material layer and b (mAh) is the initial charge capacity of the negative electrode active material layer (negative electrode intermediate layer).

[0005] However, the inventors have conducted research and found that even with the techniques described in the above documents, sufficient cycle characteristics may not be obtained in some cases.

[0006] Therefore, an object of the present invention is to provide a means for further improving the cycle characteristics of a lithium deposition type lithium secondary battery having a negative electrode intermediate layer.

[0007] One aspect of the present invention relates to a lithium secondary battery including a power generating element including: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector 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 negative electrode current collector and the solid electrolyte layer and containing a lithium-reactive material. Here, the irreversible capacity, which is the difference between the initial charge capacity and the initial discharge capacity per unit area of ​​the positive electrode active material layer, is defined as A [mAh / cm ２ ], the charge capacity which is the sum of the capacity b1 of the lithium reactive material per unit area of ​​the negative electrode intermediate layer and the capacity b2 based on the voids is B [mAh / cm ２ ], the characteristic is that A≧B is satisfied.

[0008] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (stacked-type secondary battery) according to one embodiment of the present invention.

[0009] One aspect of the present invention is a lithium secondary battery comprising a power generating element including: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode current collector, 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 negative electrode current collector and the solid electrolyte layer and containing a lithium-reactive material, wherein the irreversible capacity, which is the difference between the initial charge capacity and the initial discharge capacity per unit area of ​​the positive electrode active material layer, is expressed as A [mAh / cm ２ ], the charge capacity which is the sum of the capacity b1 of the lithium reactive material per unit area of ​​the negative electrode intermediate layer and the capacity b2 based on the voids is B [mAh / cm ２ ], the lithium secondary battery satisfies A ≥ B. According to the lithium secondary battery of this embodiment, the cycle characteristics can be further improved in a lithium deposition type lithium secondary battery having a negative electrode intermediate layer.

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicate explanations will be omitted. Furthermore, the dimensional proportions in the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0011] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. FIG. 1 shows a cross section of the stacked-type secondary battery during charging. The stacked-type secondary battery 10a shown in FIG. 1 has a structure in which a substantially rectangular power-generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior. The power-generating element 21 has a structure in which a negative electrode, a solid electrolyte layer 17, and a positive electrode are stacked. The negative electrode has a structure in which a negative electrode current collector 11′ and a negative electrode active material layer 13 made of lithium metal deposited on the surface of the negative electrode current collector 11′ are stacked. A negative electrode intermediate layer 14 is disposed so as to contact the negative electrode active material layer 13 and the solid electrolyte layer 17, respectively. The positive electrode has a structure in which a positive electrode active material layer 15 is disposed on the surface of a positive electrode current collector 11". 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 one unit cell layer 19. Therefore, it can be said that the stacked secondary battery 10a shown in FIG. 1 has a structure in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel. A negative electrode current collector 25 and a positive electrode current collector 27 that are electrically connected to the respective electrodes (negative 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 extend to the outside of the laminate film 29. A restraining pressure is applied to the stacked secondary battery 10a in the stacking direction of the power generating element 21 by a pressure member (not shown). Therefore, the volume of the power generating element 21 is kept constant.

[0012] The main components of the all-solid-state lithium secondary battery according to this embodiment will be described below.

[0013] [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 limitations on the material constituting the current collector. For example, metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins, can be used as the material constituting the current collector. There are also no particular limitations on the thickness of the current collector, but an example is 10 to 100 μm.

[0014] [Negative Electrode Active Material Layer] The all-solid-state lithium secondary battery according to this embodiment is a so-called lithium deposition type battery 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 all-solid-state 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. Although the negative electrode active material layer does not need to be present during full discharge, in some cases, a negative electrode active material layer composed of a certain amount of lithium metal may be present during full discharge. Furthermore, the thickness of the negative electrode active material layer (lithium metal layer) during full charge is not particularly limited, but is typically 0.1 to 1000 μm.

[0015] [Negative Electrode Intermediate Layer] 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, such as a material capable of absorbing lithium ions during charging or a metal capable of alloying with lithium during charging.

[0016] The material capable of absorbing lithium ions is not particularly limited, but a carbon material is preferred. Specific examples of the carbon material include carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.), carbon nanotubes (CNT), graphite, hard carbon, etc. Among these, carbon black is preferred, and at least one selected from the group consisting of acetylene black, Ketjen Black (registered trademark), furnace black, channel black, and thermal lamp black is more preferred.

[0017] Examples of metals that can be alloyed with lithium include In, Al, Si, Sn, Mg, Au, Ag, and Zn. Among these, In, Si, Sn, and Ag are preferred, and Ag is more preferred.

