Lithium ion conductive composite electrolyte, electrolyte layer using same, electrode active material layer and lithium secondary battery

Abstract

The present disclosure addresses the problem of providing a means capable of improving lithium ion conductivity, without use of a sintering process, in a lithium secondary battery using an oxide solid electrolyte. Provided is a lithium ion conductive composite electrolyte containing an oxide solid electrolyte, and a mixed salt containing first anions, second anions, and lithium ions.

Description

Lithium ion conductive composite electrolyte, electrolyte layer using the same, electrode active material layer, and lithium secondary battery

[0001] The present invention relates to a lithium ion conductive composite electrolyte, an electrolyte layer, an electrode active material layer, and a lithium secondary battery using the same.

[0002] In recent years, research and development on all-solid-state lithium secondary batteries using oxide- or sulfide-based solid electrolytes has been actively conducted. Solid electrolytes are materials primarily composed of ionic conductors that can conduct lithium ions in a solid state. Therefore, all-solid-state lithium secondary batteries have the advantage that, in principle, they do not encounter the various problems associated with flammable organic electrolytes that are present in conventional liquid-based lithium secondary batteries.

[0003] On the other hand, in all-solid-state lithium secondary batteries, lithium ion conductivity is exhibited by contact between particles via the solid electrolyte, making it difficult to ensure the desired lithium ion conductivity compared to conventional liquid-phase lithium secondary batteries. It is known that oxide-based solid electrolytes can be improved in lithium ion conductivity by adhering solid electrolytes to each other or between a solid electrolyte and an active material through a sintering process. However, this method may require subjecting the solid electrolyte or active material to high-temperature conditions exceeding 1000°C, which limits the materials and equipment that can be used and creates problems such as poor economic viability during the manufacturing process. To address these issues, for example, Japanese Patent Application Laid-Open No. 2024-4795 (corresponding to the specification of U.S. Patent Application Publication No. 2025 / 0125415) discloses a solid electrolyte that can be produced by sintering at relatively low temperatures. The solid electrolyte has the composition formula: Li ６－ｘ R １－ｙ－ａ A ｙ－ｂ M ｘ (BO ３ ) ３In the above formula, R is an element selected from the group including trivalent elements Yb, Er, Ho, and Tm, A is an element selected from Al, Fe, Mn, and Ga, M is a tetravalent element selected from the group including Zr and Ce, x and y are real numbers satisfying 0<x, y<1, and a and b are real numbers satisfying a+b=x. According to the above literature, it has been reported that a compound having the above composition formula can be produced by sintering at about 650 to 800°C and has sufficient ionic conductivity.

[0004] However, even with the techniques described in the above documents, a sintering step is still required to manufacture a secondary battery using an oxide solid electrolyte.

[0005] Therefore, an object of the present invention is to provide an electrolyte that contains an oxide solid electrolyte and has excellent lithium ion conductivity without undergoing a sintering process.

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems, and in the process have found that the above-mentioned problems can be solved by using a composite electrolyte in which a mixed salt composed of two types of anions and lithium ions is contained in an oxide solid electrolyte, thereby completing the present invention.

[0007] That is, one aspect of the present invention is a lithium ion conductive composite electrolyte containing an oxide solid electrolyte and a mixed salt containing a first anion, a second anion, and lithium ions. The oxide solid electrolyte in the lithium ion conductive composite electrolyte is characterized by not being sintered.

[0008] FIG. 1 is a cross-sectional view that schematically shows the overall structure of a stacked-type (internal parallel connection type) lithium secondary battery (hereinafter also simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention.

[0009] Hereinafter, embodiments of the present invention will be described, but 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 range "X to Y" means "X or more and Y or less."

[0010] [Lithium Ion Conductive Composite Electrolyte] A lithium ion conductive composite electrolyte (hereinafter also simply referred to as "composite electrolyte") according to one embodiment of the present invention contains an oxide solid electrolyte and a mixed salt containing a first anion, a second anion, and lithium ions, and the oxide solid electrolyte is not sintered. According to the above configuration, an electrolyte containing an oxide solid electrolyte and having excellent lithium ion conductivity without a sintering process is provided. Note that, in this specification, the first anion and the second anion have different chemical structures. The lithium ion conductive composite electrolyte according to this embodiment can improve the lithium ion conductivity of a lithium secondary battery. The mechanism by which the lithium ion conductive composite electrolyte according to this embodiment exhibits the above-described effects is not fully understood, and the present invention is not bound by any theory. However, the following mechanism is suspected. If an oxide-based solid electrolyte is not sintered, the contact between the solid electrolytes or between the solid electrolyte and the active material at the solid surface may be insufficient, resulting in reduced lithium ion conductivity. In the lithium ion conductive composite electrolyte according to the present embodiment, the mixed salt contains a first anion and a second anion having different chemical structures. Therefore, the mixed salt is less likely to crystallize, and the molten salt (also referred to as a molten salt) can maintain a liquid state (supercooled liquid state) even below its melting point. By incorporating such a mixed salt into the oxide solid electrolyte, even when there is insufficient contact between oxide solid electrolyte particles or between oxide solid electrolyte particles and active material particles, the mixed salt can penetrate into the voids between the particles and establish an ion conduction path between the particles. Therefore, when the lithium ion conductive composite electrolyte according to the present embodiment is applied to the electrolyte layer or electrode active material layer of a lithium secondary battery, the lithium ion conductivity of the lithium secondary battery can be improved, and thus the resistance can be reduced. In the lithium ion conductive composite electrolyte according to the present embodiment, it is preferable to lower the melting point of the mixed salt to make the mixed salt liquid or to maintain the liquid state of the mixed salt.

