Positive electrode active material for sodium-ion secondary battery

Abstract

This invention provides a Ni-based positive electrode active material for sodium-ion secondary batteries with excellent discharge capacity. The positive electrode active material for sodium-ion secondary batteries is characterized by comprising the general formula Na... x (Ni 1－ a M a ) y P2O z (M is at least one transition metal element selected from Fe, Cr, Mn and Co, satisfying 0.6≤x≤4, 0.3≤y≤2.7, 0≤a≤0.9, 6≤z<7.5) as shown in the crystal.

Description

[0001] This case is filed on the date of application. March 23, 2018 Application number 201880027065.1 (PCT / JP2018 / 011579) The invention is entitled "Positive Electrode Active Material for Sodium-ion Secondary Batteries". Technical Field

[0002] This invention relates to positive electrode active materials for sodium-ion batteries used in portable electronic devices or electric vehicles. Background Technology

[0003] Lithium-ion secondary batteries have established themselves as an indispensable high-capacity and lightweight power source in portable electronic devices and electric vehicles. As their positive electrode active material, active materials containing olivine-type crystals, represented by the general formula LiFePO4, have attracted attention. However, due to concerns about the rising global price of lithium as a raw material, research has recently been conducted on sodium-ion secondary batteries using sodium as a substitute element, such as Na2FeP2O7 crystals (see, for example, Non-Patent Literature 1).

[0004] However, Fe-based crystals such as Na2FeP2O7 crystals have the problem of operating voltages as low as about 3V or less. On the other hand, Ni-based cathode active materials such as Na4Ni3(PO4)2(P2O7) crystals and NaNiPO4 crystals are known, and these Ni-based cathode active materials have operating voltages as high as 5V, thus improving energy density (for example, see Patent Document 1).

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent No. 5673836

[0008] Non-patent literature

[0009] Non-patent literature 1: Journal of the Ceramic Society of Japan 120[8]344-3462012 Summary of the Invention

[0010] The technical problem that the invention aims to solve

[0011] The aforementioned Ni-based positive electrode active material, due to the Ni during the initial charging... 2+ →Ni 3+ In the oxidation reaction, deoxygenation easily occurs from the active material, and Ni does not occur during discharge. 3+ →Ni 2+The reduction reaction results in a decrease in discharge capacity. Furthermore, when the aforementioned Ni-based cathode active material is used in all-solid-state batteries, it is difficult to form a Na-ion conduction pathway between the active material and the solid electrolyte, thus resulting in low discharge capacity.

[0012] In view of the above, the object of the present invention is to provide a Ni-based sodium-ion secondary battery positive electrode active material with excellent discharge capacity.

[0013] Means for solving technical problems

[0014] The inventors of this invention, through intensive research, discovered that the above-mentioned problems can be solved by using a positive electrode active material with a specific composition containing Ni, and thus proposed this invention.

[0015] That is, the positive electrode active material for sodium-ion secondary batteries of the present invention is characterized in that it comprises the general formula Na x (Ni 1－ a M a ) y P2O z (M is at least one transition metal element selected from Fe, Cr, Mn, and Co, satisfying 0.6≤x≤4, 0.3≤y≤2.7, 0≤a≤0.9, 6≤z<7.5) The crystals shown above indicate that, since the phosphoric acid forming the framework is mainly pyrophosphoric acid (P2O7) or metaphosphoric acid (PO3), the Ni... 2+ →Ni 3+ In oxidation reactions, deoxygenation from the active material is difficult to occur. Therefore, Ni readily undergoes oxidation during discharge. 3+ →Ni 2+ The reduction reaction results in a higher discharge capacity. Furthermore, when the aforementioned active material is used in an all-solid-state battery, a Na-ion conduction pathway is easily formed between the active material and the solid electrolyte, thus further increasing the discharge capacity.

[0016] Furthermore, the positive electrode active material of the present invention has a structure that is essentially composed of only crystals and contains no amorphous material. This results in the advantage of easily maintaining a high and constant redox potential during charging and discharging, and of easily increasing the energy density.

[0017] The positive electrode active material for sodium-ion secondary batteries of the present invention preferably has a structure in which crystals belong to the triclinic space group P1 or P-1.

[0018] The positive electrode active material for sodium-ion secondary batteries of the present invention preferably has a structure in which the crystals belong to the monoclinic space group P21 / c or Cm.

[0019] The positive electrode active material for sodium-ion secondary batteries of the present invention preferably has a structure with crystals belonging to the orthorhombic space group Pcca.

