Electrolyte Fest

Abstract

Solid electrolyte satisfying formula (II): Li1+X-YAlXNbYTi2-XY(PO4)3(II) in formula (II) X and Y are real numbers satisfying X + Y ≤ 1, where the solid electrolyte has a NASICON-type crystal structure, where X 0.5 ≤ X ≤ 0.6 and Y 0.1 ≤ Y ≤ 0.3.

Description

[Technical field]

[0001] The present invention relates to a solid electrolyte and a method for producing it. [State of the art]

[0002] To popularize electric vehicles, high hopes are being placed on an air-filled battery, which has a significantly higher energy density than a lithium-ion battery. The air-filled battery uses oxygen in the air as the active material of the positive electrode.

[0003] A lithium-air battery is known to use metallic lithium, an alloy with lithium as the main component, or a compound whose main component is lithium as the active material of the negative electrode. Depending on the electrolyte type, lithium-air batteries can be broadly divided into two types: one that uses an aqueous electrolyte (solution-based electrolyte, aqueous electrolyte solution), and another that uses a non-aqueous electrolyte. Lithium-air batteries using a non-aqueous electrolyte are primarily researched and developed because the techniques for lithium-ion batteries, with the exception of those for an air electrode, can be applied to the lithium-air battery using the non-aqueous electrolyte.

[0004] At the same time, lithium-air batteries using an aqueous electrolyte are also being researched and developed, although so far only in small numbers. An aqueous lithium-air battery offers advantages over a non-aqueous one in that it is unaffected by atmospheric humidity and uses an inexpensive and non-flammable electrolyte. However, metallic lithium, the active material of the negative electrode, reacts with oxygen and water upon direct contact. To prevent this, an aqueous lithium-air battery uses a solid electrolyte with lithium-ion conductivity as a protective layer to shield the metallic lithium from the atmosphere and solutions.

[0005] A lithium-conducting Li 1+x A x Ti 2-x (PO4)3 solid electrolyte of the NASICON type (hereinafter referred to as the NASICON type solid electrolyte) is known as such a solid electrolyte (Nonpatent document 1 and the like).

[0006] The NASICON-type solid electrolyte exhibits low moisture sensitivity, can be produced in open air, and is stable while in contact with a LiCl solution. Furthermore, the NASICON-type solid electrolyte displays advantageous lithium-ion conductivity.

[0007] Non-patent document 1 describes a solid electrolyte of the NASICON type with the composition Li 1,4 Al 0,4 Ge 0,2 Ti 1,4 (PO4)3 described.

[0008] However, because many pores are open on the surface of this solid electrolyte, it has a low relative density of at most 91.2%, and a substance can permeate the solid electrolyte through these pores. Consequently, if the solid electrolyte is used in a component that comes into contact with moisture, such as a separator used, for example, to separate a negative lithium electrode from an aqueous electrolyte solution in a lithium-air battery, water can permeate the solid electrolyte.

[0009] To prevent water permeation, filling the pores of the solid electrolyte with an epoxy resin or similar material is considered. However, this presents several problems, as it requires an additional step of filling the pores of the solid electrolyte, and the epoxy resin reduces the lithium-ion conductivity compared to the unfilled state.

[0010] Secondly, non-patent document 1 describes the NASICON-type solid electrolyte with the composition Li described above. 1,4 Al 0,4 Ge 0,2 Ti 1,4 (PO4)3 described.

[0011] In cases where the solid electrolyte is actually used to manufacture the air-filled battery and is used for extended periods in a moving body such as a motor vehicle, the strength of the solid electrolyte becomes a serious issue. One indicator of strength is the three-point bending strength. However, the three-point bending strength of commonly known solid electrolytes with high lithium-ion conductivity is insufficient for use as a solid electrolyte for the aforementioned purpose.

[0012] At the same time, solid electrolytes with improved strength are being marketed, and some of these exhibit a strength of approximately 100 N / mm². 2 this represents a practically usable height. However, such solid electrolytes have a lithium-ion conductivity of approximately 1.0 × 10⁻⁶. –4 S / cm and reduce the output power below a level sufficient for an air accumulator.

[0013] The Li mentioned above, described in non-patent document 1 1,4 Al 0,4 Ge 0,2 Ti 1,4 (PO4)3 exhibits advantageous lithium ion conductivity, and it is empirically known that its strength is effectively improved when an increased amount of Al is added.