[0011] The configuration of the lithium ion conductive composite electrolyte according to this embodiment will be described in detail below.

[0012] (Oxide solid electrolyte) The oxide solid electrolyte is a solid electrolyte (lithium ion conductor) that essentially contains oxygen (O). From the viewpoint of further improving the lithium ion conductivity of the lithium ion conductive composite electrolyte, the lithium ion conductivity of the oxide solid electrolyte at room temperature (25°C) is set to 1 × 10 －５ S / cm or more, and preferably 1×10 －４ S / cm or more is more preferable, and 5×10 －４ S / cm or more is more preferable, and 1×10 －３ The lithium ion conductivity of the oxide solid electrolyte can be measured by an AC impedance method.

[0013] Examples of oxide solid electrolytes include compounds having a NASICON structure. An example of a compound having a NASICON structure is a compound represented by the general formula Li １＋ｘ Al ｘ Ge ２－ｘ (P.O. ４ ) ３ (0≦x≦2) (LAGP), a compound represented by the general formula Li １＋ｘ Al ｘ Ti ２－ｘ (P.O. ４ ) ３ (0≦x≦2) (LATP) and the like. Another example of the oxide solid electrolyte is LiLaTiO (for example, Li ０．３４ La ０．５１ TiO ３ ), LiPON (e.g., Li ２．９ P.O. ３．３ N ０．４６ ), LiLaZrO (e.g., Li ７ La ３ Zr ２ O １２ ) etc.

[0014] The solid electrolyte preferably has a particulate shape such as a spherical shape or an oval spherical shape. When the solid electrolyte is particulate, its average particle diameter (D ５０ ) is not particularly limited, but is preferably 0.01 μm or more and 40 μm or less, more preferably 0.05 μm or more and 20 μm or less, and even more preferably 0.2 μm or more and 5 μm or less. In this specification, the value of "average particle size" is the value measured as the volume-based median diameter (particle size for a cumulative distribution value of 50%) by a laser diffraction / scattering method.

[0015] The oxide solid electrolyte may be used alone or in combination of two or more kinds.

[0016] The oxide solid electrolyte has not been sintered. In this specification, "sintering" refers to a phenomenon in which powder particles are firmly adhered to each other and solidified by external energy (e.g., heat). The sintering process varies depending on the type of oxide solid electrolyte to be sintered. For example, when the sintering process is performed by heat treatment, the oxide solid electrolyte is subjected to conditions of 1000°C or higher, 800°C or higher, 600°C or higher, 500°C or higher, 400°C or higher, 300°C or higher, or 200°C or higher.

[0017] [Mixed Salt] The mixed salt essentially contains a first anion, a second anion, and a lithium ion, and may further contain a third anion and an alkali metal ion (excluding lithium ion). Here, the first anion and the second anion may each form a salt with a lithium ion as a counter ion. In other words, when focusing on the raw materials, the mixed salt can also be said to contain a mixture of a salt consisting of a first anion and a lithium ion (hereinafter simply referred to as a "first lithium salt") and a salt consisting of a second anion and a lithium ion (hereinafter simply referred to as a "second lithium salt"). Similarly, the third anion and an alkali metal ion can form a salt with each other as a counter ion. In other words, when focusing on the raw materials, the mixed salt can also be said to contain a salt consisting of a third anion and an alkali metal ion (hereinafter simply referred to as an "alkali metal salt").

[0018] (First anion and second anion) The first anion and the second anion are not particularly limited in type as long as they have different chemical structures. Examples of the first anion and the second anion include an anion represented by Formula 1: ((C ｍ F ２ｍ＋１ ) SO ２ ) ((C ｎ F ２ｎ＋１ ) SO ２ ) N － (in formula 1, m and n each independently represent an integer of 0 to 4), a fluorine-containing sulfonylimide anion represented by the following formula 2, a fluorine-containing sulfonylimide anion represented by the following formula 3, PF ６ － (hexafluorophosphate ion), BF ４ － (tetrafluoroborate ion), ClO ４ － (perchlorate ion), AsF ６ － (hexafluoroarsenate ion), CF ３ COO － (trifluoroacetate ion), B(C ２ O ４ ) ２ － (bis(oxalato)borate ion). Among these, at least one of the first anion and the second anion is preferably a fluorine-containing sulfonylimide anion represented by the above formula 1, and it is more preferable that the first anion and the second anion are fluorine-containing sulfonylimide anions represented by the above formula 1.