[0020] The positive electrode material for sodium-ion secondary batteries of the present invention is characterized in that it contains the above-mentioned active material for sodium-ion secondary batteries.

[0021] The positive electrode material for the sodium-ion secondary battery of the present invention may also contain a sodium-ion conductive solid electrolyte.

[0022] In the positive electrode material for sodium-ion secondary batteries of the present invention, the sodium-ion conductive solid electrolyte preferably contains β-alumina or NASICON (sodium superion conductor) crystals.

[0023] The positive electrode material for sodium-ion secondary batteries of the present invention preferably contains, by mass%, 30-100% of sodium-ion secondary battery positive electrode active material, 0-70% of sodium-ion conductive solid electrolyte, and 0-20% of conductive additive.

[0024] The positive electrode for sodium-ion secondary batteries of the present invention is characterized in that it uses the above-described positive electrode material for sodium-ion secondary batteries.

[0025] The sodium-ion secondary battery of the present invention is characterized in that it has the above-described positive electrode for sodium-ion secondary batteries.

[0026] Invention Effects

[0027] According to the present invention, a Ni-based sodium-ion secondary battery positive electrode active material with excellent discharge capacity can be provided. Detailed Implementation

[0028] (Positive electrode active material for sodium-ion secondary batteries)

[0029] The positive electrode active material for sodium-ion secondary batteries of the present invention is characterized in that it comprises the general formula Na x (Ni 1－a M a ) y P2O z (M is at least one transition metal element selected from Fe, Cr, Mn, and Co, satisfying 0.6≤x≤4, 0.3≤y≤2.7, 0≤a≤0.9, 6≤z<7.5) as shown in the crystal structure. The reason for specifying the crystal structure in this way will be explained below.

[0030] Na serves as the supply source for sodium ions that move between the positive and negative electrode active materials during charging and discharging. The range of x is 0.6 ≤ x ≤ 4, preferably 0.7 ≤ x ≤ 2, and particularly preferably 1 ≤ x ≤ 1.9. If x is too small, the number of Na ions participating in charging and discharging decreases, thus the discharge capacity tends to decrease. On the other hand, if x is too large, crystals such as Na3PO4 that do not participate in charging and discharging precipitate, thus the discharge capacity tends to decrease.

[0031] Ni and M, as transition metals, act as driving forces for the adsorption and release of sodium ions by inducing redox reactions through changes in their valence numbers during charging and discharging. The range of y is 0.3 ≤ y ≤ 2.7, preferably 0.4 ≤ y ≤ 2, and particularly preferably 0.7 ≤ y ≤ 1.3. When y is too small, fewer transition metal elements participate in charging and discharging, thus the discharge capacity tends to decrease. Conversely, when y is too large, NiO crystals that do not participate in charging and discharging may precipitate, thus the discharge capacity tends to decrease.

[0032] The range of a is 0 ≤ a ≤ 0.9, preferably 0 ≤ a ≤ 0.5 and 0 ≤ z ≤ 0.3, and particularly preferably a = 0. The smaller a is, the higher the redox potential, and therefore the battery's operating voltage tends to increase.

[0033] M is at least one transition metal element selected from Fe, Cr, Mn, and Co, with Co and Mn being preferred due to their high operating voltage. Fe is also preferred because it exhibits high structural stability during charge and discharge, resulting in improved cycle performance.

[0034] P2O z Because it forms a three-dimensional network structure, it has the effect of stabilizing the structure of the positive electrode active material. The range of z is 6 ≤ z < 7.5, preferably 6.3 ≤ z ≤ 7.3, and particularly preferably 6.7 ≤ z ≤ 7. If z is too small, the phosphoric acid component that does not participate in charging and discharging increases, thus the discharge capacity is prone to decrease. If z is too large, the framework component that forms the crystalline structure becomes the main body of orthophosphoric acid (PO4), therefore, deoxygenation easily occurs in the redox reaction of Ni accompanying charging and discharging, and as a result, the discharge capacity is prone to decrease.

[0035] Furthermore, x / y is preferably 0.4–10, 1.35–2, and particularly preferably 1.4–1.9. When x / y is too small or too large, the discharge capacity tends to decrease.

