Solid electrolyte and all-solid battery

Abstract

The solid electrolyte is composed of a compound represented by the following general formula (1). Li x M2(PO4) z (1) (In general formula (1), M represents at least one element having a valence number of 1 valence to 4 valence, x represents a number satisfying 1.003 ≤ x ≤ 1.900, and z represents a number satisfying 3.001 ≤ z ≤ 3.200.)

Description

Technical Field

[0001] This invention relates to solid electrolytes and all-solid batteries.

[0002] This application claims priority to Japanese Patent Application No. 2019-227465, filed on December 17, 2019, the contents of which are incorporated herein by reference. Background Technology

[0003] In recent years, significant advancements in electronic technology have enabled the miniaturization, lightweighting, thinning, and multifunctionality of portable electronic devices. Consequently, there is a strong demand for smaller, lighter, thinner batteries, which power these devices, as well as improved reliability. All-solid-state batteries using solid electrolytes have thus attracted considerable attention.

[0004] To improve the cycle characteristics and other properties of all-solid-state batteries, increasing the ionic conductivity of the solid electrolyte is effective. Therefore, phosphoric acid compounds with a NASICON-type crystalline structure are widely used as solid electrolyte materials (Patent Documents 1-4). Phosphoric acid compounds with a NASICON-type crystalline structure are usually represented as LiM2(PO4)3. Here, M is a tetravalent metal.

[0005] Furthermore, to further improve ionic conductivity, technologies are being researched that utilize metals with valences of 1 to 3 to replace a portion of M. For example, Patent Document 4 describes a method using the chemical formula Li... 1+X M y A solid electrolyte material represented by (PO4)3 (where a portion of P can be substituted with at least one selected from Si, B, and V, and M contains at least one element that is a cation with a monovalent to tetravalent charge, -0.200≤x≤0.900, 2.001≤y≤2.200). According to Patent Document 4, when M is set to contain at least one element that is a cation with a monovalent to tetravalent charge, and is set to 2.001≤y≤2.200, the ionic conductivity of the solid electrolyte can be improved. Its PO4 composition ratio is lower than that of the usual phosphoric acid compound represented by LiM2(PO4)3 with a NASICON-type crystalline structure.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2007-258165

[0009] Patent Document 2: Japanese Patent Application Publication No. 2001-143754

[0010] Patent Document 3: Japanese Patent Application Publication No. 2015-065021

[0011] Patent Document 4: International Publication No. 2017 / 183255 Summary of the Invention

[0012] [The technical problem that the invention aims to solve]

[0013] To improve cycle performance, solid electrolytes with high ionic conductivity are desired. The object of this invention is to provide a novel solid electrolyte with improved ionic conductivity, and an all-solid-state battery using this solid electrolyte exhibiting excellent cycle performance.

[0014] [Technical solutions used to solve technical problems]

[0015] Therefore, the inventors have targeted the use of the general formula Li x M2(PO4) z The solid electrolyte with a NASICON-type crystalline structure was investigated in detail regarding the composition ratio of M and PO4. The results showed that setting the PO4 composition ratio in the range of 3.001 ≤ z ≤ 3.200 improved the ionic conductivity of the solid electrolyte. Furthermore, it was confirmed that all-solid-state batteries using solid electrolytes with a PO4 composition ratio greater than 3 exhibited excellent cycle characteristics, thus completing this invention.

[0016] In other words, in order to solve the above-mentioned technical problems, the present invention provides the following technical solutions.

[0017] [1] A first method provides a solid electrolyte consisting of a compound represented by the following general formula (1).

[0018] Li x M2(PO4) z (1)

[0019] (In general formula (1), M represents at least one element with a valence of 1 to 4, x represents a number that satisfies 1.003≤x≤1.900, and z represents a number that satisfies 3.001≤z≤3.200.)

[0020] [2] Alternatively, in the general formula (1), M contains at least one element selected from Na, K, Ag, Au, Ba, Cr, Mn, Fe, Co, Ni, Pd, Pt, Sc, Y, V, Nb, Ta, Ru, Rh, Ir, Al, Ga, In, Mo, W, Tc, Re, Os, Ti, Zr, Hf, Ge, Si, Sn.

[0021] [3] It can also be that in the general formula (1), M contains at least one element with a valence of 4.

[0022] [4] In the solid electrolyte of the above manner, the general formula (1) may also be represented by the following general formula (2).

[0023] Li x M' y M” 2－y (PO4) z (2)

[0024] (In general formula (2), M' represents at least one element selected from Na, K, Ag, Au, Ba, Cr, Mn, Fe, Co, Ni, Pd, Pt, Sc, Y, V, Nb, Ta, Ru, Rh, Ir, Al, Ga, In, Mo, W, Tc, Re, and Os, M” represents at least one element with a valence of 4, x represents a number satisfying 1.003≤x≤1.900, y represents a number satisfying 0.001≤y≤1.999, and z represents a number satisfying 3.001≤z≤3.200.)

[0025] [5] In the solid electrolyte of the above manner, it may also be that M” in the general formula (2) represents at least one element selected from Ti, Zr, Hf, Ge, Si and Sn.

[0026] [6] A second approach provides an all-solid-state battery, comprising: a solid electrolyte layer containing a solid electrolyte of the approach, a positive electrode bonded to one side of the solid electrolyte layer, and a negative electrode bonded to the other side of the solid electrolyte.

[0027] [Invention Effects]

[0028] According to the present invention, a novel solid electrolyte with improved ionic conductivity and an all-solid-state battery using the solid electrolyte exhibiting excellent cycle characteristics can be provided. Attached Figure Description

[0029] Figure 1 This is a cross-sectional schematic diagram of the all-solid-state battery of this embodiment. Detailed Implementation

[0030] The present invention will now be described in detail with appropriate reference to the accompanying drawings. In the drawings used in the following description, for ease of understanding and convenience, some features may be shown as enlarged portions, and the dimensional ratios of the constituent elements may differ from the actual dimensions. The materials, dimensions, etc., exemplified in the following description are merely examples, and the present invention is not limited to these; appropriate modifications and implementations may be made without altering its spirit.