[0018] When the carbon material is in a particulate form, its average particle diameter (average primary particle diameter) is, for example, 10 nm to 200 nm, preferably 15 nm to 150 nm, and more preferably 20 nm to 100 nm. When the metal is in a particulate form, its average particle diameter is, for example, 10 nm to 500 nm, preferably 20 nm to 300 nm, more preferably 30 nm to 200 nm, and even more preferably 40 nm to 100 nm. In this specification, the average particle diameter of the carbon material and metal particles refers to the 50% cumulative diameter (D) of the particle diameters of the particles (the maximum distance between any two points on the outline of the observed particles) observed in several to several tens of fields of view when the cross section of a layer containing the particles is observed with a scanning electron microscope (SEM). ５０ )

[0019] The lithium-reactive material may be used alone or in combination with two or more. A preferred embodiment of using two or more materials in combination is a combination of a material capable of absorbing lithium ions and a metal capable of alloying with lithium. That is, according to a preferred embodiment of the present invention, the lithium-reactive material includes at least one material selected from the group consisting of materials capable of absorbing lithium ions during charging and at least one material selected from the group consisting of metal materials capable of alloying with lithium during charging. This ensures sufficient strength and lithium ion conductivity of the negative electrode intermediate layer. More specifically, it is preferable to use nanoparticles composed of In, Si, Sn, or Ag in combination with carbon black (particularly, acetylene black), and it is more preferable to use nanoparticles composed of Ag in combination with carbon black (particularly, acetylene black). When a material capable of absorbing lithium ions and a metal capable of alloying with lithium are used in combination, the compounding ratio (mass ratio) of these is not particularly limited, but the mass ratio of material capable of absorbing lithium ions to metal capable of alloying with lithium is preferably 10:1 to 1:1, and more preferably 5:1 to 2:1.

[0020] The content of the lithium-reactive material in the negative electrode intermediate layer (when two or more materials are used in combination, this refers to the total content of those materials; the same applies hereinafter) is not particularly limited, but from the viewpoint of further suppressing the precipitation and growth of dendrites, it is preferably 70% by mass or more and 100% by mass or less, more preferably 85% by mass or more and 99% by mass or less, and even more preferably 90% by mass or more and 95% by mass or less, relative to the total mass of the negative electrode intermediate layer.

[0021] The negative electrode intermediate layer may be composed solely of a lithium reactive material as long as a freestanding film can be produced using only the lithium reactive material, but may also contain a binder as necessary. The type of binder is not particularly limited, and binders known in the art can be appropriately used. Examples include polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC).

[0022] The content of the binder in the negative electrode intermediate layer is not particularly limited, but is preferably 1% by mass or more and 15% by mass or less, and more preferably 5% by mass or more and 10% by mass or less, relative to the total mass of the negative electrode intermediate layer. When the binder content is 1% by mass or more, a negative electrode intermediate layer having sufficient strength can be formed. When the binder content is 15% by mass or less, a negative electrode intermediate layer having sufficient lithium ion conductivity can be formed.

[0023] The thickness of the negative electrode intermediate layer is not particularly limited, but is preferably 1 to 30 μm, more preferably 1 to 20 μm, even more preferably 1 to 10 μm, and particularly preferably 1 to 5 μm. When the thickness of the negative electrode intermediate layer is 1 μm or more, the function of the negative electrode intermediate layer can be fully exhibited. When the thickness of the negative electrode intermediate layer is 30 μm or less, a decrease in energy density can be suppressed.

[0024] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as a main component). The solid electrolyte contained in the solid electrolyte layer is not particularly limited, and any solid electrolyte known in the art can be appropriately used. As an example, LPS (Li ２ S-P ２ S ５ ), Li ６ P.S. ５ X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S and Li ３ P.S. ４ Examples of sulfide solid electrolytes include sulfide solid electrolytes such as those listed above. These sulfide solid electrolytes are preferably used because they have excellent lithium ion conductivity and a low bulk modulus, allowing them to follow the volume changes of the electrode active material that accompany charge and discharge. These solid electrolytes may be used alone or in combination of two or more. Of course, solid electrolytes other than those listed above may also be used.

[0025] The content of the solid electrolyte 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.

[0026] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. The binder that can be used in the solid electrolyte layer is the same as that described above for the negative electrode intermediate layer.

[0027] The thickness of the solid electrolyte layer varies depending on the configuration of the intended all-solid-state lithium secondary battery, but is usually 0.1 μm or more and 1000 μm or less, and preferably 10 μm or more and 40 μm or less.

