JP7799544B2 - All solid state battery - Google Patents

Description

The present invention relates to an all-solid-state battery.

In recent years, there has been a strong desire to reduce carbon dioxide emissions in order to combat global warming. The automotive industry is pinning its hopes on reducing carbon dioxide emissions through the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs), and there has been active development of non-aqueous electrolyte secondary batteries, such as motor drive secondary batteries, which hold the key to commercializing these vehicles.

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

Currently widely used lithium secondary batteries use a flammable organic electrolyte. These liquid-based lithium secondary batteries require stricter safety measures against leakage, short circuits, overcharging, and other issues than other batteries.

In recent years, therefore, there has been active research and development into all-solid-state lithium secondary batteries that use oxide- or sulfide-based solid electrolytes. Solid electrolytes are materials primarily composed of ionic conductors that allow ionic conduction in a solid state. For this reason, all-solid-state lithium secondary batteries do not, in principle, encounter the various problems associated with flammable organic electrolytes that occur in conventional liquid-based lithium secondary batteries. Furthermore, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally leads to significant improvements in the battery's output density and energy density. All-solid-state lithium secondary batteries, which use a sulfide-based material as the positive electrode active material and metallic lithium or a lithium-containing alloy as the negative electrode active material, are promising candidates.

In all-solid-state lithium secondary batteries, the positive electrode active material layer contains a conductive additive and a solid electrolyte in addition to the positive electrode active material in order to ensure electronic and ionic conductivity. However, it is known that when charging and discharging are performed while the conductive additive and solid electrolyte are in contact, the solid electrolyte decomposes to produce a highly resistive (highly insulating) substance, increasing the resistance of the positive electrode mixture.

To prevent the above-mentioned increase in resistance of the positive electrode mixture, Patent Document 1 discloses a technology for providing a coating layer of LiF with a thickness of 5 nm or less on the conductive additive in a positive electrode mixture for an all-solid-state battery containing a positive electrode active material, a solid electrolyte, and a conductive additive. According to the disclosure in Patent Document 1, a conductive additive having this configuration supplies electrons to the positive electrode active material through the tunneling effect while not supplying electrons to the solid electrolyte, thereby suppressing the reaction between the conductive additive and the solid electrolyte and preventing an increase in resistance of the positive electrode mixture.

Japanese Patent Application Laid-Open No. 2019-194955

In all-solid-state batteries, confining pressure can be applied in the stacking direction of the battery to maintain contact between the layers, etc.

However, when the inventors applied confining pressure to an all-solid-state battery using the positive electrode composite material described in Patent Document 1, they found that it was sometimes not possible to sufficiently suppress an increase in cell resistance.

The present invention therefore aims to provide a means for suppressing the increase in resistance in all-solid-state batteries.

The inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that the above-mentioned problems could be solved by coating the surface of the conductive additive with a coating layer of a specific thickness containing an iodine compound, which led to the completion of the present invention.

That is, an all-solid-state battery according to one embodiment of the present invention comprises: a coated conductive additive in which at least a portion of the surface of the conductive additive is coated with a coating layer containing an iodine compound, the coating layer having a thickness of 150 nm or more and less than 400 nm; a positive electrode in which a positive electrode active material layer containing a sulfur-containing positive electrode active material and a sulfide solid electrolyte is disposed on the surface of a positive electrode current collector; a negative electrode; a power generating element having a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a restraining member that restrains the power generating element in the stacking direction.

The present invention provides an all-solid-state battery in which resistance increase is suppressed.

FIG. 1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium secondary battery, which is one embodiment of the all-solid-state battery according to the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 shown in FIG. FIG. 3 is a perspective view of a stacked battery according to one embodiment of the present invention. FIG. 4 is a side view seen from the direction A shown in FIG.

One aspect of the present invention is an all-solid-state battery comprising: a coated conductive additive in which at least a portion of the surface of the conductive additive is coated with a coating layer containing an iodine compound, the coating layer having a thickness of 150 nm or more and less than 400 nm; a positive electrode in which a positive electrode active material layer containing a sulfur-containing positive electrode active material and a sulfide solid electrolyte is disposed on the surface of a positive electrode current collector; a negative electrode; and a power generating element having a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a restraining member that restrains the power generating element in the stacking direction.

The following describes an embodiment of the all-solid-state battery according to the present invention, with reference to the drawings. However, the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. Note that the dimensional proportions in the drawings have been exaggerated for the sake of explanation and may differ from the actual proportions.

Figure 1 is a perspective view showing the appearance of a flat-layered all-solid-state lithium secondary battery, which is one embodiment of the all-solid-state battery according to the present invention. Figure 2 is a cross-sectional view taken along line 2-2 in Figure 1. The layered structure allows the battery to be compact and have a high capacity. This specification will be described in detail using the flat-layered, non-bipolar all-solid-state lithium secondary battery shown in Figures 1 and 2 (hereinafter simply referred to as a "layered battery"). However, when viewed from the perspective of the internal electrical connection configuration (electrode structure) of the all-solid-state lithium secondary battery according to this embodiment, it can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.