[0031] [All-solid-state battery]

[0032] Figure 1This is a magnified cross-sectional schematic diagram of the main parts of the all-solid-state battery according to the first embodiment. (As shown) Figure 1 As shown, the all-solid-state battery 10 has a stack 4. The stack 4 has multiple positive electrode layers 1, multiple negative electrode layers 2, and a solid electrolyte layer 3 located between the positive electrode layers 1 and the negative electrode layers 2. The positive electrode layer 1 is an example of a first electrode layer, and the negative electrode layer 2 is an example of a second electrode layer. Either the first electrode layer or the second electrode layer functions as a positive electrode, and the other functions as a negative electrode. The positive or negative polarity of the electrode layer varies depending on which polarity is connected to the external terminal.

[0033] The positive electrode layer 1 is connected to the positive external terminal 5, and the negative electrode layer 2 is connected to the negative external terminal 6. The all-solid-state battery 10 is a parallel type in which multiple positive electrode layers 1 connected to the positive external terminal 5 and multiple negative electrode layers 2 connected to the negative external terminal 6 are connected in parallel.

[0034] <Layered Body>

[0035] The laminate 4 has multiple positive electrode layers 1, multiple negative electrode layers 2, multiple solid electrolyte layers 3, and multiple side edge layers 7. The solid electrolyte layers 3 are located between each positive electrode layer 1 and each negative electrode layer 2. Furthermore, a side edge layer 7 with approximately the same thickness as the positive electrode layer 1 is formed at one end of the positive electrode layer 1 that is not connected to the external positive electrode terminal 5. Similarly, a side edge layer 7 with approximately the same thickness as the negative electrode layer 2 is formed at one end of the negative electrode layer 2 that is not connected to the external negative electrode terminal 6. The all-solid-state battery 10 is charged and discharged through the acceptance and donation of lithium ions between the positive electrode layer 1 and the negative electrode layer 2 via the solid electrolyte layers 3.

[0036] (Solid electrolyte layer)

[0037] Solid electrolyte layer 3 contains solid electrolyte.

[0038] Solid electrolytes are composed of compounds represented by the following general formula (1).

[0039] Li x M2(PO4) z (1)

[0040] In general formula (1), x represents a number satisfying 1.003 ≤ x ≤ 1.900. x is preferably a number satisfying 1.004 ≤ x ≤ 1.604, and more preferably a number satisfying 1.103 ≤ x ≤ 1.503. z represents a number satisfying 3.001 ≤ z ≤ 3.200. z is preferably a number satisfying 3.001 ≤ z ≤ 3.050. Furthermore, the values ​​of x and z mentioned above are calculated with the composition ratio of M set to 2.

[0041] In general formula (1), M represents at least one element having a valence of 1 to 4. M preferably includes at least one element selected from Na (sodium), K (potassium), Ag (silver), Au (gold), Ba (barium), Cr (chromium), Mn (manganese), Fe (iron), Co (copper), Ni (nickel), Pd (palladium), Pt (platinum), Sc (scandium), Y (yttrium), V (vanadium), Nb (niobium), Ta (tantalum), Ru (rubidium), Rh (rhodium), Ir (iridium), Al (aluminum), Ga (gallium), In (indium), Mo (molybdenum), W (tungsten), Tc (technetium), Re (rhenium), Os (osmium), Ti (titanium), Zr (zirconium), Hf (hafnium), Ge (germanium), Si (silicon), and Sn (tin). M is more preferably an element containing only an element having a valence of 4, or an element containing an element having a valence of 1 to 3 and an element having a valence of 4.

[0042] As elements with a valence of four, Ti, Zr, Hf, Ge, Si, and Sn can be used. These elements can be used individually or in combination. When M contains only elements with a valence of four, M preferably contains Ti alone, or at least one of Zr, Hf, Ge, and Si, and Ti.

[0043] When M contains elements having valences of 1 to 3 and elements having valences of 4, the preferred compound is one represented by the following general formula (2).

[0044] Li x M' y M” 2－y (PO4) z (2)

[0045] In general formula (2), x and z are the same as in general formula (1) above. y is a number that satisfies 0.001≤y≤1.999, preferably a number that satisfies 0.100≤y≤0.300. Furthermore, the above x and z are values ​​calculated by setting the sum of the composition ratios of M' and M” to 2.

[0046] In general formula (2), M' represents at least one element selected from Na, K, Ag, Au, Ba, Cr, Mn, Fe, Co, Ni, Pd, Pt, Sc, Y, V, Nb, Ta, Ru, Rh, Ir, Al, Ga, In, Mo, W, Tc, Re, and Os. Among these elements, at least one element selected from Ag, Au, Ba, Cr, Mn, Fe, Co, Ni, Pd, Pt, Sc, Y, V, Nb, Ta, Ru, Rh, Ir, Al, Ga, In, Mo, W, Tc, Re, and Os is preferred.

[0047] In general formula (2), M” represents at least one element with a valence of 4. M” is preferably at least one element selected from Ti, Zr, Hf, Ge, Si, and Sn.

[0048] The solid electrolyte preferably has a NASICON-type crystalline structure. Phosphoric acid compounds with a NASICON-type crystalline structure are usually represented as LiM2(PO4)3. In contrast, as can be seen from the above general formula (1), the PO4 composition ratio of the solid electrolyte of the present invention is greater than 3 of the stoichiometric composition. The reason for the improved ionic conductivity of the solid electrolyte is not yet clear, but it is believed to be as follows: Since the PO4 composition ratio as an anion is greater than the stoichiometric composition, a deformation occurs in the crystalline structure, and due to this deformation, cations (especially lithium ions) can easily move within the crystal. In addition, in the solid electrolyte, a portion of the P at the PO4 sites may be replaced by elements such as Si (silicon), B (boron), Mo (molybdenum), S (sulfur), W (tungsten), V (vanadium), etc., which together form a tetrahedral structure with oxygen.