[0019]

[0020] In the above formula 1, m and n each independently represent an integer of 0 to 4, preferably an integer of 0 to 3, more preferably an integer of 0 to 2, and even more preferably 0 or 1. When m and n are within the above ranges, the melting point of the mixed salt can be lowered.

[0021] In the lithium ion conductive composite electrolyte according to this embodiment, the mixed salt preferably contains a bis(fluorosulfonyl)imide anion in which m = n = 0 in the above formula 1 (i.e., the first anion is a bis(fluorosulfonyl)imide anion in which m = n = 0 in the above formula 1). Since lithium bis(fluorosulfonyl)imide has a low melting point of 145°C, such a configuration can further lower the melting point of the mixed salt.

[0022] In the lithium ion conductive composite electrolyte according to this embodiment, at least one of the first anion and the second anion is preferably an asymmetric anion where m≠n in Formula 1, and more preferably the first anion and the second anion are asymmetric anions where m≠n in Formula 1. By adopting such a configuration, a salt formed from the asymmetric anion is less likely to crystallize, and the melting point of the mixed salt can be lowered. In a preferred embodiment, when m≠n, at least one of m and n is 1 or less.

[0023] When the first anion is a bis(fluorosulfonyl)imide anion in which m = n = 0 in the above formula 1, the content of the first anion in the mixed salt is preferably 20 to 80 mol%, more preferably 30 to 70 mol%, and even more preferably 40 to 60 mol%, based on the total number of moles of the first anion and the second anion. This configuration can further lower the melting point of the mixed salt. In one embodiment, when the first anion is a bis(fluorosulfonyl)imide anion in which m = n = 0 in the above formula 1, the content of the first anion in the mixed salt is preferably 40 to 80 mol%, more preferably 50 to 70 mol%, and even more preferably 55 to 65 mol%, based on the total number of moles of the first anion and the second anion.

[0024] (Lithium ion) Lithium ions can be present in the mixed salt as counter ions of the first anion and the second anion. When the mixed salt consists only of the first anion, the second anion, and lithium ions, the number of moles of lithium ions is equal to the total number of moles of the first anion and the second anion.

[0025] (Tertiary Anion and Alkali Metal Ion) The mixed salt may further contain a tertiary anion and an alkali metal ion other than lithium ion (also simply referred to as "alkali metal ion"). By containing the tertiary anion and the alkali metal ion, the melting point of the mixed salt may be lowered.

[0026] The type of third anion is not particularly limited. Examples of the third anion include the same anions used as the first anion and second anion. Among these, the third anion is preferably a fluorine-containing sulfonylimide anion represented by the above formula 1. In this case, in the above formula 1, m and n are each independently preferably an integer of 0 to 3, more preferably an integer of 0 to 2, even more preferably 0 or 1, and particularly preferably 0. When m and n are within the above ranges, the melting point of the mixed salt may be lowered. The third anion may be used alone, or two or more types may be used in combination. The third anion may be the same as or different from either or both of the first anion and second anion.

[0027] The type of alkali metal ion is not particularly limited, but is preferably at least one selected from the group consisting of sodium ions, potassium ions, rubidium ions, and cesium ions. The alkali metal ions may be used alone or in combination of two or more.

[0028] When the mixed salt contains a third anion and an alkali metal ion, the content of the alkali metal ion is preferably more than 0 mol % and not more than 33 mol %, more preferably more than 1 mol % and not more than 30 mol %, even more preferably 2 mol % to 25 mol %, particularly preferably 3 mol % to 20 mol %, and most preferably 4 mol % to 15 mol % relative to the total number of moles of the lithium ion and alkali metal ion contained in the mixed salt. When the content of the alkali metal ion is within the above range, it is possible to further lower the melting point of the mixed salt while suppressing a decrease in the transport number.

[0029] The mixed salt can be prepared by mixing a salt consisting of a first anion and lithium ions (first lithium salt), a salt consisting of a second anion and lithium ions (second lithium salt), and an optionally added salt consisting of a third anion and alkali metal ions (alkali metal salt). More specifically, first, predetermined amounts of the first lithium salt, the second lithium salt, and the optionally added alkali metal salt are weighed out. These are then mixed while heating. The heating temperature is not particularly limited, but is preferably set to a temperature equal to or higher than the lower of the melting points of the first and second lithium salts, and is preferably set to a temperature equal to or higher than the reference temperature but not higher than about 15°C above the reference temperature, and more preferably set to a temperature equal to or higher than the reference temperature but not higher than about 10°C above the reference temperature. This allows one salt to melt, and then the other salt dissolves in the molten salt. This allows a mixed salt in which the ions are uniformly mixed. The mixed salt obtained in this manner may have a melting point lower than the reference temperature. That is, according to a preferred embodiment, the melting point of the mixed salt is lower than the melting points of the salt composed of the first anion and lithium ions and the salt composed of the second anion and lithium ions. This configuration allows the mixed salt to melt at a lower temperature. In this specification, the melting point can be determined as the temperature corresponding to the maximum value in the heat of fusion curve when the temperature is increased at a rate of 10°C / min using a differential scanning calorimetry (DSC) device.

