Secondary batteries

Abstract

The output of a secondary battery having an oxide-based ion conductor in the positive electrode is improved. [Solution] The battery includes a positive electrode (14), a negative electrode (12), and an electrolyte layer (15) containing 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 enables electrochemical insertion of conductive ions into the oxide-based ion conductor.

Description

Technical Field

[0001] This disclosure relates to secondary batteries.

Background Art

[0002] Patent Document 1 describes that by adding a lithium ion conductive oxide solid electrolyte to the positive electrode of a lithium ion battery, the chemical reaction between the electrolyte and the positive electrode active material can be suppressed in a high temperature environment. Furthermore, Patent Document 1 describes that by adding a lithium ion conductive oxide solid electrolyte to the positive electrode, the amount of the electrolyte can be reduced and the safety of the secondary battery can be improved.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the configuration of Patent Document 1 above, although a conductive assistant is included in the positive electrode, the electron conduction path in the positive electrode is not sufficient, and it is difficult to sufficiently improve the output of the secondary battery.

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

Means for Solving the Problems

[0006] To achieve the above object, the First, secondIn one embodiment, the device comprises 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 and negative electrodes. 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 for the electrochemical insertion of conductive ions into the oxide-based ion conductor. In a first aspect of this disclosure, the oxide-based ionic conductor is a pyrochlore-type oxide and contains a halogen element in its crystal structure. In a second aspect of this disclosure, the oxide-based ion conductor is at least one of a pyrochlore-type oxide and a perovskite-type oxide, and the amount of conductive ions that can be inserted into the oxide-based ion conductor is within 80% of the amount of conductive ions contained in the oxide-based ion conductor before the conductive ions are inserted.

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

[0008] The symbols in parentheses for each of the above components indicate their correspondence to the specific means described in the embodiments described later. [Brief explanation of the drawing]

[0009] [Figure 1] This is a cross-sectional view showing the 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] This is a conceptual diagram showing the composition of the positive electrode active material and the oxide-based ion conductor. [Figure 4] This is a conceptual diagram showing the composition of the positive electrode active material and the oxide-based ion conductor. [Figure 5] This figure shows the crystal structure of a pyrochlore-type oxide. [Figure 6] This diagram shows the manufacturing process for pyrochlore-type oxides. [Figure 7] This is a diagram illustrating the imparting of electronic conductivity to oxide-based ionic conductors. [Figure 8] This figure shows the charge and discharge curves of a battery cell equipped with an LLNOF and lithium metal electrodes. [Figure 9] This figure shows the discharge characteristics of secondary batteries in the examples and comparative examples. [Modes for carrying out the invention]

[0010] Embodiments of this disclosure will be described below with reference to the drawings. In this embodiment, the active material composite particles are applied as the positive electrode active material of the secondary battery 10. The secondary battery 10 in this embodiment is a lithium-ion battery in which lithium ions are conducted as conductive ions.

[0011] As shown in Figure 1, the secondary battery 10 comprises 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. The positive electrode 14 and the electrolyte layer 15 are in contact. The negative electrode 12 and the positive electrode 14 are connected via the electrolyte layer 15. In this embodiment, the secondary battery 10 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 comprises 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 to penetrate into the interior of both the negative electrode 12 and the positive electrode 14. The electrolyte layer 15 may also contain a solid electrolyte in part.

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

[0015] The insulating layer 15b is positioned between the negative electrode 12 and the positive electrode 14, separating them. The insulating layer 15b is an insulating ion-permeable film that prevents physical contact between the negative electrode 12 and the positive electrode 14, thereby suppressing electrical short circuits, while also allowing ions to pass through.

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

[0017] The negative electrode current collector 11 and the positive electrode current collector 13 can be made of any material suitable for use as a current collector in a lithium-ion battery. In this embodiment, Cu is used as the negative electrode current collector 11 and Al is used as the positive electrode current collector 13.

