SOLID-STATE SECONDARY BATTERY

Abstract

Solid-state electrolyte, comprising: a lithium-containing phosphoric acid compound with a cubic crystal structure, wherein the lithium-containing phosphoric acid compound, which is a cubic crystal and is contained in the solid electrolyte, is represented by: Li x M1 y M2 z M3 w P 3-w O 12 . (1), where formula (1) satisfies 0 < x ≤ 3, 0 ≤ y < 2, 0 < z ≤ 2 and 0 ≤ w < 3, M1 represents at least one element selected from the following group: Mg, Ca, Sr, Ba, V, Nb, Mn, Co, Ni, Cu, Ag, Ga, Al, In, Sc and Y M2 represents at least one element selected from the following group: Zr and Hf M3 represents at least one element selected from the following group: Si, B, S, V, Mo and W.

Description

[Technical field]

[0001] The present invention relates to a solid-state electrolyte and a solid-state secondary battery.

[0002] This patent application is based on and claims the priority advantage of Japanese patent application No. 2017-66682, filed on March 30, 2017, the entire contents of which are incorporated herein by reference. [State of the art]

[0003] In recent years, batteries have been used for a variety of purposes. For example, they have been used as mobile phone batteries and have had to be smaller and lighter, have a thinner coating, and offer improved reliability. Batteries using electrolyte solutions are prone to problems such as fluid leakage and loss. Therefore, solid-state secondary batteries, which use solid electrolytes, have attracted considerable attention.

[0004] Solid-state secondary batteries, however, have a problem in that their performance is lower than that of batteries using electrolytes. Therefore, an increase in the ionic conductivity of solid-state secondary batteries was necessary.

[0005] For example, patent specification 1 discloses a solid-state secondary battery in which the oxide-based Li 1,3 Al 0,3 Ti 1,7 (PO4)3 is used as a solid electrolyte. Furthermore, patent specification 2 discloses a solid-state secondary battery with excellent reduction resistance, in which LiZr2(PO4)3, containing Zr, is used as the solid electrolyte. Patent specification 3 further discloses a solid-state secondary battery exhibiting excellent reduction resistance, in which particles containing rhombohedral Li crystals are used. 1,55 Al 0,2 Zr 1,7 Y 0,1 Si 0,2 5P 2,75 O 12are composed of... [List of prior art][Patent literature] [Patent Specification 1] Unexamined Japanese patent application, first publication JP 2016-1595 A [Patent Specification 2] Unexamined Japanese patent application, first publication JP 2001-143754 A [Patent Specification 3] Unexamined Japanese patent application, first publication JP 2012-246196 A [Brief description of the invention][Technical problem]

[0006] However, the solid electrolytes disclosed in patent specifications 1 to 3 are triclinic or rhombohedral crystals and it cannot be determined that their conductivities are sufficient.

[0007] The invention was made in view of the above-mentioned problem and it is an object of it to provide a solid-state electrolyte with high ionic conductivity and a solid-state secondary battery in which this is used. [Solution to the problem]

[0008] The inventors have discovered that a solid-state secondary battery with excellent ion conductivity could be obtained by using a solid-state electrolyte containing a lithium-containing phosphoric acid compound that is a cubic crystal.

[0009] This means that the invention provides the following means to solve the aforementioned problem. (1) According to a first aspect, a solid electrolyte is provided comprising: a lithium-containing phosphoric acid compound with a cubic crystal structure. (2) With regard to the solid electrolyte according to the aforementioned aspect, the lithium-containing phosphoric acid compound, which is a cubic crystal and is contained in the solid electrolyte, can be represented as follows: Li x M1 y M2 z M3 w P 3-w O 12...(1), where 0 < x ≤ 3, 0 ≤ y < 2, 0 < z ≤ 2 and 0 ≤ w < 3 in formula (1) is to be satisfied, M1 is to represent at least one element selected from the group consisting of Mg, Ca, Sr, Ba, V, Nb, Mn, Co, Ni, Cu, Ag, Ga, Al, In, Sc and Y, M2 is to represent at least one element selected from the group consisting of Zr and Hf, and M3 is to represent at least one element selected from the group consisting of Si, B, S, V, Mo and W. (3) The solid electrolyte according to the aforementioned aspect may further comprise: Water equal to or greater than 0.01% by mass and water equal to or less than 20% by mass. (4) For the solid electrolyte according to the aforementioned aspect, a measurement (D50) obtained by particle size distribution measurement may be greater than or equal to 0.1 µm and less than or equal to 10 µm. (5) According to a second aspect, a solid-state secondary battery is provided, comprising: the solid-state electrolyte according to the aforementioned aspect. (6) In the case of the solid-state secondary battery according to the aforementioned second aspect, the relative density of a pair of electrode layers and a solid electrolyte layer with the solid electrolyte provided between the pair of electrode layers may be greater than or equal to 80%. [Advantageous effects of the invention]

[0010] According to the aforementioned aspects, the solid-state electrolyte can increase ionic conductivity. Furthermore, in the case of a solid-state secondary battery, where the solid-state electrolyte is used according to the aforementioned aspects, it is possible to reduce voltage drop and increase capacity. [Brief description of the drawings] Fig.Figure 1 is a schematic cross-sectional view showing enlarged main components of a solid-state secondary battery according to one embodiment. Fig. Figure 2 is a scanning electron microscope (SEM) image of a cubic monocrystalline particle contained in a solid electrolyte according to the embodiment. Fig. Figure 3A is a diagram that schematically represents an ion-conducting path in the monocrystalline particle contained in the solid electrolyte. Fig. Figure 3B is a diagram that schematically represents an ion-conducting path in a particle of random shape contained in the solid electrolyte. Fig. 4 is an X-ray diffraction (XRD) image in Example 1. [Description of embodiments]

[0011] The invention is described in detail below with reference to the drawings, if necessary. The drawings used in the following description may show enlarged portions that characterize the features of the invention to facilitate understanding, and the dimensions and other aspects of the respective components may differ from the actual dimensions. The materials, dimensions, and the like are only illustrative examples in the following description, and the invention is not limited to them and can be modified and implemented in a suitable manner without altering its core. [Solid-state secondary battery]

[0012] Fig. Figure 1 is a schematic cross-sectional view showing enlarged main components of a solid-state secondary battery according to a first embodiment. As in Fig.As shown in Figure 1, a solid-state secondary battery 10 comprises a layered body 4 with first electrode layers 1, second electrode layers 2 and a solid electrolyte 3.