[0030] The melting point of the mixed salt is not particularly limited, but is preferably 180° C. or lower, more preferably 160° C. or lower, even more preferably 140° C. or lower, and particularly preferably 130° C. or lower. The melting point of the mixed salt is preferably 10° C. or higher.

[0031] The number of types of anions in the mixed salt is not particularly limited as long as it is two or more, but is, for example, 2 to 5, preferably 2 to 4, more preferably 2 to 3, and particularly preferably 2.

[0032] Furthermore, when the first anion is a bis(fluorosulfonyl)imide anion in which m = n = 0 in the above formula 1, the content of the first anion in the mixed salt is preferably 40 to 80 mol %, more preferably 50 to 70 mol %, and even more preferably 55 to 65 mol %, relative to the total number of moles of all anions in the mixed salt.

[0033] The lithium ion conductive composite electrolyte according to this embodiment may contain anions and cations other than the first anion, the second anion, and lithium ions, as well as optional third anions and alkali metal ions, as long as the effects of the present invention are not significantly impaired. However, from the viewpoint of suppressing a decrease in the transference number, it is preferable that the electrolyte is substantially free of cations other than lithium ions and alkali metal ions, and more preferably substantially free of organic cations. In this specification, "organic cation" refers to an ion containing a carbon atom that has a positive charge. In this specification, "substantially free" means that the content is 3% by mass or less, preferably 2% by mass or less, more preferably 1% by mass or less, even more preferably 0.1% by mass or less, and particularly preferably 0% by mass.

[0034] In the lithium ion conductive composite electrolyte according to this embodiment, the content of the oxide solid electrolyte is preferably 20 to 95 mass %, more preferably 20 to 70 mass %, even more preferably 25 to 50 mass %, and even more preferably 30 mass % or more but less than 50 mass %, relative to 100 mass % of the total mass of the lithium ion conductive composite electrolyte. When the content of the oxide solid electrolyte is within the above range, high lithium ion conductivity can be ensured.

[0035] The lithium ion conductive composite electrolyte according to the present embodiment contains an oxide solid electrolyte and a mixed salt. As described above, the mixed salt contains a first anion and a second anion having different chemical structures, making it difficult to crystallize. The molten salt can maintain a liquid state (supercooled liquid state) even below its melting point. Therefore, when the mixed salt is in a liquid state, the lithium ion conductive composite electrolyte becomes a mixture of a solid and a liquid. In this case, the lithium ion conductive composite electrolyte is preferably clay-like. In this specification, "clay-like" refers to a state in which a material is plastically deformable. Here, "plastic deformation" refers to a deformation that persists even after an external force is applied to an object and then removed. Therefore, a state in which the shape changes after the external force is removed (a paste-like state) or a state in which the material crumbles without deforming when an external force is applied (a cake-like state) is not considered clay-like. The lithium ion conductive composite electrolyte can be made clay-like by changing the content of the mixed salt in the lithium ion conductive composite electrolyte or by changing the size of the solid content (particularly, the particle size of the oxide solid electrolyte). The lithium ion conductive composite electrolyte approaches a paste state as the content of the mixed salt increases and the size of the solid content decreases.

[0036] The mass ratio of the oxide solid electrolyte to the mixed salt (mass of the oxide solid electrolyte:mass of the mixed salt) is, for example, 1:9 to 9:1, preferably 3:7 to 6:4, more preferably 4:6 to 5:5, and particularly preferably 4.0:6.0 to 4.9:5.1. When the mass ratio of the oxide solid electrolyte to the mixed salt is within the above range, the lithium ion conductive composite electrolyte can have good properties, such as a clay-like state.

[0037] Furthermore, it is preferable that the volume of the mixed salt is larger than the volume of voids when the solid content (excluding the mixed salt) contained in the lithium ion conductive composite electrolyte is packed most densely. This configuration allows a sufficient amount of the mixed salt to be present in the gaps between particles of the oxide solid electrolyte, thereby further improving the lithium ion conductivity of the lithium ion conductive composite electrolyte. In this specification, the "volume of voids when the solid content (excluding the mixed salt) is packed most densely" is defined as the value obtained by placing the solid content in a 10 mL glass measuring cylinder, measuring the filling volume after tapping 200 times, and subtracting the solid content volume from the filling volume. The solid content volume can be calculated by multiplying the mass of each component contained in the solid content by the true density.

[0038] The lithium ion conductive composite electrolyte can be produced, for example, by appropriately mixing the above-mentioned mixed salt with an oxide solid electrolyte. Specifically, the lithium ion conductive composite electrolyte can be produced by mixing the above-mentioned mixed salt with an oxide solid electrolyte using a stirrer such as a hot magnetic stirrer at a temperature equal to or higher than the melting point of the mixed salt (e.g., 120 to 200°C (393 to 473K), preferably 140 to 180°C (413 to 453K)).