[0018] The negative electrode material constituting the negative electrode 12 can be any material usable as a negative electrode active material for lithium-ion batteries, such as carbon-based negative electrode materials, oxide-based negative electrode materials, or metal-based negative electrode materials. In this embodiment, graphite is used as the negative electrode material. The negative electrode 12 may also contain a conductive additive, a binder, and a polymer. Furthermore, the negative electrode 12 may also contain a solid electrolyte. If 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] During charging of the secondary battery 10, the positive electrode 14 releases lithium ions, and during discharging of the secondary battery 10, the positive electrode 14 accepts lithium ions. 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 to the positive electrode 14, the reaction between the positive electrode active material 14a and the electrolytic solution 15a can be suppressed.

[0020] The positive electrode 14 may include a conductive assistant, a binder, and a polymer. Further, the positive electrode 14 may include a solid electrolyte. When the positive electrode 14 includes 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] As the positive electrode active material 14a, any material that can be used as a positive electrode active material for a lithium ion battery can be used. For example, a layered rock salt type active material, an olivine type active material, or a spinel type active material can be used. As the layered rock salt type active material, for example, LiNi x Co y Mn z O2 (NCM), LiNi x Co y Al z O2 (NCA) and other ternary system positive electrode materials can be used. As the olivine type active material, for example, LiFePO4 (LFP), LiMn 1-x Fe x PO4 (LMFP), LiMnPO4 (LMP), LiCoPO4 (LCP), LiNiPO4 (LNP) can be used. As the spinel type active material, for example, LiMn2O4 (LMO), LiNi 0.5 Mn 1.5 O4 (LNMO) can be used.

[0022] As the oxide-based ion conductor 14b, for example, a pyrochlore type oxide, a perovskite type oxide, etc. can be used. As the pyrochlore type oxide, for example, Li 2-x La (1+x) / 3Nb2O6F(LLNOF), Li 2-x La (1+x) / 3 Ta2O6F(LLTOF) can be used. Examples of perovskite-type oxides include 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 oxide-based ionic conductor 14b. Pyrochlore-type oxides will be described in detail later.

[0023] Figure 2 is an SEM image showing the state in which the positive electrode active material 14a and oxide-based ion conductor 14b are arranged in a random structure in the positive electrode 14. Figure 2 shows the state in which the electrolyte 15a is absent. In Figure 2, the relatively large particles shown in gray (intermediate color) are the positive electrode active material 14a, and the relatively small particles shown in white are the oxide-based ion conductor 14b. In Figure 2, the black areas between the positive electrode active material 14a and oxide-based ion conductor 14b are where conductive additives and binders are present.

[0024] Figure 3 shows an example where the positive electrode 14 has a random structure in which the positive electrode active material 14a and oxide-based ion conductor 14b are randomly mixed. Figure 4 shows an example where the positive electrode 14 has a layered structure in which the outer surface of the positive electrode active material 14a is covered with oxide-based ion conductor 14b. The positive electrode active material 14a and oxide-based ion conductor 14b can be provided in any manner, such as the random structure shown in Figure 3 or the layered structure shown in Figure 4. In the positive electrode 14, it is sufficient that at least a portion of the oxide-based 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 is formed between the positive electrode active material 14a, the oxide-based ion conductor 14b, and the electrolyte 15a in the area where the positive electrode active material 14a and the oxide-based ion conductor 14b are in contact. In Figure 4, the oxide-based ion conductor 14b is shown in layers, but in reality, the oxide-based ion conductor 14b is in the form of particles, and the electrolyte 15a seeps into the gaps between the particle-like oxide-based ion conductors 14b, forming the three-phase interface.

[0026] A three-phase interface is formed between the positive electrode active material 14a, the oxide-based ion conductor 14b, and the electrolyte 15a. This allows lithium ions to be conducted between the positive electrode active material 14a and the oxide-based ion conductor 14b via the electrolyte 15a, and the oxide-based ion conductor 14b is capable of electrochemically inserting lithium ions. It is not necessarily required to change the electrode potential for lithium ion insertion into the oxide-based ion conductor 14b.

[0027] In oxide-based ionic conductor 14b, electronic conductivity is exhibited when lithium ions, which are conduction ions, are inserted into the crystal structure. Even after the onset of electronic conductivity, oxide-based ionic conductor 14b retains its ionic conductivity, becoming a mixed-electron ionic conductor possessing both electronic and ionic conductivity. The onset of electronic conductivity in oxide-based ionic conductor 14b increases the electron conduction pathways in the positive electrode 14.

