Secondary battery

Abstract

To improve output in a secondary battery having an oxide-based ion conductor in a positive electrode.SOLUTION: The secondary battery includes a positive electrode 14, a negative electrode 12, and an electrolytic layer 15 having an electrolytic solution 15a for conducting conductive ions between the positive electrode and the negative electrode. The positive electrode contains at least a positive electrode active material 14a and an oxide-based ion conductor 14b. At least a part of the oxide-based ion conductor is in contact with the positive electrode active material. The presence of the electrolytic solution between the positive electrode active material and the oxide-based ion conductor enables electrochemical insertion of conduction ions into the oxide-based ion conductor.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present disclosure relates to secondary batteries. [Background technology]

[0002] Patent Document 1 describes that adding a lithium ion conductive oxide solid electrolyte to the positive electrode of a lithium ion battery can suppress chemical reactions between the electrolyte and the positive electrode active material in a high temperature environment. Patent Document 1 also describes that adding a lithium ion conductive oxide solid electrolyte to the positive electrode can reduce the amount of electrolyte and improve the safety of the secondary battery. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 53198879 Summary of the Invention [Problem to be solved by the invention]

[0004] However, although the configuration of Patent Document 1 contains a conductive additive in the positive electrode, the electron conduction path in the positive electrode is insufficient, making it difficult to sufficiently improve the output of the secondary battery.

[0005] In view of the above, an object of the present disclosure is to improve the output of a secondary battery having an oxide-based ion conductor in the positive electrode. [Means for solving the problem]

[0006] To achieve the above object, one embodiment of the present disclosure includes a positive electrode (14), a negative electrode (12), and an electrolyte layer (15) having an electrolyte solution (15a) that conducts conductive ions between the positive electrode and the negative electrode. The positive electrode contains at least a positive electrode active material (14a) and an oxide-based ion conductor (14b). At least a portion of the oxide-based ion conductor is in contact with the positive electrode active material. The presence of the electrolyte solution between the positive electrode active material and the oxide-based ion conductor allows electrochemical insertion of conductive ions into the oxide-based ion conductor.

[0007] This allows the oxide-based ion conductor to exhibit electronic conductivity in addition to ionic conductivity. As a result, the number of electron conduction paths in the positive electrode can be increased, thereby improving the output of the secondary battery. Furthermore, since the oxide-based ion conductor has electronic conductivity, the amount of conductive additive used in the positive electrode can be reduced.

[0008] The reference numerals in parentheses for the above components indicate the corresponding relationship with the specific means described in the embodiments to be described later. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view showing a configuration of a secondary battery according to an embodiment of the present disclosure. [Figure 2] These are SEM images of the positive electrode active material and oxide-based ion conductor. [Figure 3] FIG. 2 is a conceptual diagram showing the configuration of a positive electrode active material and an oxide-based ion conductor. [Figure 4] FIG. 2 is a conceptual diagram showing the configuration of a positive electrode active material and an oxide-based ion conductor. [Figure 5] FIG. 1 is a diagram showing the crystal structure of a pyrochlore-type oxide. [Figure 6] 1A to 1C are diagrams illustrating a manufacturing process of a pyrochlore-type oxide. [Figure 7] FIG. 2 is a diagram for explaining the provision of electronic conductivity to an oxide-based ion conductor. [Figure 8] FIG. 1 shows charge / discharge curves of a battery cell comprising LLNOF and a lithium metal electrode. [Figure 9] FIG. 2 is a diagram showing the discharge characteristics of secondary batteries of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In this embodiment, the active material composite particles are used as a positive electrode active material of a secondary battery 10. The secondary battery 10 of this embodiment is a lithium ion battery in which lithium ions are conducted as conduction ions.

[0011] As shown in FIG. 1, a secondary battery 10 includes a negative electrode current collector 11, a negative electrode 12, a positive electrode current collector 13, a positive electrode 14, and an electrolyte layer 15.

[0012] An electrolyte layer 15 is sandwiched between the positive electrode 14 and the negative electrode 12. The negative electrode 12 and the electrolyte layer 15 are in contact with each other. The positive electrode 14 and the electrolyte layer 15 are in contact with each other. The negative electrode 12 and the positive electrode 14 are connected via the electrolyte layer 15. The secondary battery 10 of this embodiment is charged and discharged by lithium ions moving between the negative electrode 12 and the positive electrode 14 via the electrolyte layer 15.

[0013] The electrolyte layer 15 includes an electrolyte solution 15a and an insulating layer 15b. The electrolyte solution 15a is present from the negative electrode 12 to the positive electrode 14, and is provided so as to permeate into the inside of the negative electrode 12 and the inside of the positive electrode 14. The electrolyte layer 15 may partially contain a solid electrolyte.

[0014] The electrolyte solution 15a has lithium ion conductivity and conducts ions between the negative electrode 12 and the positive electrode 14. The electrolyte solution 15a contains a lithium salt and a solvent. The lithium salt may be a common lithium salt used in lithium ion batteries (e.g., LiPF6). The solvent constituting the electrolyte solution 15a may be an organic electrolyte solution, an ionic liquid, a gel polymer, or the like. These solvents may be used alone or in combination.

[0015] The insulating layer 15b is disposed between the negative electrode 12 and the positive electrode 14, and separates the negative electrode 12 from the positive electrode 14. The insulating layer 15b is an insulating ion-permeable membrane that prevents physical contact between the negative electrode 12 and the positive electrode 14 to suppress electrical short circuits, and also allows ions to pass through.