[0014] However, if the atomic fraction (proportion of the number of atoms) of Al in a NASICON-type solid electrolyte with such a composition exceeds 0.4, Al atoms not packed in the crystal structure are deposited as impurities, and the atomic fraction of Al cannot be increased. Accordingly, a NASICON-type solid electrolyte has a problem in that the strength and lithium-ion conductivity of the solid electrolyte itself cannot be improved. [State of the art][Non-patent document]

[0015] [Non-Patent Document 1] Zhang et al. Journal of The Electrochemical Society 162(7) A1265–A1271(2015) “Tape-Cast Water-Stable NASICON-Type High Lithium Ion Conducting Solid Electrolyte Films for Aqueous Lithium-Air Batteries” [Brief description of the invention][Problems to be solved by the invention]

[0016] In view of the circumstances described above, an object of the present invention is to provide a solid electrolyte whose relative density is improved while exhibiting advantageous lithium ion conductivity, and which can preferably be used in a lithium-air battery and the like, and a method for producing it.

[0017] Secondly, in view of the circumstances described above, an object of the present invention is to provide a solid electrolyte capable of containing Al in an atomic proportion of more than 0.4 in a composition in order to improve the lithium ion conductivity while improving the strength, and which can preferably be used in a lithium-air battery and the like, and a method for producing it. [Means of solving the problems]

[0018] The inventors of the present invention have discovered through research that the relative density of a solid electrolyte can be improved by controlling its crystal structure. The improvement in relative density described above can reduce the likelihood of water permeation into the solid electrolyte. The solid electrolyte can thus be readily applied to parts that come into contact with water. Furthermore, if the pores on the surface of the solid electrolyte are to be filled with an epoxy resin or the like, the amount of epoxy resin required can be reduced. Consequently, a decrease in lithium-ion conductivity can be suppressed.

[0019] The present invention is a solid electrolyte that fulfills formula (I): Li 1+X M1 X M2 Y Ti 2-X-Y (PO4)3 (I) (In formula (I), M1 is one or more elements selected from the set of elements.) 3+ , Cu 3+ , Co 3+ , Fe 3+ , Ni 3+ , Ga 3+ , Cr 3+ and Sc 3+ existing group, M2 is one or more elements selected from the group consisting of Si 4+ , Ge 4+ , Sn 4+ , Hf 4+ and Zr 4+ existing group, and X and Y are real numbers that satisfy X + Y ≤ 1), where the solid electrolyte has a NASICON-type crystal structure and The lattice constants of the NASICON-type crystal structure are such that the length along the a-axis is 0.8 nm or more and the length along the c-axis is 2.8 nm or less.

[0020] The X preferably satisfies 0.35 ≤ X ≤ 0.50. Furthermore, the Y preferably satisfies 0.1 ≤ Y ≤ 0.3. The relative density of the solid electrolyte is preferably 92% or more. Additionally, the lithium ion conductivity of the solid electrolyte is preferably 4.0 × 10⁻⁶. –4 S / cm or more. Furthermore, the three-point flexural strength of the solid electrolyte is preferably 70 N / mm². 2 or more.

[0021] Another aspect of the present invention is a method for producing the solid electrolyte, which satisfies formula (I): Li 1+X M1 X M2 Y pi 2-X-Y (PO4)3 (I) (In formula (I), M1 is one or more elements selected from the set of elements.) 3+ , Cu 3+ , Co 3+ , Fe 3+ , Ni 3+ , Ga 3+ , Cr 3+ and Sc 3+ existing group, M2 is one or more elements selected from the group consisting of Si 4+ , Ge 4+, Sn 4+ , Hf 4+ and Zr 4+ existing group, and X and Y are real numbers that satisfy X + Y ≤ 1), where the procedure is the steps of: Mixing a solid powder containing a composition to form the solid electrolyte; Forming a compact by pressing the mixed powder; and the Tempering of the pellet is included.

[0022] Secondly, the inventors of the present invention have found as a research result that in a solid electrolyte of the NASICON type, which, for example, has the composition Li 1,4 Al 0,4 Ge 0,2 Ti 1,4 (PO4)3, the atomic fraction of Al in the composition can preferably be increased beyond 0.4 by adding Nb instead of Ge.

[0023] In particular, a second solid electrolyte of the present invention is a solid electrolyte that satisfies formula (II): Li 1+X-YAl X Note Y Ti 2-X-Y (PO4)3 (II) (in formula (II) X and Y are real numbers that satisfy X + Y ≤ 1), where the solid electrolyte has a NASICON-type crystal structure.

[0024] In formula (II), Y preferably satisfies 0.1 ≤ Y ≤ 0.3.

[0025] Furthermore, in formula (II) X preferably satisfies 0.5 ≤ Y ≤ 0.6.