[0028] [Positive Electrode Active Material Layer] The positive electrode active material layer essentially contains a positive electrode active material, and may contain a solid electrolyte, a binder, and / or a conductive additive as necessary. The positive electrode active material layer is typically disposed on the surface of a positive electrode current collector as shown in Figure 1. However, if the positive electrode active material layer 15 itself has a certain degree of conductivity, the positive electrode active material layer itself can constitute the positive electrode without using a positive electrode current collector.

[0029] The type of positive electrode active material contained in the positive electrode active material layer is not particularly limited, but a lithium-containing metal oxide is preferred. That is, according to a preferred embodiment of the present invention, the positive electrode active material contains at least one selected from lithium-containing metal oxides. According to a more preferred embodiment of the present invention, the positive electrode active material is composed of only at least one selected from lithium-containing metal oxides. A specific example of the lithium-containing metal oxide is LiCoO ２ , LiMnO ２ , LiNiO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt active materials such as LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ Spinel-type active materials such as LiFePO ４ , LiMnPO ４ Olivine type active materials such as Li ２ FeSiO ４ , Li ２ MnSiO４ Examples of oxide active materials other than those mentioned above include Si-containing active materials such as Li ４ Ti ５ O １２ , LiVO ２ Among them, Li(Ni-Mn-Co)O ２ and those in which a part of these transition metals is substituted with other elements (NMC composite oxides) are preferably used as the positive electrode active material. These positive electrode active materials may be used alone or in combination of two or more.

[0030] 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 diameter (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. ５０ The value of can be measured by a laser diffraction scattering method.

[0031] The content of the positive electrode active material is not particularly limited, but from the viewpoint of energy density, it is, for example, 50% by mass or more and 99% by mass or less, preferably 70% by mass or more and 99% by mass or less, more preferably 80% by mass or more and 99% by mass or less, even more preferably 85% by mass or more and 97% by mass or less, and particularly preferably 87% by mass or more and 95% by mass or less, relative to the total mass of the positive electrode active material layer.

[0032] 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 above. 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 above. Examples of conductive additives include metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys or metal oxides containing these metals, carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.), carbon nanotubes (CNT), and carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.), but are not limited thereto. Furthermore, a conductive additive can also be used in which particulate ceramic materials or resin materials are coated with the above-mentioned metal materials by plating or the like.

[0033] The thickness of the positive electrode active material layer varies depending on the configuration of the intended all-solid-state lithium secondary battery, but 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.

[0034] In the all-solid-state lithium secondary battery according to this embodiment, the irreversible capacity, which is the difference between the initial charge capacity and the initial discharge capacity per unit area of ​​the positive electrode active material layer, is defined as A [mAh / cm ２ ], the charge capacity which is the sum of the capacity b1 of the lithium reactive material per unit area of ​​the negative electrode intermediate layer and the capacity b2 based on the voids is B [mAh / cm ２], the formula 1: A ≥ B is satisfied. According to the inventors' investigations, it has been found that in a lithium deposition-type lithium secondary battery having a negative electrode intermediate layer, such a configuration significantly improves cycle characteristics compared to conventional techniques. The mechanism by which the all-solid-state lithium secondary battery according to this embodiment achieves the above-described effects is not fully understood, and the present invention is not bound by any theory, but the following mechanism is presumed. That is, in a lithium deposition-type lithium secondary battery having a negative electrode intermediate layer, lithium is first stored in the negative electrode intermediate layer during charging. The storage of lithium in the negative electrode intermediate layer includes the following forms: (i) When the lithium-reactive material contained in the negative electrode intermediate layer includes a material capable of absorbing lithium ions during charging, such as a carbon material, lithium is absorbed into the material; (ii) When the lithium-reactive material contained in the negative electrode intermediate layer includes a metal capable of alloying with lithium during charging, the metal and lithium are alloyed; (iii) Lithium metal is precipitated in the voids of the negative electrode intermediate layer. After the lithium reactive material and voids in the negative electrode intermediate layer store lithium equivalent to their capacity, lithium metal (negative electrode active material layer) is deposited between the negative electrode intermediate layer and the negative electrode current collector. During subsequent discharge, lithium is first eluted from the lithium metal (negative electrode active material layer). If the lithium metal (negative electrode active material layer) disappears and the positive electrode active material is still capable of storing lithium at that point, the lithium stored in the lithium reactive material and voids in the negative electrode intermediate layer is desorbed and eluted. If the lithium capacity storable in the negative electrode intermediate layer is greater than the irreversible capacity of the positive electrode active material, repeated charge and discharge cycles result in repeated storage (absorption, alloying, and precipitation) of lithium in the negative electrode intermediate layer, followed by desorption and elution, resulting in repeated structural changes in the negative electrode intermediate layer. This leads to accelerated deterioration of the negative electrode intermediate layer and reduced cycle durability. On the other hand, in a lithium deposition-type lithium secondary battery provided with a negative electrode intermediate layer, when the above formula 1: A≧B is satisfied, the lithium stored in the negative electrode intermediate layer during the initial charge is entirely supplied by the irreversible capacity A of the positive electrode active material layer.Even after subsequent discharge, the lithium stored in the negative electrode intermediate layer does not return to the positive electrode active material layer, and remains stored in the negative electrode intermediate layer, preventing structural changes in the negative electrode intermediate layer. This reduces deterioration of the negative electrode intermediate layer due to repeated charge and discharge, improving cycle durability.