[0013] Each of the first electrode layers 1 is connected to a first external terminal 5, and each of the second electrode layers 2 is connected to a second external terminal 6. The first external terminal 5 and the second external terminal 6 are electrical contact points with the outside. (Layered body)

[0014] The layered body 4 comprises the first electrode layers 1, the second electrode layers 2, and the solid electrolyte 3. Either the first electrode layers 1 or the second electrode layers 2 act as positive electrodes, and the others act as negative electrodes. The positive and negative polarities of the electrode layers change depending on which polarity is connected to the external terminals. For the sake of clarity, it is assumed that the first electrode layers 1 act as positive electrode layers 1 and the second electrode layers 2 act as negative electrode layers 2.

[0015] In the layered body 4, the positive electrode layers 1 and the negative electrode layers 2 are alternately layered by means of the solid electrolyte 3. The solid-state secondary battery 10 is charged and discharged by the exchange of lithium ions between the positive electrode layers 1 and the negative electrode layers 2 via the solid electrolyte 3. "Solid-state electrolyte"

[0016] The solid electrolyte 3 according to the embodiment comprises a lithium-containing phosphoric acid compound with a cubic crystal structure. Although a lithium-containing phosphoric acid compound that is a rhombohedral or triclinic crystal and can be used for the solid electrolyte 3 is known, a lithium-containing phosphoric acid compound that is a cubic crystal is not known.

[0017] The lithium-containing phosphoric acid compound with a cubic crystal structure exhibits excellent ionic conductivity. This is thought to be due to the high symmetry of the cubic crystal. In the cubic crystal, the lengths of the primitive vectors (a1, a2, a3) of a unit lattice are equal and intersect perpendicularly, resulting in high symmetry.

[0018] When the solid-state secondary battery 10 is charged or discharged, ions move through the spaces between atoms contained in the crystal structure. In the highly symmetry crystal structure, the positions where these spaces exist tend to be three-dimensionally symmetric. It is therefore assumed that this is because the conducting paths of the ions introduced into the crystal are present in a three-dimensionally equivalent manner, and that this favors ionic conduction in the highly symmetry cubic crystal structure.

[0019] Meanwhile, in a rhombohedral crystal, the lengths of primitive vectors (a1, a2, a3) of a unit lattice are equal, but do not intersect perpendicularly. Similarly, in a triclinic crystal, the lengths of primitive vectors (a1, a2, a3) of a unit lattice are different and do not intersect perpendicularly. It is therefore assumed that rhombohedral and triclinic structures exhibit low symmetry and that conducting paths of ions are limited compared to those in a cubic crystal.

[0020] The solid electrolyte 3 according to the embodiment comprises a lithium-containing phosphoric acid compound. The solid electrolyte 3 according to the embodiment is preferably a material with excellent ionic conductivity and is preferably represented by the following formula (1): Li x M1 y M2 z M3 w P 3-w O 12 (1) where formula (1) satisfies 0 < x ≤ 3, 0 ≤ y < 2, 0 < z ≤ 2 and 0 ≤ w < 3, M1 in formula (1) is at least one selected from a group consisting of Mg, Ca, Sr, Ba, V, Nb, Mn, Co, Ni, Cu, Ag, Ga, Al, In, Sc and Y, M2 in formula (1) is at least one selected from a group consisting of Zr and Hf, and M3 in formula (1) is at least one selected from a group consisting of Si, B, S, V, Mo and W.

[0021] A cubic structure can be chosen for the substance represented by formula (1). Furthermore, the substance represented by formula (1) is adapted such that it is possible to easily introduce defects into the crystal structure by altering the ionic radii of constituent elements, their valences, and the like. These defects create vacancies or ions that act as charge carriers in the solid electrolyte and increase the ionic conductivity.

[0022] Furthermore, the phosphoric acid-containing compound, which is a cubic crystal, preferably consists of monocrystalline particles with regular tetrahedral particle shapes. Fig. Figure 2 is a scanning electron microscope (SEM) image of a monocrystalline particle that is a cubic crystal and is contained in the solid electrolyte according to the embodiment.

[0023] Fig.Figure 3A is a diagram that schematically represents a conducting path of an ion in the monocrystalline particle contained in the solid electrolyte, and Fig. Figure 3B is a diagram that schematically represents a conducting path of an ion in a randomly shaped particle contained in the solid electrolyte. The randomly shaped particle is obtained by sintering after pulverization and is a composite body in which a multitude of crystals are fused together.

[0024] As in Fig. As shown in Figure 3A, the monocrystalline particle 30 has a path P through which ions are guided. The path P is located between atoms contained in the monocrystalline particle 30. The path P is indeed Fig. 3A is schematically represented on one axis, but the path P actually exists three-dimensionally in the monocrystalline particle 30.

[0025] Since the monocrystalline particle 30 is designed to comprise a single crystal, the path P within the particle is uninterrupted. This means that ions can move freely within the monocrystalline particle 30 when the solid-state secondary battery 10 is charged or discharged.

[0026] Meanwhile, a multitude of grain boundaries Gs are found in the particle with a random shape 31 from a multitude of composite crystals, as is the case in Fig. Figure 3B illustrates this. Each grain G, separated by the grain boundaries Gs in the randomly shaped particle 31, has a crystalline structure. Therefore, each grain G possesses the path P through which ions are conducted. However, since the direction of path P is different for each grain G, ionic conduction is disrupted at the grain boundary Gs.

[0027] This means that the monocrystalline particle 30 in Fig.3A has a better ionic conductivity than the particle with a random shape 31 in Fig. 3B exhibits. The monocrystalline particle 30 of a grown crystal is different from the particle with a random shape 31, as it appears in Fig. 3B is shown, not amorphous.