[0016] In this embodiment, a porous separator is used as the insulating layer 15b. The separator may be made of a porous material such as polypropylene, polyethylene, or nonwoven fabric. The surface of the separator may be coated with a solid electrolyte. A polymer sheet or a solid electrolyte sheet may be used as the insulating layer 15b. The solid electrolyte sheet is a self-supporting membrane.

[0017] Any material that can be used as a current collector for a lithium ion battery can be used for the negative electrode current collector 11 and the positive electrode current collector 13. In this embodiment, Cu is used for the negative electrode current collector 11, and Al is used for the positive electrode current collector 13.

[0018] The negative electrode material constituting the negative electrode 12 can be any material that can be used as a negative electrode active material for lithium-ion batteries, such as a carbon-based negative electrode material, an oxide-based negative electrode material, or a metal-based negative electrode material. In this embodiment, graphite is used as the negative electrode material. The negative electrode 12 may contain a conductive additive, a binder, and a polymer. The negative electrode 12 may also contain a solid electrolyte. When the negative electrode 12 contains a solid electrolyte, the solid electrolyte may be mixed with the negative electrode material, or the surface of the negative electrode material may be coated with the solid electrolyte.

[0019] The positive electrode 14 releases lithium ions when the secondary battery 10 is charged and receives lithium ions when the secondary battery 10 is discharged. The positive electrode 14 includes a positive electrode active material 14a as a positive electrode material and an oxide-based ion conductor 14b having ion conductivity. The oxide-based ion conductor 14b is an oxide-based solid electrolyte having ion conductivity. By providing the oxide-based ion conductor 14b in the positive electrode 14, it is possible to suppress a reaction between the positive electrode active material 14a and the electrolyte solution 15a.

[0020] The positive electrode 14 may contain a conductive additive, a binder, and a polymer. The positive electrode 14 may further contain a solid electrolyte. When the positive electrode 14 contains a solid electrolyte, the solid electrolyte may be mixed with the positive electrode active material 14a, or the surface of the positive electrode active material 14a may be coated with the solid electrolyte.

[0021] The positive electrode active material 14a can be any material that can be used as a positive electrode active material for a lithium ion battery, such as a layered rock salt type active material, an olivine type active material, or a spinel type active material. x Co y Mn z O2(NCM), LiNi x Co y Al z Ternary positive electrode materials such as LiFePO4 (LFP), LiMnO2 (NCA) can be used. 1-x Fe x Examples of spinel active materials that can be used include LiMn2O4 (LMO), LiNi 0.5 Mn 1.5 O4(LNMO) can be used.

[0022] The oxide-based ion conductor 14b may be, for example, a pyrochlore-type oxide, a perovskite-type oxide, or the like. 2-x La (1+x) / 3 Nb2O6F(LLNOF), Li 2-x La (1+x) / 3 Ta2O6F (LLTOF) can be used. As the perovskite oxide, for example, Li 3x La 2 / 3-x TiO3(LLTO) can be used. Among these oxides, pyrochlore-type oxides have high ionic conductivity and can be suitably used as the oxide-based ionic conductor 14b. Pyrochlore-type oxides will be described in detail later.

[0023] FIG. 2 is an SEM image showing a state in which the positive electrode active material 14a and the oxide-based ion conductor 14b are randomly structured in the positive electrode 14. FIG. 2 shows a state in which the electrolyte 15a is not present. In FIG. 2, the relatively large particles shown in gray (neutral color) are the positive electrode active material 14a, and the relatively small particles shown in white are the oxide-based ion conductor 14b. In FIG. 2, the black portion between the positive electrode active material 14a and the oxide-based ion conductor 14b contains a conductive additive, a binder, and the like.

[0024] Fig. 3 shows an example of a positive electrode 14 having a random structure in which the positive electrode active material 14a and the oxide ion conductor 14b are randomly mixed. Fig. 4 shows an example of a positive electrode 14 having a layered structure in which the outer surface of the positive electrode active material 14a is covered with the oxide ion conductor 14b. The positive electrode active material 14a and the oxide ion conductor 14b can be provided in any form, such as the random structure shown in Fig. 3 or the layered structure shown in Fig. 4. In the positive electrode 14, it is sufficient that at least a portion of the oxide ion conductor 14b is in direct contact with the positive electrode active material 14a.

[0025] In the positive electrode 14, the electrolyte 15a is present between the positive electrode active material 14a and the oxide-based ion conductor 14b. Therefore, a three-phase interface among the positive electrode active material 14a, the oxide-based ion conductor 14b, and the electrolyte 15a is formed at the portion where the positive electrode active material 14a and the oxide-based ion conductor 14b are in contact with each other. While the oxide-based ion conductor 14b is illustrated as a layer in Fig. 4, in reality the oxide-based ion conductor 14b is in a particulate form, and the electrolyte 15a seeps into the gaps between the particulate oxide-based ion conductors 14b, forming the three-phase interface.

[0026] The formation of a three-phase interface among the positive electrode active material 14a, the oxide ion conductor 14b, and the electrolyte 15a allows lithium ions to be conducted between the positive electrode active material 14a and the oxide ion conductor 14b via the electrolyte 15a, and the oxide ion conductor 14b can electrochemically insert lithium ions. The insertion of lithium ions into the oxide ion conductor 14b does not necessarily require a change in the electrode potential.