[0035] In this specification, "the irreversible capacity, which is the difference between the initial charge capacity and the initial discharge capacity per unit area of ​​the positive electrode active material layer, is defined as A [mAh / cm ２ ]" is a value measured by the following method. First, a measurement cell is fabricated by sequentially stacking a SUS foil as a current collector, a positive electrode active material layer to be measured, a solid electrolyte layer (having the same configuration as the solid electrolyte layer in the Examples described later), an indium-lithium alloy as a target electrode, and a SUS foil as a current collector. While applying a confining pressure of 3 MPa using a pressure member in the stacking direction of the measurement cell, a pressure of 0.07 [mA / cm ２ ] for 25 hours. ２ ], and charging is terminated when the cell voltage reaches 3.7 V. The capacity from the start of charging to the end of charging is defined as the "initial charge capacity." ２ The discharge was stopped when the cell voltage reached 1.9 V. The capacity from the start of discharge to the end of discharge was defined as the "initial discharge capacity." The initial charge capacity [mAh / cm ２ ], the initial discharge capacity value [mAh / cm ２ ] is subtracted from the irreversible capacity A [mAh / cm ２ The value of the irreversible capacity A can be controlled by adjusting the type and amount of the positive electrode active material contained in the positive electrode active material layer.

[0036] In this specification, "capacity b1 [mAh / cm 2 ] of the lithium reactive material per unit area of ​​the negative electrode intermediate layer" means ２]" is a 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 is determined by the following method. 0.1 g of sample A, which is the lithium reactive material to be measured, is weighed out, placed in an SLD sleeve (Φ10), and clamped between hard Cr-plated SLD pins at both ends, and pressed at room temperature (25°C) at a pressure of 390 MPa for 1 minute to prepare pellet A made of sample A. In addition, Li as a solid electrolyte ６ P.S. ５ 0.1 g of Cl was weighed out, and a solid electrolyte pellet was prepared in the same manner as above. A measurement half-cell was prepared by sequentially stacking SUS foil as a current collector, pellet A, a solid electrolyte pellet, lithium metal as a counter electrode, and SUS foil as a current collector. A pressure of 3 MPa was applied to the measurement half-cell in the stacking direction using a pressure member, while a current of 1.5 [mA / cm] was applied at a temperature of 60°C. ２ ], lithium ions are transferred from the lithium metal to pellet A. The behavior of the cell voltage at this time is measured, and the current capacity [mAh] of the lithium reactive material is determined from this behavior. The cutoff voltage differs depending on the type of lithium reactive material, but the point at which the cell voltage drops sharply is taken as the cutoff voltage. The time T (h) from the start of charging to cutoff and the constant charging current of 1.5 [mA / cm ２ ] divided by the mass (0.1 g) of sample A used in the measurement is the capacity per unit mass of sample A [mA / (g cm ２ ) )]. 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 of each material calculated above [mA / (g cm ２ ) )] is the capacity b1 [mAh / cm ２ When two or more types of lithium reactive materials are contained in the negative electrode intermediate layer, the capacity per unit mass is determined for each material by the above method, and the product of this capacity and the mass of each material contained per unit area of ​​the negative electrode intermediate layer is calculated. The products calculated for all materials are then summed to obtain the capacity b1 [mAh / cm ２ ] can be obtained.

[0037] In this specification, "the capacity b2 [mAh / cm 2 ] based on the voids per unit area of ​​the negative electrode intermediate layer" means ２ ]" is a value measured by the following method. First, the volume of the voids (pores) in the negative electrode intermediate layer (total pore volume) was measured by mercury intrusion porosimetry using a mercury porosimeter. Specifically, mercury was injected into the voids in the negative electrode intermediate layer to be measured, and the volume of the mercury that was pressed in was used to calculate the volume V [cm ３ The measurement device used was a Micromeritics Autopore IV 9510, with a pore size range of 0.003 to 500 μm, a mercury contact angle of 130°, and a mercury surface tension of 485 dynes / cm. The pore volume (total pore volume) V [cm ３ ], the theoretical capacity of lithium per unit volume (2.062 × 10 −３ [mAh / cm ３ ]) and multiplied by the unit area [cm ２ ] is divided by the capacity b2 [mAh / cm ２ ]

[0038] The charge capacity B [mAh / cm 2 ] is the sum of the capacity b1 and the capacity b2. ２ The value of [Ratio of Charge Capacity to Charge Capacity] can be controlled by adjusting the type and composition of the lithium reactive material contained in the negative electrode intermediate layer.

