JP2011159528A - Lithium secondary battery and electrode for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery enhancing safety and performing more stable charging and discharging as well. <P>SOLUTION: The lithium secondary battery 10 includes a cathode 13 in which a cathode active material layer 12 is formed on a current collector 11, an anode 18 in which an anode active material layer 17 is formed on a surface of a current collector 14, an insulation layer 19 formed on the surface of the anode active material layer 17, and a nonaqueous electrolyte solution 20 interposed between the cathode 13 and the anode 18. The insulation layer 19 is formed of a garnet-type oxide which contains Zr and conducts lithium ion. The insulation layer 19 is formed of the garnet-type oxide as expressed in a composition formula Li<SB>5+X</SB>La<SB>3</SB>(Zr<SB>X</SB>, A<SB>2-X</SB>)O<SB>12</SB>. In the formula, A is an element of one kind or more selected from a group of Sc, Ti, V. Y, Nb, Hf, Ta, Al, Si, Ga and Ge, and X is 1.4≤X<2. The insulation layer 19 can be formed on the surface of the cathode active material layer 12. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery and an electrode for a lithium secondary battery.

  Conventionally, as a lithium secondary battery, a battery including a porous insulating layer containing an inorganic oxide filler made of alumina and a resin binder on either a positive electrode active material or a negative electrode active material has been proposed ( For example, see Patent Document 1). In this lithium secondary battery, lithium ions are conducted through the pores of the porous insulating layer, and in the case where an internal short circuit occurs, this porous insulating layer can suppress a direct short circuit between the positive and negative electrodes. Therefore, safety can be improved.

JP 2005-174792 A

  However, in the above-described lithium secondary battery of Patent Document 1, since an inorganic oxide porous insulating layer exists between the positive electrode and the negative electrode, an increase in the internal resistance of the battery cannot be avoided. In order to further suppress the increase in the internal resistance, the porosity of the insulating layer must be increased and the number of voids must be increased. On the other hand, when the porosity is increased, the insulating property may be lowered.

  The present invention has been made in view of such problems, and has as its main object to provide a lithium secondary battery and an electrode for a lithium secondary battery that can improve safety and perform more stable charge and discharge. To do.

  As a result of diligent research to achieve the above-described object, the inventors of the present invention provide an insulating layer formed of a garnet-type oxide containing Zr and conducting lithium ions between the positive electrode and the negative electrode. The present inventors have found that the safety can be improved and more stable charge / discharge can be performed, and the present invention has been completed.

  That is, the lithium secondary battery of the present invention is interposed between a positive electrode having a positive electrode active material that occludes and releases lithium, a negative electrode having a negative electrode active material that occludes and releases lithium, and the positive electrode and the negative electrode. An insulating layer formed of an electrolytic solution that conducts lithium and a garnet-type oxide that is present in at least one of the positive electrode surface, the negative electrode surface, and the electrolytic solution and contains Zr and conducts lithium ions And.

  An electrode for a lithium secondary battery of the present invention includes an active material layer that occludes and releases lithium, an insulating layer that is formed on the surface of the active material layer with a garnet-type oxide that contains Zr and conducts lithium ions, It is equipped with.

  The present invention can improve safety and perform more stable charge / discharge. The reason why such an effect can be obtained is assumed as follows. In the present invention, since the insulating layer is a garnet-type oxide containing Zr that does not have electronic conductivity, the thermal stability is higher than, for example, a plastic film separator, and the positive electrode and the negative electrode are short-circuited. It is easier to prevent it. In addition, since the insulating layer is a garnet-type oxide that conducts lithium ions, it is less likely to prevent occlusion / release of lithium ions by the active material as compared with those that do not have lithium ion conductivity. Therefore, it is assumed that safety can be improved and more stable charging / discharging can be performed.

2 is an explanatory diagram showing an example of a structure of a lithium secondary battery 10. FIG. It is explanatory drawing which shows an example of the structure of lithium secondary battery 10B-10D. It is a graph which shows the XRD pattern of Experimental example 1,3,5. It is a graph which shows the X value dependence of the lattice constant of Experimental examples 1-7 (except 4). It is a graph which shows the X value dependence of the lithium ion conductivity of Experimental Examples 1-7. It is explanatory drawing of the partial structure contained in the crystal structure of a garnet-type oxide. It is an explanatory view of the crystal structure of garnet-type oxide, (a) shows the whole picture, showing a state in which to expose the (b) LiO ₆ octahedra (II). It is a graph which shows X value dependence of LiO4 (I) crystal structure of Experimental example 1, _3, 5-7, (a) shows X value dependence of the sides a and b of the triangle which an oxygen ion forms. , (B) shows the X value dependency of the area of the triangle. It is a graph which shows X value dependence of the intensity | strength after normalization when each diffraction intensity of Experimental example 1, 3, 5-7 is normalized by (220) diffraction intensity. It is a graph which shows X value dependence of the intensity | strength after normalization of (024) of Experimental example 1,3,5-7. It is a graph of the Arrhenius plot of Experimental Examples 1-7. It is a graph which shows the X value dependence of the activation energy of Experimental Examples 1-7. 10 is a graph showing chemical stability in a room temperature atmosphere of Experimental Example 5. 10 is a graph showing measurement results of a potential window of Experimental Example 5.