[0027] The oxide-based ion conductor 14b exhibits electronic conductivity when lithium ions, which are conductive ions, are inserted into the crystal structure. The oxide-based ion conductor 14b maintains its ionic conductivity even after exhibiting electronic conductivity, becoming a mixed electronic ion conductor having both electronic and ionic conductivity. The oxide-based ion conductor 14b exhibits electronic conductivity, thereby increasing the number of electron conduction paths in the positive electrode 14.

[0028] To effectively conduct ions between the positive electrode active material 14a and the oxide-based ion conductor 14b via the electrolyte solution 15a, it is desirable to use a solvent that easily causes solvation as the solvent for the electrolyte solution 15a, and it is desirable to use a solvent with a high donor number. Examples of solvents with a high donor number include ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC).

[0029] In order to effectively conduct ions between the positive electrode active material 14a and the oxide-based ion conductor 14b via the electrolytic solution 15a, it is desirable to increase the concentration of the lithium salt in the electrolytic solution 15a. In this embodiment, the concentration of the lithium salt in the electrolytic solution 15a is set to be higher than 1 M (mol / L).

[0030] In the positive electrode 14, the weight ratio of the oxide-based ion conductor 14b to the positive electrode active material 14a is preferably greater than 0 wt % and 10 wt % or less. By increasing the weight ratio of the oxide-based ion conductor 14b to the positive electrode active material 14a to greater than 0 wt %, the ionic conductivity and electronic conductivity of the positive electrode 14 can be improved. A larger weight ratio of the oxide-based ion conductor 14b can improve the ionic conductivity and electronic conductivity of the positive electrode 14, but this leads to a decrease in the positive electrode active material 14a, resulting in a decrease in battery capacity and a decrease in energy density. For this reason, the weight ratio of the oxide-based ion conductor 14b to the positive electrode active material 14a is preferably 10 wt % or less.

[0031] In the positive electrode 14, the particle size of the positive electrode active material 14a is preferably larger than the particle size of the oxide-based ion conductor 14b. By making the particle size of the positive electrode active material 14a larger than the particle size of the oxide-based ion conductor 14b, the proportion of the surface area of ​​the positive electrode active material 14a that is in contact with the oxide-based ion conductor 14b can be increased, thereby enhancing the effect of improving ionic conductivity and electronic conductivity.

[0032] Here, the pyrochlore type oxide used as the oxide-based ion conductor 14b of the positive electrode 14 will be described. The pyrochlore type oxide used in this embodiment has the composition formula "Aa 2-α Ab (1+α) / 3 B2O 7-β X γ " In the above composition formula, O is an oxygen atom, and Aa, Ab, B, and X represent any element or group. Aa, Ab, and B are each different types of cations, and O and X are each different types of anion. Aa is an alkali metal cation. Pyrochlore-type oxides contain multiple cations in their composition, consisting of the alkali metal cation Aa and multiple cations other than the alkali metal cation Aa, Ab, and B. In other words, pyrochlore-type oxides contain multiple cations in their composition, including the alkali metal cation Aa.

[0033] As shown in Figure 5, pyrochlore oxides have a crystal structure in which a three-dimensional network of octahedra consisting of BO6 (NbO6) is formed. BO6 is centered around cation B, with O at each vertex, and the vertex is shared with an adjacent BO6. In the three-dimensional network consisting of BO6, a hexagonal tunnel structure is formed in which cations A and anions X are arranged.

[0034] In the above composition formula, 0.6<α<2.0, 0<β≦1, and 0<γ≦1. Changing α changes the composition ratio of Aa and Ab, and changing β and γ changes the composition ratio of O and X.

[0035] The cation Aa is an alkali metal cation. The alkali metal represented by Aa can be any of Li, Na, K, Rb, and Cs. The cation Aa may also be Mg or H, other than alkali metals. That is, the cation Aa contains at least one selected from Li, Na, K, Rb, Cs, Mg, and H. In this embodiment, Li is used as Aa. The composition ratio (2-α) of Aa is within the range of 0<(2-α)<1.4.

[0036] The cation Ab contains at least a lanthanoid. The lanthanoid represented by Ab can be at least one of La, Ce, Nd, and Sm. In this embodiment, La is used as Ab. The composition ratio (1+α) / 3 of Ab is within the range of 0.53<(1+α) / 3<1.

[0037] The basic structure of the cation Ab is a lanthanoid, and a portion of the lanthanoid constituting Ab may be substituted with an alkaline earth metal (Ca, Mg, Sr, etc.). In the pyrochlore oxide of this embodiment, the pyrochlore structure in the above composition formula, where 0.6<α<2.0 and 0<β≦1, contains a lanthanoid, which is thought to cause defects in the crystal structure and improve ionic conductivity. In this embodiment, La is used as Ab.

[0038] In the pyrochlore oxide of this embodiment, the cation A in the general pyrochlore structure formula "A2B2O7" is a composite cation of lithium metal and lanthanoid, which is thought to contribute to the improvement of the ionic conductivity of the pyrochlore oxide.

[0039] The cation B is a metal cation different from Aa and Ab, and is a transition metal or a metal selected from Groups 13 to 15 elements. B forms an octahedron surrounded by six O atoms in the crystal. The transition metal represented by B can be a Group 4 transition metal or a Group 5 transition metal, and more specifically, at least one of Nb, Ta, Ti, Zr, Hf, and V can be used. The Group 13 element represented by B can be Al, Ga, or In, the Group 14 element can be Ge or Sn, and the Group 15 element can be Sb or Bi.