As shown in Figure 1, the stacked battery 10a has a flat, rectangular shape, with a negative electrode current collector 25 and a positive electrode current collector 27 extending from both sides for extracting power. The power generating element 21 is wrapped in the battery exterior material (laminate film 29) of the stacked battery 10a, and its periphery is heat-sealed, sealing the power generating element 21 with the negative electrode current collector 25 and positive electrode current collector 27 extending to the outside.

The all-solid-state lithium secondary battery according to this embodiment is not limited to a laminated, flat shape. A wound-type all-solid-state lithium secondary battery may be cylindrical, or may be a cylindrical battery modified into a flat, rectangular shape, and is not particularly limited. The cylindrical battery may use a laminate film or a conventional cylindrical can (metal can) as its exterior material, and is not particularly limited. Preferably, the power generating element is housed inside a laminate film containing aluminum. This configuration can achieve weight reduction.

Furthermore, there are no particular limitations on how the current collectors (25, 27) shown in Figure 1 are removed. The negative electrode current collector 25 and the positive electrode current collector 27 may be pulled out from the same side, or the negative electrode current collector 25 and the positive electrode current collector 27 may each be divided into multiple pieces and removed from each side; they are not limited to what is shown in Figure 1. Furthermore, in a wound-type all-solid-state lithium secondary battery, terminals may be formed using, for example, a cylindrical can (metal can) instead of tabs.

As shown in Figure 2, the stacked battery 10a of this embodiment has a structure in which a flat, approximately rectangular power generating element 21, where the actual charge/discharge reaction takes place, is sealed inside a laminate film 29, which is the battery exterior material. Here, the power generating element 21 has a structure in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are stacked. The positive electrode has a structure in which positive electrode active material layers 15 containing positive electrode active material are disposed on both sides of a positive electrode current collector 11". The negative electrode has a structure in which negative electrode active material layers 13 containing negative electrode active material are disposed on both sides of a negative electrode current collector 11'. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are stacked in this order, with one positive electrode active material layer 15 facing the adjacent negative electrode active material layer 13 with a solid electrolyte layer 17 interposed between them. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode form a single cell layer 19. Therefore, the stacked battery 10a shown in FIG. 2 can be said to have a configuration in which multiple cell layers 19 are stacked and electrically connected in parallel. In addition, a restraining pressure is applied to the stacked battery 10a in the stacking direction of the power generation element 21 by a restraining member (pressure member) (not shown). This keeps the volume of the power generation element 21 constant.

As shown in FIG. 2, the outermost positive electrode current collectors located on both outermost layers of the power generating element 21 each have a positive electrode active material layer 15 disposed on only one side, but active material layers may be provided on both sides. In other words, instead of using a current collector exclusively for the outermost layer with an active material layer provided on only one side, a current collector with active material layers on both sides may be used as the outermost current collector. In some cases, the negative electrode active material layer 13 and the positive electrode active material layer 15 may be used as the negative electrode and positive electrode, respectively, without using current collectors (11', 11").

The negative electrode current collector 11' and the positive electrode current collector 11" are respectively fitted with a negative electrode current collector (tab) 25 and a positive electrode current collector (tab) 27 that are electrically connected to the respective electrodes (positive and negative electrodes), and are structured so as to be sandwiched between the edges of the laminate film 29, which is the battery outer casing, and extend to the outside of the laminate film 29. The positive electrode current collector 27 and the negative electrode current collector 25 may be attached to the positive electrode current collector 11" and the negative electrode current collector 11' of each electrode via a positive electrode lead and a negative electrode lead (not shown) by ultrasonic welding, resistance welding, or the like, as necessary.

Figure 3 is a perspective view of a stacked battery according to one embodiment of the present invention. Figure 4 is a side view seen from direction A shown in Figure 3.

As shown in Figures 3 and 4, the stacked battery 100 according to this embodiment includes a power generating element 21 sealed in a laminate film 29 as shown in Figures 1 and 2, two metal plates 200 sandwiching the power generating element 21 sealed in the laminate film 29, and a bolt 300 and a nut 400 as fastening members. These fastening members (bolt 300 and nut 400) function to secure the power generating element 21 sealed in the laminate film 29 in a sandwiched state between the metal plates 200. As a result, the metal plates 200 and the fastening members (bolt 300 and nut 400) function as restraining members that restrain (apply pressure to) the power generating element 21 in the stacking direction. Note that the restraining members are not particularly limited as long as they are capable of restraining the power generating element 21 in the stacking direction. A combination of a plate made of a rigid material, such as the metal plate 200, and the fastening members described above is typically used as the restraining member. Furthermore, the fastening members may not only be bolts 300 and nuts 400, but also tension plates or the like that secure the ends of the metal plates 200 so as to restrain the power generating element 21 in the stacking direction.

The lower limit of the load applied to the power generating element 21 (confining pressure in the stacking direction of the power generating element) is, for example, 0.1 MPa or more, preferably 1 MPa or more, more preferably 3 MPa or more, and even more preferably 5 MPa or more. The upper limit of the confining pressure in the stacking direction of the power generating element is, for example, 150 MPa or less, preferably 100 MPa or less, more preferably 70 MPa or less, even more preferably 40 MPa or less, and particularly preferably 10 MPa or less.

The main components of the stacked battery according to this embodiment are described below.