[0028] Note that an SEM image is used to determine whether a particle contained in the solid electrolyte 3 has a regular tetrahedral shape. The term "regular tetrahedral shape" is not limited to a perfectly regular tetrahedral shape and also includes an essentially regular tetrahedral shape. An essentially regular tetrahedral shape includes a shape with a protruding or recessed portion on part of a surface, a shape with a partially missing corner, and a composite body made up of a multitude of regular tetrahedral shapes that are partially connected.

[0029] A measured value (D50) obtained by a particle size distribution measurement performed on the solid electrolyte 3 is preferably greater than or equal to 0.1 µm and less than or equal to 10 µm, and more preferably 0.3 to 9 µm. Here, D50 is a particle diameter at a cumulative value of 50% in a distribution curve obtained by the particle size distribution measurement. The particle size distribution can be measured using a particle size distribution measuring device employing a laser diffraction scattering method (Microtrac method).

[0030] If the particles contained in the solid electrolyte 3 fall within this range, it is possible to ensure that the solid electrolyte is dense and the ionic conductivity is increased. Furthermore, the thermal conductivity becomes uniform with respect to each particle during sintering, as the particle diameters are the same, thus making it possible to lower the sintering temperature. Consequently, it is possible to prevent lithium volatilization and maintain the number of conductive charge carriers.

[0031] Furthermore, the solid electrolyte 3 preferably contains 0.01 wt% or more and 20 wt% or less bound water. The bound water is attached to components of the solid electrolyte 3 at the molecular level. The presence of the bound water contributes to a more stable cubic crystal structure. While the reason for this is not entirely clear, it is assumed that it is due to the presence of hydrogen bonds in the bound water, which is shared by the tetrahedra formed by elements P and O, an octahedron formed by elements Zr and O, and element H. These elements form hydrogen bonds within the bound water and are present in the crystal structure. Moreover, it has also been confirmed by calculation that the enthalpy of formation of the cubic crystal is stabilized by the bound water.Furthermore, results from FTIR (infrared spectroscopy) confirmed that the bound water also influences the symmetry of a PO4 bond.

[0032] If bound water is present, there is also a probability that a concentration gradient of elements will exist in the solid electrolyte between a section containing the bound water and the other sections. It is assumed that if this concentration gradient of elements exists, ions will be conducted along the gradient, thus increasing the ionic conductivity.

[0033] Whether a substance is "bound water" or not is determined by measuring the mass spectrum of the solid electrolyte 3 containing water. The amount of the detected substance (i.e., H₂O) with a m / z value of 18 is measured while the solid electrolyte 3 is heated in a vacuum and the temperature is gradually increased. Since the bound water is attached to molecules that form part of the solid electrolyte 3, it is not desorbed at a temperature of 100°C or below. Adsorbed water, however, which is only adsorbed onto the solid electrolyte 3, is desorbed at a temperature of 100°C or below. Therefore, the presence of "bound water" can be confirmed by the presence of water that has desorbed at a temperature of 100°C or above.Furthermore, a ratio between adsorbed water and bound water of the water content contained in the solid electrolyte 3 can also be obtained. "Positive electrode layer and negative electrode layer"

[0034] The positive electrode layer 1 has a Positive electrode current collector layer 1A and one Positive electrode active material layer 1B, which contains a positive electrode active material. The negative electrode layer 2 has a Negative electrode current collector layer 2A and one Negative electrode active material layer 2B, which contains a negative electrode active material.

[0035] The positive electrode current collector layer 1A and the negative electrode current collector layer 2A preferably exhibit high electrical conductivities. Therefore, it is preferable to use, for example, silver, palladium, gold, platinum, aluminum, copper, nickel, or the like for the Positive electrode current collector layer 1A and the To use negative electrode current collector layer 2A. Of these materials, copper reacts only weakly with the positive electrode active material, which Negative electrode active material and the solid electrolyte. If copper is used for the positive electrode current collector layer 1A and the By using negative electrode current collector layer 2A, the internal resistance of the solid-state secondary battery 10 can be reduced. Note that the positive electrode current collector layer 1A and the negative electrode current collector layer 2A may contain the same or different materials.

[0036] The positive electrode active material layer 1B is formed on one or both surfaces of the positive electrode current collector layer 1A. For example, if there is no negative electrode layer 2 opposite the positive electrode layer 1, which is located on the topmost layer of the solid-state secondary battery 10 in one layering direction, then the positive electrode active material layer 1B only needs to be provided on one surface on the underside of the positive electrode layer 1, located on the topmost layer of the solid-state secondary battery 10, in the layering direction. The negative electrode active material layer 2B, similar to the positive electrode active material layer 1B, is also formed on one or both surfaces of the negative electrode current collector layer 2A.

[0037] The positive electrode active material layer 1B and the negative electrode active material layer 2B contain a positive electrode active material and a negative electrode active material, respectively, that exchange electrons. In addition to these, the positive electrode active material layer 1B and the negative electrode active material layer 2B may contain an electrical conductivity enhancer, a binder, and the like. The positive electrode active material and the negative electrode active material are preferably configured such that lithium ions can be effectively intercalated and deintercalated.

[0038] Preferably, for example, a transition metal oxide or a transition metal composite oxide is used for the positive electrode active material and the negative electrode active material. In particular, a lithium-manganese composite oxide Li₂Mn₂ can be used. a Ma 1-aO3 (0.8 ≤ a ≤ 1, Ma = Co, Ni), lithium cobalt dioxide (LiCoO2), lithium nickel dioxide (LiNiO2), lithium manganese spinel (LiMn2O4), a compound metal oxide, represented by the formula: LiNi x Co y Mn z O2 (x + y + z = 1, 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1), a lithium vanadium compound (LiV2O5), olivine-like LiMbPO4 (where Mb represents one or more types of elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al and Zr), lithium vanadium phosphate (Li3V2(PO4)3 or LiVOPO4, a positive electrode of Li-excess mixed crystal represented by Li2MnO3-LiMcO2 (Mc = Mn, Co, Ni), lithium titanate (Li4Ti5O 12 ), a compound metal oxide consisting of Li s Ni t Co u Al v O2 (0.9 < s < 1.3, 0.9 < t + u + v < 1.1) is represented, or similar.