[0049] There are no particular limitations on the shape of the solid electrolyte. The shape of the solid electrolyte can be, for example, spherical, ellipsoidal, needle-like, plate-like, scaly, tubular, wire-like, rod-like, or amorphous. The average particle size (D50) of the solid electrolyte is, for example, 0.1 μm or more and 10 μm or less, or 0.3 μm or more and 9 μm or less. D50 is the diameter of the particles whose cumulative value in the distribution curve obtained by particle size distribution measurement is 50%. The particle size distribution is measured using a particle size distribution measuring device, for example, using laser diffraction scattering (Microtrac method).

[0050] Solid electrolytes can be manufactured by methods including, for example, weighing a Li source, an M source, and a PO4 source in a manner that constitutes the target composition, mixing them to obtain a mixed powder, and then calcining and firing the obtained mixed powder. The resulting calcined product (solid electrolyte) can also be pulverized to form a powder.

[0051] There are no particular restrictions on the materials used as Li, M, and PO4 sources. Carbonates, nitrates, oxides, hydroxides, chlorides, and phosphates can be used as Li and M sources. Phosphates also function as PO4 sources. Phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate can be used as PO4 sources.

[0052] The mixing of the Li source, M source, and PO4 source can be carried out in a dry or wet manner. Furthermore, regarding mixing, it is preferable to use a mixing device with a pulverizing function to mix the Li source, M source, and PO4 source simultaneously.

[0053] The calcination temperature of the mixed powder can be any temperature above 600°C, with no particular limitation, but calcination at a temperature below the sintering temperature is preferred. The atmosphere during calcination is also not particularly limited; atmospheric, nitrogen, argon, or oxygen atmospheres are acceptable. Furthermore, during calcination, the mixed powder can be molded into square or round shapes using a molding machine, and then calcined under pressure of 0.1–300 MPa using uniaxial pressure calcination (hot pressing) or hot isostatic pressing (HIP). By calcining under pressure, a solid electrolyte of a phosphoric acid compound with a NASICON-type crystalline structure is obtained while maintaining a high proportion of phosphoric acid. Even after subsequent calcination, the NASICON-type crystalline structure can be maintained in the calcined mixed powder while preserving a high proportion of phosphoric acid.

[0054] Regarding the firing temperature, firing can be carried out in the range of 600–1500°C. There are no particular limitations on the firing atmosphere; it can be carried out under the same atmosphere as calcination. Even without uniaxial pressure firing or hot isostatic pressing, a solid electrolyte with a NASICON-type crystalline structure that retains a higher proportion of phosphoric acid can be obtained during firing. Similar to calcination, firing can also be performed using uniaxial pressure firing or hot isostatic pressing.

[0055] (Positive electrode layer and negative electrode layer)

[0056] For example, multiple positive electrode layers 1 and negative electrode layers 2 are each present in the laminate 4. The positive electrode layer 1 is bonded to one main surface of the solid electrolyte layer 3, and the negative electrode layer 2 is bonded to the other main surface of the solid electrolyte layer 3.

[0057] The positive electrode layer 1 has a positive current collector layer 1A and a positive active material layer 1B. The negative electrode layer 2 has a negative current collector layer 2A and a negative active material layer 2B.

[0058] The positive electrode current collector layer 1A and the negative electrode current collector layer 2A contain conductive materials. Preferably, the positive electrode current collector layer 1A and the negative electrode current collector layer 2A contain more than 50% conductive materials. Examples of conductive materials include silver, palladium, gold, platinum, aluminum, copper, nickel, and carbon. In particular, copper is less reactive with the positive electrode active material, the negative electrode active material, and the solid electrolyte; for example, when copper is used in the positive electrode current collector layer 1A and the negative electrode current collector layer 2A, the internal resistance of the all-solid-state battery 10 can be reduced. Furthermore, there are no restrictions on the conductive materials as long as they do not decompose within the battery's operating voltage range. Additionally, the materials constituting the positive electrode current collector layer 1A and the negative electrode current collector layer 2A can be the same or different.

[0059] The positive current collector layer 1A may also contain the positive active material described later. The negative current collector layer 2A may also contain the negative active material described later. There is no particular limitation on the content ratio of active materials in each current collector layer, as long as it functions as a current collector. The volume ratio of conductive material to positive active material in the positive current collector layer 1A is, for example, in the range of 90:10 to 70:30. Similarly, the volume ratio of conductive material to negative active material in the negative current collector layer 2A is, for example, in the range of 90:10 to 70:30. When the positive current collector layer 1A and the negative current collector layer 2A contain positive active material and negative active material respectively, the adhesion between the positive current collector layer 1A and the positive active material layer 1B, and the adhesion between the negative current collector layer 2A and the negative active material layer 2B, are improved.

[0060] The positive electrode active material layer 1B is formed on one or both sides of the positive electrode current collector layer 1A. Alternatively, the positive electrode active material layer 1B may not be present on the side of the positive electrode current collector layer 1A where the opposing negative electrode layer 2 is not located. Similarly, the negative electrode active material layer 2B is formed on one or both sides of the negative electrode current collector layer 2A. Alternatively, the negative electrode active material layer 2B may not be present on the side of the negative electrode current collector layer 2A where the opposing positive electrode layer 1 is not located. For example, the uppermost or lowermost positive electrode layer 1 or negative electrode layer 2 of the laminate 4 may not have a positive electrode active material layer 1B or a negative electrode active material layer 2B on one side.

[0061] The positive electrode active material layer 1B and the negative electrode active material layer 2B contain compounds capable of adsorbing and releasing lithium ions as active materials for the positive and negative electrodes, respectively. In addition to the active materials, the positive electrode active material layer 1B and the negative electrode active material layer 2B may also contain conductive additives, ion-conducting additives, binders, etc. Preferably, the positive and negative electrode active materials enable the effective insertion and detachment of lithium ions.