[Current collector]
The current collector has a function of mediating the transfer of electrons from the electrode active material layer. There are no particular limitations on the material constituting the current collector. For example, metals and conductive resins can be used as the material constituting the current collector.

Specific examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. Other materials that may be used include clad materials of nickel and aluminum, and clad materials of copper and aluminum. Foils in which the metal surface is coated with aluminum may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the standpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering.

Another example of a conductive resin is a resin in which a conductive filler is added to a non-conductive polymer material.

The current collector may have a single layer structure made of a single material, or a laminate structure made of an appropriate combination of layers made of these materials. From the perspective of reducing the weight of the current collector, it is preferable for it to contain at least a conductive resin layer made of a conductive resin. Furthermore, from the perspective of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on part of the current collector. Furthermore, if the negative electrode active material layer and positive electrode active material layer described below are themselves conductive and can perform a current collecting function, it is not necessary to use a current collector as a separate component from these electrode active material layers. In such a configuration, the negative electrode active material layer described below directly constitutes the negative electrode, and the positive electrode active material layer described below directly constitutes the positive electrode.

[Cathode active material layer]
The positive electrode active material layer essentially contains a positive electrode active material containing sulfur, a sulfide solid electrolyte, and a coating conductive additive.

(Positive electrode active material containing sulfur)
The type of sulfur-containing positive electrode active material is not particularly limited, but includes elemental sulfur (S) and lithium sulfide (Li ₂ S), as well as particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and can be any material that can release lithium ions during charging and absorb lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur. Examples of organic sulfur compounds include disulfide compounds, sulfur-modified polyacrylonitrile, sulfur-modified polyisoprene, rubeanic acid (dithiooxamide), polycarbonate, etc., as typified by the compounds described in International Publication No. 2010/044437. On the other hand, inorganic sulfur compounds are preferred because of their excellent stability, and specific examples include elemental sulfur (S), lithium sulfide (Li ₂ S), S-carbon composite, TiS ₂ , TiS ₃ , TiS ₄ , NiS, NiS ₂ , CuS, FeS ₂ , Li ₂ S, MoS ₂ , MoS ₃ , MnS, MnS ₂ , CoS, CoS ₂ , etc. Among these, S, lithium sulfide (Li ₂ S), S-carbon composite, TiS ₂ , TiS ₃ , TiS ₄ , FeS ₂ , and MoS ₂ are preferred, elemental sulfur (S), lithium sulfide (Li ₂ S), TiS ₂ , and FeS ₂ are more preferred, and elemental sulfur (S) and lithium sulfide (Li ₂ S) are particularly preferred from the viewpoint of high capacity. As the elemental sulfur (S), α-sulfur, β-sulfur, or γ-sulfur having an _S8 structure can be used.

The positive electrode active material layer may further contain a sulfur-free positive electrode active material in addition to the sulfur-containing positive electrode active material. Examples of sulfur-free positive electrode active materials include layered rock salt active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO ₂ , and Li(Ni—Mn—Co)O ₂ , spinel active materials such as LiMn ₂ O ₄ and LiNi _0.5 Mn _1.5 O ₄ , olivine active materials such as LiFePO ₄ and LiMnPO ₄ , and Si-containing active materials such as Li ₂ FeSiO ₄ and Li ₂ MnSiO ₄ . Examples of oxide active materials other than those mentioned above include Li ₄ Ti ₅ O ₁₂ .

In some cases, two or more types of positive electrode active materials may be used in combination. Of course, positive electrode active materials other than those listed above may also be used. However, the proportion of the content of the sulfur-containing positive electrode active material in the total amount (100% by mass) of the positive electrode active material is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 40 to 99% by mass, and more preferably in the range of 50 to 90% by mass.

(Sulfide solid electrolyte)
Examples of the sulfide solid electrolyte include LiI-Li ₂ S-SiS ₂ , LiI-Li ₂ SP ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , Li ₂ S-P ₂ S ₅ , LiI-Li ₃ PS ₄ , LiI-LiBr-Li ₃ PS ₄ , Li ₃ PS ₄ , Li ₂ SP ₂ S ₅ , Li ₂ SP ₂ S ₅ -LiI, Li ₂ SP ₂ S ₅ -Li ₂ O, Li ₂ SP ₂ S ₅ -Li ₂ O-LiI, Li ₂ S-SiS ₂ , Li ₂ S-SiS ₂ -LiI, Li ₂ S—SiS ₂ —LiBr, Li ₂ S—SiS ₂ —LiCl, Li ₂ S—SiS ₂ —B ₂ S ₃ —LiI, Li ₂ S—SiS ₂ —P ₂ S ₅ —LiI, Li ₂ S—B ₂ S ₃ , Li ₂ S—P ₂ S ₅ —Z _m S _n (where m and n are positive numbers, and Z is Ge, Zn, or Ga), Li ₂ S—GeS ₂ , Li ₂ S—SiS ₂ —Li ₃ PO ₄ , Li ₂ S—SiS ₂ —Li _x MO _y (where x and y are positive numbers, and M is any of P, Si, Ge, B, Al, Ga, and In.) The term "Li ₂ S-P ₂ S ₅ " means a sulfide solid electrolyte obtained using a raw material composition containing Li ₂ S and P ₂ S ₅ , and the same applies to other terms.