[0026] Another aspect of the present invention is a method for producing the second solid electrolyte, which satisfies formula (I): Li 1+X-Y Al X Note Y Ti 2-X-Y (PO4)3 (II) (in formula (II) X and Y are real numbers that satisfy X + Y ≤ 1), where the procedure is the steps of: Mixing a solid powder containing a composition to form the solid electrolyte; Forming a compact by pressing the mixed powder; and the Tempering of the pellet is included. [Advantageous effects of the invention]

[0027] The present invention makes available the solid electrolyte, whose relative density is improved while exhibiting advantageous lithium ion conductivity, and which can preferably be used in a lithium-air battery and the like, and a method for producing it.

[0028] Secondly, the present invention makes available a solid electrolyte capable of containing Al in an atomic proportion of more than 0.4 in a composition to improve lithium ion conductivity while improving strength, and which can preferably be used in a lithium-air battery and the like, and the method for producing it. [Brief description of the drawings]

[0029] Fig.Figure 1 is a schematic view illustrating a NASICON-type crystal structure.

[0030] Fig. 2 is a graphic showing the X-ray diffraction patterns of Li 1,5 Al 0,5 Ge 0,2 Ti 1,3 (PO4)3 samples illustrated (corresponding to the case where X = 0.5 and Y = 0.2).

[0031] Fig. Figure 3 is a diagram showing the relative densities of Li 1,5 Al 0,5 Ge 0,2 Ti 1,3 (PO4)3 pellets illustrated (corresponding to the case where X = 0.5 and Y = 0.2) that were sintered at different temperatures.

[0032] Fig. 4 is a diagram showing the lithium ion conductivity and the relative density of Li 1+X Al X Ge 0,2 Ti 1,8-X (PO4)3, measured at 25°C as a function of X, illustrated.

[0033] Fig.Figure 5 is a diagram showing the X-dependence of the three-point bending strength in relation to the Al content of Li 1+X Al X Ge 0,2 Ti 1,8-X (PO4)3 (which corresponds to Y = 0.2), which was sintered for seven hours at 900°C, is illustrated.

[0034] Fig. Figure 6 is a diagram showing the lithium ion conductivity and the relative density of Li 1+X Al X Ge 0,3 Ti 1,7-X (PO4)3 (which corresponds to Y = 0.3) is illustrated as a function of X.

[0035] Fig. 7 is a diagram showing the variation of the grid parameters of Li 1+X Al X Ge 0,2 Ti 1,8-X (PO4)3 illustrated as a function of X.

[0036] Fig. Figure 8 includes graphs illustrating X-ray diffraction patterns of samples obtained by varying X, which represents the atomic fraction of Al in the case where Li 1+X-Y Al X Note Y Ti2-X-Y (PO4)3 Y = 0.1 to 0.3.

[0037] Fig. Figure 9 includes diagrams illustrating the variation of X, which represents the atomic fraction of Al, and the variation of the three-point bending strength, the relative density, and the lithium ion conductivity in the case where Li 1+X-Y Al X Note Y Ti 2-X-Y (PO4)3 Y = 0.1 to 0.3. [Mode for carrying out the invention]

[0038] A preferred embodiment of a solid electrolyte in the present invention and a method for producing it are described in more detail below.

[0039] A first solid electrolyte in the present invention is a solid electrolyte satisfying the following formula (I): Li 1+X-Y M1 X M2 Y Ti 2-X-Y (PO4)3 (I).

[0040] M1 in formula (I) is a metallic element and one or more metallic elements selected from the group consisting of Al 3+ , Cu 3+ , Co 3+ , Fe 3+ , Ni 3+ , Ga 3+ , Cr 3+ and Sc 3+ The valences listed here are the valences of the metal element M1 when the element forms the resulting solid electrolyte of the NASICON type. Of these elements, Al 3+ especially preferred.

[0041] M2 is a metal element and one or more elements selected from the group consisting of Si 4+ , Ge 4+ , Sn 4+ , Hf 4+ and Zr 4+ The valences listed here are the valences of the metal element M2 when the element forms a solid electrolyte of the NASICON type. Of these elements, Ge 4+ especially preferred.

[0042] X and Y in formula (I) are real numbers that satisfy X + Y ≤ 1. By satisfying X + Y ≤ 1, the strength and lithium-ion conductivity of the solid electrolyte can be improved. Furthermore, 0 < Y < X is preferred. By including the metal element (M2) with a valence of 4, the carrier ions in the solid electrolyte can be increased and the lithium-ion conductivity improved. By satisfying Y < X, the crystal lattice constants of the solid electrolyte can be suitably adjusted, and consequently, the relative density of the solid electrolyte can be improved.

[0043] It should be noted that X (the atomic fraction of metal element M1 in the solid electrolyte) preferably satisfies 0.35 ≤ X ≤ 0.50. This is preferred because it allows for the preservation of desired lengths along the a-axis and the c-axis, a reduction in crystal grain boundary resistance, and the maintenance of high lithium-ion conductivity. Furthermore, it makes it possible to improve the three-point bending strength. Y (the atomic fraction of metal element M2 in the solid electrolyte) preferably satisfies 0.1 ≤ Y ≤ 0.3. This is preferred because it allows for a reduction in the lattice constant along the c-axis and consequently an improvement in the relative density of the solid electrolyte.