  The lithium secondary battery of the present invention conducts lithium by interposing between a positive electrode having a positive electrode active material that occludes and releases lithium, a negative electrode having a negative electrode active material that occludes and releases lithium, and the positive electrode and the negative electrode. And an insulating layer formed of a garnet-type oxide that is present in at least one of the surface of the positive electrode, the surface of the negative electrode, and the electrolytic solution and contains Zr and conducts lithium ions. A garnet-type oxide that conducts lithium ions has a low electronic conductivity, a wide potential window, a high lithium ion conductivity, and is stable even at high temperatures, and is preferable as an insulating layer. For example, the insulating layer formed of a garnet-type oxide that conducts lithium ions may be formed on the surface of the positive electrode active material layer, or may be formed on the surface of the negative electrode active material layer. Alternatively, it may be present as a layered body (plate-shaped body) in the electrolytic solution. Furthermore, the lithium secondary battery of the present invention includes a separator between the positive electrode and the negative electrode, and the insulating layer formed of the garnet-type oxide that conducts lithium ions is either between the separator and the electrode. It is good also as what is formed in the position. In this way, the insulating layer may be formed at any position between the positive electrode and the negative electrode. In this way, the short circuit of the electrodes can be prevented by the thermally and chemically stable garnet oxide. In the present embodiment, for convenience of explanation, an example in which an insulating layer formed of a garnet oxide that conducts lithium ions is formed on the surface of the negative electrode active material layer will be mainly described.

The positive electrode of the lithium secondary battery of the present invention is, for example, a mixture of a positive electrode active material, a conductive material, and a binder, and an appropriate solvent is added to form a paste-like positive electrode material, which is applied to the surface of the current collector. It may be dried and compressed to increase the electrode density as necessary. As the positive electrode active material, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, or the like can be used. Specifically, transition metal sulfides such as TiS ₂ , TiS ₃ , MoS ₃ , FeS ₂ , Li _(1-x) MnO ₂ (0 <x <1, etc., the same shall apply hereinafter), Li _(1-x) Mn Lithium manganese composite oxide such as ₂ O ₄ , lithium cobalt composite oxide such as Li _(1-x) CoO ₂ , lithium nickel composite oxide such as Li _(1-x) NiO ₂ , lithium such as LiV ₂ O ₃ Vanadium composite oxides, transition metal oxides such as V ₂ O _{5, and the} like can be used. Of these, lithium transition metal composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiV ₂ O ₃ are preferable. The conductive material is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the positive electrode. For example, graphite such as natural graphite (scale-like graphite, scale-like graphite) or artificial graphite, acetylene black, carbon black, What mixed 1 type (s) or 2 or more types, such as ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) can be used. Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coatability. The binder serves to bind the active material particles and the conductive material particles. For example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluorine-containing resin such as fluorine rubber, or polypropylene, Thermoplastic resins such as polyethylene, ethylene-propylene-dienemer (EPDM), sulfonated EPDM, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. In addition, an aqueous dispersion of cellulose or styrene butadiene rubber (SBR), which is an aqueous binder, can also be used. Examples of the solvent for dispersing the positive electrode active material, the conductive material, and the binder include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, and N, N-dimethylaminopropyl. Organic solvents such as amine, ethylene oxide, and tetrahydrofuran can be used. Moreover, a dispersing agent, a thickener, etc. may be added to water, and an active material may be slurried with latex, such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more. Examples of the application method include roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain an arbitrary thickness and shape. Current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, and aluminum, copper, etc. for the purpose of improving adhesion, conductivity, and oxidation resistance. A surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group. The thickness of the current collector is, for example, 1 to 500 μm.

  The negative electrode of the lithium secondary battery of the present invention is prepared by, for example, mixing a negative electrode active material, a conductive material, and a binder, and adding a suitable solvent to form a paste-like negative electrode material on the surface of the current collector. It may be dried and compressed to increase the electrode density as necessary. Examples of negative electrode active materials include inorganic compounds such as lithium, lithium alloys and tin compounds, carbonaceous materials capable of occluding and releasing lithium ions, and conductive polymers. Among these, carbonaceous materials are used from the viewpoint of safety. It is preferable to see. The carbonaceous material is not particularly limited, and examples thereof include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Of these, graphites such as artificial graphite and natural graphite have an operating potential close to that of metallic lithium, can be charged and discharged at a high operating voltage, and suppresses self-discharge when a lithium salt is used as an electrolyte salt. In addition, the irreversible capacity during charging can be reduced, which is preferable. In addition, as the conductive material, binder, solvent, and the like used for the negative electrode, those exemplified for the positive electrode can be used. The negative electrode current collector includes copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., as well as improved adhesion, conductivity and reduction resistance. For the purpose, for example, a copper surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. The shape of the current collector can be the same as that of the positive electrode.