The sulfide solid electrolyte may have, for example, a Li ₃ PS ₄ skeleton, a Li ₄ P ₂ S ₇ skeleton, or a Li ₄ P ₂ S ₆ skeleton. Examples of sulfide solid electrolytes having a Li ₃ PS ₄ skeleton include LiI-Li ₃ PS ₄ , LiI-LiBr-Li ₃ PS ₄ , and Li ₃ PS ₄ . Examples of sulfide solid electrolytes having a Li ₄ P ₂ S ₇ skeleton include Li-P-S solid electrolytes known as LPS (for example, Li ₇ P ₃ S ₁₁ ). Examples of sulfide solid electrolytes that may be used include LGPS, which is represented by Li _(4-x) Ge _(1-x) P _x S ₄ (where x satisfies 0<x<1). Among these, the sulfide solid electrolyte is preferably a sulfide solid electrolyte containing P element, and more preferably a material containing Li ₂ S—P ₂ S ₅ as a main component. Furthermore, an example of a sulfide solid electrolyte containing a halogen is an argyrodite-type solid electrolyte (Li ₆ PS ₅ X (X is Cl, Br, or I)), which is also a material that can be preferably used.

The sulfide solid electrolyte may be sulfide glass, crystallized sulfide glass, or a crystalline material obtained by a solid-phase method. Sulfide glass can be obtained, for example, by mechanical milling (ball mill, etc.) a raw material composition. Crystallized sulfide glass can be obtained, for example, by heat treating sulfide glass at a temperature equal to or higher than the crystallization temperature. The ionic conductivity (e.g., Li ion conductivity) of the sulfide solid electrolyte at room temperature (25°C) is preferably, for example, 1×10 ⁻⁵ S/cm or more, and more preferably 1×10 ⁻⁴ S/cm or more. The ionic conductivity value of the solid electrolyte can be measured by an AC impedance method.

The positive electrode active material layer may further include a solid electrolyte other than the sulfide solid electrolyte. Examples of solid electrolytes other than the sulfide solid electrolyte include oxide solid electrolytes. Specific examples of oxide solid electrolytes include compounds having a NASICON structure, such as a compound (LAGP) represented by the general formula Li _1+x Al _x Ge _2-x (PO ₄ ) ₃ (0≦x≦2) and a compound (LATP) represented by the general formula Li _1+x Al _x Ti _2-x (PO ₄ ) ₃ (0≦x≦2). Other specific examples of oxide solid electrolytes include LiLaTiO (for example, Li _0.34 La _0.51 TiO ₃ ), LiPON (for example, Li _2.9 PO _3.3 N _0.46 ), and LiLaZrO (for example, Li ₇ La ₃ Zr ₂ O ₁₂ ).

In some cases, two or more types of solid electrolytes may be used in combination. Of course, solid electrolytes other than those mentioned above may also be used. However, the content of the sulfide solid electrolyte in the positive electrode active material layer relative to the total amount of solid electrolytes (100% by mass) is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

The content of the sulfide solid electrolyte in the positive electrode active material layer is not particularly limited, but is preferably in the range of 1 to 60 mass%, more preferably in the range of 10 to 50 mass%, and even more preferably in the range of 15 to 40 mass%.

(Coating conductive additive)
The stacked battery according to the present embodiment is characterized in that the cathode active material layer contains a conductive additive, at least a portion of whose surface is coated with a coating layer containing an iodine compound, and the coating layer has a thickness of 150 nm or more and less than 400 nm. By adopting such a configuration, an increase in resistance of the all-solid-state battery can be suppressed.

As mentioned above, Patent Document 1 discloses a technology for providing a coating layer (coating layer) of LiF with a thickness of 5 nm or less on the conductive additive in a cathode composite for an all-solid-state battery containing a cathode active material, a solid electrolyte, and a conductive additive. However, when confining pressure was applied to an all-solid-state battery using the cathode composite, an increase in cell resistance could not be sufficiently suppressed in some cases. This is thought to be due to the following mechanism. According to the technology described in Patent Document 1, because the coating layer is thin, at 5 nm or less, cracks occur in the coating layer when confining pressure is applied. The conductive additive and solid electrolyte come into contact in the cracks in the coating layer, and when charge/discharge reactions are carried out in this state, a decomposition reaction of the solid electrolyte occurs. The product of the decomposition reaction has high resistance, resulting in an increase in cell resistance.

On the other hand, the coated conductive additive according to this embodiment uses an iodine compound as the material for the coating layer, and by making the coating layer 150 nm or thicker, cracks in the coating layer are less likely to occur even when confining pressure is applied. This suppresses decomposition of the sulfide solid electrolyte and reduces increases in resistance due to the generation of reaction products. Furthermore, by making the coating layer less than 400 nm thick, the functionality of the conductive additive can be maintained.

The content of the coating conductive additive in the positive electrode active material layer is not particularly limited, but is preferably in the range of 1 to 40 mass%, more preferably in the range of 5 to 35 mass%, and even more preferably in the range of 10 to 30 mass%.