[0044] The first solid electrolyte of the present invention has a NASICON-type crystal structure. More specifically, the solid electrolyte of the present invention has a rhombohedral (hexagonal) structure, which is illustrated by the space group R-3c. This crystal structure is described in Fig. 1 illustrates. The in Fig. Figure 1 illustrates the crystal structure of LiM2(PO4)3. It should be noted that such basic concepts of crystal structure are also the same as those of the second solid electrolyte.

[0045] Li(1) in Fig. 1 means fixed Li, which is irrelevant for ionic conduction, and Li(2) means mobile Li, which is relevant for ionic conduction.

[0046] It should be noted that “R” in R-3c, which expresses the space group, denotes the rhombohedral structure. “3” signifies the symmetry operation of performing a rotational inversion (a “–” of x, y, and z, respectively) by 120°. “c” signifies a c / 2 glide reflection (translation) along the c-axis.

[0047] In the drawings, the length along the a-axis is referred to as a (directions of the x-axis and the y-axis in the drawing), and the length along the c-axis is referred to as c (direction of the z-axis in the drawing).

[0048] In the first solid electrolyte of the present invention, the lattice constants of the NASICON-type crystal structure are such that the length along the a-axis is 0.8 nm or more and the length along the c-axis is 2.8 nm or less.

[0049] The length along the c-axis is 2.8 nm or less, preferably 2.5 nm or less, and more preferably 2.2 nm or less, as described above. The inventors of the present invention have found that the relative density can be improved by adjusting the lattice constant within such a range. It should be noted that the lower limit of the length along the c-axis is defined as the range in which the NASICON-type crystal structure can be obtained, and that it varies depending on the atomic composition used.

[0050] The length along the a-axis is 0.8 nm or more, preferably 0.82 nm or more, and more preferably 0.85 nm or more. The inventors of the present invention have found that the relative density can be improved by adjusting the lattice constant within such a range. It should be noted that the upper limit of the length along the a-axis is defined as the range in which the NASICON-type crystal structure can be obtained, and that it varies depending on the atomic composition used.

[0051] The relative density of the first solid electrolyte of the present invention is preferably 92% or more and preferably 95% or more.

[0052] If the relative density is in such a range, it is possible to reduce the possibility of water permeating into the solid electrolyte and also to reduce the amount of epoxy resin used in cases where, for example, pores on the surface of the solid electrolyte are filled with epoxy resin.

[0053] The relative density here refers to the ratio between the density calculated based on the lattice constants of the solid electrolyte used as a sample and the density based on the volume and mass of the solid electrolyte. The closer the ratio is to 100%, the fewer pores are present in the solid electrolyte.

[0054] Furthermore, the lithium ion conductivity of the first solid electrolyte of the present invention is preferably 4.0 × 10 –4 S / cm or more, and preferably 6.0 × 10 –4 S / cm or more.

[0055] If the lithium ion conductivity is in such a range, the performance of a separator of a battery and the like can be improved, for example, by applying the solid electrolyte to the separator of the battery and the like.

[0056] Furthermore, the three-point bending strength of the first solid electrolyte of the present invention is 70 N / mm². 2 or more, preferably 80 N / mm 2 or more, and preferably is 85 N / mm 2 or more.

[0057] In general, the solid electrolyte is a ceramic material and tends to break under stress.

[0058] However, the durability of a solid electrolyte with such three-point flexural strength can be improved.

[0059] The first solid electrolyte of the present invention can be produced by a step of mixing a solid powder comprising a composition to form the solid electrolyte described above, a step of pressure forming the mixed powder to form pellets, and a step of tempering the pellets obtained. In other words, the method for producing the first solid electrolyte of the present invention includes at least the steps described above.

[0060] In particular, analytical-grade Li2CO3, TiO2, GeO2, Al2O3 and NH4H2PO4 are used in quantities sufficient to support a lithium ion conductivity. 1+X Al X Ge Y Ti 2-X-Y (PO4)3 solid electrolytes of the NASICON type are subjected to ball milling with zirconium dioxide balls in a zirconium dioxide container, resulting in the mixed powder.

[0061] Next, the mixed powder is pressure-molded into pellets, and calcination is carried out at a relatively low temperature (500 to 800°C, for example 600°C).

[0062] The calcined pellets are then ground again and subjected to ball milling once more.

[0063] The resulting powder is pressure-molded into pellets under hydrostatic pressure (production of pressed pellets).