[0028] In order to effectively conduct ions between the positive electrode active material 14a and the oxide-based ion conductor 14b via the electrolyte 15a, it is desirable to use a solvent that readily undergoes solvation as the solvent for the electrolyte 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] Furthermore, in order to effectively conduct ions between the positive electrode active material 14a and the oxide-based ion conductor 14b via the electrolyte 15a, it is desirable to increase the concentration of lithium salt in the electrolyte 15a. In this embodiment, the concentration of lithium salt in the electrolyte 15a is higher than 1 M (mol / L).

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

[0031] In the positive electrode 14, it is desirable that the particle size of the positive electrode active material 14a is 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 that of the oxide-based ion conductor 14b, the proportion of the surface area of ​​the positive electrode active material 14a in contact with the oxide-based ion conductor 14b can be increased. Therefore, the effect of improving ionic conductivity and electronic conductivity can be enhanced.

[0032] Here, we will describe the pyrochlore-type oxide used as the oxide-based ion conductor 14b of the positive electrode 14. The pyrochlore-type oxide used in this embodiment has the composition formula "Aa 2-α Ab (1+α) / 3 B2O 7-β X γIt has a pyrochlore structure represented by the above compositional formula. In the above compositional 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 anions. 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 Ab and B other than the alkali metal cation Aa. 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-type oxides have a crystalline structure in which a three-dimensional octahedral network of BO6 (NbO6) is formed. In BO6, cation B is at the center, with oxygen atoms at its vertices, and these vertices are shared with adjacent BO6 molecules. Within the three-dimensional network of BO6 molecules, hexagonal tunnel structures are formed in which cation A and anion X are arranged.

[0034] In the above compositional formula, 0.6 < α < 2.0, 0 < β ≤ 1, and 0 < γ ≤ 1. A change in α alters the compositional ratio of Aa and Ab, and a change in β and γ alters the compositional ratio of O and X.

[0035] Cation Aa is an alkali metal cation. Any of Li, Na, K, Rb, or Cs can be used as the alkali metal represented by Aa. Alternatively, Mg or H, which are not alkali metals, may be used as cation Aa. In other words, 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 in the range of 0 < (2-α) < 1.4.

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

[0037] The basic structure of the cation Ab consists of lanthanides, and some of the lanthanides constituting Ab may be substituted with alkaline earth metals (such as Ca, Mg, and Sr). In this embodiment, the pyrochlore-type oxide has a pyrochlore structure where 0.6 < α < 2.0 and 0 < β ≤ 1. It is thought that the inclusion of lanthanides in this pyrochlore structure creates defects in the crystal structure, thereby improving ionic conductivity. In this embodiment, La is used as Ab.

[0038] In this embodiment, the pyrochlore-type oxide has a composite cation in which cation A in the typical pyrochlore structure's composition formula "A2B2O7" is composed of lithium metal and a lanthanide. This is thought to contribute to the improved ionic conductivity of the pyrochlore-type oxide.

[0039] Cation B is a metallic cation distinct from Aa and Ab, and is a transition metal or a metal selected from group 13 to 15 elements. In a crystal, B forms an octahedron surrounded by six oxygen atoms. As the transition metal represented by B, group 4 or group 5 transition metals can be used, and more specifically, at least one of Nb, Ta, Ti, Zr, Hf, or V can be used. As group 13 elements represented by B, Al, Ga, and In can be used; as group 14 elements, Ge and Sn can be used; and as group 15 elements, Sb and Bi can be used.

[0040] As described above, the oxide-based ion conductor 14b of this embodiment exhibits electronic conductivity through the insertion of lithium ions, which are conduction ions, into its crystal structure. When the electrode potential of the oxide-based ion conductor 14b is reduced, the lithium ion insertion and deinsertion reaction of the oxide-based ion conductor 14b is promoted. In pyrochlore-type oxides, the electrode potential at which the lithium ion insertion and deinsertion reaction occurs changes depending on the type of cation B. In pyrochlore-type oxides, the effect of exhibiting electronic conductivity can be greatly enhanced when Nb is used as the cation metal represented by B in the composition formula.