[0040] As described above, the oxide-based ion conductor 14b of this embodiment exhibits electronic conductivity by inserting lithium ions, which are conductive ions, into its crystal structure. When the electrode potential of the oxide-based ion conductor 14b is lowered, the lithium ion insertion / extraction reaction of the oxide-based ion conductor 14b is promoted. The electrode potential at which the lithium ion insertion / extraction reaction occurs in a pyrochlore-type oxide varies depending on the type of cation B. When Nb is used as the cation metal represented by B in the composition formula, the effect of the pyrochlore-type oxide in exhibiting electronic conductivity can be enhanced.

[0041] The anion X is an anion that can substitute for the O atoms that make up the pyrochlore structure. X has different electronegativity and polarizability from the O atoms. The anion represented by X can be at least one of O, F, Cl, Br, I, S, OH, and P. The composition ratio γ of X is in the range of 0<γ≦1, and at least a portion of the O atoms that make up the pyrochlore structure are substituted with X.

[0042] The pyrochlore oxide of this embodiment has a defect structure in which lattice defects are present in the crystal due to some of the O atoms constituting the pyrochlore structure being substituted with anions that have different electronegativity and polarizability from the O atoms. It is believed that the pyrochlore oxide of this embodiment has improved ionic conductivity due to the presence of a defect structure in the pyrochlore structure.

[0043] The pyrochlore oxide is preferably a halogen-containing oxide in which a halogen element is used as the anion X. Pyrochlore oxides containing a halogen element have defects in the crystal structure, which facilitates the insertion and desorption of Li, making it easier for the pyrochlore oxide to exhibit electronic conductivity. Among halogen elements, it is particularly desirable to use F as the anion X.

[0044] In the pyrochlore oxide of this embodiment, a defect structure is formed in which a portion of Aa and Ab is missing. The compositional formula of a general pyrochlore structure is "A2B2O7," and the composition ratio of the cation A is 2. In contrast, in the pyrochlore oxide of this embodiment, the composition ratios of Aa and Ab are "2-α" and "(1+α) / 3," respectively, and since 0.6<α<2.0, the sum of the composition ratios of Aa and Ab is less than 2. In other words, in the crystal structure of the pyrochlore oxide of this embodiment, at least a portion of Aa and Ab is missing. The composition ratio corresponding to the missing portions of Aa and Ab is (2α-1) / 3.

[0045] In addition to the deviation in the composition ratio, a defect structure can also be formed by making the sum of the valences of the cations consisting of Aa, Ab, and B and the anions consisting of O and X in the above composition formula negative.

[0046] Furthermore, the pyrochlore oxide of this embodiment is a mixed anion compound in which the pyrochlore structure contains multiple anions such as O and X. Because the anion represented by X is present in the BO6 coordination octahedron structure, the alkali metal Aa can be positioned in the center of the space between the BO6 coordination octahedron and the BO6 coordination octahedron, rather than moving closer to it. This is thought to be why the pyrochlore oxide of this embodiment exhibits high ionic conductivity when used in an electric field, such as in a battery.

[0047] Furthermore, since the α, β, and γ in the composition formula affect lattice defects and ionic conductivity, it is desirable to use them within appropriate ranges. Large values ​​of α, β, and γ increase the defect concentration in the crystal lattice, but if they exceed a certain amount, the concentration of the alkali metal represented by Aa decreases, resulting in a decrease in ionic conductivity. For this reason, it is desirable to control α within the range of 0.6<α<2.0, β within the range of 0<β≦1, and γ within the range of 0<γ≦1.

[0048] As a pyrochlore-type oxide, "Li 1.25 La 0.58 An example is a pyrochlore oxide represented by Nb2O6F(LLNOF). LLNOF uses Li as the cation Aa, La as the cation Ab, Nb as the cation B, and F as the anion X, with α=0.75, β=1, and γ=1.

[0049] The pyrochlore oxide of this embodiment has a surface area of ​​1×10 -3 The pyrochlore-type oxide of this embodiment has an ionic conductivity significantly higher than that of other oxide-type solid electrolytes such as garnet-type oxides.

[0050] 6 shows the method for producing a pyrochlore oxide according to this embodiment, which includes a first mixing step S10, a first firing step S11, a second mixing step S12, a forming step S13, and a second firing step S14, performed in this order.

[0051] First, a lanthanum source, a lithium source, and a niobium source are prepared as raw materials for the pyrochlore oxide, and a first mixing step S10 is performed in which these are mixed. Metal oxides, metal carbonates, etc. can be used as the lanthanum source, lithium source, and niobium source. In this embodiment, La2O3 is used as the lanthanum source, Li2CO3 as the lithium source, and Nb2O5 as the niobium source. In the first mixing step, La2O3, Li2CO3, and Nb2O5 are mixed in a predetermined ratio.

[0052] Next, the mixture prepared in the first mixing step is fired in the first firing step S11. In the first firing step S11, two firing steps are performed. In the first step, the mixture is pre-fired in air at 500°C for 6 hours. Pre-fire removes moisture and other substances from the mixture, increasing its reactivity. Following pre-fire, the mixture is fired in air at 1200°C for 4 hours. This produces a precursor of the target product, Li. 0.5 La 0.5 Nb2O6 is obtained.

[0053] Next, a fluorine source is prepared as a raw material and mixed with the precursor in a second mixing step S12. Metal fluorides can be used as the fluorine source. In this embodiment, LiF and LaF3 are used as the fluorine source. LiF is a fluorine source and a lithium source, and LaF3 is a fluorine source and a lanthanum source. In the second mixing step, LiF and LaF3 are mixed with the precursor in a predetermined ratio.

