Lithium-ion conductor and lithium secondary battery using same

Abstract

The present invention provides a means capable of improving electrochemical stability and purity in an LGPS type lithium-ion conductor. More specifically, the present invention relates to a lithium-ion conductor represented by composition formula 1. Composition formula 1: Li11.4-x-4y Si3-x+yP x S11.4-z Cl0.6 Oz, as defined in the description.

Description

Lithium ion conductor and lithium secondary battery using the same

[0001] The present invention relates to a lithium ion conductor and a lithium secondary battery using the same.

[0002] In recent years, research and development on all-solid-state lithium secondary batteries, which use lithium ion conductors (solid electrolytes) capable of conducting lithium ions in solids, has been actively pursued. All-solid-state lithium secondary batteries have the advantage that, in principle, they do not encounter the various problems associated with flammable organic electrolytes, as 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 power density and energy density.

[0003] As a lithium ion conductor, an LGPS type having a composition of Li-MP-S-X (M=Ge, Si, Sn; X=Cl, O) is known. ９．５４ Si １．７４ P １．４４ S １１．７ Cl ０．３ (LSiPSCl) exhibits extremely high ionic conductivity. However, due to its low electrochemical stability, it has the problem of side reactions occurring at the interface between the solid electrolyte and the active material, resulting in increased resistance. Furthermore, LSiPSCl is prone to the formation of impurity phases, which causes a problem of reduced ionic conductivity.

[0004] To address these issues, Yuxiang Li et al., Oxygen Substitution for Li-Si-PS-Cl Solid Electrolytes toward Purified Li 10 GeP2S 12 In "Li-Type Phase with Enhanced Electrochemical Stabilities for All-Solid-State Batteries," Chem. Mater. 2020, 32, 8860-8867, some of the sulfur (S) in LSiPSCl was replaced with oxygen (O). ９．５４ Si １．７４ P１．４４ S １１．７－ｚ Cl ０．３ O ｚ (0<z≦0.6) (LSiPSClO ｚ ) discloses that such oxygen substitution improves electrochemical stability and purity.

[0005] However, according to the investigations of the present inventors, the LSiPSClO ｚ However, there are cases where a lithium ion conductor having sufficient electrochemical stability cannot be obtained by this method, and further improvement has been desired.

[0006] Therefore, an object of the present invention is to provide a means for improving the electrochemical stability and purity of an LGPS-type lithium ion conductor.

[0007] The present inventors have conducted extensive research to solve the above problems, and in the process have found that the above problems can be solved by incorporating silicon (Si) as M and chlorine (Cl) and oxygen (O) as X in the Li-MP-S-X structure and controlling the composition within a predetermined range, which has led to the completion of the present invention.

[0008] That is, the lithium ion conductor according to one embodiment of the present invention is characterized in that it has a composition represented by the following composition formula 1.

[0009]

[0010] In the above composition formula 1, x is 1.5 or more and 1.6 or less, y is 0.1 or more and 0.2 or less, and z is more than 1.15 and 1.2 or less.

[0011] Fig. 1 is a cross-sectional view schematically showing the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (stacked-type secondary battery) according to one embodiment of the present invention. Fig. 2 shows diffraction patterns obtained by X-ray diffraction (XRD) measurement of the lithium ion conductors obtained in Comparative Example 4 and Example 3.

[0012] 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 forms. Therefore, the present invention is not limited to the configurations described in the embodiments below, and can be modified as appropriate based on the claims. Note that the range "X to Y" means "X or more and Y or less."

[0013] [Lithium ion conductor] One embodiment of the present invention is a lithium ion conductor represented by the following composition formula 1:

[0014]