The insulating layer of the lithium secondary battery of the present invention may be formed on the surface of the negative electrode active material layer. The garnet-type oxide that conducts lithium ions contained in this insulating layer preferably contains at least Zr, more preferably contains at least Zr and Nb, and contains at least Zr, Nb, and La. More preferably. In this way, the conductivity of lithium ions can be further increased. The garnet-type oxide that conducts lithium ions has the composition formula Li _{5 + X} La ₃ (Zr _X , A _2−X ) O ₁₂ (where A is Sc, Ti, V, Y, Nb, Hf, Ta). , Al, Si, Ga, and Ge, one or more elements selected from the group consisting of X, X may be represented by 1.4 ≦ X <2). Since the garnet-type oxide used here satisfies X ≦ 1.4 ≦ X <2, the lithium ion conductivity is higher than that of the known garnet-type oxide Li ₇ La ₃ Zr ₂ O ₁₂ (that is, X = 2). Increases and the activation energy also decreases. For example, when A is Nb, the lithium ion conductivity is 2.5 × 10 ⁻⁴ Scm ⁻¹ or more and the activation energy is 0.34 eV or less. Therefore, according to the lithium secondary battery containing this oxide, lithium ions are easily conducted, the resistance is lowered, and the output of the battery is improved. Further, since the activation energy is small, that is, the rate of change in lithium ion conductivity with respect to temperature is small, the output of the battery is stabilized. Further, it is more preferable that X satisfies 1.6 ≦ X ≦ 1.95 because lithium ion conductivity is higher and activation energy is lower. Furthermore, if X satisfies 1.65 ≦ X ≦ 1.9, the lithium ion conductivity is almost maximized and the activation energy is almost minimized. As A, Nb or Ta having an ion radius equivalent to that of Nb is preferable.

Alternatively, in the garnet-type oxide that conducts lithium ions contained in the insulating layer of the present invention, the Zr site of the composition formula Li ₇ La ₃ Zr ₂ O ₁₂ is substituted with an element having an ionic radius different from that of Zr, and ( 220) The intensity after normalization of (024) diffraction when the diffraction intensity is normalized to 1 may be 9.2 or more. (024) When the intensity after diffraction standardization exceeds 9.2, the triangle formed by the LiO ₄ (I) tetrahedral oxygen ions approaches an equilateral triangle, and the area of the triangle increases. Compared with the garnet-type oxide Li ₇ La ₃ Zr ₂ O ₁₂ (that is, X = 2), the lithium ion conductivity increases and the activation energy also decreases. For example, when A is Nb, the lithium ion conductivity is 2.5 × 10 ⁻⁴ Scm ⁻¹ or more and the activation energy is 0.34 eV or less. Therefore, when this oxide is used for a lithium secondary battery, lithium ions are easily conducted, so that the output of the battery is improved. Further, since the activation energy is small, that is, the rate of change in lithium ion conductivity with respect to temperature is small, the output of the battery is stabilized. Moreover, if the intensity | strength after the normalization of (024) diffraction is 10.0 or more, since lithium ion conductivity is higher and activation energy becomes lower, it is more preferable. Further, it is more preferable that the intensity after normalization of (024) diffraction is 10.2 or more because the lithium ion conductivity is almost maximum and the activation energy is almost minimum. The element having an ionic radius different from that of Zr includes one or more elements selected from the group consisting of Sc, Ti, V, Y, Nb, Hf, Ta, Al, Si, Ga, and Ge. Among these, Nb and Ta having the same ion radius as Nb are preferable.

  Here, the garnet-type oxide that conducts lithium ions only needs to have a garnet-type structure. For example, the garnet-type oxide may include a part of other structures, or the peak position of X-ray diffraction may shift. For example, it may include a distorted structure as viewed from the garnet. In addition, as shown by the composition formula, the garnet-type oxide that conducts lithium ions may contain some other elements and structures.

  The insulating layer of the lithium secondary battery of the present invention may be a layer in which particles of garnet-type oxide that conduct lithium ions are fixed, or the fixed layer may be a fired fired layer, or it conducts lithium ions. It is good also as a plate-shaped body layer which shape | molded and baked the garnet-type oxide and crimped | bonded to the surface of the active material layer. When the insulating layer is a fixed layer, it can be produced by a sintering method, a sol-gel method, an electrostatic coating method, a doctor blade method, a screen printing method, a slurry cast method, powder pressure bonding, or the like. When the insulating layer is a fired body, the garnet-type oxide powder may be pressure-molded and fired, or the garnet-type oxide powder may be cast as a slurry and fired. . It is preferable that the insulating layer be a fired body because it is stronger and easily prevents a short circuit between the positive electrode and the negative electrode. Although the thickness of this insulating layer depends on the size of the lithium secondary battery, it is preferably not more than the thickness T of the active material layer, preferably not less than 5% of the thickness T. The thickness may be 0.1 μm or more and 5 mm or less. The thickness of the insulating layer may be set empirically in accordance with the safety and stability of the lithium secondary battery.