[0039] In the all-solid-state lithium secondary battery according to this embodiment, it is essential to satisfy the above formula 1: A≧B, but it is preferable to satisfy formula 2: 2.0B>A≧B, it is preferable to satisfy formula 3: 1.5B>A≧B, and it is even more preferable to satisfy formula 4: 1.5B>A>B. By satisfying 2.0B>A (more preferably 1.5B>A), the cycle characteristics can be further improved. In addition, since the irreversible capacity A of the negative electrode intermediate layer does not become too large relative to the charge capacity B, a lithium secondary battery having a high energy density can be obtained. By satisfying A>B, structural changes in the negative electrode intermediate layer are even less likely to occur, and therefore a further improvement in the cycle characteristics can be obtained.

[0040] In the all-solid-state lithium secondary battery according to this embodiment, the ratio of the thickness of the negative electrode intermediate layer to the thickness of the positive electrode active material layer is preferably 0.005 or more and 0.040 or less, more preferably 0.010 or more and 0.040 or less, even more preferably 0.015 or more and 0.035 or less, and even more preferably 0.020 or more and 0.030 or less. By adopting such a configuration, the cycle characteristics can be further improved.

[0041] [Positive current collector plate and negative current collector plate] The material constituting the current collector plate (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plate. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive current collector plate 27 and the negative current collector plate 25 may be made of the same material or different materials.

[0042] [Positive Electrode Lead and Negative Electrode Lead] Although not shown in the drawings, the current collectors (11", 11') and the current collector plates (27, 25) may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent materials of the positive electrode and the negative electrode lead, materials used in known lithium ion secondary batteries can be similarly adopted. Note that the portion removed from the exterior is preferably covered with a heat-resistant, insulating heat-shrinkable tube or the like so as to prevent contact with peripheral devices or wiring, etc., causing electrical leakage and affecting products (for example, automobile parts, particularly electronic devices, etc.).

[0043] [Battery Exterior Material] As the battery exterior material, a known metal can case can be used. Alternatively, a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power-generating element can be used, as shown in Figures 1 and 2 . 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 thereto. 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 equipment such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable as the exterior material because it allows for easy adjustment of the collective pressure applied to the power-generating element from the outside.

[0044] The all-solid-state lithium secondary battery according to the present embodiment has a configuration in which a plurality of unit cell layers are connected in parallel, and therefore has high capacity and excellent cycle durability, and is therefore suitable for use as a power source for driving EVs and HEVs.

[0045] Although one embodiment of the lithium secondary battery of the present invention has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0046] For example, the type of battery to which the lithium secondary battery according to the present invention is applied includes a bipolar battery including a bipolar electrode having a positive electrode active material layer electrically coupled to one surface of a current collector and a negative electrode active material layer electrically coupled to the opposite surface of the current collector.

[0047] Furthermore, 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 preferably an amount that allows the shape of the solid electrolyte layer formed by the solid electrolyte to be maintained and prevents leakage of the liquid electrolyte (electrolytic solution).

[0048] The following embodiments are also included within the scope of the present invention: a lithium secondary battery according to claim 1 having the features of claim 2; a lithium secondary battery according to claim 1 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 any one of claims 1 to 4 having the features of claim 5; a lithium secondary battery according to any one of claims 1 to 5 having the features of claim 6; a lithium secondary battery according to any one of claims 1 to 6 having the features of claim 7; and a lithium secondary battery according to any one of claims 1 to 7 having the features of claim 8.

[0049] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following, the operations were carried out in a glove box with a dew point of -68°C or less. Furthermore, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0050] <Examples of Preparation of Evaluation Cell> [Example 1] (Preparation of Positive Electrode) In a glove box with an argon atmosphere having a dew point of −68° C. or less, LiNi as a positive electrode active material was ０．８ Mn ０．１ Co ０．１ O ２ , acetylene black as a conductive additive, and Li as a solid electrolyte. ６ P.S. ５ Cl was weighed out to a mass ratio of 90:1:9. These were mixed using an agate mortar and then further stirred and mixed using a planetary ball mill. 2 parts by mass of styrene-butadiene rubber (SBR) as a binder was added to 100 parts by mass of the obtained mixed powder, and mesitylene was added as a solvent and mixed to prepare a positive electrode active material slurry. The positive electrode active material slurry was applied to the surface of an aluminum foil as a positive electrode current collector, dried, and pressed to obtain a positive electrode having a positive electrode active material layer (thickness 120 μm) on the surface of the positive electrode current collector.