[0036] general formula Na x (Ni 1－a M a ) y P2O z The crystals shown preferably have any one of the following crystal structures: triclinic, monoclinic, or orthorhombic. More specifically, the general formula Na... x (Ni1－a M a ) y P2O z The crystals shown preferably have a structure belonging to the triclinic space group P1 or P-1, a structure belonging to the monoclinic space group P21 / c or Cm, or a structure belonging to the orthorhombic space group Pcca. In particular, structures belonging to the triclinic space group P1 or P-1 with excellent structural stability are preferred due to their superior discharge capacity. As specific examples of crystals, the following crystals can be cited (in parentheses, the general formula is normalized so that the coefficient of P becomes 2, and both the crystal structure and theoretical capacity are indicated).

[0037] Na4Ni5(PO4)2(P2O7)2(=Na 1.33 Ni 1.67 P2O 7.33 Monoclinic P21 / c, theoretical capacity 116mAh / g)

[0038] Na 3.64 Ni 2.18 (P₂O₇)₂(=Na) 1.82 Ni 1.09 P2O7, triclinic P-1 crystal, theoretical capacity 104 mAh / g)

[0039] Na 3.12 Ni 2.44 (P₂O₇)₂(=Na) 1.56 Ni 1.22 P2O7, triclinic P-1 crystal, theoretical capacity 116mAh / g)

[0040] Na 5.6 Ni4P8O 28 (=Na 1.4 NiP2O7, triclinic P-1 crystal, theoretical capacity 103 mAh / g)

[0041] Na2NiP2O7 (triclinic P-1 crystal, theoretical capacity 96mAh / g)

[0042] Na3Ni2P5O 16 (=Na 1.2 Ni 0.8 P2O 6.4 Theoretical capacity 90mAh / g

[0043] NaNi(PO3)3(=Na 0.67 Ni 0.67 P2O6, orthorhombic Pcca, theoretical capacity 84mAh / g)

[0044] Na4Ni(PO3)6(=Na 1.33 Ni 0.33P2O6, triclinic P-1 crystal, theoretical capacity 43 mAh / g)

[0045] Among them, Na 3.64 Ni 2.18 (P₂O₇)₂, Na 3.12 Ni 2.44 (P2O7)2 and Na2NiP2O7 are preferred due to their excellent discharge capacity, especially Na 3.64 Ni 2.18 (P2O7)2 is preferred due to its high capacity.

[0046] The positive electrode active material of the present invention can also be coated with conductive carbon or composited with conductive carbon. This increases electronic conductivity and facilitates improved high-speed charge-discharge characteristics. As the conductive carbon, highly conductive carbon blacks such as acetylene black and Ketjen black, carbon powders such as graphite, and carbon fibers can be used. Among these, acetylene black, which has high electronic conductivity, is preferred.

[0047] One method for coating positive electrode active materials with conductive carbon is to carbonize the organic compound by mixing the positive electrode active material with an organic compound that serves as a conductive carbon source and then firing it in an inert or reducing atmosphere. The organic compound can be any raw material that is a carbon residue left during the heat treatment process; glucose, citric acid, ascorbic acid, phenolic resin, and surfactants are preferred, with surfactants that readily adsorb onto the surface of the positive electrode active material being particularly preferred. The surfactant can be any of cationic, anionic, amphoteric, or nonionic surfactants, with nonionic surfactants that exhibit excellent adsorption properties onto the surface of the positive electrode active material being particularly preferred.

[0048] The mixing ratio of the positive electrode active material to the conductive carbon is preferably 80–99.5:0.5–20 by mass, more preferably 85–98:2–15. If the content of conductive carbon is too low, there is a tendency for decreased electronic conductivity. On the other hand, if the content of conductive carbon is too high, the content of the positive electrode active material is relatively reduced, thus there is a tendency for a decrease in discharge capacity.

[0049] Furthermore, when the surface of the positive electrode active material is coated with conductive carbon, the thickness of the conductive carbon film is preferably 1–100 nm, and particularly preferably 5–80 nm. If the thickness of the conductive carbon film is too small, the conductive carbon film will disappear during charging and discharging, and the battery characteristics will easily deteriorate. On the other hand, if the thickness of the conductive carbon film is too large, a decrease in discharge capacity and voltage is likely to occur.

[0050] The 1300-1400 cm⁻¹ region of the positive electrode active material for sodium-ion secondary batteries in this invention was determined by Raman spectroscopy. －1The peak intensity D relative to 1550–1650 cm⁻¹ －1 The ratio of peak intensity G (D / G) is preferably 1 or less, particularly preferably 0.8 or less, and the peak intensity is between 800 and 1100 cm⁻¹. －1 The ratio of peak intensity F to peak intensity G (F / G) is preferably 0.5 or less, particularly preferably 0.1 or less. When these peak intensity ratios meet the above range, the electronic conductivity of the positive electrode active material tends to increase.