[0015] In the above composition formula 1, x is 1.5 or more and 1.6 or less, y is 0.1 or more and 0.2 or less, and z is greater than 1.15 and 1.2 or less. According to the present invention, the electrochemical stability and purity of an LGPS-type lithium ion conductor can be improved. The mechanism by which the lithium ion conductor according to this embodiment exhibits the above-described effects is not completely clear, and the present invention is not bound by any theory. However, the following mechanism is presumed. In the above composition formula 1, z corresponds to the amount of oxygen atoms (O) present in the lithium ion conductor. In particular, in an LGPS-type lithium ion conductor, the electrochemical stability of the lithium ion conductor can be improved by substituting some of the sulfur atoms (S) with oxygen atoms. The reason why the electrical stability of the lithium ion conductor is improved by substituting sulfur atoms with oxygen atoms is presumed to be because the Si / P-O bond energy is higher than the Si / P-S bond energy in an LGPS-type solid. On the other hand, if the amount of oxygen atoms substituted is too large, an impurity phase other than the LGPS-type phase may be formed. The presence of an impurity phase is believed to reduce lithium ion conductivity. Furthermore, there is a possibility that many oxygen atoms may be inserted into the impurity phase rather than the LGPS-type lithium ion conductor phase, which may result in insufficient improvement in electrochemical stability. Furthermore, in the lithium ion conductor according to the present embodiment, the LGPS-type phase is believed to be further stabilized by controlling the values ​​of x and y in addition to the value of z in the composition formula 1 within the above ranges. One reason for this stabilization may be that, for example, by controlling the composition to the above range, the ratio of the volume of all cations to the volume of all anions contained in the lithium ion conductor is appropriately controlled, making it easier to form an LGPS-type crystal structure. It is presumed that this makes it difficult for an impurity phase to form, resulting in a lithium ion conductor with high purity and lithium ion conductivity.

[0016] In composition formula 1, z is preferably greater than 1.18 and not greater than 1.2. When z is within the above range, the electrochemical stability of the lithium ion conductor can be further improved.

[0017] Furthermore, x and y relate to the abundance ratios of Li, Si, and P in the lithium ion conductor. By controlling x and y within the above ranges, the purity of the lithium ion conductor can be improved. In composition formula 1, y is preferably 0.1 or more and 0.15 or less. When y is within the above range, the electrochemical stability is further improved.

[0018] In this specification, the purity of a lithium ion conductor is evaluated by X-ray diffraction (XRD) measurement. Specifically, when peaks other than those attributable to LGPS-type peaks are observed in a spectrum showing a diffraction pattern obtained under the conditions of X-ray diffraction (XRD) measurement described in the Examples, the lithium ion conductor is determined to have an impurity phase. For example, if the peak intensity of a peak appearing at a diffraction angle (2θ) of 29.3° to 29.9° attributable to an LGPS-type peak is taken as 100, when the peak intensity of a peak other than the LGPS-type peak (i.e., a peak of an impurity phase) is 4 or more, the lithium ion conductor is determined to have an impurity phase. When the peak intensity is less than 4, the lithium ion conductor is determined to be a single phase. When the purity of a lithium ion conductor decreases, the lithium ion conductivity of the lithium ion conductor may decrease.

[0019] Impurity phases that may occur during the production of lithium ion conductors include, for example, argyrodite-type solids (Li-P-S-Cl) and Li ３＋ｘ Si ｘ P １－ｘ S ４ Under the above measurement conditions, the argyrodite-type solid may exhibit a peak at a diffraction angle of, for example, about 25.0° to 26.0°. ３＋ｘ Si ｘ P １－ｘ S ４ The type solid may exhibit a peak at a diffraction angle of, for example, about 17.9° to 19.5°.

[0020] The lithium ion conductivity of the lithium ion conductor at 25° C. is, for example, 4.0 mS / cm or more, preferably 4.5 mS / cm or more, more preferably 5.0 mS / cm or more, even more preferably 5.2 mS / cm or more, and particularly preferably 5.5 mS / cm or more. There is no particular upper limit to the lithium ion conductivity of the lithium ion conductor, but it is, for example, 20 mS / cm or less. The lithium ion conductivity of the lithium ion conductor is a value measured by the method described in the examples.

[0021] The electrochemical stability of a lithium ion conductor is evaluated by the magnitude of the oxidation current value obtained by cyclic voltammetry. Specifically, the larger the oxidation current value obtained by the measurement method described in the Examples, the more easily the lithium ion conductor is oxidized and the lower its electrochemical stability. The oxidation current value of the lithium ion conductor obtained by the above measurement is, for example, 1.0 μA or less, preferably 0.60 μA or less, more preferably 0.55 μA or less, and even more preferably 0.50 μA or less. The lower limit of the oxidation current is not particularly limited, but is, for example, more than 0.00 μA or 0.01 μA or more.