  The insulating layer of the lithium secondary battery of the present invention is preferably a porous insulating layer that is porous. In this way, lithium ions can be moved through the holes formed in the porous insulating layer. For example, when the garnet-type oxide has a low lithium ion conductivity, the insulating layer may have a high porosity, and the movement of the lithium ions may depend on the porous pores. Alternatively, when the lithium ion conductivity of the garnet-type oxide is high, the insulating layer may be low in porosity, and the movement of lithium ions may depend on the garnet-type oxide itself. The insulating layer can be made porous by, for example, adding a porous material for forming pores at the time of forming the insulating layer, and then removing the porous material. For example, carbon particles or the like may be added as a porous material and removed during firing, or a porous material that dissolves in an organic solvent may be added and then dissolved in an organic solvent to remove it. Also good. The porosity of the porous insulating layer is, for example, preferably 5% by volume to 80% by volume, and more preferably 10% by volume to 50% by volume. If the porosity is 5% by volume or more, it is preferable because it easily contains an electrolytic solution, and if it is 80% by volume or less, insulation can be further ensured.

  As the electrolytic solution of the lithium secondary battery of the present invention, a non-aqueous electrolytic solution containing a supporting salt or an aqueous electrolytic solution can be used. Examples of the solvent for the nonaqueous electrolytic solution include carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t -Chain carbonates such as butyl carbonate, di-i-propyl carbonate, t-butyl-i-propyl carbonate, cyclic esters such as γ-butyllactone and γ-valerolactone, methyl formate, methyl acetate, ethyl acetate, Chain esters such as methyl butyrate, ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; Examples include furans such as lan, methyltetrahydrofuran, sulfolanes such as sulfolane and tetramethylsulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, the combination of cyclic carbonates and chain carbonates is preferable. According to this combination, not only the cycle characteristics representing the battery characteristics in repeated charge and discharge are excellent, but also the viscosity of the electrolyte, the electric capacity of the obtained battery, the battery output, etc. should be balanced. it can.

The supporting salt contained in the lithium secondary battery of the present invention is, for example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , Examples include LiSbF ₆ , LiSiF ₆ , LiAlF ₄ , LiSCN, LiClO ₄ , LiCl, LiF, LiBr, LiI, and LiAlCl ₄ . Among these, from the group consisting of inorganic salts such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , and organic salts such as LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) _3. It is preferable from the viewpoint of electrical characteristics to use a combination of one or two or more selected salts. The supporting salt preferably has a concentration in the non-aqueous electrolyte of 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. If the concentration of the supporting salt is 0.1 mol / L or more, a sufficient current density can be obtained, and if it is 5 mol / L or less, the electrolytic solution can be made more stable. Moreover, you may add flame retardants, such as a phosphorus type and a halogen type, to this non-aqueous electrolyte.

  The lithium secondary battery of the present invention may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the lithium secondary battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or a thin fine olefin resin such as polyethylene or polypropylene is used. A porous membrane is mentioned. These may be used alone or in combination. For example, when a resin separator is used in combination with an insulating layer, for example, when the temperature rises, the ion conduction can be blocked (shutdown effect) due to melting of the separator, and the safety can be further improved.

  The shape of the lithium secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing used for an electric vehicle etc. FIG. 1 is a schematic diagram showing an example of a lithium secondary battery 10 of the present invention. The lithium secondary battery 10 includes a positive electrode 13 in which a positive electrode active material layer 12 is formed on a current collector 11, a negative electrode 18 in which a negative electrode active material layer 17 is formed on the surface of a current collector 14, and a negative electrode active material layer 17. An insulating layer 19 formed on the surface and a non-aqueous electrolyte solution 20 interposed between the positive electrode 13 and the negative electrode 18 are provided. The insulating layer 19 is made of a garnet-type oxide containing Zr and conducting lithium ions. In this way, the insulating layer 19 can enhance safety and perform more stable charge / discharge.