[0051] The shape of the positive electrode active material for sodium-ion secondary batteries is not particularly limited, but it is preferred when it is in powder form because it provides more sites for sodium ion adsorption and release. In this case, the average particle size is preferably 0.1–20 μm, 0.3–15 μm, or 0.5–10 μm, and particularly preferably 0.6–5 μm. Furthermore, the maximum particle size is preferably 150 μm or less, 100 μm or less, or 75 μm or less, and particularly preferably 55 μm or less. If the average particle size or the maximum particle size is too large, the sites for sodium ion adsorption and release decrease during charging and discharging, thus tending to reduce the discharge capacity. On the other hand, if the average particle size is too small, the powder dispersion becomes poor during paste formation, making it difficult to manufacture a uniform electrode.

[0052] Here, the average particle size and the maximum particle size represent the median particle size D of the primary particles, respectively. 50 (50% cumulative diameter) and D 99 (99% cumulative volume diameter) refers to the value measured by a laser diffraction particle size distribution measuring device.

[0053] The positive electrode active material for sodium-ion secondary batteries of the present invention can be manufactured, for example, by a solid-state reaction method. Alternatively, conductivity can be imparted by adding conductive carbon to the obtained positive electrode active material while simultaneously pulverizing and mixing it. Examples of methods for simultaneous pulverization and mixing include using conventional pulverizers such as mortars, grinders, ball mills, attritors, vibratory ball mills, satellite ball mills, planetary ball mills, jet mills, and bead mills. Among these, a planetary ball mill is preferred. Planetary ball mills generate very high impact energy efficiently through the rotation of the grinding jar and the revolution of the base plate. Therefore, conductive carbon can be uniformly dispersed in the positive electrode active material, improving electronic conductivity.

[0054] Alternatively, the positive electrode active material can be mixed with an organic compound that serves as a conductive carbon source, and then fired in an inert or reducing atmosphere to carbonize the organic compound, thereby coating the positive electrode active material with conductive carbon.

[0055] (Positive electrode material for sodium-ion secondary batteries)

[0056] By mixing the positive electrode active material for sodium-ion secondary batteries of the present invention with conductive additives, binders, etc., a positive electrode material for sodium-ion secondary batteries can be obtained.

[0057] Examples of conductive additives include highly conductive carbon blacks such as acetylene black and Ketjen black, and conductive carbons in powder or fibrous form such as graphite. Among these, acetylene black, which can improve conductivity by adding a small amount, is preferred.

[0058] Adhesives are components added to bond the materials constituting the positive electrode material together, preventing the detachment of the positive electrode active material due to volume changes during charging and discharging. Specific examples of adhesives include: thermoplastic linear polymers such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluoropolymers, and styrene-butadiene rubber (SBR); thermosetting resins such as thermosetting polyimides, polyamide-imides, polyamides, phenolic resins, epoxy resins, urea-formaldehyde resins, melamine resins, unsaturated polyester resins, and polyurethanes; cellulose derivatives such as carboxymethyl cellulose (including carboxymethyl cellulose salts such as sodium carboxymethyl cellulose, hereinafter the same), hydroxypropyl methyl cellulose, hydroxypropyl cellulose, hydroxyethyl cellulose, ethyl cellulose, and hydroxymethyl cellulose; and water-soluble polymers such as polyvinyl alcohol, polyacrylamide, polyvinylpyrrolidone, and their copolymers. Among these, thermosetting resins, cellulose derivatives, and water-soluble polymers are preferred for their excellent adhesive properties, and more preferably, industrially widely used thermosetting polyimides or carboxymethyl cellulose. Carboxymethyl cellulose, in particular, is preferred due to its low cost and low environmental impact, as it does not require organic solvents when making electrode forming pastes. These binders can be used alone or in combination of two or more.