[0022] The method for producing the lithium ion conductor is not particularly limited, and a conventionally known method for producing an LGPS-type solid (for example, the method described in the above-mentioned document) can be adopted. Specifically, for example, it can be produced by the method described in the Examples. That is, the raw material (Li ２ S, P ２ S ５ , SiS ２ , LiCl, GeS ２ and P ２ O ５ ) are weighed and mixed. The resulting mixture is mechanically pulverized using a pulverizer such as a planetary ball mill. The pulverized mixture is formed into pellets, which are gradually heated to an appropriate temperature, for example, 400°C to 500°C (673 to 773K), heated at the same temperature for a certain period of time, and then allowed to cool to room temperature (for example, 25°C (298K)), thereby producing a lithium ion conductor.

[0023] [Lithium Secondary Battery] A lithium secondary battery according to one embodiment of the present invention includes a power generating element including a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode active material layer containing a negative electrode active material, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte. The battery is also characterized in that at least one of the positive electrode active material layer and the solid electrolyte layer contains the lithium ion conductor described above. As described above, the lithium ion conductor has high purity and lithium ion conductivity while also exhibiting excellent electrochemical stability. Therefore, by using the lithium ion conductor as the solid electrolyte of a lithium secondary battery, the battery's performance, such as the capacity characteristics and / or high-rate characteristics, can be improved. The lithium ion conductor may be contained only in the positive electrode active material layer, only in the solid electrolyte layer, or in both the positive electrode active material layer and the solid electrolyte layer.

[0024] An embodiment of the present invention will be described below 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.

[0025] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. The stacked-type structure allows the battery to be compact and have a high capacity. In this specification, the stacked-type lithium ion secondary battery (hereinafter also simply referred to as a "stacked-type battery") shown in FIG. 1 will be described in detail as an example.

[0026] As shown in FIG. 1 , the stacked battery 10 a of this embodiment has a structure in which a flat, approximately rectangular power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior material. Here, the power generating element 21 has a configuration in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are laminated. The positive electrode has a structure in which positive electrode active material layers 15 containing a 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 a 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 laminated in this order so that one positive electrode active material layer 15 and the adjacent negative electrode active material layer 13 face each other with the solid electrolyte layer 17 interposed therebetween. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode constitute one cell layer 19. Therefore, the stacked battery 10a shown in FIG. 1 can be said to have a configuration in which a plurality of 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 power generation element 21 by a pressure member (not shown). Therefore, the volume of the power generation element 21 is kept constant.

[0027] The main components of the lithium secondary battery according to this embodiment will be described below.

[0028] [Current Collector] The current collector (negative electrode current collector, positive electrode current collector) has the function of mediating the transfer of electrons from the 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.

[0029] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as a main component). The solid electrolyte may contain the above-mentioned lithium ion conductor.

[0030] The solid electrolyte other than the lithium ion conductor contained in the solid electrolyte layer is not particularly limited, and any solid electrolyte known in the art can be appropriately used. ２ S-P ２ S ５ ), Li ６ P.S. ５ X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S and Li ３ P.S. ４ These sulfide solid electrolytes are preferably used because they have excellent lithium ion conductivity and a low bulk modulus, allowing them to follow the volume change of the electrode active material during charge and discharge.

[0031] The content of the lithium ion conductor is preferably 50 to 100 mass %, and more preferably 90 to 100 mass %, based on the total mass of the solid electrolyte present in the solid electrolyte layer. The content of the lithium ion conductor in the solid electrolyte layer is preferably 50 to 100 mass %, and more preferably 90 to 100 mass %, based on the total mass of the solid electrolyte layer. The content of the solid electrolyte in the solid electrolyte layer is preferably 50 to 100 mass %, and more preferably 90 to 100 mass %, based on the total mass of the solid electrolyte layer.

[0032] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. The type of binder is not particularly limited, and binders known in the art can be appropriately used. Examples include polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and carboxymethyl cellulose. The content of the binder in the solid electrolyte layer is not particularly limited, but is, for example, 1 to 20 mass%. The solid electrolyte layer may not contain a binder, and may contain only a lithium ion conductor as the solid electrolyte, for example.