[0039] Furthermore, the negative electrode active material and the positive electrode active material can be selected depending on the solid electrolyte 3.

[0040] In a case where the compound of formula (1) is used for the solid electrolyte 3, it is preferable, for example, to use one of LiVOPO4 and Li3V2(PO4)3, or both, for the positive electrode active material and the negative electrode active material. This is because the bond at the interface between the positive electrode active material layer 1B or the negative electrode active material layer 2B and the solid electrolyte 3 is thus strengthened. This is also because the contact area at the interface between the positive electrode active material layer 1B or the negative electrode active material layer 2B and the solid electrolyte 3 can be increased.

[0041] There is no clear demarcation between the active materials contained in the positive electrode active material layer 1B and the negative electrode active material layer 2B, and it is possible to use a compound that has a higher potential than the positive electrode active material and a compound that has a lower potential than the negative electrode active material by comparing the potentials of the two compounds.

[0042] Furthermore, each of the positive electrode current collector layer 1A and the negative electrode current collector layer 2A can contain the positive electrode active material and the negative electrode active material, respectively. The amount of active material contained in each of the current collectors is not subject to any particular restrictions as long as they function as current collectors. For example, the volume ratio of the positive electrode current collector / positive electrode active material or the negative electrode current collector / negative electrode active material is preferably in the range of 90 / 10 to 70 / 30.

[0043] Adhesion between the positive electrode current collector layer 1A and the positive electrode active material layer 1B, as well as between the Negative electrode current collector layer 2A and the The negative electrode active material layer 2B is traversed by each Positive electrode current collector layer 1A and each The negative electrode current collector layer 2A is reinforced, which Contains positive electrode active material and negative electrode active material. (Connections)

[0044] It is preferable to use a material with high electrical conductivity for the first external terminal 5 and the second external terminal 6 of the solid-state secondary battery 10. For example, silver, gold, platinum, aluminum, copper, tin, or nickel can be used. The first external terminal 5 and the second external terminal 6 can have a single layer or multiple layers. (protective layer)

[0045] Furthermore, the solid-state secondary battery 10 can have a protective layer that electrically, physically, and chemically protects the layered body 4 and the terminals on the outer circumference of the layered body 4. A material included in the protective layer preferably has excellent insulating properties, durability, and moisture resistance, and is preferably environmentally friendly. For example, glass, ceramic, thermosetting resin, or photocurable resin is preferably used. One type of material can be used alone, or a variety of materials can be used together for the protective layer. Although the protective layer can also have a single layer, it preferably comprises a variety of layers. Among these, an organic-inorganic hybrid layer is preferably used, in which a thermosetting resin and a ceramic powder are mixed.

[0046] As described above, the solid-state electrolyte according to the embodiment contains a lithium-containing phosphoric acid compound, which is a cubic crystal. The ion-conducting pathways are present in the cubic crystal in a three-dimensionally equivalent manner with high symmetry, resulting in excellent ionic conductivity. Furthermore, the solid-state secondary battery produced using this electrolyte can incorporate an active material that reduces voltage drop across the electrolyte and provides energy that contributes to the battery's capacity. Therefore, the capacity of the solid-state secondary battery can be increased. “Manufacturing process” (process for the production of a lithium-containing phosphoric acid compound that is a cubic crystal)

[0047] The lithium-containing phosphoric acid compound, which is a cubic crystal, can be produced using various methods. For example, a solid-phase process, a liquid-phase process, a water heating process, a reflux process, or similar methods can be used.

[0048] In the case of a liquid-phase synthesis process, for example, a compound containing elements present in the solid electrolyte is dissolved in water. At this point, the amount of compound to be dissolved in water is determined based on the elemental ratio of the resulting solid electrolyte. Subsequently, a nucleus of a single crystal is generated, and the monocrystalline particles 30 are produced by gradually lowering the temperature of the solvent. The lithium-containing phosphoric acid compound, which is a cubic crystal, can be selectively produced by adjusting the temperature from which the solvent is gradually cooled. Furthermore, the lithium-containing phosphoric acid compound of a cubic crystal can also be selectively produced by adjusting the temperature during heating in solid-phase processes, water heating processes, reflux processes, and the like. (Method for manufacturing the solid-state secondary battery)

[0049] A simultaneous firing process or a sequential firing process can be used as a method for producing the solid-state secondary battery 10.

[0050] The simultaneous firing process is a method for layering materials that form the respective layers and producing a layered body by firing them together. The sequential firing process is a method in which the respective layers are produced one after the other, and the firing process is carried out each time the individual layers are produced. The number of work steps for the solid-state secondary battery 10 can be further reduced by using the simultaneous firing process. Furthermore, the resulting layered body 4 is finer when using the simultaneous firing process. The following describes the use of the simultaneous firing process as an example.

[0051] The simultaneous firing process comprises a process for producing pastes of the respective materials contained in the layered body 4, a process for applying and drying the pastes to produce green films, and a process for layering the green films and simultaneously firing the produced layered film.

[0052] First, the respective materials for the positive electrode current collector layer 1A, the positive electrode active material layer 1B, the solid electrolyte 3, the negative electrode active material layer 2B and the negative electrode current collector layer 2A, which are contained in the layered body 4, are prepared in the form of pastes.

[0053] A method for preparing these materials in paste form is not subject to any particular restrictions. For example, pastes can be obtained by mixing powders of the respective materials in a carrier. Here, "carrier" generally refers to media in the liquid phase. The carriers contain solvents and binders. The paste for the positive electrode current collector layer 1A, the paste for the positive electrode active material layer 1B, the paste for the solid electrolyte 3, the paste for the negative electrode active material layer 2B, and the paste for the negative electrode current collector layer 2A are produced using such a method.