[0062] The positive and negative electrode active materials are, for example, transition metal oxides and transition metal composite oxides. Specifically, the positive and negative electrode active materials are, for example, lithium manganese composite oxides (Li₂Mn). a Ma 1－a O3 (0.8≤a≤1, Ma=Co, Ni), lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese spinel (LiMn2O4), and other materials with the general formula LiNi x Co y Mn zComposite metal oxides represented by O2 (x+y+z=1, 0≤x≤1, 0≤y≤1, 0≤z≤1), lithium vanadium compounds (LiV2O5), olivine-type LiMbPO4 (where Mb is one or more elements selected from Co (cobalt), Ni (nickel), Mn (manganese), Fe (iron), Mg (magnesium), Nb (niobium), Ti (titanium), Al (aluminum), Zr (zirconium), lithium vanadium phosphate (Li3V2(PO4)3 or LiVOPO4), solid solution cathodes of Li excess systems represented by Li2MnO3-LiMcO2 (Mc=Mn, Co, Ni), lithium titanate (Li4Ti5O) 12 Titanium oxide (TiO2), with Li s Ni t Co u Al v O2 (0.9 < s < 1.3, 0.9 < t + u + v < 1.1) represents composite metal oxides, etc.

[0063] As the positive and negative active materials of this embodiment, it is preferred to include a phosphoric acid compound as the main component, for example, lithium vanadium phosphate (LiVOPO4, Li3V2(PO4)3, Li4(VO)(PO4)2), lithium vanadium pyrophosphate (Li2VOP2O7, Li2VP2O7), and Li9V3(P2O7)3(PO4)2, and particularly preferred is one or both of LiVOPO4 and Li3V2(PO4)3.

[0064] In this embodiment, the main component refers to the proportion of phosphoric acid compound greater than 50 parts by weight when the total amount of positive and negative active materials in the positive and negative active material layers is set at 100 parts by weight. Preferably, the proportion of phosphoric acid compound is 80 parts by weight or more.

[0065] Alternatively, for these positive and negative electrode active materials, some of their elements may be replaced with different types of elements, or their stoichiometric composition may have changed. LiVOPO4 and Li3V2(PO4)3 preferably have lithium defects, and more preferably are Li... x VOPO4 (0.94≤x≤0.98) or Li x V2(PO4)3(2.8≤x≤2.95).

[0066] In addition, as negative electrode active materials, for example, Li metal, Li-Al alloy, Li-In alloy, carbon, silicon (Si), and silicon oxide (SiO) can be used. x Lithium titanate (Li4Ti5O) 12 ), titanium dioxide (TiO2).

[0067] Here, there is no clear distinction between the active materials constituting the positive electrode active material layer 1B and the negative electrode active material layer 2B. The potentials of the two compounds in the positive and negative electrode active material layers are compared, and the compound exhibiting a higher potential is used as the positive electrode active material, while the compound exhibiting a lower potential is used as the negative electrode active material. Furthermore, if a compound possesses both lithium-ion release and lithium-ion storage capabilities, the same material can be used as the active material constituting both the positive electrode active material layer 1B and the negative electrode active material layer 2B. By using the same material for both the positive electrode active material layer 1B and the negative electrode active material layer 2B, a non-polar all-solid-state battery is created. Therefore, no orientation needs to be specified when mounting it on a circuit board, thus facilitating installation.

[0068] Examples of conductive additives include carbon materials such as carbon black, acetylene black, Ketjen black, carbon nanotubes, graphite, graphene, and activated carbon; and metallic materials such as gold, silver, palladium, platinum, copper, and tin.

[0069] As an ion-conducting aid, for example, a solid electrolyte can be used. Specifically, the same material as the solid electrolyte contained in the solid electrolyte layer 3 can be used as the solid electrolyte.

[0070] As adhesives, both organic and inorganic adhesives can be used. Examples of organic adhesives include polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), polyacrylate (PAA), polyimide (PI), and polyamide-imide (PAI). Examples of inorganic adhesives include lithium halides, silicate compounds, phosphate compounds, and low-melting-point glass.

[0071] <External terminal>

[0072] The positive external terminal 5 and the negative external terminal 6 can be made of materials with excellent conductivity, for example. The positive external terminal 5 and the negative external terminal 6 preferably contain, for example, any one of silver, gold, platinum, aluminum, copper, tin, and nickel. The positive external terminal 5 and the negative external terminal 6 can be a single layer or multiple layers.

[0073] <Protective Layer>

[0074] The all-solid-state battery 10 may also have a protective layer on its outer periphery that provides electrical, physical, and chemical protection to the laminate 4 and terminals. The protective layer is preferably made of a material that exhibits excellent insulation, durability, moisture resistance, and environmental safety. Examples of materials for the protective layer include glass, ceramics, thermosetting resins, and photocurable resins. Only one type of material may be used in the protective layer, or multiple materials may be used in combination. The protective layer may be a single layer or multiple layers. The protective layer is preferably an organic-inorganic mixture formed by mixing powders of thermosetting resin and ceramic.

[0075] Next, the manufacturing method of the all-solid-state battery of this embodiment will be described.

[0076] The all-solid-state battery 10 can be manufactured using either a simultaneous firing method or a sequential firing method. The simultaneous firing method involves firing all the materials stacked together. The sequential firing method involves firing each layer as it is stacked. The simultaneous firing method is simpler than the sequential firing method. Furthermore, the laminate 4 manufactured using the simultaneous firing method is denser than the laminate 4 manufactured using the sequential firing method. The following explanation will use the simultaneous firing method as an example.