  In the lithium secondary battery of the present embodiment described in detail above, since the insulating layer is a garnet-type oxide containing Zr that does not have electronic conductivity, for example, thermal stability compared to a plastic film separator. And it is easier to prevent a short circuit between the positive electrode and the negative electrode. In addition, since the insulating layer is a garnet-type oxide that conducts lithium ions, it is less likely to prevent occlusion / release of lithium ions as an active material compared to a material that does not have lithium ion conductivity. A garnet-type oxide that conducts lithium ions is preferable because it has a wide potential window, high lithium ion conductivity, and is stable even at high temperatures. Therefore, safety can be improved and more stable charging / discharging can be performed.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  For example, in the above-described embodiment, the insulating layer is formed on the surface of the negative electrode active material layer. However, as shown in FIG. 2, the insulating layer may be formed between the positive electrode and the negative electrode. . FIG. 2 is an explanatory diagram showing an example of the structure of the lithium secondary batteries 10B to 10D. The lithium secondary battery 10 </ b> B includes an insulating layer 19 </ b> B formed on the surface of the positive electrode active material layer 12 instead of the surface of the negative electrode active material layer 17. Even if it does in this way, while improving safety | security, more stable charge / discharge can be performed. The lithium secondary battery 10 </ b> C includes an insulating layer 19 </ b> C formed on the surface of the positive electrode active material layer 17 and an insulating layer 19 formed on the surface of the negative electrode active material layer 17. Even if it does in this way, while improving safety | security, more stable charge / discharge can be performed. The lithium secondary battery 10 </ b> D includes a positive electrode 13 in which the positive electrode active material layer 12 is formed on the current collector 11, a negative electrode 18 in which the negative electrode active material layer 17 is formed on the surface of the current collector 14, and a negative electrode active material layer 17. A separator 21 is provided between the insulating layer 19 and the positive electrode 13 formed on the surface. Even if it does in this way, while improving safety | security, more stable charge / discharge can be performed.

  In the above-described embodiment, the present invention has been described as a lithium secondary battery. However, for example, an electrode for a lithium secondary battery having an insulating layer formed on the surface of the active material layer may be used. Even if it does in this way, when it uses as a lithium secondary battery, while improving safety | security, more stable charge / discharge can be performed.

  Below, the example which produced the lithium secondary battery of this invention concretely is demonstrated as an experiment example.

[Production of garnet-type oxide]
Garnet-type oxides Li _{5 + X} La ₃ (Zr _X , Nb _2−X ) O ₁₂ (X = 0 to 2) are composed of Li ₂ CO ₃ , La (OH) ₃ , ZrO ₂ , and Nb ₂ O ₅ . The starting material was used for synthesis. Here, the values of X in Experimental Examples 1 to 7 were set to X = 0, 1.0, 1.5, 1.625, 1.75, 1.875, and 2.0, respectively (see Table 1). First, starting materials were weighed so as to have a stoichiometric ratio, and mixed and pulverized in ethanol with a planetary ball mill (300 rpm / zirconia balls) for 1 hour. After the mixed powder of the starting material was separated from the balls and ethanol, calcination was performed in an air atmosphere at 950 ° C. for 10 hours in an Al ₂ O ₃ crucible. Thereafter, in order to make up for the loss of Li in the main sintering, the calcined powder was mixed with Li _{5 + X} La ₃ (Zr _X , Nb _2−X ) O ₁₂ (X = 0 to 2) in the composition. 10 at. Li ₂ CO ₃ was excessively added so as to be a%. This mixed powder was treated in a planetary ball mill (300 rpm / zirconia ball) for 1 hour in ethanol for mixing. The obtained powder was again calcined again at 950 ° C. for 10 hours under atmospheric conditions to obtain a calcined body. Then, after this calcined body was molded, main sintering was performed under the conditions of 1200 ° C. and 36 hours in the air to prepare garnet-type oxides (Experimental Examples 1 to 7).

[Measurement and results of physical properties of garnet oxide]
1. Relative density The measured density of each sample was calculated by dividing the dry weight measured with an electronic balance by the volume determined from the actual size measured with calipers. The theoretical density was calculated, and the value obtained by dividing the measured density by the theoretical density and multiplying by 100 was taken as the relative density (%). The relative densities of Experimental Examples 1 to 7 were 88 to 92%.

2. Phase and lattice constant The phase and lattice constant of each sample were determined from the XRD measurement results. The XRD measurement was performed using an XRD measuring device (D8ADVANCE, manufactured by Bruker), and the sample powder was CuKα, 2θ: 10 to 120 °, 0.01 ° step / 1 sec. It measured on condition of this. Crystal structure analysis was performed using a crystal structure analysis program: Rietan-2000 (Matter. Sci. Forum, p321-324 (2000), 198). As representative examples, XRD patterns of Experimental Examples 1, 3, 5 and 7, that is, Li _{5 + X} La ₃ (Zr _X , Nb _2−X ) O ₁₂ (X = 0, 1.5, 1.75, 2) are illustrated. 3 shows. FIG. 3 shows that each sample does not contain impurities and is a single phase. Moreover, the X value dependence of the lattice constant calculated | required from the XRD pattern about Experimental example 1-3, 5-7 is shown in FIG. FIG. 4 shows that the lattice constant increases as the ratio of Zr increases. This ionic radius of _{^{Zr 4+ (r Zr4 + = 0.79Å}} ) is larger than the ionic radius of the _{^{Nb 5+ (r Nb5 + = 0.69Å}} ). Since the lattice constant is continuously changed, it is considered that Nb is substituted for the Zr site (it is considered that full solid solution is possible).