[0054] Next, the mixed powder of the precursor, LiF, and LaF3 is processed into a pellet shape and a molding step S13 is performed in which the mixed powder is pressed at 100 MPa, thereby molding the mixture of the precursor, LiF, and LaF3 into a pellet shape.

[0055] Next, a second firing step S14 is performed to fire the mixture of the precursor, LiF, and LaF. In the second firing step S14, the mixture of the precursor, LiF, and LaF is heated to 1000°C for 6 hours in a nitrogen atmosphere. In the second firing step S14, firing may be performed in a sealed state or in a state covered with mother powder to prevent composition deviation due to volatilization of the Li and F elements.

[0056] By cooling the product of the second firing step, the composition formula "Li 1.25 La 0.58 The pyrochlore-type oxide expressed as Nb2O6F(LLNOF) is obtained. The pyrochlore-type oxide produced is in the form of particles.

[0057] By changing the mixing ratio of La2O3, Li2CO3, Nb2O5, LiF, and LaF3 in the above manufacturing process, 2-α La (1+α) / 3 NbO 7-β F γ It is possible to obtain a pyrochlore-type solid electrolyte represented by the formula "α, β, and γ." By changing the mixing ratio of La2O3, Li2CO3, Nb2O5, LiF, and LaF3, it is possible to adjust the α, β, and γ in the composition formula. Furthermore, part of the material sublimes during firing. Therefore, it is possible to adjust the α, β, and γ by changing the firing conditions, furnace atmosphere, and furnace size in the first and second firing steps.

[0058] Next, the development of electronic conductivity of the oxide-based ion conductor 14b will be described with reference to Fig. 7. In the example shown in Fig. 7, LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) was used, and Li was used as the oxide ion conductor 14b. 1.25 La 0.58 Nb2O6F (LLNOF) is used.

[0059] In FIG. 7, the left side shows the initial state before electrolyte 15a is injected into positive electrode 14, and the right side shows the state in use after electrolyte 15a has been injected into positive electrode 14.

[0060] In the initial state, the electronic conductivity of LLNOF is 8.3 × 10 -8 S / cm, which is close to the value of an insulator. 1+x Al x Ti 2-x The electronic conductivity of (PO4)3(LATP) is 8.5×10 -8 The electronic conductivity of LLNOF in the initial state and LATP was measured using pellets formed by sintering the respective powders.

[0061] When the electrolyte solution 15a is injected into the positive electrode 14, the positive electrode active material 14a and the oxide ion conductor 14b react locally at the contact area. By injecting the electrolyte solution 15a into the positive electrode 14, a three-phase interface is formed among the positive electrode active material 14a, the oxide ion conductor 14b, and the electrolyte solution 15a at the area where the positive electrode active material 14a and the oxide ion conductor 14b are in direct contact with each other.

[0062] The formation of this three-phase interface causes lithium ions to be desorbed from the positive electrode active material 14a and electrochemically inserted into the crystalline structure of the oxide-based ion conductor 14b via the electrolyte 15a. The desorption of lithium ions from the positive electrode active material 14a and the insertion of lithium ions into the oxide-based ion conductor 14b proceed simply by injecting the electrolyte 15a into the positive electrode 14, and do not necessarily require a change in the electrode potential.

[0063] NCM811 has a composition formula of "LiNi 0.8 Co 0.1 Mn 0.1 O2" to "Li 1-α Ni 0.8 Co 0.1 Mn 0.1 O2". LLNOF changes its composition to "Li 1.25 La0.58 Nb2O6F" to "Li 1.25+α La 0.58 As lithium ions are inserted into LLNOF, the Nb in LLNOF is reduced and its oxidation state changes from "+5" to "5-δ+". LLNOF exhibits electronic conductivity due to the insertion of lithium ions.

[0064] LLNOF has an electronic conductivity of 4.8×10 due to lithium ion insertion. -3 S / cm. That is, the oxide ion conductor 14b of this embodiment exhibits high electronic conductivity in addition to high ionic conductivity due to the insertion of lithium ions. The electronic conductivity of LLNOF after lithium ion insertion was measured after the potential of the LLNOF pellet was set to 2.5 V or less and lithium ion insertion and desorption were performed.

[0065] In this embodiment, the concentration of the electrolyte solution 15a is set to be higher than 1 M, which allows lithium ions to be inserted into the oxide ion conductor 14b effectively. Furthermore, in this embodiment, a solvent with a high donor number is used, which allows lithium ions to be inserted into the oxide ion conductor 14b effectively.

[0066] After injecting the electrolyte solution 15a into the positive electrode 14, the secondary battery 10 is overdischarged to reduce the electrode potential to 2.5 V or less, thereby improving the electronic conductivity of the oxide-based ion conductor 14b. By reducing the electrode potential to 2.5 V or less, lithium ions desorbed from the positive electrode active material 14a can be inserted into the oxide-based ion conductor 14b even in areas where the positive electrode active material 14a and the oxide-based ion conductor 14b are not in direct contact and no three-phase interface is formed. As a result, the proportion of the oxide-based ion conductor 14b that exhibits electronic conductivity can be increased.

[0067] The lower the electrode potential, the more lithium ions are inserted into the oxide-based ion conductor 14b. 1.25+α La 0.58On the other hand, if the electrode potential is too low, the oxide ion conductor 14b will be irreversibly deteriorated. For this reason, it is desirable to carry out the lithium ion insertion / extraction reaction into the oxide ion conductor 14b at an electrode potential of 0.5 V or higher.