[0039] <Lithium Secondary Battery> By applying the above-described lithium ion conductive composite electrolyte to the electrolyte layer and / or electrode active material layer of a lithium secondary battery, it is possible to improve the lithium ion conductivity of the lithium secondary battery. That is, according to the present invention, there is also provided a lithium secondary battery including: an electrolyte layer containing the lithium ion conductive composite electrolyte; an electrode active material layer containing the lithium ion conductive composite electrolyte, an electrode active material, and a conductive additive; and a lithium secondary battery including battery elements having: a positive electrode having a positive electrode active material layer, a negative electrode having a negative electrode active material layer, and an electrolyte layer interposed between the positive electrode and the negative electrode, wherein at least one of the electrolyte layers is the electrolyte layer and / or at least one of the positive electrode active material layer and the negative electrode active material layer is the electrode active material layer.

[0040] Hereinafter, an embodiment of a lithium secondary battery according to one aspect 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 duplicated explanations will be omitted. Furthermore, the dimensional proportions in the drawings are exaggerated for the sake of explanation and may differ from the actual proportions.

[0041] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) lithium secondary battery (hereinafter also simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. The stacked-type secondary battery 10a shown in FIG. 1 has a structure in which a substantially rectangular battery element 21, in which charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior. Here, the battery element 21 has a structure in which a negative electrode, an 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 are disposed on the surface of the negative electrode current collector 11'. The positive electrode has a structure in which a positive electrode active material layer 15 are 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 electrolyte layer 17, the positive electrode active material layer 15, and the positive electrode current collector 11" constitute one single cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can be said to have a configuration 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 extended 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 battery elements 21 by a pressure member (not shown). Therefore, the volume of the battery elements 21 is kept constant.

[0042] The main components of the lithium secondary battery according to this embodiment will be described below.

[0043] [Current Collector] The current collector (negative electrode current collector, positive electrode current collector) has the function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There are no particular restrictions on the material that constitutes the current collector. Examples of materials that can be used for the current collector include metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins. There are also no particular restrictions on the thickness of the current collector, but an example is 10 to 100 μm.

[0044] [Negative Electrode Active Material Layer] The negative electrode active material layer essentially contains a negative electrode active material. The negative electrode active material layer is not particularly limited, but preferably contains the lithium ion conductive composite electrolyte. The negative electrode active material layer is typically disposed on the surface of a negative electrode current collector as shown in FIG. 1 . However, if the negative electrode active material layer 13 itself has a certain degree of conductivity, the negative electrode active material layer itself can constitute the negative electrode without using a negative electrode current collector.

[0045] The type of negative electrode active material is not particularly limited, and examples include carbon materials, metal oxides, and metal active materials. Alternatively, lithium-containing active materials such as lithium metal and lithium-containing alloys may be used as the negative electrode active material. Examples of lithium-containing alloys include alloys of Li with at least one of In, Al, Si, Sn, Mg, Au, Ag, and Zn. When lithium metal or a lithium-containing alloy is used as the negative electrode active material, the lithium secondary battery is preferably a so-called lithium deposition type in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during the charging process. The layer of lithium metal deposited on the negative electrode current collector during this charging process becomes the negative electrode active material layer. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharging process progresses. The negative electrode active material layer need not be present during full discharge; however, in some cases, a negative electrode active material layer composed of a certain amount of lithium metal may be present during full discharge.

[0046] The negative electrode active material layer contains the lithium ion conductive composite electrolyte and a conductive additive as needed, which can improve the lithium ion conductivity and electrical conductivity of the negative electrode active material layer.

[0047] Examples of conductive additives that can be used in the negative electrode active material layer include, but are not limited to, 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.). In addition, particulate ceramic materials or resin materials coated with the above-mentioned metal materials by plating or the like can also be used as conductive additives.

[0048] The negative electrode active material layer may further 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 styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), and carboxymethyl cellulose (CMC). Among these, styrene-butadiene rubber, tetrafluoroethylene, and polyvinylidene fluoride are preferred, and tetrafluoroethylene and polyvinylidene fluoride are more preferred. These binders may be used alone or in combination of two or more.

[0049] The thickness of the negative electrode active material layer (in the case of a lithium deposition type, the thickness at full charge) differs depending on the configuration of the intended lithium secondary battery, but is preferably within the range of, for example, 0.1 to 1000 μm.

[0050] [Electrolyte Layer] The electrolyte layer is interposed between the negative electrode and the positive electrode and essentially contains an electrolyte. The electrolyte is not particularly limited, but preferably contains the lithium ion conductive composite electrolyte. By using such a configuration, the lithium ion conductivity of the electrolyte layer can be improved.