The conductive additives constituting the coated conductive additive are not particularly limited, but examples 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.). Furthermore, particulate ceramic materials or resin materials coated with the above metal materials by plating or other methods can also be used as conductive additives. Among these conductive additives, from the standpoint of electrical stability, it is preferable to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, copper, titanium, and carbon. It is more preferable to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, and carbon. It is even more preferable to include at least one selected from the group consisting of carbon. These conductive additives may be used alone or in combination of two or more.

The conductive additive is preferably in the form of particles or fibers. When the conductive additive is in the form of particles, the shape of the particles is not particularly limited and may be any shape, such as powder, sphere, rod, needle, plate, column, irregular shape, scale, or spindle shape.

When the conductive additive is particulate, its average particle size (primary particle size) is not particularly limited, but from the perspective of the electrical characteristics of the battery, it is preferably 0.01 to 10 μm. In this specification, "particle size of conductive additive" refers to the longest distance L between any two points on the contour line of the conductive additive. The value of "average particle size of conductive additive" is calculated as the average particle size of particles observed within several to several tens of fields of view using an observation method such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

The coating layer contains an iodine compound (excluding the above-mentioned sulfide solid electrolyte). The iodine compound is not particularly limited, but preferably contains lithium in addition to iodine, from the viewpoint of further suppressing _the increase in resistance. Specific examples of iodine compounds include lithium iodide (LiI), _Li7P2S8I , _{LixPSyI ,} etc. Among these, from the viewpoint of further exerting _the effect of suppressing the increase in resistance, the iodine compound preferably contains lithium iodide, and more preferably is lithium iodide.

The coating layer need only cover at least a portion of the surface of the conductive additive, but from the perspective of further suppressing an increase in resistance in the all-solid-state battery, it is preferable that a larger proportion of the conductive additive's surface is covered by the coating layer. Specifically, the proportion of the area covered by the coating layer to the surface area of the conductive additive is preferably 50% or more, more preferably 70% or more, even more preferably 80% or more, even more preferably 90% or more, particularly preferably 95% or more, and most preferably 100%.

The content of the iodine compound in the coating layer is not particularly limited, but the proportion of the iodine compound in the total amount (100% by mass) of the coating layer is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

The thickness of the coating layer must be in the range of 150 nm or more and less than 400 nm, preferably in the range of 180 nm or more and 300 nm or less, and more preferably in the range of 200 nm or more and 300 nm or less. When the thickness of the coating layer is within the above range, the increase in resistance of the all-solid-state battery can be further suppressed. Note that, in this specification, the thickness of the coating layer is the value measured by the method described in the Examples below.

The ratio of the mass of the coating layer to the mass of the conductive additive contained in the coated conductive additive (mass of coating layer/mass of conductive additive) is preferably 0.1 to 1, more preferably 0.2 to 0.8, and even more preferably 0.2 to 0.5. When the ratio of the mass of the coating layer to the mass of the conductive additive is within the above range, the increase in resistance of the all-solid-state battery can be further suppressed.

There are no particular restrictions on the manufacturing method of the coated conductive additive, but one example is a method in which the conductive additive is impregnated in an iodine compound solution and then the solvent is removed.

The solvent used for the iodine compound solution is not particularly limited, but from the standpoints of solubility, operability, safety, etc., alcohols are suitable, and alcohols having 1 to 4 carbon atoms are preferred. Examples of alcohols having 1 to 4 carbon atoms include methanol, ethanol, 1-propanol, 2-propanol, n-butanol, 2-butanol, isobutanol, and tert-butanol. Of these, methanol, ethanol, 1-propanol, and 2-propanol are preferred, with methanol and ethanol being more preferred, and ethanol being particularly preferred.

The concentration of the iodine compound solution is preferably 1 to 200 mg/mL, and more preferably 10 to 100 mg/mL. The thickness of the coating layer of the coated conductive additive can be controlled by adjusting the amount or concentration of the iodine compound solution.

The specific method for removing the solvent after impregnating the conductive aid into the iodine compound solution is not particularly limited, but it is preferable to remove the solvent while stirring the dispersion containing the iodine compound solution and the conductive aid. The solvent is preferably removed under reduced pressure at a temperature of 100°C or less. The solid content after solvent removal can be crushed as needed to obtain the coated conductive aid.

The thickness of the positive electrode active material layer varies depending on the intended stacked battery configuration, but is preferably within the range of 0.1 to 1000 μm, for example.

[Solid electrolyte layer]
The solid electrolyte layer is a layer interposed between the above-described positive electrode active material layer and negative electrode active material layer, and essentially contains a solid electrolyte. The sulfide solid electrolyte and oxide solid electrolyte described in the section on the positive electrode active material layer can be appropriately used as the solid electrolyte contained in the solid electrolyte layer. Among these, the solid electrolyte layer preferably contains a sulfide solid electrolyte. The solid electrolyte layer may further contain a binder in addition to the above-described solid electrolyte. There are no particular restrictions on the blending amounts of each component contained in the solid electrolyte layer, but the blending amount of the binder is preferably 2 to 8 mass% relative to the total amount of the solid electrolyte layer (100 mass%).