[0051] (Preparation of Solid Electrolyte Layer) In a glove box with an argon atmosphere having a dew point of −68° C. or less, Li as a solid electrolyte was ６ P.S. ５A solid electrolyte slurry was prepared by adding 2 parts by mass of SBR as a binder to 100 parts by mass of Cl, and adding mesitylene as a solvent and mixing them. The solid electrolyte slurry was applied to the surface of a stainless steel foil as a support and dried to obtain a solid electrolyte layer (thickness: 30 μm).

[0052] (Preparation of Negative Electrode Intermediate Layer) Silver nanoparticles (average particle diameter (D50): 60 nm) and acetylene black (average particle diameter (D50): 35 nm) were weighed and mixed at a mass ratio of 1:3. 0.5 parts by mass of SBR as a binder was added to 5 parts by mass of the obtained mixture, and mesitylene was added as a solvent and mixed to prepare a lithium-reactive material slurry. The lithium-reactive material slurry was applied to the surface of a stainless steel foil as a negative electrode current collector and dried to obtain a negative electrode intermediate layer (thickness 10.0 μm).

[0053] (Preparation of Evaluation Cell) A positive electrode active material layer formed on the surface of a stainless steel foil (positive electrode current collector) and a solid electrolyte layer formed on the surface of the stainless steel foil were stacked so that the exposed surface of the positive electrode active material layer and the exposed surface of the solid electrolyte layer faced each other, and then transferred by cold isostatic pressing (CIP; 700 MPa, 25 ° C., 1 minute). After peeling off the stainless steel foil adjacent to the solid electrolyte layer, the solid electrolyte layer and the negative electrode intermediate layer formed on the surface of the stainless steel foil (negative electrode current collector) were stacked so that the exposed surface of the solid electrolyte layer and the exposed surface of the negative electrode intermediate layer faced each other, and then transferred by cold isostatic pressing (CIP; 80 ° C., 500 MPa, 1 minute). Finally, an aluminum positive electrode tab and a nickel negative electrode tab were bonded to each of the stainless steel foil (positive electrode current collector) and the stainless steel foil (negative electrode current collector) using an ultrasonic welding machine, and the resulting laminate was placed inside an aluminum laminate film and vacuum sealed to obtain an evaluation cell, which is a lithium deposition-type all-solid-state lithium secondary battery.

[0054] In the evaluation cell, the initial charge capacity per unit area of ​​the positive electrode active material layer was 8.18 [mAh / cm ２ ], and the initial discharge capacity per unit area is 7.63 [mAh / cm ２ ], and the irreversible capacity A, which is the difference between the initial charge capacity and the initial discharge capacity, is 0.55 [mAh / cm２ The capacity b1 of the mixture of silver nanoparticles and acetylene black contained per unit area of ​​the negative electrode intermediate layer was 0.24 [mAh / cm ２ ], and the capacity b2 based on the voids per unit area of ​​the negative electrode intermediate layer is 0.31 [mAh / cm ２ ], and the sum of these, the charge capacity B, is 0.55 [mAh / cm ２ The thickness of the negative electrode intermediate layer was 3.5 μm, and the thickness of the positive electrode active material layer was 120 μm. The relationship between the irreversible capacity A of the positive electrode active material layer and the charge capacity B of the negative electrode intermediate layer was A=B, and the ratio of the thickness of the negative electrode intermediate layer to the thickness of the positive electrode active material layer (thickness ratio) was 0.029.

[0055] [Example 2] In the above (production of the negative electrode intermediate layer), the coating amount of the lithium reactive material slurry was adjusted so that the thickness of the negative electrode intermediate layer after drying was 8.3 µm. An evaluation cell of this example was obtained in the same manner as in Example 1.

[0056] In the evaluation cell, the initial charge capacity per unit area of ​​the positive electrode active material layer was 8.18 [mAh / cm ２ ], and the initial discharge capacity per unit area is 7.63 [mAh / cm ２ ], and the irreversible capacity A, which is the difference between the initial charge capacity and the initial discharge capacity, is 0.55 [mAh / cm ２ The capacity b1 of the mixture of silver nanoparticles and acetylene black contained per unit area of ​​the negative electrode intermediate layer was 0.20 [mAh / cm ２ ], and the capacity b2 based on the voids per unit area of ​​the negative electrode intermediate layer is 0.25 [mAh / cm ２ ], and the sum of these, the charge capacity B, is 0.45 [mAh / cm ２ The thickness of the negative electrode intermediate layer was 2.9 μm, and the thickness of the positive electrode active material layer was 120 μm. The relationship between the irreversible capacity A of the positive electrode active material layer and the charge capacity B of the negative electrode intermediate layer was A=1.2B, and the ratio of the thickness of the negative electrode intermediate layer to the thickness of the positive electrode active material layer (thickness ratio) was 0.024.