[0051] When the electrolyte layer contains the lithium ion conductive composite electrolyte, it is preferable that the volume of the mixed salt is larger than the volume of voids when the solid content (excluding the mixed salt) contained in the electrolyte layer is most densely packed. By adopting such a configuration, a sufficient amount of the mixed salt is present in the gaps between particles of the oxide solid electrolyte, thereby further improving the lithium ion conductivity of the electrolyte layer.

[0052] The content of the electrolyte in the 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.

[0053] The electrolyte layer may further contain a binder in addition to the electrolyte. The binder that can be used in the electrolyte layer is the same as that described above for the negative electrode active material layer.

[0054] The thickness of the electrolyte layer varies depending on the intended configuration of the lithium secondary battery, but is usually 0.1 to 1000 μm, preferably 10 to 40 μm.

[0055] [Positive Electrode Active Material Layer] The positive electrode active material layer essentially contains a positive electrode active material. The positive electrode active material layer is not particularly limited, but preferably contains the lithium ion conductive composite electrolyte. The positive electrode active material layer is typically disposed on the surface of a positive electrode current collector as shown in FIG. 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.

[0056] 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. Specific examples of lithium-containing metal oxides include 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 , LiMnPO４ Olivine type active materials such as Li ２ FeSiO ４ , Li ２ MnSiO ４ 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.

[0057] In addition, a sulfur-based positive electrode active material is also used as a preferred embodiment. Examples of the sulfur-based positive electrode active material include particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and any material can be used as long as it is capable of releasing lithium ions during charging and absorbing lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur.

[0058] The positive electrode active material layer contains the lithium ion conductive composite electrolyte and a conductive additive as needed. This configuration improves the lithium ion conductivity and electrical conductivity of the positive electrode active material layer. The conductive additive that can be used in the positive electrode active material layer is the same as that described for the negative electrode active material layer.

[0059] The positive electrode active material layer may further contain a binder as needed. The binder that can be used in the positive electrode active material layer is the same as that described above for the negative electrode active material layer.

[0060] The thickness of the positive electrode active material layer varies depending on the configuration of the intended lithium secondary battery, but is, for example, 0.1 to 1000 μm, preferably 30 to 300 μm, more preferably 50 to 200 μm, and even more preferably 70 to 150 μm.

[0061] The mixed salt contained in the lithium secondary battery becomes solid at a temperature lower than its melting point, which may make it difficult for the mixed salt to fill the gaps between particles. Even in such a case, for example, by raising the temperature of the lithium secondary battery above its melting point to melt (liquefy) the mixed salt, the gaps between particles can be filled with the mixed salt again, thereby suppressing or eliminating the decrease in lithium ion conductivity due to poor contact between particles.

[0062] [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.

[0063] [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.).

[0064] [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 battery element, as shown in FIG. 1, can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited 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 as the exterior material because it allows for easy adjustment of the collective pressure applied to the battery element from the outside.

[0065] The 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.