[0041] Anion X is a substituteable anion for the oxygen (O) atoms that make up the pyrochlore structure. X has different electronegativity and polarizability from the oxygen atom. At least one of O, F, Cl, Br, I, S, OH, or P can be used as the anion represented by X. The composition ratio γ of X is in the range of 0 < γ ≤ 1, and at least some of the oxygen atoms that make up the pyrochlore structure are substituted with X.

[0042] The pyrochlore-type oxide of this embodiment has a defect structure in which lattice defects are included in the crystal, as some of the oxygen atoms constituting the pyrochlore structure are replaced by anions with different electronegativity and polarizability from the oxygen atoms. It is believed that the ionic conductivity of the pyrochlore-type oxide of this embodiment is improved because the pyrochlore structure contains defect structures.

[0043] Pyrochlore-type oxides are preferably halogen-containing oxides in which a halogen element is used as anion X. Pyrochlore-type oxides containing halogen elements facilitate the formation of defects in the crystal structure, making it easier for Li to be inserted and removed, thus promoting the electronic conductivity of pyrochlore-type oxides. Among halogen elements, the use of F as anion X is particularly desirable.

[0044] In the pyrochlore-type oxide of this embodiment, a portion of Aa and Ab is missing as a defect structure. The composition formula of a typical pyrochlore structure is "A2B2O7", and the composition ratio of cation A is 2. In contrast, in the pyrochlore-type 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-type oxide of this embodiment, a portion of at least one 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 deviations in compositional ratios, defect structures can also be formed by making the sum of the valencies of the cations consisting of Aa, Ab, and B and the anions consisting of O and X in the above compositional formula negative.

[0046] Furthermore, the pyrochlore-type oxide of this embodiment is a complex anionic compound in which multiple anions such as O and X are contained in the pyrochlore structure. Because the anion represented by X is present in the BO6 coordination octahedron structure, the alkali metal Aa can be located in the center of the space between the BO6 coordination octahedron and the BO6 coordination octahedron without being confined to it. Therefore, it is believed that the pyrochlore-type oxide of this embodiment exhibits high ionic conductivity when used with an electric field applied, such as in a battery.

[0047] Furthermore, since α, β, and γ in the above compositional formula affect lattice defects and ionic conductivity, it is desirable to use them within an appropriate range. Larger values ​​of α, β, and γ increase the defect concentration in the crystal lattice, but beyond a certain amount, the concentration of alkali metals represented by Aa decreases, and the ionic conductivity declines. 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 0.58An example of a pyrochlore-type oxide represented as "Nb2O6F(LLNOF)" is used. In LLNOF, Li is used as cation Aa, La as cation Ab, Nb as cation B, and F as anion X, with α=0.75, β=1, and γ=1.

[0049] The pyrochlore-type oxide of this embodiment is 1 × 10 -3 Ionic conductivity of S / cm or higher has been achieved. The pyrochlore-type oxide of this embodiment exhibits significantly higher ionic conductivity than other oxide-type solid electrolytes such as garnet-type oxides.

[0050] Figure 6 shows the method for producing pyrochlore-type oxide according to this embodiment. In the method for producing pyrochlore-type oxide, the first mixing step S10, the first calcination step S11, the second mixing step S12, the molding step S13, and the second calcination step S14 are carried out in order.

[0051] First, a lanthanum source, a lithium source, and a niobium source are prepared as raw materials for the pyrochlore-type oxide, and a first mixing step S10 is performed to mix them. Metal oxides and metal carbon oxides 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 predetermined ratios.

[0052] Next, the first calcination step S11 is performed to calcine the mixture prepared in the first mixing step. The first calcination step S11 consists of two stages. In the first stage, the mixture is calcined in air at 500°C for 6 hours. Calcination removes moisture and other substances from the mixture, thereby increasing its reactivity. Following the calcination, the mixture is calcined in air at 1200°C for 4 hours. This process yields Li, a precursor of the target product. 0.5 La 0.5 Nb2O6 is obtained.