[0077] First, a paste is prepared for each layer constituting the laminate 4. The materials forming the positive electrode current collector layer 1A, positive electrode active material layer 1B, solid electrolyte layer 3, negative electrode active material layer 2B, negative electrode current collector layer 2A, and side edge layer 7 are each pasted. There are no particular limitations on the paste-forming method. For example, powders of each material can be mixed into a carrier to obtain a paste. The carrier is a general term for the medium in the liquid phase. The carrier includes solvents and binders.

[0078] For the laminate, it is preferable to prepare the positive electrode active material layer unit and the negative electrode active material layer unit as described below, and to fabricate using the positive electrode active material layer unit and the negative electrode active material layer unit.

[0079] The positive electrode active material layer unit can be fabricated in the following order.

[0080] First, a solid electrolyte layer is applied to a PET film using a doctor blade to form a sheet, which is then dried to create a green solid electrolyte sheet. Next, a portion of the obtained green solid electrolyte sheet is screen-printed with a paste to form a positive electrode active material layer, which is then dried to create the positive electrode active material layer.

[0081] Next, a paste for the positive current collector layer is screen-printed onto the obtained positive electrode active material layer and dried to form the positive electrode current collector layer. Then, a paste for the positive electrode active material layer is screen-printed again onto the obtained positive electrode current collector layer and dried to form the positive electrode active material layer. As described above, a positive electrode layer is formed on a portion of the solid electrolyte layer. Next, a paste for the side edge layer is screen-printed onto the solid electrolyte layer where no positive electrode layer has been formed and dried to form the side edge layer. Then, a positive electrode unit is fabricated by peeling off the PET film. In the positive electrode unit, a positive electrode layer 1 (positive electrode active material layer 1B / positive electrode current collector layer 1A / positive electrode active material layer 1B) and a side edge layer 7 are formed on the solid electrolyte layer 3.

[0082] Furthermore, the side edge layer 7 eliminates the steps between the solid electrolyte layer 3 and the positive electrode layer 1, and between the solid electrolyte layer 3 and the negative electrode layer 2, on the end faces of the positive electrode layer 1 and the negative electrode layer 2 where they do not extend. As the side edge layer 7, a material with low electronic conductivity is required; for example, a solid electrolyte can be used. As described above, the side edge layer 7 can also be formed separately. Alternatively, instead of forming it separately, the portion generated by the deformation of the solid electrolyte layer 3 during lamination can be used as the side edge layer 7, thereby placing it side-by-side with the positive electrode layer 1 and the negative electrode layer 2.

[0083] The negative electrode unit is fabricated in the same order. In the negative electrode unit, a negative electrode layer 2 (negative electrode active material layer 2B / negative electrode current collector layer 2A / negative electrode active material layer 2B) and a side edge layer 7 are formed on the solid electrolyte layer 3.

[0084] Next, positive and negative electrode units are stacked. The positive and negative electrode layers are biased such that one end of each layer is not aligned, so that the positive and negative electrode layers are stacked in a comb-like manner, separated by a solid electrolyte layer. This creates a laminated substrate comprising multiple positive electrode layers 1, multiple negative electrode layers 2, and a solid electrolyte layer 3 located between the positive and negative electrode layers 1 and 2. Furthermore, outer layers can be provided on the two main surfaces of the top and bottom layers of the laminated substrate, as needed. The outer layers can be made of the same material as the solid electrolyte and can be formed by stacking solid electrolyte green sheets.

[0085] The above-described method of stacking positive and negative electrode units is useful in manufacturing a parallel-type all-solid-state battery 10 in which multiple positive electrode layers 1 connected to the positive electrode external terminal 5 and multiple negative electrode layers 2 connected to the negative electrode external terminal 6 are connected in parallel. In manufacturing a series-type all-solid-state battery in which the positive and negative electrode layers are connected in series, the layers can be stacked without bias, as long as one end of each positive and negative electrode layer is facing each other.

[0086] Next, the fabricated laminated substrates are pressed together. Pressing is performed while heating at a low temperature. The heating temperature is set, for example, to 40–95°C.

[0087] The fabricated stack is cut into sheets using a cutting device, and then, after debinding as needed, it is fired to produce the stack of all-solid-state batteries.

[0088] The unsintered laminated sheet is sintered to obtain the laminate 4 of the all-solid-state battery of this embodiment. For example, sintering is performed in a nitrogen atmosphere at a temperature range of 600°C to 1500°C. The sintering time is set to, for example, 0.1 to 3 hours.

[0089] As described above, a debinding treatment can also be performed as a separate step from the firing process, prior to the firing process. By heating and decomposing the binder components contained in the laminate 4 before firing, the vigorous decomposition of the binder components during the firing process can be suppressed. Regarding the debinding treatment, for example, it can be performed in a nitrogen atmosphere at a temperature ranging from 300°C to 800°C for 0.1 to 10 hours. If a reducing atmosphere is used, firing can also be performed in, for example, an argon atmosphere or a nitrogen-hydrogen mixed gas atmosphere, instead of a nitrogen atmosphere.

[0090] For laminate 4, it can also be placed together with abrasive materials such as alumina into a cylindrical container for tumbling grinding. Through grinding, the corners of laminate 4 are chamfered. Grinding can also be performed by methods such as sandblasting.

[0091] Finally, a positive external terminal 5 and a negative external terminal 6 are installed onto the laminate 4. The positive external terminal 5 and the negative external terminal 6 are formed in electrical contact with the positive current collector layer 1A or the negative current collector layer 2A, respectively. For example, the positive external terminal 5 is connected to the positive current collector layer 1A exposed from the side of the laminate 4, and the negative external terminal 6 is connected to the negative current collector layer 2A exposed from the side of the laminate 4. The positive external terminal 5 and the negative external terminal 6 can be manufactured by methods such as sputtering, dipping, or spraying.