[0066] The following items are also included within the scope of the present invention: Item 1: A lithium ion conductive composite electrolyte comprising an oxide solid electrolyte and a mixed salt containing a first anion, a second anion, and lithium ions, wherein the oxide solid electrolyte is not sintered; Item 2: The lithium ion conductive composite electrolyte according to Item 1, wherein the melting point of the mixed salt is lower than the melting point of the salt composed of the first anion and lithium ions and the melting point of the salt composed of the second anion and lithium ions; Item 3: The lithium ion conductive composite electrolyte according to Item 1 or 2, wherein the melting point of the mixed salt is 180° C. or lower (preferably 160° C. or lower, more preferably 140° C. or lower, even more preferably 130° C. or lower, particularly preferably 10° C. or higher and 130° C. or lower); Item 4: The first anion and the second anion are each represented by Formula 1: ((C ｍ F ２ｍ＋１ ) SO ２ ) ((C ｎ F ２ｎ＋１ ) SO ２ ) N －(In Formula 1, m and n are each independently an integer of 0 to 4 (preferably 0 and 1)), the lithium ion conductive composite electrolyte according to any one of Items 1 to 3; Item 5: The lithium ion conductive composite electrolyte according to Item 4, wherein the first anion is a bis(fluorosulfonyl)imide anion in which m = n = 0 in Formula 1; Item 6: The lithium ion conductive composite electrolyte according to Item 4 or Item 5, wherein the second anion is a bis(fluorosulfonyl)imide anion in which m = n = 1 in Formula 1; Item 7: The lithium ion conductive composite electrolyte according to any one of Items 4 to 6, wherein at least one of the first anion and the second anion is an asymmetric anion in Formula 1 in which m ≠ n (preferably at least one of m and n is 1 or less); Item 8: The lithium ion conductive composite electrolyte according to any one of Items 4 to 7, wherein the content of the first anion is 20 to 80 mol % (preferably 40 to 60 mol %) relative to the total number of moles of the first anion and the second anion; Item 9: The lithium ion conductive composite electrolyte according to any one of Items 5 to 7, wherein the first anion is a bis(fluorosulfonyl)imide anion in which m = n = 0 in Formula 1, and the content of the first anion is 50 to 70 mol % (preferably 55 to 65 mol %) relative to the total number of moles of the first anion and the second anion; Item 10: The lithium ion conductive composite electrolyte according to any one of Items 1 to 9, wherein the mixed salt further contains a third anion and an alkali metal ion (excluding lithium ion); Item 11: The lithium ion conductive composite electrolyte according to Item 10, wherein the alkali metal ion is at least one selected from the group consisting of sodium ion, potassium ion, rubidium ion, and cesium ion; Item 12: The lithium ion conductive composite electrolyte according to Item 9 or Item 10, wherein the content of the alkali metal ions is more than 0 mol % and 33 mol % or less with respect to the total number of moles of the lithium ions and the alkali metal ions contained in the mixed salt;Item 13: The lithium ion conductive composite electrolyte according to any one of Items 1 to 12, which is substantially free of organic cations (for example, imidazolium cations, pyridinium cations, quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations); Item 14: The lithium ion conductive composite electrolyte according to any one of Items 1 to 13, which is clay-like; Item 15: The lithium ion conductive composite electrolyte according to any one of Items 1 to 14, in which the content of the oxide solid electrolyte is 20 to 95% by mass (preferably 20 to 70% by mass, and more preferably 30% by mass or more and less than 50% by mass) relative to 100% by mass of the total mass of the lithium ion conductive composite electrolyte; Item 16: The lithium ion conductive composite electrolyte according to any one of Items 1 to 15, in which the volume of the mixed salt is larger than the volume of voids when the solid content (excluding the mixed salt) contained in the lithium ion conductive composite electrolyte is most densely packed; Item 17: An electrolyte layer comprising the lithium ion conductive composite electrolyte according to any one of Items 1 to 16; Item 18: An electrolyte layer according to Item 17, wherein the volume of the mixed salt is larger than the volume of voids when the solid content (excluding the mixed salt) contained in the electrolyte layer is most densely packed; Item 19: An electrode active material layer comprising the lithium ion conductive composite electrolyte according to any one of Items 1 to 16, an electrode active material, and a conductive additive; Item 20: A lithium secondary battery comprising a battery element having: a positive electrode having a positive electrode active material layer; a negative electrode having a negative electrode active material layer; and an electrolyte layer interposed between the positive electrode and the negative electrode, wherein at least one of the electrolyte layers is the electrolyte layer according to Item 17 or 18, and / or at least one of the positive electrode active material layer and the negative electrode active material layer is the electrode active material layer according to Item 19;

[0067] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. Note that the following operations were carried out in a glove box with an argon atmosphere at a dew point of -68°C or less. Furthermore, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0068] <Examples of Preparation of Lithium Ion Conductive Composite Electrolyte> [Example 1] (Preparation of Mixed Salt) Lithium bis(fluorosulfonyl)imide (chemical formula: LiN(SO )) was used as a salt composed of a first anion and lithium ions. ２ F) ２ , manufactured by Nippon Shokubai Co., Ltd., melting point 418 K (145°C), hereinafter also referred to as "LiFSA"), and lithium bis(trifluoromethanesulfonyl)imide (chemical formula: LiN(SO ２ CF ３ ) ２ , manufactured by Kishida Chemical Co., Ltd., melting point 511 K (238°C), hereinafter also referred to as "LiTFSA") were used. － " is a bis(fluorosulfonyl)imide anion (i.e., N(SO ２ F) ２ － ), "TFSA － " is a bis(trifluoromethanesulfonyl)imide anion (i.e., N(SO ２ CF ３ ) ２ － ) means

[0069] A total of 2 g of LiFSA and LiTFSA were weighed out so that the molar ratio of LiFSA:LiTFSA was 6:4, and the mixture was placed in a container with LiFSA on the bottom and LiTFSA on the top, and heated to 155°C (428K) using a hot magnetic stirrer. After maintaining the temperature for 10 minutes, a stirrer was placed in the container, and the mixture was stirred at 50 to 100 rpm (1 rpm = 0.017 s) using a hot magnetic stirrer at 155°C (428K). －１ ) for 1 hour, and the liquid mixed salt (Li([FSA] ０．６ [TFSA] ０．４ ), melting point 391 K (118° C.). When the liquid mixed salt was stored at room temperature, it maintained a supercooled liquid state for about one month.

[0070] (Preparation of Electrolyte) The mixed salt (Li([FSA] ０．６ [TFSA]０．４ )) and a stirrer were placed in a container, and Li as an unsintered oxide solid electrolyte was added. １．３ Al ０．３ Ti １．７ (P.O. ４ ) ３ (manufactured by Toshima Manufacturing Co., Ltd., average particle size: 1 μm, also referred to as "LATP" in this specification) was added to LATP:Li([FSA] ０．６ [TFSA] ０．４ ) = 0.9:1 (mass ratio), and the mixture was heated to 155°C (428K) using a hot magnetic stirrer, and then stirred at the same temperature using the hot magnetic stirrer at a rotation speed of 50 to 300 rpm for 1 hour, thereby obtaining a lithium ion conductive composite electrolyte of Example 1.