[0053] Next, a second mixing step S12 is performed in which a fluorine source is prepared as a raw material and mixed with the precursor. Metal fluorides can be used as the fluorine source. In this embodiment, LiF and LaF3 are used as the fluorine source. LiF is both a fluorine source and a lithium source, and LaF3 is both 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 mixture of the precursor, LiF, and LaF3 is processed into pellets and subjected to a molding process S13 under pressure of 100 MPa. This process molds the mixture of the precursor, LiF, and LaF3 into pellets.

[0055] Next, a second calcination step S14 is performed in which the precursor, LiF, and LaF3 mixture is calcined. In the second calcination step S14, the precursor, LiF, and LaF3 mixture is heated at 1000°C for 6 hours under a nitrogen atmosphere. In the second calcination step S14, in order to suppress compositional shifts due to the volatilization of Li and F elements, calcination may be performed in a sealed state or in a state covered with mother powder.

[0056] By cooling the product from the second calcination process, the composition formula "Li 1.25 La 0.58 A pyrochlore-type oxide represented as "Nb2O6F(LLNOF)" is obtained. The resulting pyrochlore-type oxide is particulate.

[0057] By changing the mixing ratio of La2O3, Li2CO3, Nb2O5, LiF, and LaF3 in the above manufacturing process, "Li 2-α La (1+α) / 3 Nb2O 7-β F γ A pyrochlore-type solid electrolyte represented by can be obtained. By changing the mixing ratio of La2O3, Li2CO3, Nb2O5, LiF, and LaF3, α, β, and γ in the composition formula can be adjusted. In addition, a portion of the material sublimes during firing. Therefore, α, β, and γ can also be adjusted by changing the firing conditions, firing furnace atmosphere, and firing furnace size in the first and second firing processes.

[0058] Next, the manifestation of the electronic conductivity of the oxide-based ion conductor 14b will be explained using Figure 7. In the example shown in Figure 7, LiNi is used as the positive electrode active material 14a. 0.8 Co 0.1 Mn 0.1 Using O2 (NCM811), Li is used as the oxide-based ion conductor 14b. 1.25 La 0.58 Nb2O6F(LLNOF) is used.

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

[0060] In its initial state, the electronic conductivity of LLNOF is 8.3 × 10⁻⁶. -8 The value is S / cm, which is close to that of an insulator. Incidentally, the oxide solid electrolyte used in Patent Document 1 is Li 1+x Al x Ti 2-x The electronic conductivity of (PO4)3(LATP) is 8.5 × 10⁻⁶. -8 The value is S / cm. The initial state of LLNOF and the electronic conductivity of LATP were measured using pellets formed by sintering the respective powders.

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

[0062] The formation of this three-phase interface causes lithium ions to desorb from the positive electrode active material 14a, and these lithium ions are electrochemically inserted into the crystal 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 it is not necessarily required to change the electrode potential.

[0063] NCM811 is formed by the desorption of lithium ions, and its chemical formula is "LiNi 0.8 Co 0.1 Mn 0.1 From "O2" to "Li 1-α Ni 0.8 Co 0.1 Mn 0.1 It changes to "O2". LLNOF's compositional formula changes to "Li" due to lithium ion insertion. 1.25 La 0.58 From "Nb2O6F" to "Li 1.25+α La 0.58 The structure changes to "Nb2O6F". Upon lithium ion insertion into LLNOF, the Nb in LLNOF is reduced, and its oxidation state changes from "+5" to "5-δ+". LLNOF exhibits electronic conductivity through lithium ion insertion.

[0064] LLNOF has an electronic conductivity of 4.8 × 10⁻⁶ through lithium-ion insertion. -3 The conductivity improved to S / cm. In other words, the oxide-based ion conductor 14b of this embodiment exhibits high electronic conductivity in addition to high ionic conductivity through lithium ion insertion. The electronic conductivity of LLNOF after lithium ion insertion was measured after lithium ion insertion and deinsertion were performed with the potential of the LLNOF pellet set to 2.5V or less.

[0065] In this embodiment, the concentration of the electrolyte 15a is higher than 1M, which allows lithium ion insertion into the oxide-based ion conductor 14b to proceed effectively. Furthermore, in this embodiment, the use of a solvent with a high donor number also allows lithium ion insertion into the oxide-based ion conductor 14b to proceed effectively.