[0092] The solid electrolyte layer 3 of the all-solid-state battery 10 of this embodiment is composed of a compound represented by the above general formula (1), and has a large proportion of PO4 sites relative to M, thus improving ionic conductivity. In the all-solid-state battery 10 of this embodiment, the ionic conductivity of the solid electrolyte layer 3 is improved, thus exhibiting excellent cycle characteristics and isoelectric properties.

[0093] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the structures and combinations thereof in each embodiment are merely examples, and structural additions, omissions, substitutions, and other modifications can be made without departing from the spirit of the present invention.

[0094] Example

[0095] [Example 1]

[0096] (Preparation of solid electrolytes)

[0097] Li₂CO₃ (lithium carbonate), TiO₂ (titanium oxide), Al₂O₃ (aluminum oxide), and NH₄H₂PO₄ (ammonium dihydrogen phosphate) were prepared as initial materials. The prepared Li₂CO₃, TiO₂, Al₂O₃, and NH₄H₂PO₄ were weighed in a Li:Al:Ti:PO₄ ratio of 1.303:0.3000:1.700:3.001 (Li:Al:Ti:PO₄). Next, the weighed Li₂CO₃, TiO₂, Al₂O₃, and NH₄H₂PO₄ were added to a zirconia mill and mixed for 16 hours to obtain a mixed powder. Then, 0.2 g of the obtained mixed powder was placed into a circular mold with a diameter of 12 mm and milled at 2.0 t / cm². 2 The pressure is applied to form a circular molded body. Multiple molded bodies are prepared and subjected to a pressure of 10 MPa using a hot press while being calcined at 700°C for 1 hour in a nitrogen atmosphere. A portion of the calcined molded body is pulverized to obtain calcined powder of the solid electrolyte. The remaining portion of the calcined molded body is then calcined at 850°C for 2 hours in a nitrogen atmosphere to obtain a sintered body of the solid electrolyte.

[0098] [Examples 2-7, Comparative Examples 1 and 2]

[0099] Li₂CO₃, TiO₂, Al₂O₃, and NH₄H₂PO₄ were mixed in the proportions shown in Table 1 below, with Li, Al, Ti, and PO₄ in the same manner. Otherwise, the process was the same as in Example 1, yielding solid electrolyte sintered bodies of Examples 2-7 and Comparative Example 2. Furthermore, in Comparative Example 1, pressure was not applied using a hot press during the calcination of the formed body; otherwise, the process was the same as in Example 1, yielding calcined powder and sintered bodies of the solid electrolyte.

[0100] [Table 1]

[0101]

[0102] [evaluate]

[0103] (composition)

[0104] A portion of the sintered solid electrolyte was pulverized and dissolved in nitric acid. The concentrations of Li, Al, Ti, and P in the resulting solution were determined by ICP-N spectrophotometry. Then, the composition ratio of Li, Al, Ti, and PO4 in the solid electrolyte was calculated based on the concentrations of each element. Furthermore, when calculating the composition ratio, the general formula (2) Li...x M' y M” 2－y (PO4) z In this process, the sum of the composition ratios of M' and M” is set to 2 for conversion, thereby calculating the above x and z. The results are shown in Table 2 below.

[0105] (X-ray diffraction pattern)

[0106] The X-ray diffraction pattern of the solid electrolyte sintered body was determined using CuKα rays. Analysis of the obtained X-ray diffraction pattern revealed the same pattern as that of LiTi2(PO4)3 (lithium titanium phosphate) on ICDD card 35-0754, thus confirming a NASICON-type crystal structure. The results are shown in Table 2 below.

[0107] (Ion conductivity)

[0108] Gold electrodes are formed by sputtering gold onto both sides of the solid electrolyte sintered body. Furthermore, during gold sputtering, the sides of the sintered body are shielded with tape to prevent the gold electrodes on both sides from conducting.

[0109] Then, the aforementioned solid electrolyte sintered bodies were placed in a fixture for measuring ionic conductivity, and the ionic conductivity of each solid electrolyte sintered body was measured using a potentiostat equipped with a frequency response analyzer by electrochemical impedance spectroscopy. The measurements were performed under conditions of a frequency range of 7 MHz to 0.1 Hz, an amplitude of 10 mV, and a temperature of 25 °C. The results are shown in Table 2 below.

[0110] (Charge-discharge cycle characteristics of all-solid-state batteries)

[0111] An all-solid-state battery was fabricated in the following order. A paste containing each material was prepared to form a positive electrode current collector layer, a positive electrode active material layer, a solid electrolyte layer, a negative electrode active material layer, a negative electrode current collector layer, and a side edge layer. Next, a solid electrolyte green sheet was fabricated, and an electrode layer and a side edge layer were formed on this green sheet to fabricate an electrode unit. Then, the electrode units were alternately stacked, thereby fabricating the all-solid-state battery of this embodiment. Hereinafter, the manufacturing method will be described in more detail, using Example 1 as an example.

[0112] (Preparation of positive and negative electrode active materials)

[0113] Li3V2(PO4)3, prepared using the following method, was used as both the positive and negative electrode active materials. The preparation method involved using Li2CO3, V2O5, and NH4H2PO4 as initial materials, wet-mixing them in a ball mill for 16 hours, and then calcining the resulting powder at 700°C for 2 hours in a nitrogen-hydrogen mixed gas. After wet pulverization in a ball mill, the calcined material was dehydrated and dried to obtain the calcined powders of the positive and negative electrode active materials. The X-ray diffraction pattern of the prepared calcined powder was determined by X-ray diffraction, and the composition was determined by ICP-N (Inductively Coupled Phosphorus) spectroscopy. The results confirmed that it was Li3V2(PO4)3 with a NASICON-type crystalline structure.

[0114] (Preparation of paste for positive electrode active material layer and paste for negative electrode active material layer)

[0115] Regarding the paste for the positive electrode active material layer and the paste for the negative electrode active material layer, 15 parts of ethyl cellulose as a binder and 65 parts of dihydroterpineol as a solvent are added to 100 parts of calcined Li3V2(PO4)3 powder, and then mixed and dispersed to produce the paste for the positive electrode active material layer and the paste for the negative electrode active material layer.