3. Lithium ion conductivity Lithium ion conductivity is calculated from the resistance value obtained from the arc of the Nyquist plot using an AC impedance analyzer (frequency: 0.1 Hz to 1 MHz, amplitude voltage: 100 mV) in a thermostatic chamber. did. An Au electrode was used as a blocking electrode when measuring with an AC impedance analyzer. The Au electrode was formed by baking a commercially available Au paste at 850 ° C. for 30 minutes. FIG. 5 shows the X value dependence of lithium ion conductivity at 25 ° C. of Experimental Examples 1 to 7, that is, Li _{5 + X} La ₃ (Zr _X , Nb _2−X ) O ₁₂ (X = 0 to 2). From FIG. 5, when X is 1.4 ≦ X <2, the lithium ion conductivity is higher than that of the known Li ₇ La ₃ Zr ₂ O ₁₂ (that is, X = 2, Experimental Example 7). When 1.6 ≦ X ≦ 1.95, the value is higher than that of Experimental Example 7, and when X is in the range of 1.65 ≦ X ≦ 1.9, the maximum value (6 × 10 ⁻⁴ Scm ^−1). You can see that Above 1. As described above, since the relative density of each sample was 88 to 92%, it is considered that the change in lithium ion conductivity according to the X value is not an influence of the density.

Here, the reason why lithium ion conductivity is improved by adding an appropriate amount of niobium will be considered. As shown in FIG. 6, the crystal structure of the garnet-type oxide includes tetrahedral LiO ₄ (I) in which lithium ions are 4-coordinated with oxygen ions, and lithium ions are 6-coordinated with oxygen ions. Octahedral LiO ₆ (II), dodecahedron LaO ₈ (I) in which lanthanum ions are 8-coordinated with oxygen ions, and octahedral ZrO in which zirconium ions are 6-coordinated with oxygen ions ₆ and included. An overall image of this crystal structure is shown in FIG. In the crystal structure of FIG. 7A, the octahedral LiO ₆ (II) is surrounded by the octahedral ZrO ₆ and the dodecahedron LaO ₈ , so that it cannot be seen. FIG. 7B shows a state where LaO ₈ (I) is deleted from the crystal structure of FIG. 7A to expose octahedral LiO ₆ (II). Thus, the lithium ions that are six-coordinated are in a position surrounded by six oxygen ions, three lanthanum ions, and two zirconium ions, and probably have almost no lithium ion conductivity. It is thought that it does not contribute. On the other hand, the tetracoordinated lithium ions form a tetrahedron with the oxygen ions at the vertices. FIG. 8 shows changes in the LiO ₄ (I) tetrahedral structure obtained from the Rietveld structural analysis. The distance between oxygen ions forming the LiO ₄ (I) tetrahedron has two lengths. Here, a long side is a, and a short side is b. As shown in FIG. 8A, the long side a shows an almost constant value regardless of the amount of Nb replacement, whereas the short side b becomes longer by replacing Nb with an appropriate amount. Yes. That is, the triangle formed by the oxygen ions is replaced with an appropriate amount of Nb, and the area increases while approaching the regular triangle (see FIG. 8B). From this, it is considered that when an appropriate amount of Nb is substituted with Zr, the structure around the conducting lithium ions (tetrahedron formed by oxygen ions) is optimized, and the effect of facilitating the movement of lithium ions is obtained. Even if the element substituted for Zr is an element other than Nb, such as Sc, Ti, V, Y, Hf, Ta, etc., the same effect can be obtained because the same structural change is expected.

Here, the intensity of the diffraction peak of XRD changes reflecting the LiO ₄ (I) tetrahedral structure. That is, by replacing the Zr site with Nb, the triangle forming the LiO ₄ (I) tetrahedron changes as described above, and naturally the intensity ratio of each diffraction peak of XRD also changes. FIG. 9 shows the X-value dependency of the normalized intensity of each diffraction when the intensity of (220) diffraction of each sample of Experimental Examples 1 to 3, 5, and 7 is normalized to 1. As a typical peak, pay attention to the intensity after normalization of (024) diffraction (see FIG. 10). Regarding the diffraction, normalization corresponding to 1.4 ≦ X <2 in which lithium ion conductivity is higher than that of the known Li ₇ La ₃ Zr ₂ O ₁₂ (that is, X = 2, Experimental Example 7). The post-intensity is 9.2 or higher, and the normalized strength corresponding to 1.6 ≦ X ≦ 1.95, which further increases the lithium ion conductivity, is 10.0 or higher, and the lithium ion conductivity is almost a maximum value. It can be seen that the normalized strength corresponding to 1.65 ≦ X ≦ 1.9 is 10.2 or more.