[0066] After injecting electrolyte 15a into the positive electrode 14, the secondary battery 10 is over-discharged to reduce the electrode potential to 2.5V or less, thereby improving the electronic conductivity of the oxide-based ion conductor 14b. By reducing the electrode potential to 2.5V or less, even in areas where the positive electrode active material 14a and the oxide-based ion conductor 14b are not in direct contact and a three-phase interface is not formed, lithium ions detached from the positive electrode active material 14a can be inserted into the oxide-based ion conductor 14b. As a result, the proportion of the oxide-based ion conductor 14b exhibiting electronic conductivity can be increased.

[0067] The lower the electrode potential, the more lithium ions are inserted into the oxide-based ion conductor 14b, and "Li 1.25+α La 0.58 The α of "Nb2O6F" increases. On the other hand, if the electrode potential is made too low, the oxide-based ion conductor 14b will degrade irreversibly. For this reason, it is desirable to perform the lithium ion insertion / deinsertion reaction into the oxide-based ion conductor 14b at an electrode potential of 0.5V or higher.

[0068] Figure 8 shows the charge-discharge characteristics of a battery cell containing an LLNOF. In the example shown in Figure 8, a battery cell composed of lithium metal, electrolyte, and LLNOF was used, and lithium ions were inserted into and removed from the LLNOF by changing the electrode potential. The potential V on the vertical axis of Figure 8 represents "V vs. Li + It is / Li」

[0069] In Figure 8, the downward-sloping solid line shows the capacitance when the potential is reduced to 0.5V, and the upward-sloping solid line shows the capacitance when the potential is increased after being reduced to 0.5V. In Figure 8, the downward-sloping dashed line shows the capacitance when the potential is reduced to 0.25V, and the upward-sloping dashed line shows the capacitance when the potential is increased after being reduced to 0.25V.

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

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

[0072] When the potential is reduced to 0.25V, the irreversible capacitance, which is the difference between the capacitance at the reduced potential and the capacitance at the increased potential, increases, indicating a greater degree of LLNOF degradation. 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 is reduced to 0.5V, the irreversible capacitance, which is the difference between the capacitance at the reduced potential and the capacitance at the increased potential, decreases, indicating a lesser degree of LLNOF degradation. Therefore, it is desirable to perform lithium ion insertion into the LLNOF at a potential of 0.5V or higher.

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

[0074] Next, the discharge characteristics of the secondary battery 10 when the type and particle size of the positive electrode active material 14a and oxide-based ion conductor 14b are different will be explained using the examples and comparative examples shown in Figure 9. In the example shown in Figure 9, a negative electrode 12 made of graphite was used, and an electrolyte 15 was used which was a solvent in which EC and DEC were mixed in a 1:1 ratio, and LiPF6 was added at a concentration higher than 1M.

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

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

[0077] In Examples 1-7 and 9, Li was used as the oxide-based ion conductor 14b. 1.25 La 0.58 Nb2O6F(LLNOF) was used, and in Example 8, La 0.57 Li 0.29 TiO3 (LLTO) was used, and 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 oxide-based ion conductor 14b is not provided on the positive electrode 14.

[0078] LLNOF in Examples 1-7 and 9, and LLTO in Example 7, are oxide-based ion conductors 14b into which lithium ions can be electrochemically inserted. LATP in Comparative Example 2 is an oxide-based ion conductor into which lithium ions cannot be electrochemically inserted.

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

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

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

[0082] As shown in Figure 9, when comparing Examples 1-8, which use NCM811 as the positive electrode active material 14a, with Comparative Examples 1 and 2, all of Examples 1-8, which use an oxide-based ion conductor 14b (LLNOF) into which lithium ions can be inserted, show discharge characteristics exceeding 100%. This is thought to be because the oxide-based ion conductor 14b exhibits electronic conductivity in addition to ionic conductivity, resulting in improved discharge characteristics. In contrast, Comparative Example 2, which uses an oxide-based ion (LATP) into which lithium ions cannot be inserted, shows discharge characteristics below 100%.