[0054] Green films are then produced. The green films are obtained by applying the prepared pastes in a desired sequence to a base material such as polyethylene terephthalate (PET) or the like, drying the pastes as needed, and peeling off the base material. There are no particular restrictions on the method for applying the pastes. For example, a known method such as screen printing, coating, transfer, or a doctor blade application can be used. The monocrystalline particle 30 can be oriented or aligned during the application of the pastes.

[0055] Each of the manufactured green films is layered according to a desired sequence and number of layers. Alignment, cutting, or similar operations are performed as needed, resulting in a layered body. In the case of manufacturing a battery with parallel or series-parallel mixed connections, it is preferable to align and stack the layers in such a way that an end face of the positive electrode current collector layer does not coincide with an end face of the negative electrode current collector layer.

[0056] A positive electrode active material layer unit and a negative electrode active material layer unit, described below, can be fabricated before the layered body is fabricated, and the layered body can be fabricated afterwards.

[0057] First, the paste for the solid electrolyte 3 is formed into a film on the PET film using a doctor blade process and dried, thus forming the solid electrolyte 3. The paste for the positive electrode active material layer 1B is then screen-printed onto the resulting solid electrolyte 3 and dried, thus forming the positive electrode active material layer 1B.

[0058] The paste for the positive electrode current collector layer 1A is then screen-printed onto the prepared paste for the positive electrode active material layer 1B and dried, thus forming the positive electrode current collector layer 1A. The paste for the positive electrode active material layer 1B is again screen-printed onto the resulting positive electrode current collector layer 1A and dried, thus forming the positive electrode active material layer 1B. The positive electrode active material layer unit is then produced by peeling off the PET film. The positive electrode active material layer unit contains the solid electrolyte 3, the positive electrode active material layer 1B, the positive electrode current collector layer 1A, and the positive electrode active material layer 1B in that order.

[0059] The negative electrode active material layer unit is also manufactured using a similar process. In the negative electrode active material layer unit, the solid electrolyte 3, the negative electrode active material layer 2B, the negative electrode current collector layer 2A, and the negative electrode active material layer 2B are layered in this order.

[0060] A positive electrode active material layer unit and a negative electrode active material layer are layered. At this point, the one positive electrode active material layer unit and the one negative electrode active material layer are layered such that the solid electrolyte 3 in the positive electrode active material layer unit is in contact with the negative electrode active material layer 2B in the negative electrode active material layer unit, or the positive electrode active material layer 1B in the positive electrode active material layer unit is in contact with the solid electrolyte 3 in the negative electrode active material layer unit.In this way, a layered body is obtained in which the positive electrode active material layer 1B, the positive electrode current collector layer 1A, the positive electrode active material layer 1B, the solid electrolyte 3, the negative electrode active material layer 2B, the negative electrode current collector layer 2A, the negative electrode active material layer 2B, and the solid electrolyte 3 are layered in that order. The respective units are stacked in a different manner such that the positive electrode current collector layer 1A in the first positive electrode active material layer unit extends only to one end face, and the negative electrode current collector layer 2A in the second negative electrode active material layer unit extends only to the other end face. Films of solid electrolyte 3 with a predetermined thickness are further stacked onto both surfaces of the stacked units, thus producing the layered body.

[0061] The manufactured layered body is pressure-bonded. Pressure bonding is carried out while the layered body is heated, and the heating temperature is set, for example, to 40 to 95°C.

[0062] The pressure-bonded layered body is heated and fired at 600°C to 1000°C, for example in a nitrogen atmosphere, resulting in a sintered body. The firing time is set, for example, to 0.1 to 3 hours.

[0063] The sintered body can be placed in a cylindrical container along with a polishing material such as aluminum oxide and subjected to drum polishing. This allows the edges of the layered body to be chamfered. Alternatively, the layered body can be polished by sandblasting. This method is preferred because only specific sections can be polished.

[0064] In the aforementioned sintered body, the relative density of the pair of electrode layers and the solid electrolyte layer containing the solid electrolyte 3, which is provided between the pair of electrode layers, can be greater than or equal to 80%. This is because diffusion pathways of mobile ions in the crystal tend to be better connected, and ionic conductivity is further increased when the relative density is higher. (Training the connection)

[0065] The first external terminal 5 and the second external terminal 6 are attached to the layered body 4. The first external terminal 5 and the second external terminal 6 are configured such that they are each in electrical contact with the positive electrode current collector layer 1A and the negative electrode current collector layer 2A, respectively. For example, the first external terminal 5 and the second external terminal 6 can be formed on the positive electrode current collector layer 1A and the negative electrode current collector layer 2A, which are exposed on the side faces of the layered body 4, using a known method such as sputtering, dipping, or spray coating. If the first external terminal 5 and the second external terminal 6 are formed only on certain sections, they are formed, for example, by applying a cover with a strip.

[0066] Although the embodiments of the invention have been described in detail above with reference to the drawings, the respective configurations, their combinations and the like are only examples in each embodiment and additions, omissions, replacements and other changes to the configurations can be made without deviating from the core of the invention. [Examples](Example 1)

[0067] LiZr2(PO4)3 was prepared using the following procedure. First, each of LiOH·H2O, ZrO(NO3)2, and NH4(H2PO4) was weighed out in a specific weight ratio and dissolved in water. The respective solutions were mixed, heated to 120°C after adjusting the pH, and then gradually cooled. X-ray diffraction was then performed on the prepared compound. Fig. Figure 4 shows an X-ray diffraction (XRD) image in Example 1. As in Fig.As shown in Figure 4, the solid electrolyte in Example 1 has a peak in a position corresponding to a cubic crystal structure, and it has been confirmed that the solid electrolyte is a cubic crystal.