[0064] The pellets are then further sintered (tempered) at 900 to 1200°C. Setting the tempering temperature to 900°C or higher can reduce the generation of impurities in the manufacturing steps and thereby improve the relative density of the solid electrolyte. Simultaneously, setting the tempering temperature to 1200°C or lower can reduce the evaporation of lithium compounds and thus improve the relative density of the solid electrolyte.

[0065] It should be noted that calcination can be omitted at a relatively low temperature.

[0066] The second solid electrolyte of the present invention is a solid electrolyte that satisfies the following formula (II): Li 1+X-Y Al X Note Y Ti 2-X-Y (PO4)3 (II).

[0067] X and Y in the formula are real numbers that satisfy X + Y ≤ 1.

[0068] The solid electrolyte has a NASICON-type crystal structure.

[0069] In formula (II), Y preferably satisfies 0.1 ≤ Y ≤ 0.3. Setting Y within this range allows the atomic fraction X of Al to be increased to over 0.4.

[0070] In particular, adjusting the Nb atomic fraction Y in formula (II) within the range 0.1 ≤ Y ≤ 0.3 allows adjusting the Al atomic fraction X in formula (II) within a range of 0.5 ≤ X ≤ 0.6.

[0071] Furthermore, in formula (II), X preferably satisfies 0.5 ≤ Y ≤ 0.6. Setting X within this range promotes sintering and improves the strength of the solid electrolyte. Additionally, setting X within this range improves the relative density of the solid electrolyte and enhances its water-repellent properties.

[0072] The solid electrolyte with the increased Al atom content described above can achieve a strength of 100 N / mm². 2 or more, as illustrated in an example described below, and can also have a high lithium ion conductivity of 5.0 × 10 –4 exhibit S / cm or more.

[0073] The reason for obtaining such effects is assumed to be as follows: Ge, which has a valence of 4, is replaced by Nb, which has a higher valence of 5. This increases the solubility limit of Al, which has a low valence of 3, thus allowing for an increase in the amount of Al in the structure. Consequently, it is assumed that the proportion of reaction intermediates, including Al with a relatively low melting point, is increased during sintering, and sintering is promoted, resulting in improvements in strength and lithium-ion conductivity. In the second solid electrolyte of the present invention, sintering is indeed promoted, and the density is indeed increased.

[0074] The solid electrolyte of the present invention can be produced by a step of mixing a solid powder containing a composition to form the solid electrolyte described above, a step of pressure forming the mixed powder to form pellets, and a step of tempering the pellets obtained. In other words, the method for producing the second solid electrolyte of the present invention includes at least the steps described above.

[0075] In particular, analytical-grade Li2CO3, TiO2, Nb2O5, Al2O3 and NH4H2PO4 are used in quantities sufficient to support a lithium ion conductivity. 1+X-Y Al X Note Y Ti 2-X-Y (PO4)3 solid electrolytes of the NASICON type are subjected to ball milling with zirconium dioxide balls in a zirconium dioxide container, resulting in the mixed powder.

[0076] Next, the mixed powder is pressure-molded into pellets, and calcination is carried out at a relatively low temperature (500 to 800°C, for example 600°C).

[0077] The calcined pellets are then ground again and subjected to ball milling once more.

[0078] The resulting powder is pressure-molded into pellets under hydrostatic pressure (production of pressed pellets).

[0079] The pellets are then further sintered (tempered) at 900 to 1200°C. Setting the tempering temperature to 900°C or higher can reduce the generation of impurities in the manufacturing steps and thereby improve the relative density of the solid electrolyte. Setting the tempering temperature to 1200°C or lower can reduce the evaporation of lithium compounds and thereby improve the relative density of the solid electrolyte. It should be noted that sintering can be adequately promoted at a temperature of 900 to 1000°C.

[0080] It should be noted that calcination can be omitted at a relatively low temperature. [Example]

[0081] An example of the first solid electrolyte of the present invention is described below. The present invention is not limited to the following example. Example (first solid electrolyte): Production of lithium ion-conducting Li 1+X Al X Ge Y Ti 2-X-Y (PO4)3 solid electrolytes of the NASICON type

[0082] A lithium-ion conductive Li 1+X Al X Ge Y Ti 2-X-Y (PO4)3 solid electrolyte of the NASICON type was prepared by a conventionally known solid-phase reaction. The preparation was carried out within the ranges X = 0.30 to 0.55 and Y = 0.1 to 0.3.

[0083] Analytical-grade Li2CO3, TiO2, GeO2, Al2O3 and NH4H2PO4 in appropriate quantities were subjected to two hours of ball milling with zirconium dioxide balls at 400 / min in a zirconium dioxide container, using high energy mechanical milling (HEMM) with a planetary micromill (Fritsch Pulverisette 7), resulting in a mixed powder.