[0068] Figure 8 shows the charge-discharge characteristics of a battery cell with LLNOF. In the example shown in Figure 8, a battery cell composed of lithium metal, electrolyte, and LLNOF was used, and the electrode potential was changed to insert and extract lithium ions into the LLNOF. The potential V on the vertical axis of Figure 8 is expressed as "V vs. Li + / Li".

[0069] In Figure 8, the solid line sloping downward to the right indicates the capacity when the potential is lowered to 0.5 V, and the solid line sloping upward to the right indicates the capacity when the potential is lowered to 0.5 V and then increased. In Figure 8, the dashed line sloping downward to the right indicates the capacity when the potential is lowered to 0.25 V, and the dashed line sloping upward to the right indicates the capacity when the potential is lowered to 0.25 V and then increased.

[0070] As shown in Figure 8, the capacity increases when the potential is lowered below 2 V. This increase in capacity reflects the intercalation of lithium ions into LLNOF and the formation of a Li film on the surface of the Li metal side of LLNOF. The increase in capacity is greater when the potential is lowered to 0.25 V than when it is lowered to 0.5 V.

[0071] By increasing the potential from 0.25 V or 0.5 V, the lithium ions inserted into LLNOF are deintercalated. The capacity at this potential increase is thought to correspond to the amount of lithium ions inserted into LLNOF. The electronic conductivity of LLNOF, which is manifested by lithium ion insertion, is maintained even after the lithium ions are deintercalated from LLNOF.

[0072] When the potential was lowered to 0.25 V, the irreversible capacity, which is the difference between the capacity when the potential was lowered and the capacity when the potential was raised, increased, indicating a greater degree of degradation of the LLNOF. This is thought to be due to the irreversible desorption of some of the O and F elements contained in the LLNOF as a result of the increased amount of lithium ions inserted into the LLNOF. On the other hand, when the potential was lowered to 0.5 V, the irreversible capacity, which is the difference between the capacity when the potential was lowered and the capacity when the potential was raised, decreased, indicating a smaller degree of degradation of the LLNOF. For this reason, it is desirable to insert lithium ions into LLNOF at a potential of 0.5 V or higher.

[0073] When lithium ions are inserted into LLNOF at a potential of 0.5 V or higher, the composition formula of LLNOF becomes "Li 2.25 La 0.58 Nb2O6F', that is, 'Li 1.25+α La 0.58 The α of Nb2O6F is 1, and the amount of lithium inserted into LLNOF, "1," is 80% of the initial amount of lithium, "1.25." Therefore, by keeping the amount of lithium that can be inserted into LLNOF at 80% or less of the initial amount of lithium, irreversible degradation of LLNOF can be suppressed.

[0074] Next, the discharge characteristics of secondary batteries 10 with different types and particle sizes of positive electrode active material 14a and oxide ion conductor 14b will be described using examples and comparative examples shown in Fig. 9. In the example shown in Fig. 9, negative electrode 12 made of graphite was used, and electrolyte 15 was used in which LiPF6 was added at a concentration of more than 1M to a solvent in which EC and DEC were mixed in a 1:1 ratio.

[0075] The discharge characteristics in FIG. 9 are the dischargeable time until the lower limit voltage is reached when the secondary battery 10 is discharged at 10 C, and are shown as relative values ​​with the value of Comparative Example 1 set to 100%.

[0076] In Examples 1 to 8 and Comparative Examples 1 and 2, the positive electrode active material 14a was LiNi 0.8 Co 0.1 Mn0.1 O2 (NCM811) was used, and in Example 9 and Comparative Example 3, LiMn 0.6 Fe 0.4 PO4 (LMFP) is used.

[0077] In Examples 1 to 9, the oxide ion conductor 14b was Li 1.25 La 0.58 In Example 8, Nb2O6F (LLNOF) was used, and in Example 9, La 0.57 Li 0.29 In Comparative Example 2, Li 1.4 Al 0.4 Ti 1.6 (PO4)3(LATP) is used. In Comparative Examples 1 and 3, the positive electrode 14 does not include the oxide ion conductor 14b.

[0078] The LLNOFs of Examples 1 to 7 and 9 and the LLTO of Example 7 are oxide ion conductors 14b into which lithium ions can be electrochemically inserted. The LATP of Comparative Example 2 is an oxide ion conductor into which lithium ions cannot be electrochemically inserted.

[0079] The amount of oxide ion conductor 14b added was 3 wt% in Examples 1, 5 to 9 and Comparative Example 2, 5 wt% in Example 2, 7 wt% in Example 3, and 10 wt% in Example 4. The amount of oxide ion conductor 14b added is the weight ratio of oxide ion conductor 14b to positive electrode active material 14a.

[0080] In Examples 1 to 4 and 8, the particle diameter of the positive electrode active material 14a was 5 μm, and the particle diameter of the oxide ion conductor 14b was 0.1 μm. In Example 5, the particle diameter of the positive electrode active material 14a was 5 μm, and the particle diameter of the oxide ion conductor 14b was 0.8 μm. In Example 6, the particle diameter of the positive electrode active material 14a was 5 μm, and the particle diameter of the oxide ion conductor 14b was 4 μm. In Example 9, the particle diameter of the positive electrode active material 14a was 1 μm, and the particle diameter of the oxide ion conductor 14b was 0.1 μm. In all of Examples 1 to 6, 8 and 9, the particle diameter of the positive electrode active material 14a was greater than the particle diameter of the oxide ion conductor 14b.