[0068] Then, the particle size distribution, ionic conductivity, and bound water content of the resulting solid electrolyte were measured. The particle size distribution was measured using a particle size distribution analyzer with a laser diffraction scattering technique (Microtrac method). To measure the ionic conductivity, a disk-shaped and sintered electrolyte was used. This solid electrolyte sintered electrolyte was impregnated in water for 24 hours to maximize water absorption. The bound water content was adjusted by varying the time the electrolyte was treated at a temperature of 100°C or higher. Electrodes were then sputtered onto both surfaces of the resulting solid electrolyte sintered electrolyte, and measurements were taken using an impedance analyzer (Solartron Analytical; model no. 1).S11260) was used at an amplitude of 50 mV and a frequency of 0.5 Hz to 1 MHz. The bound water content was measured using a TDS meter (EMD-WA1000, manufactured by ESCO Ltd.). The results are shown in Table 1. (Comparative examples 1 to 3)

[0069] Comparison examples 1 to 3 differed from example 1 in that the crystal phases of the solid electrolyte sintered body were changed. The measurement was performed while the other conditions were set to the same as in example 1. The results are shown in Table 1.

[0070] In comparative example 1, a solid electrolyte sintered body was obtained from a rhombohedral crystal by preparing a mixed solution, adjusting the pH, then heating the mixed solution to 120°C and gradually cooling the mixed solution in the same way as in example 1.

[0071] In comparative example 2, a solid electrolyte sintered body was produced using a flux process. In comparative example 2, the solid electrolyte sintered body was obtained from a triclinic crystal by setting a melting temperature of 1200°C and then gradually cooling the mixture.

[0072] A solid electrolyte sintered body in comparative example 3 was also produced using the flux method. In comparative example 3, the solid electrolyte sintered body, which was a monoclinic crystal, was obtained by setting a melting temperature of 900°C and then gradually cooling the mixture. [Table 1] Main crystal phase Content of bound water (wt%) composition Grain size distribution D50 (µm) Ionic conductivity (S / cm) Example 1 cubic crystal 7,05 LiZr 2,0 P 3,0 O 12 1,4 2,10 × 10 -5 Comparative example 1 Rhombohedral crystal 7,05 LiZr 2,0 P 3,0 O 12 1,4 2,36 × 10 -6 Comparative example 2 Triclinic crystal 7,05 LiZr 2,0 P 3,0 O 12 1,4 1,25 × 10 -7 Comparative example 3 Monoclinic crystal 7,05 LiZr 2,0 P 3,0 O 12 1,4 1,12 × 10 -8

[0073] The solid electrolyte sintered body in Example 1, which mainly has a cubic crystal structure, has an ionic conductivity that is an order of magnitude or more higher than that of the solid electrolyte sintered bodies in comparative examples 1 to 3, which mainly had other crystal structures. The sintered body in Example 1 thus has excellent ionic conductivity. (Examples 2 to 32)

[0074] Examples 2 to 32 differed from Example 1 in that the compositions of the monocrystalline particles were changed. The measurement was performed while the other conditions were set to the same as in Example 1. The composition was changed by exchanging some of the elements. The results are presented in Tables 2 to 4. (Comparative examples 4 and 5)

[0075] Comparison example 4 differed from example 3 in that one crystal phase of the solid electrolyte sintered body was a rhombohedral crystal.