[0057] [Example 3] An evaluation cell for this example was obtained in the same manner as in Example 1, except that in the above (production of a negative electrode intermediate layer), the coating amount of the lithium-reactive material slurry was adjusted so that the thickness of the negative electrode intermediate layer after drying was 5.7 µm.

[0058] In the evaluation cell, the initial charge capacity per unit area of ​​the positive electrode active material layer was 8.18 [mAh / cm ２ ], and the initial discharge capacity per unit area is 7.63 [mAh / cm ２ ], and the irreversible capacity A, which is the difference between the initial charge capacity and the initial discharge capacity, is 0.55 [mAh / cm ２ The capacity b1 of the mixture of silver nanoparticles and acetylene black contained per unit area of ​​the negative electrode intermediate layer was 0.14 [mAh / cm ２ ], and the capacity b2 based on the voids per unit area of ​​the negative electrode intermediate layer is 0.18 [mAh / cm ２ ], and the sum of these, the charge capacity B, is 0.32 [mAh / cm ２ The thickness of the negative electrode intermediate layer was 2.0 μm, and the thickness of the positive electrode active material layer was 120 μm. The relationship between the irreversible capacity A of the positive electrode active material layer and the charge capacity B of the negative electrode intermediate layer was A=1.7 B, and the ratio of the thickness of the negative electrode intermediate layer to the thickness of the positive electrode active material layer (thickness ratio) was 0.017.

[0059] [Example 4] An evaluation cell for this example was obtained in the same manner as in Example 1, except that in the above (production of a negative electrode intermediate layer), the coating amount of the lithium-reactive material slurry was adjusted so that the thickness of the negative electrode intermediate layer after drying was 5.1 µm.

[0060] In the evaluation cell, the initial charge capacity per unit area of ​​the positive electrode active material layer was 8.18 [mAh / cm ２ ], and the initial discharge capacity per unit area is 7.63 [mAh / cm ２ ], and the irreversible capacity A, which is the difference between the initial charge capacity and the initial discharge capacity, is 0.55 [mAh / cm ２ The capacity b1 of the mixture of silver nanoparticles and acetylene black contained per unit area of ​​the negative electrode intermediate layer was 0.11 [mAh / cm ２], and the capacity b2 based on the voids per unit area of ​​the negative electrode intermediate layer is 0.13 [mAh / cm ２ ], and the sum of these, the charge capacity B, is 0.24 [mAh / cm ２ The thickness of the negative electrode intermediate layer was 1.8 μm, and the thickness of the positive electrode active material layer was 120 μm. The relationship between the irreversible capacity A of the positive electrode active material layer and the charge capacity B of the negative electrode intermediate layer was A=2.3 B, and the ratio of the thickness of the negative electrode intermediate layer to the thickness of the positive electrode active material layer (thickness ratio) was 0.015.

[0061] Comparative Example 1 An evaluation cell for this comparative example was obtained in the same manner as in Example 1, except that in the above (production of a negative electrode intermediate layer), the coating amount of the lithium-reactive material slurry was adjusted so that the thickness of the negative electrode intermediate layer after drying was 16.3 μm.

[0062] In the evaluation cell, the initial charge capacity per unit area of ​​the positive electrode active material layer was 8.18 [mAh / cm ２ ], and the initial discharge capacity per unit area is 7.63 [mAh / cm ２ ], and the irreversible capacity A, which is the difference between the initial charge capacity and the initial discharge capacity, is 0.55 [mAh / cm ２ The capacity b1 of the mixture of silver nanoparticles and acetylene black contained per unit area of ​​the negative electrode intermediate layer was 0.39 [mAh / cm ２ ], and the capacity b2 based on the voids per unit area of ​​the negative electrode intermediate layer is 0.50 [mAh / cm ２ ], and the sum of these, the charge capacity B, is 0.89 [mAh / cm ２ The thickness of the negative electrode intermediate layer was 5.7 μm, and the thickness of the positive electrode active material layer was 120 μm. The relationship between the irreversible capacity A of the positive electrode active material layer and the charge capacity B of the negative electrode intermediate layer was A=0.6B, and the ratio of the thickness of the negative electrode intermediate layer to the thickness of the positive electrode active material layer (thickness ratio) was 0.048.

[0063] Comparative Example 2 An evaluation cell for this comparative example was obtained in the same manner as in Example 1, except that in the above (preparation of the positive electrode active material layer), the coating amount of the positive electrode active material slurry was adjusted so that the thickness of the positive electrode active material layer after the pressing treatment was 81.0 μm.