[0059] The positive electrode active material for sodium-ion secondary batteries of the present invention has a relatively high operating potential (e.g., 5V) during charge and discharge, making it suitable for solid-state sodium-ion secondary batteries using a solid electrolyte that does not decompose during charge and discharge. When using the positive electrode active material for sodium-ion secondary batteries of the present invention in solid-state sodium-ion secondary batteries, a sodium-ion conductive solid electrolyte is preferably added as a component of the positive electrode material. The sodium-ion conductive solid electrolyte is the component that carries sodium ion conduction between the positive and negative electrodes in an all-solid-state secondary battery. When the sodium-ion conductive solid electrolyte is β-alumina or NASICON crystal, it is preferred due to its excellent sodium ion conductivity. β-alumina exists in two crystalline forms: β-alumina (theoretical formula: Na₂O·11Al₂O₃) and β'⁻ aluminum oxide (theoretical formula: Na₂O·5.3Al₂O₃). Since β'⁻ aluminum oxide is metastable, it is commonly used with Li₂O and MgO added as stabilizers. Because β'⁻ aluminum oxide has a higher sodium ion conductivity than β-alumina, it is preferable to use β'⁻ aluminum oxide alone or a mixture of β'⁻ aluminum oxide and β-alumina, and more preferably to stabilize β'⁻ aluminum oxide (Na₂O·11Al₂O₃) with Li₂O. 1.6 Li 0.34 Al 10.66 O 17 ) or MgO stabilized β” alumina ((Al 10.32 Mg 0.68 O 16 (Na) 1.68 O)).

[0060] As a NASICON crystal, Na3Zr2Si2PO is preferred. 12 Na 3.2 Zr 1.3 Si 2.2 P 0.8 O 10.5 Na3Zr 1.6 Ti 0.4 Si2PO 12 Na3Hf2Si2PO 12 Na 3.4 Zr 0.9 Hf 1.4 Al 0.6 Si 1.2 P 1.8 O 12 Na3Zr 1.7 Nb 0.24 Si2PO 12 Na 3.6 Ti 0.2 Y 0.8 Si 2.8 O9, Na3Zr 1.88 Y0.12 Si2PO 12 Na5YSi4O 12 Na 3.12 Zr 1.88 Y 0.12 Si2PO 12 Na 3.6 Zr 0.13 Yb 1.67 Si 0.11 P 2.9 O 12 etc., especially Na 3.12 Zr 1.88 Y 0.12 Si2PO 12 Sodium is preferred due to its excellent ion conductivity.

[0061] The average particle size D of sodium ion-conducting solid electrolytes 50 The average particle size D of the sodium ion-conducting solid electrolyte is 0.3–25 μm, preferably 0.5–20 μm, and particularly preferably 1.2–15 μm. 50 When the particle size is too small, it is not only difficult to mix uniformly with the positive electrode active material, but also prone to decreased ion conductivity due to hygroscopicity or carbonation. As a result, internal resistance increases, and charge / discharge voltage and discharge capacity tend to decrease. On the other hand, the average particle size D of sodium ion-conductive solid electrolytes... 50 When the value is too large, it significantly hinders the softening and flow of the positive electrode active material during sintering to form the positive electrode layer. Therefore, there is a tendency for the smoothness of the obtained positive electrode layer to deteriorate, resulting in a decrease in mechanical strength or an increase in internal resistance.

[0062] The composition of the positive electrode material is preferably selected appropriately according to the type of electrolyte used. For example, in sodium-ion secondary batteries using aqueous or non-aqueous liquid electrolytes, the composition, by mass%, preferably contains 70-95% positive electrode active material, 1-15% conductive additive, and 3-15% binder; more preferably, it contains 80-95% positive electrode active material, 2-10% conductive additive, and 3-10% binder. If the content of positive electrode active material is too low, the discharge capacity of the sodium-ion secondary battery tends to decrease; if it is too high, the content of conductive additive and binder is relatively reduced, thus the electronic conductivity and cycle characteristics tend to decrease. If the content of conductive additive is too low, the electronic conductivity deteriorates; if it is too high, the adhesion between the components of the positive electrode material decreases, resulting in increased internal resistance, thus tending to decrease charge / discharge voltage and discharge capacity. If the content of binder is too low, the adhesion between the components of the positive electrode material decreases, and the cycle characteristics tend to decrease; if it is too high, the electronic conductivity decreases, thus the fast charge / discharge characteristics tend to decrease.

[0063] In the case of solid-state sodium-ion secondary batteries using sodium-ion conductive solid electrolytes as the electrolyte, the preferred composition (by mass%) is 30–100% positive electrode active material, 0–70% solid electrolyte, and 0–20% conductive additive; more preferably, it contains 34.5–94.5% positive electrode active material, 5–65% solid electrolyte, and 0.5–15% conductive additive; and even more preferably, it contains 40–92% positive electrode active material, 7–50% solid electrolyte, and 1–10% conductive additive. If the content of positive electrode active material is too low, the discharge capacity of the sodium-ion secondary battery tends to decrease. If the content of conductive additives or solid electrolyte is too high, the adhesion between the components of the positive electrode material decreases, thereby increasing the internal resistance. Therefore, the charge / discharge voltage and discharge capacity tend to decrease.