4). Activation energy (Ea)
The activation energy (Ea) is calculated using the Arrhenius equation: σ = Aexp (−Ea / kT) (σ: conductivity, A: frequency factor, k: Boltzmann constant, T: absolute temperature) Obtained from the slope. As a representative example, the temperature dependence (Arrhenius plot) of the lithium ion conductivity of Li _{5 + X} La ₃ (Zr _X , Nb _2−X ) O ₁₂ (X = 0 to 2) in Experimental Examples 1 to 7 is shown in FIG. Show. FIG. 11 also shows glass ceramics Li _{1 + X} Ti ₂ Si _X P _3-X O ₁₂ .AlPO ₄ (Ohara electrolyte, X = 0. 4) shows the temperature dependence of the lithium ion conductivity of Li _1.5 Al _0.5 Ge _1.5 (PO ₄ ) ₃ (LAGP) (all are literature values). FIG. 12 shows the X value dependency of the activation energy Ea (25 ° C.) obtained from the Arrhenius plot for Experimental Examples 1-7. From FIG. 12, when X is 1.4 ≦ X <2, the activation energy Ea (that is, less than 0.34 eV) is lower than Li ₇ La ₃ Zr ₂ O ₁₂ (that is, X = 2, Experimental Example 7). Therefore, it can be said that the lithium ion conductivity takes a stable value in a wide temperature range. In addition, when X is 1.5 ≦ X ≦ 1.9, the activation energy is 0.32 eV or less, and particularly when X is 1.75, the minimum value is 0.3 eV. The value of 0.3 eV is equivalent to the lowest value among the existing Li ion conductive oxides (Ohara electrolyte: 0.3 eV, LAGP: 0.31 eV).

5. Chemical Stability The chemical stability of the garnet-type oxide Li _6.75 La ₃ Zr _1.75 Nb _0.25 O ₁₂ (that is, X = 1.75, Experimental Example 5) in the room temperature atmosphere was examined. Specifically, it was performed by confirming the presence or absence of a change over time (0 to 7 days) in lithium ion conductivity of Li _6.75 La ₃ Zr _1.75 Nb _0.25 O ₁₂ left in the atmosphere. The result is shown in FIG. Since the bulk resistance component is constant regardless of the time it has been left in the air, it can be said that the garnet-type oxide is stable in the air at room temperature.

6). Potential window The potential window of the garnet-type oxide Li _6.75 La ₃ Zr _1.75 Nb _0.25 O ₁₂ (that is, X = 1.75, Experimental Example 5) was examined. The potential window was formed by bonding gold on one side of a bulk pellet of Li _6.75 La ₃ Zr _1.75 Nb _0.25 O ₁₂ and Li metal on the other side, and 0 to 5.5 V (vs. Li ⁺ ) and −0.5 V to 9. The potential was swept (1 mV / sec.) In the range of 5 V (vs. Li ⁺ ). The measurement results are shown in FIG. Even when the potential was scanned in the range of 0 to 5.5 V, no current flowed. From this, it can be said that Li _6.75 La ₃ Zr _1.75 Nb _0.25 O ₁₂ is stable in the range of 0 to 5.5V. When the scanning potential was expanded to -0.5 to 9V, an oxidation / reduction current flowed around 0V. This is probably due to the oxidation and reduction of lithium. Further, a slight oxidation current began to flow at about 7 V or more. However, because the amount of flowing oxidation current is very weak and there is no visual change in color, the flowing oxidation current is not the decomposition of the electrolyte, but the decomposition of trace amounts of impurities and grain boundaries contained in the ceramics. I think that is the cause.