[0033] The thickness of the solid 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.

[0034] [Positive Electrode Active Material Layer] The positive electrode active material layer essentially contains a positive electrode active material, and may contain a solid electrolyte, a binder, a conductive additive, and the like, as necessary.

[0035] In addition, examples of the positive electrode active material contained in the positive electrode active material layer other than the above include LiCoO ２ , LiMnO ２ , LiNiO ２ , LiVO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt active materials such as LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ Spinel-type active materials such as LiFePO ４ , LiMnPO ４ Olivine type active materials such as Li ２ FeSiO ４ , Li ２ MnSiO ４ In addition, examples of oxide active materials other than those mentioned above include Li ４ Ti ５ O １２ Among them, Li(Ni-Mn-Co)O ２Also, those in which part of these transition metals has been replaced with other elements (hereinafter also simply referred to as "NMC composite oxides") are preferably used as the positive electrode active material.

[0036] Furthermore, in one preferred embodiment, a sulfur-based positive electrode active material is used. 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.

[0037] The content of the positive electrode active material in the positive electrode active material layer is preferably 50 to 100 mass %, more preferably 55 to 95 mass %, and even more preferably 60 to 90 mass %, relative to the total mass of the positive electrode active material layer.

[0038] The solid electrolyte contained in the positive electrode active material layer may include the lithium ion conductor. The solid electrolyte other than the lithium ion conductor contained in the positive electrode active material layer is not particularly limited, and a known solid electrolyte may be used as appropriate. For example, a solid electrolyte other than the lithium ion conductor described in the solid electrolyte layer may be used.

[0039] The content of the lithium ion conductor in the solid electrolyte contained in the positive electrode active material layer is not particularly limited and may be, for example, 50 to 100 mass % or 90 to 100 mass % relative to the total mass of the solid electrolyte contained in the positive electrode active material layer. In one embodiment, the content may be 0 to 50 mass % or 0 to 10 mass %. The content of the lithium ion conductor in the positive electrode active material layer is not particularly limited and may be, for example, 50 to 100 mass % or 90 to 100 mass % relative to the total mass of the positive electrode active material layer. In one embodiment, the content may be 0 to 50 mass % or 0 to 10 mass %. The content of the solid electrolyte in the positive electrode active material layer is, for example, 1 to 70 mass %, preferably 5 to 50 mass %, relative to the total mass of the positive electrode active material layer.

[0040] The binder used in the positive electrode active material layer is not particularly limited, and any known binder can be used as appropriate. For example, the binder described above for the solid electrolyte layer can be used. The content of the binder in the positive electrode active material layer is not particularly limited, and is, for example, 1 to 10 mass %.

[0041] The conductive additive used in the positive electrode active material layer is not particularly limited, and may be, for example, carbon such as carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). The content of the conductive additive in the positive electrode active material layer is not particularly limited, and is, for example, 1 to 30 mass%.

[0042] The thickness of the positive electrode active material 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.

[0043] [Negative Electrode (Negative Electrode Active Material Layer)] In the lithium secondary battery according to the above embodiment, 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 examples include carbon materials, metal oxides, and metal active materials. Furthermore, a lithium-containing metal may be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is a lithium-containing active material, and examples thereof include metallic lithium and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li with at least one of In, Al, Si, and Sn. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than those specifically listed above may also be used. The negative electrode active material preferably contains metallic lithium from the viewpoint of improving the battery capacity. Furthermore, the negative electrode active material may consist solely of metallic lithium.

[0044] The negative electrode active material may be in the form of particles (spherical, fibrous), thin film, etc. When the negative electrode active material is in the form of particles, the average particle diameter is preferably in the range of, for example, 1 nm to 100 μm.

[0045] 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 mass %. The negative electrode active material layer may further contain a solid electrolyte, a conductive additive, and / or a binder, and specific and preferred forms thereof may be the same as those described in the section on the positive electrode active material layer above.

[0046] The thickness of the negative electrode active material layer varies depending on the intended configuration of the lithium secondary battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[0047] [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.

[0048] [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.).