[0116] (Preparation of a paste for a solid electrolyte layer)

[0117] 100 parts of calcined powder of the solid electrolyte prepared in Example 1 were mixed with 100 parts of ethanol and 200 parts of toluene as solvents using a ball mill. Then, 16 parts of polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate were added and mixed to prepare a paste for the solid electrolyte layer.

[0118] (Fabrication of solid electrolyte layer sheets)

[0119] The solid electrolyte layer paste is formed into a sheet on a PET film using a doctor blade method, resulting in a solid electrolyte layer sheet with a thickness of 15 μm.

[0120] (Preparation of paste for positive electrode current collector layer and paste for negative electrode current collector layer)

[0121] As positive and negative current collectors, Cu powder and Li3V2(PO4)3 calcined powder are mixed in a volume ratio of 80 / 20, and then 10 parts of ethyl cellulose as a binder and 50 parts of dihydroterpineol as a solvent are added and mixed and dispersed to prepare a paste for positive and negative current collector layers.

[0122] (Preparation of the side edge layer paste)

[0123] Add 100 parts of ethanol and 100 parts of toluene as solvents to 100 parts of the calcined powder of the solid electrolyte prepared in Example 1, and perform wet mixing using a ball mill. Then, further add 16 parts of polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mix to prepare a paste for the side edge layer.

[0124] (Preparation of paste for external terminals)

[0125] Silver powder, epoxy resin, and solvent are mixed and dispersed to produce a thermosetting paste for external terminals.

[0126] (Fabrication of the positive electrode unit)

[0127] A positive electrode active material layer paste is screen-printed onto the aforementioned solid electrolyte green sheet to form a positive electrode active material layer with a thickness of 5 μm, and then dried at 80°C for 10 minutes. Next, a positive electrode current collector layer paste is screen-printed onto the aforementioned positive electrode active material layer to form a positive electrode current collector layer with a thickness of 5 μm, and then dried at 80°C for 10 minutes. Further, a positive electrode active material layer paste is screen-printed onto the aforementioned positive electrode current collector layer to form another positive electrode active material layer with a thickness of 5 μm, and then dried at 80°C for 10 minutes, thus fabricating a positive electrode layer on the solid electrolyte green sheet. Next, a side edge layer paste is screen-printed into the areas of the solid electrolyte green sheet where the positive electrode layer has not yet formed, forming a side edge layer with a height approximately the same as the aforementioned positive electrode layer, and then dried at 80°C for 10 minutes, thereby fabricating a positive electrode unit.

[0128] (Fabrication of the negative electrode unit)

[0129] For the negative electrode unit, the negative electrode unit is made in the same way as the positive electrode unit.

[0130] (Creating a layered body)

[0131] The positive and negative electrode cells were peeled off from the PET film and stacked with one end of the positive electrode layer and the other end of the negative electrode layer not aligned. The solid electrolyte layer sandwiched between the positive and negative electrode layers was stacked in such a way that when one layer was considered a solid electrolyte layer, there were 50 solid electrolyte layers, thus creating a laminated substrate. Next, multiple solid electrolyte green sheets were stacked as outer layers on the top and bottom surfaces of the laminated substrate, forming 500 μm outer layers. These outer layers were then hot-pressed using a molding machine and cut to create an unfired all-solid-state battery laminate. Next, the laminate was debonded and fired to obtain the all-solid-state battery laminate. The firing process was carried out in a nitrogen atmosphere, with the temperature increased at a rate of 200°C / hour to a firing temperature of 850°C, held at this temperature for 2 hours, and then removed after natural cooling.

[0132] (Fabrication of external terminals)

[0133] External terminal paste is applied to the end face of the stack of the above-mentioned solid-state battery after firing, and then heat-cured at 150°C for 30 minutes to form a pair of external terminals.

[0134] (Charge-discharge cycle test)

[0135] For the all-solid-state batteries fabricated in this embodiment and the comparative example, charge-discharge cycle characteristics were evaluated under the charge-discharge conditions shown below. The charge-discharge current is recorded using the C(C) rate. The C rate is recorded as nC (or current value [A]) (n is a numerical value), which refers to the current that can charge and discharge the nominal capacity (μAh) at a rate of 1 / n(h). For example, 1C is the charge-discharge current that can charge the nominal capacity in 1 hour, and 2C is the charge-discharge current that can charge the nominal capacity in 0.5 hours. For example, in the case of an all-solid-state battery with a nominal capacity of 100 μAh, the current at 0.1C is 10 μA (calculated as 100 μA × 0.1 = 10 μA). Similarly, the current at 0.2C is 20 μA, and the current at 1C is 100 μA.

[0136] Regarding the charge-discharge cycle test conditions, at an environment of 25°C, constant current charging (CC charging) is performed at a constant current rate of 0.2C until the battery voltage reaches 1.6V. Then, constant current discharging is performed at a constant current rate of 0.2C until the battery voltage reaches 0V (CC discharging). The above charging and discharging is recorded as one cycle, and the discharge capacity retention rate after repeating this cycle for 500 cycles is evaluated as the charge-discharge cycle characteristic. Furthermore, the charge-discharge cycle characteristic of this embodiment is calculated according to the following formula.

[0137] Discharge capacity retention after 500 cycles (%) = (Discharge capacity after 500 cycles ÷ Discharge capacity after 1 cycle) × 100

[0138] [Table 2]

[0139]

[0140] According to the results in Table 2, the ionic conductivity of the solid electrolyte sintered bodies of Examples 1-7, where the Al+Ti composition ratio is 2 and the PO4 content exceeds 3, is higher than that of Comparative Example 1, where the PO4 content is 3. In particular, the ionic conductivity of the solid electrolyte sintered bodies with PO4 content in the range of 3.001 to 3.050 is especially high. Furthermore, it can be seen that the cycle characteristics of all-solid-state batteries using this as a solid electrolyte are improved. The detailed reason is not yet clear, but the inventors believe the reason is as follows: In the solid electrolytes obtained in Examples 1-7, the excess PO4 causes deformation in the crystal structure, thereby increasing the ionic conductivity. On the other hand, the ionic conductivity of the solid electrolyte sintered body of Comparative Example 2, where the PO4 content exceeds 3.200, begins to decrease, and the cycle characteristics of all-solid-state batteries using this solid electrolyte tend to decrease slightly.