[0081] In Example 7, the particle diameter of the positive electrode active material 14a is 5 μm, and the particle diameter of the oxide ion conductor 14b is 8 μm. In Comparative Example 2, the particle diameter of the positive electrode active material 14a is 5 μm, and the particle diameter of the oxide ion conductor 14b is 1 μm.

[0082] 9, comparing Examples 1 to 8, which used NCM811 as the positive electrode active material 14a, with Comparative Examples 1 and 2, all of Examples 1 to 8, which used an oxide-based ion conductor 14b (LLNOF) capable of inserting lithium ions, had discharge characteristics exceeding 100%. This is thought to be because the oxide-based ion conductor 14b exhibits electronic conductivity in addition to ionic conductivity, thereby improving the discharge characteristics. In contrast, Comparative Example 2, which used an oxide-based ion conductor (LATP) that cannot insert lithium ions, had discharge characteristics below 100%.

[0083] In Examples 1 to 7, the type and particle size of the positive electrode active material 14a and the type of the oxide-based ion conductor 14b are the same in combination. In Examples 1 to 7, there is a tendency for the discharge characteristics to improve as the particle size of the oxide-based ion conductor 14b becomes smaller. In particular, in Examples 1 to 6, where the particle size of the positive electrode active material 14a is greater than the particle size of the oxide-based ion conductor 14b, excellent discharge characteristics are obtained.

[0084] Comparing Example 9, which uses LMFP as the positive electrode active material 14a, with Comparative Example 3, Comparative Example 3, which does not include the oxide ion conductor 14b, had a discharge characteristic of 100%, whereas Example 9, which includes LLTO as the oxide ion conductor 14b, had a discharge characteristic of 115%. In other words, Example 9 has the effect of improving the discharge characteristic by using the oxide ion conductor 14b (LLTO) into which lithium ions can be inserted.

[0085] According to the present embodiment described above, the presence of the electrolyte solution 15a between the positive electrode active material 14a and the oxide-based ion conductor 14b while at least a portion of the oxide-based ion conductor 14b is in contact with the positive electrode active material 14a allows the oxide-based ion conductor 14b to electrochemically intercalate lithium ions. This allows the oxide-based ion conductor 14b to exhibit electronic conductivity in addition to ionic conductivity. As a result, the number of electronic conduction paths in the positive electrode 14 can be increased, thereby improving the output of the secondary battery 10. Furthermore, the electronic conductivity of the oxide-based ion conductor 14b allows the amount of conductive additive used in the positive electrode 14 to be reduced.

[0086] Furthermore, according to this embodiment, by using an oxide containing a halogen as the oxide-based ion conductor 14b, defects are generated in the crystal structure, which facilitates the insertion and desorption of Li, and makes it easier to exhibit electronic conductivity.

[0087] Furthermore, according to this embodiment, the oxide-based ion conductor 14b maintains its ionic conductivity even after exhibiting electronic conductivity, and thus can have both ionic and electronic conductivity. Therefore, the oxide-based ion conductor 14b can improve the ionic and electronic conductivity of the positive electrode 14, thereby improving the output of the secondary battery 10.

[0088] According to the present embodiment, the potential at which lithium ions are inserted into and extracted from the oxide ion conductor 14b is set to 2.5 V (vs. Li + / Li) or less, the proportion of the oxide-based ion conductor 14b that exhibits electronic conductivity can be increased, thereby further improving the electronic conductivity of the oxide-based ion conductor 14b.

[0089] Furthermore, according to this embodiment, by setting the amount of lithium inserted into the oxide ion conductor 14b to 80% or less of the amount of lithium initially contained, it is possible to prevent the potential at which lithium ions are inserted and extracted from the oxide ion conductor 14b from becoming excessively low, and to prevent irreversible deterioration of the oxide ion conductor 14b.

[0090] (Other embodiments) The present disclosure is not limited to the above-described embodiments, and various modifications can be made as follows without departing from the spirit of the present disclosure. Furthermore, the means disclosed in the above-described embodiments may be combined as appropriate within the scope of feasibility.

[0091] For example, in the above embodiment, an example was described in which the active material composite particle of the present disclosure is applied to a lithium ion battery in which the conductive ion is lithium ion, but the active material composite particle of the present disclosure may also be applied to a secondary battery in which the conductive ion is different. Specifically, the active material composite particle of the present disclosure can be applied to a potassium ion battery in which potassium ions are conductive, a sodium ion battery in which sodium ions are conductive, or the like.

[0092] Furthermore, in the above embodiment, an example was described in which the active material composite particles of the present disclosure were applied to a secondary battery 10 in which a negative electrode 12 was previously provided, but the active material composite particles of the present disclosure may also be applied to an anode-free battery. In an anode-free battery, in the initial state, the negative electrode 12 is not formed on the negative electrode current collector 11, and lithium ions that migrate from the positive electrode 14 during charging cause lithium metal to deposit on the negative electrode current collector 11, forming the negative electrode 12. Then, the lithium metal that constitutes the negative electrode 12 migrates to the positive electrode 14 as lithium ions during discharge.

[0093] The active material composite particles 140 of the present disclosure may also be applied to a bipolar battery. A bipolar battery has a structure in which multiple battery cells are stacked and connected in series, and adjacent battery cells share a current collector. In other words, the current collector in contact with the positive electrode of one adjacent battery cell is in contact with the negative electrode of the other adjacent battery cell.