[0084] Next, the mixed powder was pressure-molded into pellets at 150 MPa and subjected to calcination at 600°C for four hours.

[0085] The calcined pellets were ground again and subjected to ball milling again using HEMM.

[0086] The resulting powder was pressure-molded into pellets under hydrostatic pressure at 150 MPa.

[0087] The pellets were then sintered for seven hours at different temperatures (850 to 1000°C). Analysis of test results

[0088] Fig. Figure 2 illustrates the X-ray diffraction pattern of Li 1,5 Al 0,5 Ge 0,2 Ti 1,3(PO4)3 samples (corresponding to the case where Y = 0.2) were sintered for seven hours at various temperatures, using an internal silicon standard to measure the lattice constants. An impurity phase of AlPO4 was observed in the sample sintered at 850°C. At the low sintering temperature of 850°C, the reaction was incomplete. All diffraction lines of the samples sintered at 900, 950, and 1000°C could be identified as NASICON-type structures. In other words, it could be determined that the samples sintered at 900, 950, and 1000°C exhibited NASICON-type structures.

[0089] Fig. Figure 3 illustrates the relative densities of Li 1,5 Al 0,5 Ge 0,2 Ti 1,3(PO4)3 pellets (corresponding to X = 0.5 and Y = 0.2) were sintered at different temperatures. The sample sintered at 850°C, containing the impurity phase, exhibited a relatively low density of 87%. The highest relative density of 95.5% was observed in the sample sintered at 900°C, and the relative density decreased with increasing sintering temperature. This decrease in relative density is thought to be due to the evaporation of lithium compounds at these higher temperatures.

[0090] The relative density of each of the sintered samples was determined based on the ratio between the density calculated from the lattice constants and the density calculated from the volume and mass of the sintered main body.

[0091] The results described above indicate that a temperature of 900 to 1200°C is preferred as the sintering temperature, and a temperature of 900 to 1000°C is even more preferred.

[0092] The lithium ion conductivity, relative density, and three-point flexural strength of a lithium sintered for seven hours at 900°C 1+X Al X Ge 0,2 Ti 1,8X (PO4)3 systems (corresponding to Y = 0.2) were each tested as a function of X.

[0093] Fig. Figure 4 illustrates the lithium ion conductivity and the relative density of Li 1+X Al X Ge 0,2 Ti 1,8X (PO4)3, measured at 25°C as a function of X. The highest lithium ion conductivity of 1.0 × 10 –3 S / cm and the highest relative density of 95.8% at 25°C are achieved with Li 1,45 Al 0,45 Ge 0,2 Ti 1,35 (PO4)3 observed.

[0094] The lithium ion conductivity of each of the sintered pellets (with a diameter of about 12 mm and a thickness of 1 mm) with electrodes onto which gold was sputtered was measured at a bias voltage of 10 mV in a frequency range of 0.1 Hz to 1 MHz using an impedance phase analyzer (Solartron 1260).

[0095] The relative density of each sintered sample was determined based on the ratio between the density calculated from the lattice constants and the density calculated from the volume and mass of the sintered main body.

[0096] Fig. Figure 5 illustrates the X-dependence of the three-point bending strength with respect to the Al content of Li 1+X Al X Ge 0,2 Ti 1,8X (PO4)3 (corresponding to Y = 0.2), sintered for seven hours at 900°C. The highest flexural strength is 90 N / mm². 2 will be at Li 1,45 Al 0,45 Ge 0,2 Ti1,35 (PO4)3 was observed with a relative density of 95.8%. The flexural strength is higher than 65 N / mm². 2 , the flexural strength of Li 1,4 Al 0,4 Ge 0,2 Ti 1,4 (PO4)3, which was produced by foil casting using a powder obtained in a sol-gel process (Zhang et al., 2015).

[0097] It should be noted that the three-point bending strength of each sintered pellet (with a thickness of approximately 0.24 mm and a width of approximately 15 mm) was measured using a material testing device (Shimadzu EZ-SX 500 N) at room temperature.

[0098] The Fig. 4 and Fig. It can be seen from paragraph 5 that the preferred range of X is 0.35 to 0.50.

[0099] Fig. Figure 6 illustrates the lithium ion conductivity and the relative density of Li 1+X Al X Ge 0,3 Ti 1,7-X(PO4)3 (which corresponds to Y = 0.3) as a function of X. The highest relative density of 96.3% is found at Li 1,45 Al 0,45 Ge 0,3 Ti 1,25 (PO4)3 observed. From Fig. 6 also shows that a favorable lithium ion conductivity and relative density are obtained when X is between 0.4 and 0.50.