[0064] In the evaluation cell, the initial charge capacity per unit area of ​​the positive electrode active material layer was 5.74 [mAh / cm ２ ], and the initial discharge capacity per unit area is 5.69 [mAh / cm ２ ], and the irreversible capacity A, which is the difference between the initial charge capacity and the initial discharge capacity, is 0.05 [mAh / cm ２ The capacity b1 of the mixture of silver nanoparticles and acetylene black contained per unit area of ​​the negative electrode intermediate layer was 0.24 [mAh / cm ２ ], and the capacity b2 based on the voids per unit area of ​​the negative electrode intermediate layer is 0.31 [mAh / cm ２ ], and the sum of these, the charge capacity B, is 0.55 [mAh / cm ２ The thickness of the negative electrode intermediate layer was 3.5 μm, and the thickness of the positive electrode active material layer was 81.0 μm. The relationship between the irreversible capacity A of the positive electrode active material layer and the charge capacity B of the negative electrode intermediate layer was A=0.1 B, and the ratio of the thickness of the negative electrode intermediate layer to the thickness of the positive electrode active material layer (thickness ratio) was 0.043.

[0065] <Cycle Test> A cycle test was performed on the evaluation cell prepared above, applying a restraining pressure of 3 MPa in the stacking direction of the evaluation cell using a pressure member. First, the cell was charged at a rate of 0.01 C for 25 hours in a thermostatic chamber set at 60°C, followed by constant-current, constant-voltage (CCCV) charging at 0.05 C and 4.3 V (4.15 V for the evaluation of Comparative Example 2) with a cutoff current value of 0.01 C. Subsequently, constant-current (CC) discharging was performed at 0.1 C to 2.5 V. This charge / discharge was considered the first cycle, and the discharge capacity at this time was defined as the initial discharge capacity. Next, for the second and subsequent cycles, constant-current (CC) charging / discharging was repeated at 0.5 C in a cell voltage range from 2.5 V to 4.3 V (4.15 V for the evaluation of Comparative Example 2) up to 200 cycles. The discharge capacity at the 200th cycle was measured, and the ratio of the discharge capacity at the 200th cycle to the initial discharge capacity, expressed as a percentage, was defined as the capacity retention rate (%). The results are shown in Table 1 below.

[0066]

[0067] As shown in Table 1, it is clear that the present invention can further improve the cycle characteristics of a lithium deposition type lithium secondary battery provided with a negative electrode intermediate layer.

[0068] REFERENCE SIGNS LIST 10a laminated secondary battery, 11' negative electrode current collector, 11" positive electrode current collector, 13 negative electrode active material layer, 14 negative electrode intermediate layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 19 single cell layer, 21 power generating element, 25 negative electrode current collector, 27 positive electrode current collector, 29 laminate film.

Claims

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; a negative electrode intermediate layer interposed between the negative electrode current collector and the solid electrolyte layer and including a lithium reactive material; A lithium secondary battery comprising a power generating element having The irreversible capacity, which is the difference between the initial charge capacity and the initial discharge capacity per unit area of ​​the positive electrode active material layer, is defined as A [mAh / cm ２ ], the charging capacity which is the sum of the capacity b1 of the lithium reactive material per unit area of ​​the negative electrode intermediate layer and the capacity b2 based on the voids is B [mAh / cm ２ ], A ≧ B is satisfied. 　 The irreversible capacity A [mAh / cm ２ ] and the charging capacity B [mAh / cm ２ 2.0B>A≧B. 　 The irreversible capacity A [mAh / cm ２ ] and the charging capacity B [mAh / cm ２ 2. The lithium secondary battery according to claim 1 , wherein A satisfies 1.5B>A≧B. 　 The irreversible capacity A [mAh / cm ２ ] and the charging capacity B [mAh / cm ２ 2. The lithium secondary battery according to claim 1 , wherein the relationship 1.5B>A>B is satisfied. 　 The lithium secondary battery according to claim 1 or 2, wherein the positive electrode active material comprises at least one selected from lithium-containing metal oxides.

3. The lithium secondary battery according to claim 1, wherein the lithium reactive material comprises at least one selected from the group consisting of a material capable of absorbing lithium ions during charging and a metal material capable of alloying with lithium during charging.

3. The lithium secondary battery according to claim 1, wherein a content of the positive electrode active material with respect to a total mass of the positive electrode active material layer is from 70% by mass to 99% by mass.

3. The lithium secondary battery according to claim 1, wherein a ratio of a thickness of the negative electrode intermediate layer to a thickness of the positive electrode active material layer is 0.005 or more and 0.040 or less.

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