[0071] Comparative Example 1 Without adding any mixed salt, LATP (manufactured by Ohara Corporation, average particle size: 1 μm) was used as an unsintered oxide solid electrolyte as is for this comparative example.

[0072] <Measurement of ionic conductivity> 100 mg of the electrolyte of Example 1 was placed on a cylindrical stainless steel (SUS) electrode. Another cylindrical stainless steel electrode was then placed on top to sandwich the electrolyte, and the electrodes were pressed at a pressure of 40 MPa using a uniaxial press to produce a laminate consisting of SUS / electrolyte / SUS. A laminate was also obtained using the electrolyte of Comparative Example 1 in the same manner.

[0073] The laminates of Example 1 and Comparative Example 1 obtained above were subjected to AC impedance measurement (PEIS). From the results of this AC impedance measurement, the ionic conductivity values ​​for lithium ions of the electrolytes of Example 1 and Comparative Example 1 were calculated. The measurements were carried out on a hot magnetic stirrer set at 80°C, with a measurement frequency of 10 mHz to 50 kHz, an applied voltage of OCV, and a voltage amplitude of 500 mV. As a result, the ionic conductivity of the electrolyte of Example 1 was 5.6 × 10 －６ In the measurement of Comparative Example 1, the electrolyte was not conductive and measurement was not possible.

[0074] From the above results, it was found that the electrolyte of Example 1 had sufficient ionic conductivity, even though it contained an oxide solid electrolyte and did not undergo a sintering process.

[0075] This application is based on Japanese Patent Application No. 2024-131944, filed on August 8, 2024, the disclosure of which is hereby incorporated by reference in its entirety.

[0076] 10a: laminated secondary battery, 11': negative electrode current collector, 11'': positive electrode current collector, 13: negative electrode active material layer, 15: positive electrode active material layer, 17: electrolyte layer, 19: single cell layer, 21: battery element, 25: negative electrode current collector, 27: positive electrode current collector, 29: laminate film.

Claims

1. A lithium ion conductive composite electrolyte comprising: an oxide solid electrolyte; and a mixed salt containing a first anion, a second anion, and lithium ions, wherein the oxide solid electrolyte has not been sintered.

2. The lithium ion conductive composite electrolyte according to claim 1, wherein the melting point of the mixed salt is lower than the melting point of the salt formed from the first anion and lithium ions and the melting point of the salt formed from the second anion and lithium ions.

3. The first anion and the second anion are each represented by Formula 1: ((C ｍ F ２ｍ＋１ ) SO ２ ) ((C ｎ F ２ｎ＋１ ) SO ２ ) N － 3. The lithium ion conductive composite electrolyte according to claim 2, represented by the formula (1): wherein m and n are each independently an integer of 0 to 4.

4. The lithium ion conducting composite electrolyte of claim 3, wherein the first anion is a bis(fluorosulfonyl)imide anion in which m=n=0 in formula 1.

5. The lithium ion conductive composite electrolyte according to claim 3, wherein at least one of the first anion and the second anion is an asymmetric anion where m≠n in formula 1.

6. The lithium ion conductive composite electrolyte according to claim 4, wherein the content of the first anion is 20 to 80 mol % based on the total number of moles of the first anion and the second anion.

7. The lithium ion conducting composite electrolyte of claim 1, wherein the mixed salt further comprises a third anion and an alkali metal ion (but excluding lithium ion).

8. The lithium ion conductive composite electrolyte according to claim 7, wherein the content of the alkali metal ions is more than 0 mol % and not more than 33 mol % based on the total number of moles of the lithium ions and the alkali metal ions contained in the mixed salt.

9. The lithium ion conducting composite electrolyte of claim 1, which is substantially free of organic cations.

10. The lithium ion conducting composite electrolyte of claim 1, which is clay-like.

11. The lithium ion conductive composite electrolyte according to claim 10, wherein the content of the oxide solid electrolyte is 20 to 95 mass % relative to 100 mass % of the total mass of the lithium ion conductive composite electrolyte.

12. An electrolyte layer comprising the lithium ion conducting composite electrolyte of claim 1.

13. The electrolyte layer according to claim 12, wherein the volume of the mixed salt is larger than the volume of voids when the solid content (excluding the mixed salt) contained in the electrolyte layer is packed most densely.

14. An electrode active material layer comprising the lithium ion conductive composite electrolyte according to claim 1, an electrode active material, and a conductive additive.

15. A lithium secondary battery comprising a battery element having: a positive electrode having a positive electrode active material layer; a negative electrode having a negative electrode active material layer; and an electrolyte layer interposed between the positive electrode and the negative electrode, wherein at least one of the electrolyte layers is the electrolyte layer defined in claim 12, and / or at least one of the positive electrode active material layer and the negative electrode active material layer is the electrode active material layer defined in claim 14.

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