The thickness of the solid electrolyte layer varies depending on the configuration of the intended stacked battery, but from the perspective of improving the volumetric energy density of the battery, it is preferably 600 μm or less, more preferably 500 μm or less, and even more preferably 400 μm or less. On the other hand, there is no particular lower limit on the thickness of the solid electrolyte layer, but it is preferably 10 μm or more, more preferably 50 μm or more, and even more preferably 100 μm or more.

[Negative electrode active material layer]
The negative electrode active material layer 13 contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but includes carbon materials, metal oxides, and metal active materials. Examples of carbon materials include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. Examples of metal oxides include Nb ₂ O ₅ and Li ₄ Ti ₅ O _12. Furthermore, silicon-based negative electrode active materials and tin-based negative electrode active materials may also be used. Silicon and tin belong to Group 14 elements and are known to be negative electrode active materials that can significantly improve the capacity of non-aqueous electrolyte secondary batteries. These elements can absorb and release a large number of charge carriers (such as lithium ions) per unit volume (mass), making them high-capacity negative electrode active materials. Here, it is preferable to use Si as the silicon-based negative electrode active material. Similarly, it is also preferable to use silicon oxides such as SiO _x (0.3≦x≦1.6) disproportionated into two phases, a Si phase and a silicon oxide phase. In this case, the range of x is more preferably 0.5≦x≦1.5, and even more preferably 0.7≦x≦1.2. Furthermore, an alloy containing silicon (silicon-containing alloy-based negative electrode active material) may be used. On the other hand, negative electrode active materials containing tin element (tin-based negative electrode active materials) include simple Sn, tin alloys (Cu—Sn alloys, Co—Sn alloys), amorphous tin oxide, tin silicon oxide, etc. Among these, an example of amorphous tin oxide is SnB _0.4 P _0.6 O _3.1 . An example of tin silicon oxide is SnSiO _3. Furthermore, a metal containing lithium may be used as the negative electrode active material. Such negative electrode active materials are not particularly limited as long as they are active materials containing lithium, and examples include lithium metal and lithium-containing alloys. The lithium-containing alloy is not particularly limited, and examples thereof include alloys of Li and at least one of In, Al, Si, Sn, Mg, Au, Ag, and Zn. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than those described above may also be used. The negative electrode active material preferably includes lithium metal, a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and particularly preferably includes lithium metal or a lithium-containing alloy. When the negative electrode active material includes lithium metal or a lithium-containing alloy, the secondary battery according to this embodiment may be a so-called lithium deposition type in which lithium metal or a lithium-containing alloy is deposited on the negative electrode current collector during charging. In this case, the layer of lithium metal or a lithium-containing alloy deposited on the negative electrode current collector during charging constitutes the negative electrode active material layer of the stacked battery according to this embodiment. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and the thickness of the negative electrode active material layer decreases as the discharging process progresses. The negative electrode active material layer does not need to be present during full discharge, but in some cases, a negative electrode active material layer made of a certain amount of lithium metal or lithium-containing alloy may be present during full discharge.

The negative electrode active material may be, for example, particulate (spherical, fibrous), thin film, or the like. When the negative electrode active material is particulate, its average particle diameter is preferably, for example, 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.

The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably in the range of 40 to 100% by mass, and more preferably in the range of 50 to 90% by mass. If the negative electrode active material contained in the negative electrode active material layer is in particulate form, the negative electrode active material layer may further contain a solid electrolyte, a conductive additive (including a coated conductive additive), and/or a binder. Specific and preferred forms of these may be similar to those described above in the section on the positive electrode active material layer.

The thickness of the negative electrode active material layer (in the case of lithium deposition-type secondary batteries, the thickness when fully charged) varies depending on the configuration of the intended stacked battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[Positive electrode current collector plate and negative electrode current collector plate]
The material constituting the current collector plate 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 the constituent material 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 electrode current collector plate and the negative electrode current collector plate may be made of the same material or different materials.

[Positive electrode lead and negative electrode lead]
The current collector and the current collecting plate may be electrically connected via a positive electrode lead or a negative electrode lead. Materials used in known lithium secondary batteries may be used as the constituent materials of the positive electrode and negative electrode lead. It is preferable that the portion removed from the outer casing be covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, resulting in electrical leakage and affecting the product (e.g., automotive parts, particularly electronic devices).

[Battery exterior material]
As the battery exterior material, a known metal can case can be used, or a bag-shaped case using an aluminum-containing laminate film that can cover the power generating element can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of its 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 for the exterior body because it can easily adjust the collective pressure applied to the power generating element from the outside.

The stacked 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 stacked battery according to this embodiment is suitable for use as a power source for driving EVs and HEVs.

The above describes one embodiment of an all-solid-state battery, but the present invention is not limited to the configuration described in the above embodiment and can be modified as appropriate based on the claims.

The present invention will be explained in more detail below using examples. However, the technical scope of the present invention is not limited to the following examples. Furthermore, all instruments and devices used in the glove box below were thoroughly dried beforehand.