[0076] Furthermore, comparison example 5 differed from example 22 in that one crystal phase of the solid electrolyte sintered body was a rhombohedral crystal. The measurement was performed while the other conditions were set to the same as those in example 1. The results are presented in Tables 2 and 3. [Table 2] Main crystal phase Content of bound water (wt%) composition Grain size distribution D50 (µm) Ionic conductivity (S / cm) Example 2 Cubic crystal 7,05 Li 1,02 Y 0,02 Zr 1,98 P 3,0 O 12 1,4 3,21 × 10 -5 Example 3 Cubic crystal 7,05 Li 1,1 Y 0,1 Zn 1,9 Q 3,0 O 12 1,4 5,36 × 10 -5 Example 4 Cubic crystal 7,05 Li 1,18 Y 0,18 Zr 1,82 P 3,0 O 12 1,4 5,78 × 10 -5 Example 5 Cubic crystal 7,05 Li 1,26 Y 0,26 Zr 1,74 P 3,0 O 12 1,4 1,40 × 10 -4 Example 6 Cubic crystal 7,05 Li 1,33 Y 0,33 Zr 1,67 P 3,0 O 12 1,4 8,21 × 10 -5 Example 7 Cubic crystal 7,05 Li 1,7 Y 0,7 Zr 1,3 P 3,0 O 12 1,4 5,29 × 10 -5 Example 8 Cubic crystal 7,05 Li 2,0 Y 1,0 Zn 1,0 Q 3,0 O 12 1,4 8,21 × 10 -6 Example 9 Cubic crystal 7,05 Li 1,01 Zr 2,0 Yes 0,01 P 2,99 ON 12 1,4 5,32 × 10 -5 Example 10 Cubic crystal 7,05 Li 1,1 Zr 2,0 Yes 0,1 P 2,9 ON 12 1,4 7,35 × 10 -5 Example 11 Cubic crystal 7,05 Li 1,2 Zr 2,0 Yes 0,2 P 2,8 ON 12 1,4 8,10 × 10 -6 Example 12 Cubic crystal 7,05 Li 2, 0Zr 2, 0Si 1, 0P 2,0 About 12 1,4 1,01 × 10 -5 Example 13 Cubic crystal 7,05 Li 2,5 Zr 2,0 Yes 1,5 P 1,5 ON 12 1,4 8,92 × 10 -6 Comparison example 4 Rhombohedral crystal 7,05 Li 1,1 Y0,1Zr 1,9 P 3,0 O 12 1,4 1,21 × 10 -6 [Table 3] Main crystal phase Content of bound water (wt%) composition Grain size distribution D50 (µm) Ionic conductivity (S / cm) Example 14 Cubic crystal 7,05 Li 0,9 Ca 0,05 Zr 2,0 P 3,0 O12 1,4 3,29 × 10 -5 Example 15 Cubic crystal 7,05 Li 0,8 Ca 0,1 Zr 2,0 P 3,0 ON 12 1,4 5,21 × 10 -5 Example 16 Cubic crystal 7,05 Li 0,6 Ca 0,2 Zr 2,0 P 3,0 ON 12 1,4 3,21 × 10 -5 Example 17 Cubic crystal 7,05 Li 0,5 Ca 0,25 Zr 2,0 P 3,0 O12 1,4 2,91 × 10 -5 Example 18 Cubic crystal 7,05 Li 0,4 Ca 0,3 Zr 2,0 P 3,0 ON 12 1,4 8,21 × 10 -6 Example 19 Cubic crystal 7,05 Li 0,2 Ca 0,4 Zr 2,0 P 3,0 ON 12 1,4 6,21 × 10 -6 Example 20 Cubic crystal 7,05 Li 1,26 SC 0,26 Zr 1,74 P 3,0 ON 12 1,4 8,41 × 10 -5 Example 21 Cubic crystal 7,05 Li 1,26 A1 0,26 Zr 1,74 P 3, 0O 12 1,4 7,34 × 10 -5 Example 22 Cubic crystal 7,05 Li 1,2 Ca 0,1 Zr 0,9 P 3,0 ON 12 1,4 1,14 × 10 -4 Example 23 Cubic crystal 7,05 Li 1,52 Ca 0,26 Zr 1,74 P 3,0 ON 12 1,4 8,92 × 10 -5 Example 24 Cubic crystal 7,05 Li 1,52 Mg 0,26 Zr 1,74 P 3,0 ON 12 1,4 6,31 × 10 -6 Example 25 Cubic crystal 7,05 Li 1,52 Sr 0,26 Zr 1,74 P 3,0 O 12 1,4 7,63 × 10 -5 Example 26 Cubic crystal 7,05 Li 1,52 Ba 0,26 Zn 1,74 Q 3.0 O 12 1,4 5,90 × 10 -5 Example 27 Cubic crystal 7,05 Li 0,74 Nb 0,26 Zr 1,74 P 3,0 O 12 1,4 1,81 × 10 -4 Comparison example 5 Rhombohedral crystal 7,05 Li 1,2 Ca 0,1 Zr 0,9 P 3,0 ON 12 1,4 4,20 × 10 -6 [Table 4] Main crystal phase Content of bound water (wt%) composition Grain size distribution D50 (µm) Ionic conductivity (S / cm) Example 28 cubic crystal 7,05 Li 1,4 Zr 2,0 B 0,2 P 2,8 ON 12 1,4 6,11 × 10 -5 Example 29 cubic crystal 7,05 Li 0,8 Zr 2,0 Mo 0,2 P 2,8 ON 12 1,4 9,35 × 10 -5 Example 30 cubic crystal 7,05 Li 0,8 Mg 0‚1 Zr 2,0 P 3,0 ON 12 1,4 1,36 × 10 -5 Example 31 cubic crystal 7,05 Li 0,8 Mg 0‚1 Zr 2,0 P 3,0 ON 12 1,4 6,22 × 10 -5 Example 32 cubic crystal 7,05 Li 0,8 Mg 0‚1 Zr 2,0 P 3,0 ON 12 1,4 2,81 × 10 -5

[0077] As described in Examples 2 to 32, the solid-state electrolyte sintered bodies made of cubic crystals still exhibited excellent ionic conductivity even after changes in composition. Furthermore, in comparisons between Example 3 and Example 4, as well as between Example 22 and Example 5, the ionic conductivity was increased by the cubic crystal phase. Thus, solid-state electrolyte sintered bodies with excellent ionic conductivities are obtained. (Examples 33 to 42)

[0078] Examples 33 to 42 differed from Example 1 only in that the amounts of bound water contained in the solid electrolyte sintered bodies were changed. The measurement was performed while the other conditions were set to the same as in Example 1. The results are shown in Table 5. [Table 5] Main crystal phase Content of bound water (wt%) composition Grain size distribution D50 (µm) Ionic conductivity (S / cm) Example 33 cubic crystal 0 LiZr 2,0 P 3,0 O 12 1,4 2,44 × 10 -6 Example 34 cubic crystal 0,01 LiZr 2,0 P 3,0 O 12 1,4 1,10 × 10 -5 Example 35 cubic crystal 0,38 LiZr 2,0 P 3,0 O 12 1,4 1,33 × 10 -5 Example 36 cubic crystal 1,86 LiZr 2,0 P 3,0 O 12 1,4 1,40 × 10 -5 Example 37 cubic crystal 3,66 LiZr 2,0 P 3,0 O 12 1,4 1,64 × 10 -5 Example 38 cubic crystal 6,23 LiZr 2,0 P 3,0 O 12 1,4 2,01 × 10 -5 Example 1 cubic crystal 7,05 LiZr 2,0 P 3,0 O 12 1,4 2,10 × 10 -5 Example 39 cubic crystal 10,22 LiZr 2,0 P 3,0 O 12 1,4 1,21 × 10 -5 Example 40 cubic crystal 18,55 LiZr 2,0 P 3,0 O 12 1,4 8,92 × 10 -6 Example 41 cubic crystal 20,99 LiZr 2,0 P 3,0 O 12 1,4 5,92 × 10 -6 Example 42 cubic crystal 23,29 LiZr 2,0 P 3,0 O 12 1,4 4,28 × 10 -6

[0079] All solid electrolyte sintered bodies in Examples 33 to 42 had better ionic conductivities than those in the comparative Examples 1 to 3, where the crystal phases were different. In Example 1 and in Examples 34 to 40, where the bound water content ranged from 0.01 wt% or greater to 20 wt% or less, the ionic conductivity values ​​were particularly high, and excellent ionic conductivities were achieved. (Examples 43 to 47)