[Experimental Example 8]
A lithium secondary battery having an insulating layer formed using the garnet-type oxide that conducts lithium ions produced as described above was produced. As the positive electrode active material, a layered Ni-based active material, LiNi _0.80 Co _0.15 Al _0.05 O ₂ having an average particle size of about 10 μm was used. 85% by weight of this positive electrode active material, 10% by weight of carbon black as a conductive material, and 5% by weight of polyvinylidene fluoride as a binder were mixed to prepare a positive electrode mixture. This positive electrode mixture is dispersed with N-methyl-2-pyrrolidone (NMP) to form a paste, and this positive electrode mixture paste is applied to and dried on an aluminum foil having a thickness of 20 μm, and the density is increased by roll pressing. It was. The positive electrode was 30 mm × 30 mm, and the amount of positive electrode active material deposited was about 7 mg / cm ² per side. Next, artificial graphite was used as the negative electrode active material. The negative electrode active material was mixed with 95% by weight and 5% by weight of polyvinylidene fluoride as a binder to prepare a negative electrode mixture. This negative electrode mixture was dispersed with N-methyl-2-pyrrolidone (NMP) to obtain a paste. This negative electrode mixture paste was coated and dried on a copper foil current collector having a thickness of 10 μm, and was densified by roll pressing to obtain a negative electrode. The negative electrode was 30 mm × 30 mm, and the amount of negative electrode active material deposited was about 5 mg / cm ² per side. Next, a plate-like porous insulating layer was prepared using the garnet-type oxide Li _6.75 La ₃ Zr _1.75 Nb _0.25 O ₁₂ (that is, X = 1.75, Experimental Example 5). The calcined body of the garnet-type oxide before main sintering was pulverized so that the average particle size was about 0.6 μm. This average particle diameter is obtained by measuring the short diameter and the long diameter of each particle in the region observed using an electron microscope, and setting the average value of the short diameter and the long diameter as one particle diameter. Was calculated by calculating. Using the garnet-type oxide calcined particles, carbon particles (average particle size of 40 nm) are blended at 1% by weight with respect to the garnet-type oxide, and a flat plate (30 mm × 30 mm) mold is used. Molded. This compact was sintered at 1200 ° C. for 36 hours in the atmosphere to produce a porous insulator formed of a garnet oxide. This porous insulator was pressure-bonded to the surface of the negative electrode (the surface of the negative electrode active material layer) to produce the lithium secondary battery electrode of the present invention. Then, a negative electrode, an insulating layer, a separator (manufactured by Tonen Tapils, PE 25 μm thickness), and a positive electrode were stacked in this order, and the lithium secondary battery of Experimental Example 8 was fabricated by filling the electrolyte solution. The electrolytic solution used was LiPF ₆ dissolved in a mixed solvent (volume ratio 3: 7) of ethylene carbonate (EC) and diethyl carbonate (DEC) at a concentration of 1 mol / L.

[Experimental Example 9]
The lithium secondary battery obtained through the same steps as in Experimental Example 8 was used, except that the slurry containing the garnet-type oxide that conducts lithium ions was used, and an insulating layer was applied to the surface of the negative electrode active material layer. The secondary battery was designated as Experimental Example 9. In forming the insulating layer, first, 72 parts by weight of a garnet-type oxide that conducts lithium ions having an average particle diameter of 0.6 μm, 3 parts by weight of polyvinylidene fluoride, and 25 parts by weight of NMP were mixed to prepare a slurry. The slurry was applied to the surface of the negative electrode active material layer so as to have a thickness of 20 μm, and dried to prepare an electrode for a lithium secondary battery of the present invention in which an insulating layer was formed on the surface of the negative electrode active material layer. .

[Experimental Example 10]
Obtained through the same steps as in Experimental Example 8 except that the above-prepared slurry containing garnet-type oxide that conducts lithium ions was used, and the surface of the negative electrode active material layer was electrostatically coated to form a 6 μm insulating layer. The obtained lithium secondary battery was named experimental example 10.

(Charge / discharge test)
About the produced battery, the conditioning which performs charging / discharging 5 cycles with an electric current of 0.2C (100mA) as an upper limit of 4.1V and a minimum of 3.0V was performed. Next, the battery was charged over 7 hours to a charging upper limit voltage of 4.1 V by a constant current / low voltage charging method with a current density of 0.2 mA / cm ² under a temperature condition of 20 ° C. Next, discharging was performed to a discharge lower limit voltage of 3.0 V at a constant current of a current density of 0.1 mA / cm ² . The batteries of Experimental Examples 8 to 10 were all charged and discharged satisfactorily and were found to have good battery characteristics.

  10, 10B, 10C, 10D Lithium secondary battery, 11, 14 current collector, 12 positive electrode active material layer, 13, 33 positive electrode, 17 negative electrode active material layer, 18 negative electrode, 19, 19B, 19C insulating layer, 20 non-aqueous Electrolyte, 21 separator.

Claims (4)

A positive electrode having a positive electrode active material that absorbs and releases lithium;
A negative electrode having a negative electrode active material that absorbs and releases lithium; and
An electrolytic solution that is interposed between the positive electrode and the negative electrode and conducts lithium;
An insulating layer that is present in at least one of the surface of the positive electrode, the surface of the negative electrode, and the electrolyte, and is formed of a garnet-type oxide that contains Zr and conducts lithium ions;
Rechargeable lithium battery.   The lithium secondary battery according to claim 1, wherein the insulating layer is further formed of a garnet-type oxide containing Nb. The insulating layer has a composition formula Li _{5 + X} La ₃ (Zr _X , A _2−X ) O ₁₂ (where A is Sc, Ti, V, Y, Nb, Hf, Ta, Al, Si, Ga, and 3. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is formed of a garnet-type oxide represented by one or more elements selected from the group consisting of Ge, wherein X is 1.4 ≦ X <2). .   An electrode for a lithium secondary battery, comprising: an active material layer that occludes and releases lithium; and an insulating layer that is formed on a surface of the active material layer with a garnet-type oxide that contains Zr and conducts lithium ions.