<Example of test cell fabrication>
[Comparative Example 1]
(Preparation of positive electrode material)
In a glove box in an argon atmosphere with a dew point of −68° C. or lower, 40 g of 5 mm diameter zirconia balls, 0.1 g of sulfur (manufactured by Aldrich), 0.08 g of sulfide solid electrolyte (manufactured by Ampcera, Li ₆ PS ₅ Cl), and 0.02 g of conductive additive (manufactured by Lion Specialty Chemicals Co., Ltd., Ketjenblack (registered trademark) EC600JD, primary particle diameter 34.0 nm) were placed in a 45 mL zirconia container and processed at 370 rpm for 6 hours in a planetary ball mill (manufactured by Fritsch, Premium line P-7) to obtain a powdered positive electrode material.

(Production of test cells (all-solid-state lithium secondary batteries))
The battery was fabricated in a glove box with an argon atmosphere at a dew point of -68 ° C. or less. A SUS cylindrical convex punch (10 mm diameter) was inserted into one side of a Macor cylindrical tube jig (tube inner diameter 10 mm, outer diameter 23 mm, height 20 mm), and 80 mg of sulfide solid electrolyte (Ampcera, Li ₆ PS ₅ Cl) was placed from the top of the cylindrical tube jig. Then, another SUS cylindrical convex punch was inserted to sandwich the solid electrolyte, and a solid electrolyte layer with a diameter of 10 mm and a thickness of approximately 0.6 mm was formed in the cylindrical tube jig by pressing it at a pressure of 75 MPa for 3 minutes using a hydraulic press. Next, the cylindrical convex punch inserted from above was removed, and 7.5 mg of the prepared positive electrode material was placed on one side of the solid electrolyte layer in the cylindrical tube. A cylindrical convex punch (also serving as a positive electrode current collector) was inserted again from above and pressed at a pressure of 300 MPa for 3 minutes to form a positive electrode active material layer with a diameter of 10 mm and a thickness of approximately 0.06 mm on one side of the solid electrolyte layer. Next, the lower cylindrical convex punch (also serving as a negative electrode current collector) was removed, and a lithium foil (manufactured by Nilaco Corporation, thickness 0.20 mm) punched to a diameter of 8 mm and an indium foil (manufactured by Nilaco Corporation, thickness 0.30 mm) punched to a diameter of 9 mm were stacked as negative electrodes. The cylindrical tube was inserted from the bottom of the jig so that the indium foil was positioned on the solid electrolyte layer side. The cylindrical convex punch was then inserted again and restrained in a metal casing at a pressure equivalent to 100 MPa. In this manner, a test cell (all-solid-state lithium secondary battery) of this comparative example was produced, in which the negative electrode current collector (punch), lithium-indium negative electrode, solid electrolyte layer, positive electrode active material layer, and positive electrode current collector (punch) were laminated in this order.

[Comparative Example 2]
(Preparation of coated conductive assistant)
Lithium iodide was dissolved in ethanol to a concentration of 20 mg/mL, and 1.00 g of a conductive additive (Ketjenblack (registered trademark) EC600JD, primary particle size 34.0 nm, manufactured by Lion Specialty Chemicals Co., Ltd.) was added to 10 mL of this solution and stirred, and the ethanol was distilled off under reduced pressure to obtain a coated conductive additive having a coating layer made of lithium iodide.

The test cell for this comparative example was prepared in the same manner as in Comparative Example 1, except that in the above (preparation of the positive electrode material), 0.02 g of the above-mentioned coated conductive additive was used instead of the conductive additive that was not coated with lithium iodide.

[Example 1]
In the above (Preparation of coated conductive additive), 1.00 g of a conductive additive (Ketjenblack (registered trademark) EC600JD, primary particle size 34.0 nm, manufactured by Lion Specialty Chemicals Corporation) was added to 15 mL of a 20 mg/mL lithium iodide solution and stirred, and the ethanol was distilled off under reduced pressure to obtain a coated conductive additive having a coating layer made of lithium iodide.

In the above (Preparation of Positive Electrode Material), 0.1 g of sulfur (manufactured by Aldrich), 0.08 g of sulfide solid electrolyte (manufactured by Ampcera, Li ₆ PS ₅ Cl), and 0.02 g of coating conductive additive were treated in a planetary ball mill at 370 rpm for 6 hours in an argon atmosphere glove box with a dew point of −68° C. or less, to obtain a powdered positive electrode material.

Other than this, the test cell for this example was prepared using the same method as in Comparative Example 2.

[Example 2]
In the above (Preparation of coated conductive additive), 1.00 g of a conductive additive (Ketjenblack (registered trademark) EC600JD, primary particle size 34.0 nm, manufactured by Lion Specialty Chemicals Corporation) was added to 20 mL of a 20 mg/mL lithium iodide solution and stirred, and the ethanol was distilled off under reduced pressure to obtain a coated conductive additive having a coating layer made of lithium iodide.

In the above (Preparation of Positive Electrode Material), 0.1 g of sulfur (manufactured by Aldrich), 0.08 g of sulfide solid electrolyte (manufactured by Ampcera, Li ₆ PS ₅ Cl), and 0.02 g of coating conductive additive were treated in a planetary ball mill at 370 rpm for 6 hours in an argon atmosphere glove box with a dew point of −68° C. or less, to obtain a powdered positive electrode material.