[0094] The secondary battery disclosed in this specification has the following features. (Item 1) A positive electrode (14); a negative electrode (12); an electrolyte layer (15) having an electrolyte solution (15a) that conducts conduction ions between the positive electrode and the negative electrode; the positive electrode contains at least a positive electrode active material (14a) and an oxide ion conductor (14b); At least a portion of the oxide-based ion conductor is in contact with the positive electrode active material, a secondary battery in which the electrolyte exists between the positive electrode active material and the oxide-based ion conductor so that the conductive ions can be electrochemically inserted into the oxide-based ion conductor; (Item 2) 2. The secondary battery according to item 1, wherein the oxide-based ion conductor contains a halogen element in its crystal structure. (Item 3) 3. The secondary battery according to item 1 or 2, wherein the oxide-based ionic conductor is a mixed-electron ionic conductor having ionic conductivity and electronic conductivity when the conductive ions are inserted. (Item 4) the conductive ions are lithium ions; The oxide-based ionic conductor has a resistance of 2.5V (vs. Li + 4. The secondary battery according to any one of items 1 to 3, wherein the insertion of the conduction ions is performed at a potential of not more than 1 / Li. (Item 5) 5. The secondary battery according to any one of items 1 to 4, wherein the amount of the conductive ions that can be inserted into the oxide-based ion conductor is within 80% of the amount of the conductive ions contained in the oxide-based ion conductor before the conductive ions are inserted. (Item 6) the oxide-based ion conductor is a pyrochlore-type oxide, The composition formula of the pyrochlore oxide is Aa 2-α Ab (1+α) / 3 B2O 7-β X γ wherein Aa is an alkali metal, Ab contains at least a lanthanoid, B is a cation different from Aa and Ab, X is an anion that can be substituted for an O atom constituting the pyrochlore oxide, and in the composition formula, α is in the range of 0.6<α<2.0, β is in the range of 0<β≦1, and γ is in the range of 0<γ≦1, and the secondary battery according to any one of items 1 to 5 contains a defect structure. (Item 7) 7. The secondary battery according to item 6, wherein the cationic metal represented by B in the composition formula of the pyrochlore oxide is Nb. (Item 8) 8. The secondary battery according to any one of items 1 to 7, wherein the positive electrode active material has a particle size larger than that of the oxide-based ion conductor. (Item 9) 9. The secondary battery according to any one of items 1 to 8, wherein the weight ratio of the oxide-based ion conductor to the positive electrode active material in the positive electrode is greater than 0 wt % and less than or equal to 10 wt %. (Item 10) 10. The secondary battery according to any one of items 1 to 9, wherein a three-phase interface is formed between the positive electrode active material, the oxide ion conductor, and the electrolyte at a portion where the positive electrode active material and the oxide ion conductor are in contact with each other. [Explanation of symbols]

[0095] 14 Positive electrode 14a Cathode active material 14b Oxide-based ionic conductors 15 Electrolyte layer 15a Electrolyte

Claims

1. A positive electrode (14); A negative electrode (12), an electrolyte layer (15) having an electrolyte solution (15a) that conducts conduction ions between the positive electrode and the negative electrode; The positive electrode contains at least a positive electrode active material (14a) and an oxide-based ion conductor (14b), At least a portion of the oxide-based ion conductor is in contact with the positive electrode active material, a secondary battery in which the electrolyte exists between the positive electrode active material and the oxide-based ion conductor so that the conductive ions can be electrochemically inserted into the oxide-based ion conductor;

2. 2. The secondary battery according to claim 1, wherein the oxide-based ion conductor contains a halogen element in its crystal structure.

3. 2. The secondary battery according to claim 1, wherein the oxide-based ionic conductor is a mixed electronic ionic conductor having ionic conductivity and electronic conductivity in a state in which the conductive ions are inserted.

4. the conductive ions are lithium ions; The oxide-based ionic conductor has a resistance of 2.5 V (vs. Li ＋ 2. The secondary battery according to claim 1, wherein the insertion of the conductive ions occurs at a potential of not more than 1000 volts / Li.

5. 2. The secondary battery according to claim 1, wherein the amount of the conductive ions that can be inserted into the oxide-based ion conductor is within 80% of the amount of the conductive ions contained in the oxide-based ion conductor before the conductive ions are inserted.

6. the oxide-based ion conductor is a pyrochlore-type oxide, The composition formula of the pyrochlore oxide is Aa ２－α Ab （１＋α）／３ B ２ O ７－β X γ wherein Aa is an alkali metal, Ab contains at least a lanthanoid, B is a cation different from Aa and Ab, X is an anion that can be substituted for an O atom that constitutes the pyrochlore oxide, and in the composition formula, α is in the range of 0.6<α<2.0, β is in the range of 0<β≦1, and γ is in the range of 0<γ≦1, and the secondary battery according to claim 1 contains a defect structure.

7. 7. The secondary battery according to claim 6, wherein the cationic metal represented by B in the composition formula of the pyrochlore-type oxide is Nb.

8. The secondary battery according to claim 1 , wherein the positive electrode active material has a particle size larger than that of the oxide-based ion conductor.

9. 2. The secondary battery according to claim 1, wherein the weight ratio of the oxide-based ion conductor to the positive electrode active material in the positive electrode is greater than 0 wt % and is 10 wt % or less.

10. 2. The secondary battery according to claim 1, wherein a three-phase interface is formed between the positive electrode active material, the oxide-based ion conductor, and the electrolyte at a portion where the positive electrode active material and the oxide-based ion conductor are in contact with each other.

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