[0100] The lithium ion conductivity of each of the sintered pellets (with a diameter of about 12 mm and a thickness of 1 mm) with electrodes onto which gold was sputtered was measured at a bias voltage of 10 mV in a frequency range of 0.1 Hz to 1 MHz using an impedance phase analyzer (Solartron 1260).

[0101] The relative density of each sintered sample was determined based on the ratio between the density calculated from the lattice constants and the density calculated from the volume and mass of the sintered main body.

[0102] Fig.Figure 7 illustrates the variation of the lattice parameters of Li 1+X Al X Ge 0,2 Ti 1,8-X (PO4)3 as a function of X. It should be noted that the lattice parameters are based on the in Fig. The X-ray diffraction patterns illustrated in Figure 1 could be determined. In particular, the crystal structures of the sintered samples were determined by X-ray diffraction (XRD) analysis at a sampling rate of 0.02° s. –1 analyzed in a 2θ range from 10° to 90° using a Rigaku RINT2500 diffractometer with Cu-Kα radiation.

[0103] It is agreed that the lattice parameters corresponding to the region of X, which is considered in advance to be the preferred region, are the lattice constants of the NASICON-type crystal structure such that the length along the a-axis is 0.8 nm or more and the length along the c-axis is 2.8 nm or less.

[0104] An example of the second solid electrolyte of the present invention is described below. The present invention is not limited to the following example. Example (second solid electrolyte) Production of lithium ion conductive Li 1+X-1 Al X Note Y Ti 2-X-Y (PO4)3 solid electrolytes of the NASICON type

[0105] A lithium-ion conductive Li 1+X-Y Al X Note Y Ti 2-X-Y (PO4)3 solid electrolyte of the NASICON type was prepared by a conventionally known solid-phase reaction. The preparation was carried out within the ranges X = 0.35 to 0.6 and Y = 0.1 to 0.3.

[0106] Analytical-grade Li2CO3, TiO2, Nb2O5, Al2O3 and NH4H2PO4 in appropriate quantities were subjected to two hours of ball milling with zirconium dioxide balls at 400 / min in a zirconium dioxide container, using high-energy mechanical milling (HEMM) with a planetary micromill (Fritsch Pulverisette 7), resulting in a mixed powder.

[0107] Next, the mixed powder was pressure-molded into pellets at 150 MPa and subjected to calcination at 600°C for four hours.

[0108] The calcined pellets were ground again and subjected to ball milling again using HEMM.

[0109] The resulting powder was pressure-molded into pellets under hydrostatic pressure at 150 MPa.

[0110] The pellets were then sintered for seven hours at 900°C. Analysis of test results

[0111] Fig.Figure 8 illustrates the X-ray diffraction patterns of samples in which Y is in Li 1+X-Y Al X Note Y Ti 2-X-Y (PO4)3 is set to 0.1, 0.2 and 0.3 and X of Al is set within a range of 0.35 to 0.6.

[0112] Table 1 illustrates the results for the three-point flexural strength, relative density, and lithium ion conductivity in each of the examples where Y is in Li 1+X-Y Al X Note Y Ti 2-X-Y (PO4)3 is set to 0.1, 0.2 and 0.3 and X of Al is set within a range of 0.35 to 0.6, as described above.

[0113] The relative density of each of the sintered samples was determined based on the ratio between the density calculated from the lattice constants and the density calculated from the volume and mass of the sintered main body.

[0114] It should be noted that the lithium ion conductivity of each of the sintered pellets (with a diameter of about 12 mm and a thickness of 1 mm) with electrodes onto which gold was sputtered was measured at a bias voltage of 10 mV in a frequency range of 0.1 Hz to 1 MHz using an impedance phase analyzer (Solartron 1260).

[0115] The test results are listed in Table 1. [Table 1] Note Al Contaminants strength [N / mm 2 ] density [%] conductivity [10 –4 S / cm] 0,1 0,35 44 77,5 1,95 0,4 27 82,0 2,41 0,45 35 88,25 3,15 0,5 Available 85 87,5 3,72 0,55 Available 104 89,25 2,00 0,6 Available 67 0,2 0,35 26 78,1 1,20 0,4 42,5 84,2 1,90 0,45 46 93,1 2,25 0,5 98,5 94,7 5,60 0,55 110 96,2 5,90 0,6 Available 108 95,8 3,00 0,3 0,35 26 80 0,5 0,4 25,5 82,0 1,65 0,45 28,5 83 2,65 0,5 46 89,8 3,80 0,55 55,5 94,5 2,65 0,6 80 97 1,75

[0116] In the test results from Table 1, the three-point flexural strength, relative density, and lithium-ion conductivity are advantageous when, in the Nb range, Y ≤ 0.3 and Al ≤ 0.5 ≤ X ≤ 0.6. In other words, advantageous lithium-ion conductivity can be maintained when the Al content is increased to improve the three-point flexural strength.