[0049] [Battery Exterior Material] As the battery exterior material, a known metal can case can be used. Alternatively, a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power generating element 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 achieving high output and excellent cooling performance, making it suitable for use in batteries for large equipment such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable as the exterior material because it allows for easy adjustment of the collective pressure applied to the power generating element from the outside.

[0050] 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 driving power source for EVs and HEVs.

[0051] Although one embodiment of the secondary battery of the present invention has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0052] For example, the type of battery to which the lithium ion conductor according to the present invention can be applied includes a bipolar battery including a bipolar electrode having a positive electrode active material layer electrically bonded to one surface of a current collector and a negative electrode active material layer electrically bonded to the opposite surface of the current collector.

[0053] Furthermore, the lithium secondary battery according to the present embodiment does not have to be an all-solid-state type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolytic solution). There is no particular limitation on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferably an amount that allows the shape of the solid electrolyte layer formed by the solid electrolyte to be maintained and prevents leakage of the liquid electrolyte (electrolytic solution).

[0054] The following items are also included in the scope of the present invention: Item 1: A lithium ion conductor represented by the following composition formula 1:

[0055]

[0056] In the composition formula 1, x is 1.5 or more and 1.6 or less, y is 0.1 or more and 0.2 or less, and z is more than 1.15 and 1.2 or less; Item 2: The lithium ion conductor according to Item 1, in which z is more than 1.18 and 1.2 or less in the composition formula 1; Item 3: The lithium ion conductor according to Item 1 or 2, in which y is 0.1 or more and 0.15 or less in the composition formula 1; Item 4: The lithium ion conductor according to any one of Items 1 to 3, which contains an LGPS-type lithium ion conductor; Item 5: The lithium ion conductor according to any one of Items 1 to 4, which consists solely of an LGPS-type lithium ion conductor; Item 6: A lithium ion conductor according to any one of Items 1 to 5, wherein, when the peak intensity of a peak appearing at 29.3° to 29.9°, which is assigned to an LGPS-type peak in a spectrum obtained by X-ray diffraction measurement under the following conditions, is taken as 100, the peak intensity of a peak other than an LGPS-type peak (i.e., a peak of an impurity phase) is less than 4: [X-ray diffraction measurement] Measurement apparatus: SmartLab (manufactured by Rigaku Corporation) Measurement angle range: 2θ = 10° to 60° Step width (2θ / θ): 0.01° Radiation source: CuKα1 (λ = 1.5405 Å); Item 7: A lithium secondary battery comprising a power generating element including a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode active material layer containing a negative electrode active material, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, wherein at least one of the positive electrode active material layer and the solid electrolyte layer contains the lithium ion conductor according to any one of Items 1 to 6.

[0057] 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.

[0058] <Examples of Preparation of Lithium Ion Conductor> [Example 1] Li ２ S (purity>99.9%, manufactured by Mitsuwa Chemical Co., Ltd.), P ２ S ５ (purity>99%, manufactured by Sigma-Aldrich), SiS ２ (purity >99%, manufactured by Mitsuwa Chemicals Co.), LiCl (purity >99%, manufactured by Sigma-Aldrich), and P ２ O ５ (purity > 99.99%, manufactured by Kojundo Chemical Laboratory Co., Ltd.) was weighed out in total 2 g of raw materials so that the molar ratio of Li:Si:P:S:Cl:O was 9.5:1.6:1.5:10.2:0.6:1.2 (in this example, Li ２ 0.7868g of S, ２ S ５ 0.4314 g of SiS ２ 0.5615 g, LiCl 0.0968 g, P ２ O ５ 0.1296 g of the mixture was weighed out and stirred in a mortar for 15 minutes. The resulting mixture was placed in a zirconia pot containing zirconia balls (φ10 mm) and mechanically pulverized for 40 hours at 380 rpm using a planetary ball mill. The pulverized mixture was pressed into a disk (φ10 mm, thickness 1 mm), heated to 400-475°C over 3 hours, maintained at that temperature for an additional 8 hours, and then naturally cooled to 25°C. This yielded the lithium ion conductor of this example.

[0059] [Example 2] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５ The lithium ion conductor of this example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=9.1:1.7:1.5:10.2:0.6:1.2.