[0083] Examples 1 to 7 use the same combination of positive electrode active material 14a type and particle size and oxide-based ion conductor 14b type. 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 decreases. In particular, high discharge characteristics were obtained in Examples 1 to 6, where the particle size of positive electrode active material 14a > particle size of oxide-based ion conductor 14b.

[0084] Comparing Example 9, which uses LMFP as the positive electrode active material 14a, with Comparative Example 3, which does not have an oxide ion conductor 14b, the discharge characteristics were 100%, whereas in Example 9, which uses LLTO as the oxide ion conductor 14b, the discharge characteristics were 115%. In other words, in Example 9, the effect of improving discharge characteristics is obtained by using an oxide-based ion conductor 14b (LLTO) that can insert lithium ions.

[0085] According to the embodiment described above, with at least a portion of the oxide-based ion conductor 14b in contact with the positive electrode active material 14a, and with the presence of an electrolyte 15a between the positive electrode active material 14a and the oxide-based ion conductor 14b, the oxide-based ion conductor 14b is able to electrochemically insert 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 electron conduction paths in the positive electrode 14 can be increased, and the output of the secondary battery 10 can be improved. Furthermore, because the oxide-based ion conductor 14b possesses electronic conductivity, the amount of conductive additive used in the positive electrode 14 can be reduced.

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

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

[0088] Furthermore, according to this embodiment, the potential for lithium ion insertion and deinsertion into the oxide-based ion conductor 14b is set to 2.5V (vs. Li + By setting the ratio to less than or equal to / Li, the proportion of oxide-based ionic conductor 14b that exhibits electronic conductivity can be increased. This further improves the electronic conductivity of oxide-based ionic conductor 14b.

[0089] Furthermore, according to this embodiment, by setting the amount of lithium inserted into the oxide-based ion conductor 14b to 80% or less of the initially contained lithium amount, it is possible to suppress the potential for lithium ion insertion and deinsertion into the oxide-based ion conductor 14b from becoming excessively low, thereby suppressing irreversible degradation of the oxide-based ion conductor 14b.

[0090] (Other embodiments) This disclosure is not limited to the embodiments described above, and can be modified in various ways without departing from the spirit of this disclosure. Furthermore, the means disclosed in each of the embodiments described above may be combined as appropriate to the extent that they are feasible.

[0091] For example, in the above embodiment, an example was described in which the active material composite particles of this disclosure were applied to a lithium-ion battery in which the conductive ions are lithium ions, but they may also be applied to secondary batteries with different conductive ions. Specifically, the active material composite particles of this disclosure can be applied to potassium-ion batteries in which potassium ions conduct, sodium-ion batteries in which sodium ions conduct, and the like.

[0092] Furthermore, although the above embodiment describes an example in which the active material composite particles of the present disclosure are applied to a secondary battery 10 that is pre-equipped with a negative electrode 12, the active material composite particles of the present disclosure may also be applied to an anode-free battery. In an anode-free battery, the negative electrode 12 is not formed on the negative electrode current collector 11 in the initial state, and lithium metal is deposited on the negative electrode current collector 11 by lithium ions that move from the positive electrode 14 during charging, thereby forming the negative electrode 12. Then, the lithium metal constituting the negative electrode 12 moves to the positive electrode 14 as lithium ions during discharge.

[0093] Furthermore, the active material composite particles 140 of this disclosure may 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. That is, the current collector that contacts the positive electrode of one adjacent battery cell contacts the negative electrode of the other adjacent battery cell.