[0141] [Examples 8-14]

[0142] Li₂CO₃, TiO₂, Al₂O₃, and NH₄H₂PO₄ were mixed in the proportions of Li, Al, Ti, and PO₄ as described in Table 3 below, except that the process was the same as in Example 1, to obtain a solid electrolyte sintered body. Regarding the obtained solid electrolyte sintered body, its composition, X-ray diffraction pattern, ionic conductivity, and cycle characteristics of the all-solid-state battery were evaluated in the same manner as in Example 1.

[0143] [Table 3]

[0144]

[0145] [Table 4]

[0146]

[0147] According to the results in Table 4, the solid electrolyte sintered body with a Li content in the range of 1.203 to 1.403 as a composition ratio has particularly high ionic conductivity, and the all-solid-state battery using this solid electrolyte sintered body has particularly high cycle characteristics.

[0148] [Examples 15-18, Comparative Examples 3-6]

[0149] Using Na₂CO₃ (sodium carbonate), CoO (cobalt(II) oxide), Y₂O₃ (yttrium oxide), or ZrO₂ (zirconia) instead of Al₂O₃, these compounds were mixed in the proportions of Na, Co, Y, or Zr shown in Table 5 below, otherwise, the process was the same as in Example 1, to obtain a solid electrolyte sintered body. The composition, X-ray diffraction pattern, and ionic conductivity of the obtained solid electrolyte sintered body were evaluated in the same manner as in Example 1. The results are shown in Table 6 below.

[0150] [Table 5]

[0151]

[0152] [Table 6]

[0153]

[0154] According to the results in Table 6, when Na (a monovalent element), Co (a divalent element), Y (a trivalent element), and Zr (a tetravalent element) are used to replace Al (a trivalent element), the ionic conductivity is also improved in solid electrolyte sintered bodies with a PO4 content exceeding 3.

[0155] [Examples 19-22]

[0156] Using ZrO2 (zirconia), HfO2 (hafnium oxide), GeO2 (germanium oxide), or SnO2 (tin oxide (IV)) instead of TiO2, these compounds were mixed in the proportions of Zr, Hf, Ge, or Sn shown in Table 7 below. Otherwise, the process was the same as in Example 3, yielding solid electrolyte powder and solid electrolyte sintered bodies. The composition, X-ray diffraction pattern, and ionic conductivity of the resulting solid electrolyte sintered bodies were evaluated in the same manner as in Example 1. Furthermore, in the analysis of the X-ray diffraction patterns, the solid electrolyte of Example 19 exhibited the same X-ray diffraction pattern as LiZr2(PO4)3 (lithium zirconium phosphate) on ICDD card 072-7742; the solid electrolyte of Example 20 exhibited the same X-ray diffraction pattern as LiHf2(PO4)3 (lithium hafnium phosphate) on ICDD card 004-0755; the solid electrolyte of Example 21 exhibited the same X-ray diffraction pattern as LiGe2(PO4)3 (lithium germanium phosphate) on ICDD card 080-1992; and the solid electrolyte of Example 22 exhibited the same X-ray diffraction pattern as LiSn2(PO4)3 (lithium tin phosphate) on ICDD card 087-2078. Therefore, NASICON-type crystal structures were confirmed. These results, along with those of Example 3, are shown in Table 8 below.

[0157] [Table 7]

[0158]

[0159] [Table 8]

[0160]

[0161] As shown in Table 8, even when Zr, Hf, Ge, and Sn are used to replace Ti, the ionic conductivity is improved in solid electrolyte sintered bodies with a PO4 content exceeding 3%.

[0162] [Symbol Explanation]

[0163] 1. Positive electrode layer, 1A. Positive current collector layer, 1B. Positive active material layer, 2. Negative electrode layer, 2A. Negative current collector layer, 2B. Negative active material layer, 3. Solid electrolyte layer, 4. Stacked layer, 5. Positive external terminal, 6. Negative external terminal, 7. Side edge layer, 10. All-solid-state battery.

Claims

1. A solid electrolyte, wherein, the solid electrolyte is composed of a compound of a NASICON type represented by the following general formula (2), Li x M' y M" 2－y (PO4) z (2) in the general formula (2), M' represents at least one element selected from the group consisting of Na, K, Ag, Au, Ba, Cr, Mn, Fe, Co, Ni, Pd, Pt, Sc, Y, V, Nb, Ta, Ru, Rh, Ir, Al, Ga, In, Mo, W, Tc, Re, Os, M" represents at least one element having a valence number of 4, x represents a number satisfying 1.003≤x≤1.900, y represents a number satisfying 0.001≤y≤1.999, and z represents a number satisfying 3.001≤z≤3.

200.

2. The solid electrolyte according to claim 1, wherein, in the general formula (2), M" represents at least one element selected from the group consisting of Ti, Zr, Hf, Ge, Si, and Sn.

3. The solid electrolyte according to claim 1, wherein, in the general formula (2), (M', M") represents any one combination of (Al, Ti), (Na, Ti), (Co, Ti), (Y, Ti), (Zr, Ti), (Al, Zr), (Al, Hf), (Al, Ge), and (Al, Sn).

4. An all-solid battery, wherein, provided are a solid electrolyte layer containing the solid electrolyte according to any one of claims 1 to 3, a positive electrode joined to one face of the solid electrolyte layer, and a negative electrode joined to the other face of the solid electrolyte layer.

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