[0060] [Example 3] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５The lithium ion conductor of this example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=9.4:1.5:1.6:10.2:0.6:1.2.

[0061] [Example 4] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５ The lithium ion conductor of this example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=9.0:1.6:1.6:10.2:0.6:1.2.

[0062] [Comparative Example 1] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５ The lithium ion conductor of this comparative example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=9.7:1.55:1.5:10.2:0.6:1.2.

[0063] [Comparative Example 2] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５ The lithium ion conductor of this comparative example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=9.6:1.45:1.6:10.2:0.6:1.2.

[0064] [Comparative Example 3] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５The lithium ion conductor of this comparative example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=9.6:1.7:1.4:10.2:0.6:1.2.

[0065] [Comparative Example 4] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５ The lithium ion conductor of this comparative example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=9.3:1.4:1.7:10.2:0.6:1.2.

[0066] [Comparative Example 5] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５ The lithium ion conductor of this comparative example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=9.2:1.8:1.4:10.2:0.6:1.2.

[0067] [Comparative Example 6] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５ The lithium ion conductor of this comparative example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=8.9:1.75:1.5:10.2:0.6:1.2.

[0068] [Comparative Example 7] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５The lithium ion conductor of this comparative example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=8.8:1.65:1.6:10.2:0.6:1.2.

[0069] [Comparative Example 8] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５ The lithium ion conductor of this comparative example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=9.24:1.74:1.44:10.5:0.6:0.9.

[0070] [Comparative Example 9] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５ The lithium ion conductor of this comparative example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=9.25:1.6:1.55:10.15:0.6:1.25.

[0071] [Comparative Example 10] Li ２ S, P ２ S ５ , SiS ２ , LiCl and P ２ O ５ The lithium ion conductor of this comparative example was obtained in the same manner as in Example 1, except that the above components were weighed out so as to have a molar ratio of Li:Si:P:S:Cl:O=9.25:1.6:1.55:10.25:0.6:1.15.

[0072] <Evaluation of Lithium Ion Conductors (Presence or Absence of Impurity Phase)> The presence or absence of an impurity phase was examined by X-ray diffraction (XRD) measurement for the lithium ion conductors obtained in Examples 1 to 4 and Comparative Examples 1 to 10. The conditions for the XRD measurement were as follows.

[0073] (XRD measurement conditions) Measuring device: SmartLab (manufactured by Rigaku Corporation) Measuring angle range: 2θ=10° to 60° Step width (2θ / θ): 0.01° Radiation source: CuKα1 (λ=1.5405 Å).

[0074] 2 shows the diffraction patterns of the lithium ion conductors obtained in Example 3 and Comparative Example 4. As shown in FIG. 2, the diffraction patterns of the lithium ion conductors of Example 3 and Comparative Example 4 were confirmed to be similar to the diffraction patterns of known LGPS-type lithium ion conductors, although the peaks were slightly shifted. When the diffraction pattern of Comparative Example 4 was compared with the diffraction pattern of Example 3, only peaks derived from the LGPS-type phase were observed in Example 3, whereas in Comparative Example 4, only peaks derived from the Li ３＋ｘ Si ｘ P １－ｘ S ４ The peaks due to the LGPS-type crystal structure (the peaks indicated by ● in FIG. 2) were further observed. This indicates that the lithium ion conductor of Example 3 is a single phase consisting of the LGPS-type phase, whereas the lithium ion conductor obtained in Comparative Example 4 contains an impurity phase, Li, in addition to the LGPS-type phase. ３＋ｘ Si ｘ P １－ｘ S ４ In this way, the diffraction patterns of the lithium ion conductors of the examples and comparative examples were confirmed, and the purity was evaluated by assigning a grade of ◯ to a case in which only a peak of a single phase consisting of the LGPS-type phase was observed, and an x ​​to a case in which peaks of impurity phases were observed in addition to the LGPS-type phase. The results are shown in Table 1 below.

[0075] <Measurement of lithium ion conductivity of lithium ion conductor> The lithium ion conductivity of the lithium ion conductors of Examples 1 to 4 and Comparative Examples 1 to 10 was measured. Specifically, Au powder / LGPS-type lithium ion conductor / Au powder was compression-molded at a pressure of 570 MPa to prepare pellets. The lithium ion conductivity (25°C) of the pellets was measured by an AC impedance method. A frequency response analyzer (FRA) was used for the measurement, and the measurement conditions were an applied voltage of 0.01 V and a measurement frequency range of 1 to 10. ６The measurements were made in Hz and the results are shown in the table.