[0117] Fig.Figure 9 illustrates the variation of the three-point flexural strength, the relative density and the lithium ion conductivity in the case where the Nb content Y is set to a fixed value of 0.2 and the Al content X is varied.

[0118] It is agreed that in the range where the Al content X 0.5 ≤ X ≤ 0.6, advantageous three-point flexural strength and advantageous lithium ion conductivity are obtained, while keeping the relative density at a constant level. [Industrial applicability]

[0119] The first solid electrolyte of the present invention has a high relative density, and the probability of water permeating the first solid electrolyte can be reduced. The first solid electrolyte can therefore be advantageously applied to a component that comes into contact with water. Furthermore, it is possible to reduce the amount of epoxy resin filling the pores and to achieve high lithium-ion conductivity. Accordingly, the first solid electrolyte can preferably be used as a solid electrolyte for a lithium-air battery.

[0120] The second solid electrolyte of the present invention is prepared such that it contains Al in an atomic proportion of more than 0.4, thereby improving the lithium-ion conductivity and also the strength. The second solid electrolyte can thus have an advantageous relative density and is preferably used in a lithium-air battery and the like. QUOTES INCLUDED IN THE DESCRIPTION

[0121] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited non-patent literature

[0122] Zhang et al. Journal of The Electrochemical Society 162(7) A1265–A1271(2015) “Tape-Cast Water-Stable NASICON-Type High Lithium Ion Conducting Solid Electrolyte Films for Aqueous Lithium-Air Batteries”

[0015] Zhang et al., 2015

[0096]

Claims

[1] Solid electrolyte that satisfies formula (I): Li 1+X M1 X M2 Y Ti 2-X-Y (PO4)3 (I) (in formula (I) M1 is one or more elements selected from the group consisting of Al, Cu, Co, Fe, Ni, Ga, Cr and Sc, M2 is one or more elements selected from the group consisting of Si, Ge, Sn, Hf, Zr and Nb, and X and Y are real numbers satisfying X + Y ≤ 1), where the solid electrolyte has a NASICON-type crystal structure and the lattice constants of the NASICON-type crystal structure are such that the length along the a-axis is 0.8 nm or more and the length along the c-axis is 2.8 nm or less. [2] Solid electrolyte according to claim 1, wherein X 0.35 ≤ X ≤ 0.50 satisfies. [3] Solid electrolyte according to claim 1 or 2, wherein Y 0.1 ≤ Y ≤ 0.3 is satisfied. [4] Solid electrolyte according to claim 1 or 2, having a relative density of 92% or more. [5] Solid electrolyte according to claim 1 or 2, having a lithium ion conductivity of 4.0 × 10 –4 S / cm or more. [6] Solid electrolyte according to claim 1 or 2, having a three-point bending strength of 40 N / mm² 2 or more. [7] Method for producing a solid electrolyte that satisfies formula (I): Li 1+X M1 X M2 Y Ti 2-X-Y (PO4)3 (I) (in formula (I) M1 is one or more elements selected from the group consisting of Al, Cu, Co, Fe, Ni, Ga, Cr and Sc, M2 is one or more elements selected from the group consisting of Si, Ge, Sn, Hf, Zr and Nb, and X and Y are real numbers satisfying X + Y ≤ 1), wherein the procedure comprises the steps of: Mixing a solid powder containing a composition to form the solid electrolyte; Forming a compact by pressing the mixed powder; and the Tempering of the pellet is included. [8] Method for producing a solid electrolyte according to claim 7, wherein the sintering temperature in the tempering step of the compact is 900 to 1200°C. [9] Solid electrolyte that satisfies formula (II): Li 1+X-Y Al X Note Y Ti 2-X-Y (PO4)3 (II) (in formula (II) X and Y are real numbers that satisfy X + Y ≤ 1), where the solid electrolyte has a NASICON-type crystal structure. [10] Solid electrolyte according to claim 9, wherein Y 0.1 ≤ Y ≤ 0.3 is satisfied. [11] Solid electrolyte according to claim 9 or 10, wherein X ≤ 0.5 ≤ X ≤ 0.

6. [12] Method for producing a solid electrolyte that satisfies formula (II): Li 1+X-Y Al X Note Y Ti 2-X-Y (PO4)3 (II) (in formula (II) X and Y are real numbers that satisfy X + Y ≤ 1), where the procedure is the steps of: Mixing a solid powder containing a composition to form the solid electrolyte; Forming a compact by pressure forming the mixed powder and the Tempering of the pellet is included. [13] Method for producing a solid electrolyte according to claim 12, wherein the sintering temperature in the tempering step of the compact is 900 to 1000°C.