Other than this, the test cell for this example was prepared using the same method as in Comparative Example 2.

[Example 3]
In the above (Preparation of coated conductive additive), 1.00 g of a conductive additive (Ketjenblack (registered trademark) EC600JD, primary particle size 34.0 nm, manufactured by Lion Specialty Chemicals Corporation) was added to 30 mL of a 20 mg/mL lithium iodide solution and stirred, and the ethanol was distilled off under reduced pressure to obtain a coated conductive additive having a coating layer made of lithium iodide.

In the above (Preparation of Positive Electrode Material), 0.1 g of sulfur (manufactured by Aldrich), 0.08 g of sulfide solid electrolyte (manufactured by Ampcera, Li ₆ PS ₅ Cl), and 0.02 g of coated conductive additive were treated in a planetary ball mill at 370 rpm for 6 hours in an argon atmosphere glove box with a dew point of −68° C. or less, to obtain a powdered positive electrode material.

Other than this, the test cell for this example was prepared using the same method as in Comparative Example 2.

[Comparative Example 3]
In the above (Preparation of coated conductive additive), 1.00 g of a conductive additive (Ketjenblack (registered trademark) EC600JD, primary particle size 34.0 nm, manufactured by Lion Specialty Chemicals Corporation) was added to 40 mL of a 20 mg/mL lithium iodide solution and stirred, and the ethanol was distilled off under reduced pressure to obtain a coated conductive additive having a coating layer made of lithium iodide.

In the above (Preparation of Positive Electrode Material), 0.1 g of sulfur (manufactured by Aldrich), 0.08 g of sulfide solid electrolyte (manufactured by Ampcera, Li ₆ PS ₅ Cl), and 0.02 g of coating conductive additive were treated in a planetary ball mill at 370 rpm for 6 hours in an argon atmosphere glove box with a dew point of −68° C. or less, to obtain a powdered positive electrode material.

Other than this, the test cell for this comparative example was prepared using the same method as in Comparative Example 2.

<Observation of the coating layer of the conductive additive coating>
For the test cell prepared above, the positive electrode active material layer was thinned using a focused ion beam processing device (FEI Nova200 NanoLab), and its cross-section was observed and its composition was analyzed using TEM-DEX (JEOL JEM-F200). The thickness of the coating layer covering the surface of the conductive additive was determined from cross-sectional images of the particles in several fields of view. The area where I was detected by EDX was determined as the area covered by the coating layer. The thickness of the coating layer was then measured at 20 different points, and the arithmetic average value was calculated to determine the thickness of the coating layer. The results are shown in Table 1 below.

<Measurement of DC Resistance (DCR)>
The direct current resistance (DCR) of the test cells prepared in each of the above examples and comparative examples was measured using a charge-discharge tester (HJ-SD8, manufactured by Hokuto Denko Corporation) in a constant temperature bath set at 25°C.

First, a 3.1 V constant current constant voltage (CCCV) charge was performed at a current density of 0.1 mA/ ^cm² with a current cutoff of 0.05 mA/ ^cm² . Then, the battery was discharged at a rate of 0.03 C according to the battery design. The voltage range was 1.1 to 2.5 V, the current rate during charge and discharge was 0.05 C, and the cutoff current during charge was 0.01 C. The capacity value (mAh/g) per mass of the positive electrode active material was calculated from the charge/discharge capacity value obtained after five cycles of the above charge/discharge cycle and the mass of the positive electrode active material contained in the positive electrode. Next, a constant current discharge was performed at a current density of 0.2 mA/ ^cm² to 50% capacity (SOC50%) of the capacity value calculated in this way (100%). After a 30-minute rest, the battery was discharged for 10 seconds at a discharge rate of 1 C, and the direct current resistance (DCR) was calculated from the voltage drop and current value at that time according to Ohm's law. The results are shown in Table 1 below. The direct current resistance (DCR) shown in Table 1 is a relative value when the value for Comparative Example 1 is set to 100. For Comparative Example 3, measurement of the direct current resistance (DCR) was impossible due to the high resistance.

The results shown in Table 1 show that the present invention can suppress the increase in resistance of all-solid-state batteries.

In particular, Examples 1 and 2, in which the coating layer thickness was in the range of 180 nm or more and 300 nm or less, were shown to be able to maintain even lower resistance.

10a, 100 stacked battery,
11′ negative electrode current collector,
11” positive electrode current collector,
13 negative electrode active material layer,
15 positive electrode active material layer,
17 solid electrolyte layer,
19 cell layer,
21 power generating element,
25 negative electrode current collector plate,
27 positive electrode current collector plate,
29 Laminating film,
200 metal plate,
300 volts,
400 nuts.

Claims (2)

a positive electrode including a conductive additive, at least a portion of the surface of which is coated with a coating layer containing lithium iodide , the coating layer having a thickness of 193 nm or more and 388 nm or less , a positive electrode active material layer containing sulfur, and a sulfide solid electrolyte, and the positive electrode active material layer is disposed on the surface of a positive electrode current collector;
a negative electrode;
a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte;
and a restraining member that restrains the power generating element in a stacking direction. The all-solid-state battery according to claim 1 , wherein the coating layer has a thickness of 200 nm or more and 300 nm or less.