[0076] <Measurement of Oxidation Current (Cyclic Voltammetry)> (Preparation of Evaluation Cell) A stainless steel cylindrical convex punch (10 mm diameter, working electrode) was inserted into one side of a cylindrical tube jig (tube inner diameter 10 mm, outer diameter 23 mm, height 20 mm) manufactured by Macor, and 100 mg of the measurement target (LGPS-based lithium ion conductor) was placed from the upper side of the cylindrical tube jig. Then, another stainless steel cylindrical convex punch was inserted to sandwich the measurement target, and the measurement target was pressed at a pressure of 300 MPa using a hydraulic press for 1 minute to form a pellet of the measurement target with a diameter of 10 mm and a thickness of approximately 0.6 mm in the cylindrical tube jig. The lower stainless steel cylindrical convex punch was removed, and indium foil (9 mm diameter) and lithium foil (5 mm diameter, counter electrode / reference electrode) were inserted from the bottom of the cylindrical tube jig. The stainless steel cylindrical convex punch was then inserted again, and pressed at a pressure of 75 MPa for 10 seconds, to produce a test cell in which the working electrode (stainless steel), the object to be measured, and the counter electrode / reference electrode (lithium-indium) were stacked in this order.

[0077] (Measurement of Oxidation Current) The oxidation current was measured using cyclic voltammetry. The test cell was placed in a thermostatic chamber set at 25°C, and after the cell temperature became constant, oxidation was performed from the open circuit voltage (OCV) to 4.4 V vs. Li-In at a sweep rate of 10 mV / s, followed by reduction to 1.6 V vs. Li-In. In the first cycle, the current value at which the current reached a maximum within the voltage range of 2.5 to 4.0 V vs. Li-In was taken as the oxidation current. The results are shown in Table 1.

[0078]

[0079] From the results shown in Table 1, the lithium ion conductors of Examples 1 to 4, which are represented by the above composition formula 1 and satisfy the conditions of x being 1.5 or more and 1.6 or less, y being 0.1 or more and 0.2 or less, and z being more than 1.15 and 1.2 or less, exhibited good lithium ion conductivity, and the oxidation current observed in the region of 2.5 to 4.0 V was suppressed. In Comparative Examples 1 to 7, in which the values ​​of x and / or y were outside the range, an impurity phase was observed in addition to the LGPS-type lithium ion conductor phase, and compared to the lithium ion conductors of Examples 1 to 4, the lithium ion conductivity was low and the oxidation current value was equal to or larger. Furthermore, it was found that the lithium ion conductor of Comparative Example 8, in which the values ​​of x and z, which indicate the amount of oxygen substitution, were outside the range, formed a single phase and had good lithium ion conductivity, but had a large oxidation current value and low electrochemical stability. Furthermore, it was found that the lithium ion conductors of Comparative Examples 9 and 10, in which the value of z was outside the range, contained an impurity phase, and had lower lithium ion conductivity, larger oxidation current values, and lower electrochemical stability than the lithium ion conductors of Examples 1 to 4.

[0080] 10a: laminated 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: single cell layer, 21: power generating element, 25: negative electrode current collector, 27: positive electrode current collector, 29: laminate film.

Claims

1. A lithium ion conductor represented by the following composition formula 1: In the above composition formula 1, x is 1.5 or more and 1.6 or less, y is 0.1 or more and 0.2 or less, and z is more than 1.15 and 1.2 or less.

2. The lithium ion conductor according to claim 1, wherein in said composition formula 1, z is greater than 1.18 and not greater than 1.

2.

3. The lithium ion conductor according to claim 2, wherein in said composition formula 1, y is 0.1 or more and 0.15 or less.

4. A lithium secondary battery comprising a power generating element including: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode active material layer containing a negative electrode active material; and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, wherein at least one of the positive electrode active material layer and the solid electrolyte layer contains the lithium ion conductor according to any one of claims 1 to 3.

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