[0094] The characteristics of the secondary battery disclosed herein are as follows: (Item 1) Positive electrode (14), Negative electrode (12), The system comprises an electrolyte layer (15) having an electrolyte (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. A secondary battery in which the electrolyte is present between the positive electrode active material and the oxide-based ion conductor such that the conduction ions can be electrochemically inserted into the oxide-based ion conductor. (Item 2) The oxide-based ion conductor is a secondary battery as described in item 1, wherein the oxide-based ion conductor contains a halogen element in its crystal structure. (Item 3) The secondary battery according to item 1 or 2, wherein the oxide-based ion conductor is a mixed-electron ion conductor having ionic conductivity and electronic conductivity when the conduction ions are inserted. (Item 4) The aforementioned conducting ion is a lithium ion, The aforementioned oxide-based ionic conductor is 2.5V(vs.Li + A secondary battery according to any one of items 1 to 3, wherein the insertion of the conduction ions is performed at a potential of less than or equal to / Li. (Item 5) The secondary battery according to any one of items 1 to 4, wherein the amount of conduction ions that can be inserted into the oxide-based ion conductor is within 80% of the amount of conduction ions contained in the oxide-based ion conductor before the conduction ions are inserted. (Item 6) The aforementioned oxide-based ionic conductor is a pyrochlore-type oxide. The compositional formula of the pyrochlore-type oxide is Aa 2-α Ab (1+α) / 3 B2O 7-β X γ A secondary battery according to any one of items 1 to 5, wherein Aa is an alkali metal, Ab contains at least a lanthanide, B is a cation different from Aa and Ab, X is an anion that can be replaced by an O atom constituting the pyrochlore-type 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, γ is in the range of 0 < γ ≤ 1, and the secondary battery contains a defect structure. (Item 7) The secondary battery described in item 6, wherein the cation metal represented by B in the compositional formula of the pyrochlore-type oxide is Nb. (Item 8) The positive electrode active material is a secondary battery according to any one of items 1 to 7, wherein the particle size is larger than that of the oxide-based ion conductor. (Item 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 10 wt% or less. (Item 10) A secondary battery according to any one of items 1 to 9, wherein a three-phase interface is formed in the portion where the positive electrode active material and the oxide-based ion conductor are in contact, comprising the positive electrode active material, the oxide-based ion conductor, and the electrolyte. [Explanation of symbols]

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

Claims

1. Positive electrode (14) and, Negative electrode (12), The device comprises an electrolyte layer (15) having an electrolyte (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 electrolyte is present between the positive electrode active material and the oxide-based ion conductor such that the conduction ions can be electrochemically inserted into the oxide-based ion conductor. The oxide-based ion conductor is a pyrochlore-type oxide and is a secondary battery containing halogen elements in its crystal structure.

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

3. Positive electrode (14) and, Negative electrode (12), The device comprises an electrolyte layer (15) having an electrolyte (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 electrolyte is present between the positive electrode active material and the oxide-based ion conductor such that the conduction ions can be electrochemically inserted into the oxide-based ion conductor. The oxide-based ionic conductor is at least one of a pyrochlore-type oxide and a perovskite-type oxide. A secondary battery in which the amount of conduction ions that can be inserted into the oxide-based ion conductor is within 80% of the amount of conduction ions contained in the oxide-based ion conductor before the conduction ions are inserted.

4. The aforementioned conducting ion is a lithium ion, The aforementioned oxide-based ionic conductor is 2.5V (vs. Li ＋ The secondary battery according to any one of claims 1 to 3, wherein the insertion of the conduction ions is performed at a potential of less than or equal to / Li.

5. The compositional formula of the pyrochlore-type oxide is Aa ２－α Ab （１＋α）／３ B ２ O ７－β X γ A secondary battery according to any one of claims 1 to 3, wherein Aa is an alkali metal, Ab contains at least a lanthanide, B is a cation different from Aa and Ab, X is an anion that can be replaced with an O atom constituting the pyrochlore-type 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, γ is in the range of 0 < γ ≤ 1, and includes a defect structure.

6. The secondary battery according to claim 5, wherein the cation metal represented by B in the composition formula of the pyrochlore-type oxide is Nb.

7. The secondary battery according to any one of claims 1 to 3, wherein the positive electrode active material has a particle size larger than that of the oxide-based ion conductor.

8. The secondary battery according to any one of claims 1 to 3, wherein in the positive electrode, the weight ratio of the oxide-based ion conductor to the positive electrode active material is greater than 0 wt% and 10 wt% or less.

9. The secondary battery according to any one of claims 1 to 3, wherein a three-phase interface is formed in the portion where the positive electrode active material and the oxide-based ion conductor are in contact, comprising the positive electrode active material, the oxide-based ion conductor, and the electrolyte.

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