[0080] Examples 43 to 47 differed from Example 1 only in that the particle size distributions of the particles contained in the solid electrolyte sintered bodies were modified. The particle size distributions of the particles contained in the solid electrolyte sintered bodies were produced by pulverizing and filtering the solid electrolytes. The measurement was performed while the other conditions were set to the same as in Example 1. The results are shown in Table 6. [Table 6] Main crystal phase Content of bound water (wt%) composition Grain size distribution D50 (µm) Ionic conductivity (S / cm) Example 43 cubic crystal 7,05 LiZr 2,0 P 3,0 O 12 0,32 3,31 × 10 -5 Example 1 cubic crystal 7,05 LiZr 2,0 P 3,0 O 12 1,4 2,10 × 10 -5 Example 44 cubic crystal 7,05 LiZr 2,0 P 3,0 O 12 2,6 1,16 × 10 -5 Example 45 cubic crystal 7,05 LiZr 2,0 P 3,0 O 12 8,6 9,29 × 10 -6 Example 46 cubic crystal 7,05 LiZr 2,0 P 3,0 O 12 12,3 4,47 × 10 -6 Example 47 cubic crystal 7,05 LiZr 2,0 P 3,0 O 12 15,2 3,40 × 10 -6

[0081] All solid electrolyte sintered bodies in Example 1 and in Examples 44 to 47 exhibited excellent ionic conductivities. Particularly excellent ionic conductivities were achieved in Example 1 and in Examples 43 to 45, where the value of D50 was in the range of 0.1 µm or more to 10 µm or less. (Example 48)

[0082] Example 48 differed from Example 1 in that Hf was used instead of Zr in LiZr₂(PO₄)₃ and that the composition of the solid electrolyte was fixed as LiHf₂(PO₄)₃. For a solid electrolyte in Example 48, LiOH·H₂O, HfCl₄, and NH₄(H₂PO₄) were weighed out in a specific ratio and dissolved in water. The respective solutions were mixed, heated to 120°C after adjusting the pH, and then gradually cooled. As a result, it was confirmed that a crystalline phase of the solid electrolyte, represented by LiHf₂(PO₄)₃, was a cubic crystal. Furthermore, the ionic conductivity of the solid electrolyte sintered body was measured in the same manner as in Example 1. Accordingly, the ionic conductivity was 3.15 × 10⁻⁶. -5 S / cm. The ionic conductivity was 6.5 × 10 in one case where the crystal phase of LiHf2(PO4)3 was a rhombohedral crystal. -6S / cm. It was confirmed that the ionic conductivity was increased in a case where the crystal phase of LiHf2(PO4)3 was a tetragonal crystal.

[0083] Solid-state secondary batteries were fabricated using the solid electrolytes in Example 1, Example 5, Example 11, Example 22, Example 27, and Example 43, as well as Comparative Example 1 and Comparative Example 5, from the aforementioned solid electrolytes. Li3V2(PO4)3 was used as the positive electrode active material, and Li4Ti5O 12 was used as the negative electrode active material. The discharge capacities of the solid-state secondary batteries were then measured. These capacities were measured at a constant current of 2 µA. Charging and discharging voltages were set to 2.8 V and 1.8 V, respectively. The results are shown in Table 7. [Table 7] Main crystal phase Content of bound water (wt%) composition Particle size distribution D50 (4 m ) Discharge capacity (µAh) Example 1 cubic crystal 7,05 LiZr 2,0 P 3,0 O 12 1,4 9,9 Example 5 cubic crystal 7,05 Li 1,26 Y 0,26 Zr 1,74 P 3,0 O12 1,4 12,2 Example 11 cubic crystal 7,05 Li 1,2 Y 2,0 Yes 0,2 P 2,8 About 12 1,4 11,1 Example 22 cubic crystal 7,05 Li 1,2 Ca 0,1 Zr 0,9 P 3,0 ON 12 1,4 12,1 Example 27 cubic crystal 7,05 Lio, 26 Nbo, 26 Zr1, 74 P3.0O 12 1,4 12,5 Example 43 cubic crystal 7,05 LiZr 2,0 P 3,0 O 12 0,32 10,2 Comparative example 1 Rhombohedral crystal 7,05 LiZr 2,0 P 3,0 O 12 1,4 4,7 Comparative example 5 Rhombohedral crystal 7,05 Li 1,2 Ca 0,1 Zr 0,9 P 3,0 ON 12 1,2 0,8

[0084] In a comparison between Example 1 and Comparative Example 1, as well as between Example 22 and Comparative Example 5, it was found that the discharge capacity was increased by using a solid electrolyte with a cubic crystal structure. Furthermore, high-capacity solid-state batteries were obtained regardless of changes in composition, particle size distribution, and the like. [List of reference symbols] 1 Positive electrode layer 1A Positive electrode current collector layer 1B Positive electrode active material layer 2 Negative electrode layer 2A negative electrode current collector layer 2B Negative electrode active material layer 3 Solid-state electrolyte 4-layered body 5 First external connection 6 Second external connection 30 Monocrystalline particles 31 particles with a random shape P path G grain Gs grain boundary

Claims

[1] Solid electrolyte, comprising: a lithium-containing phosphoric acid compound with a cubic crystal structure, wherein the lithium-containing phosphoric acid compound, which is a cubic crystal and is contained in the solid electrolyte, is represented by: Li x M1 y M2 z M3 w P 3-w O 12 . (1), where formula (1) satisfies 0 < x ≤ 3, 0 ≤ y < 2, 0 < z ≤ 2 and 0 ≤ w < 3, M1 represents at least one element selected from the following group: Mg, Ca, Sr, Ba, V, Nb, Mn, Co, Ni, Cu, Ag, Ga, Al, In, Sc and Y M2 represents at least one element selected from the following group: Zr and Hf M3 represents at least one element selected from the following group: Si, B, S, V, Mo and W. [2] Solid electrolyte according to claim 1, further comprising: 0.01% by mass or more up to 20% by mass or less bound water. [3] Solid electrolyte according to claim 1 or 2, wherein a measured value (D50) obtained by particle size distribution measurement is greater than or equal to 0.1 µm and less than or equal to 10 µm. [4] Solid-state secondary battery, comprising: the solid electrolyte according to any one of claims 1 to 3. [5] Solid-state secondary battery according to claim 4, wherein the relative density of a pair of electrode layers and a solid electrolyte layer with the solid electrolyte provided between the pair of electrode layers is greater than or equal to 80%.

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