[0064] The components of the cathode material can be mixed using conventional pulverizers such as rotary mixers, drum mixers, mortars, grinders, ball mills, ultrafine grinders, vibratory ball mills, satellite ball mills, planetary ball mills, jet mills, and bead mills. In particular, the use of a planetary ball mill allows for the uniform dispersion of the constituent materials.

[0065] The positive electrode material for sodium-ion secondary batteries of the present invention can be used as a positive electrode for sodium-ion secondary batteries by coating it onto a current collector made of metal foil such as aluminum, copper, or gold and then drying it. Alternatively, after the positive electrode material for sodium-ion secondary batteries of the present invention is formed into a sheet, a current collector including a metal coating can be formed by sputtering, electroplating, or the like.

[0066] (Sodium-ion secondary battery)

[0067] In addition to the positive electrode for sodium-ion secondary batteries described above, the sodium-ion secondary battery of the present invention also has a negative electrode as the counter electrode and an electrolyte.

[0068] The negative electrode contains an active material capable of adsorbing and releasing sodium ions during charge and discharge. Examples of suitable active materials include metallic materials such as Na, Sn, Bi, Zn, Sn-Cu alloys, and Bi-Cu alloys; carbon materials such as hard carbon; and oxide materials containing Ti and / or Nb. Oxide materials containing Ti and / or Nb are preferred due to their high safety and abundant resources. Particularly preferred materials are those with a redox potential of 1.5V (vs. Na / Na) during charge and discharge. + Oxide materials with crystal phases such as Na4TiO(PO4)2 and Na5Ti(PO4)3 are used. In this case, the operating voltage of the sodium-ion secondary battery increases, and the precipitation of metallic Na dendritic crystals during repeated charge and discharge can be suppressed.

[0069] As the electrolyte, aqueous electrolytes, non-aqueous electrolytes, and solid electrolytes can be used. Non-aqueous electrolytes or solid electrolytes have a wider potential window and produce almost no gases during charging and discharging, thus improving the safety of sodium-ion secondary batteries. Among these, non-flammable solid electrolytes are preferred.

[0070] Aqueous electrolytes contain water-soluble electrolyte salts. Examples of electrolyte salts include NaNO3, Na2SO4, NaOH, NaCl, and CH3COONa. These electrolyte salts can be used alone or in combination of two or more. The concentration of the electrolyte salt is usually adjusted appropriately within the range of 0.1M to the saturation concentration.

[0071] Furthermore, when using an aqueous electrolyte, the redox potential of the positive electrode active material for the sodium-ion secondary battery of the present invention can be used as long as it is within the potential window of water.

[0072] Non-aqueous electrolytes include organic solvents and / or ionic liquids that serve as non-aqueous solvents, and electrolyte salts dissolved in such non-aqueous solvents. There are no particular limitations on the organic solvents that serve as non-aqueous solvents, but examples include propylene carbonate (PC), ethylene carbonate (EC), 1,2-dimethoxyethane (DME), γ-butyrolactone (GBL), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeHF), 1,3-dioxolane, sulfolane, acetonitrile (AN), diethyl carbonate (DEC), dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and dipropyl carbonate (DPC). These non-aqueous solvents can be used alone or in mixtures of two or more. Propylene carbonate, which exhibits excellent low-temperature properties, is preferred.

[0073] Ionic liquids are not particularly limited in their ability to dissolve the electrolyte salts used. Specific examples include: aliphatic quaternary ammonium salts such as N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide [TMPA-TFSI], N-methyl-N-propylpiperidine bis(trifluoromethanesulfonyl)imide [PP13-TFSI], N-methyl-N-propylpyrrolidine bis(trifluoromethanesulfonyl)imide [P13-TFSI], and N-methyl-N-butylpyrrolidine bis(trifluoromethanesulfonyl)imide [P14-TFSI]; 1-methyl-3-ethylimidazolium tetrafluoroborate [EMIBF4], 1-methyl... Alkyl imidazolium quaternary salts, such as 1-allyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)imide [abbreviation: EMITFSI], 1-allyl-3-ethylimidazolium bromide [abbreviation: AEImBr], 1-allyl-3-ethylimidazolium tetrafluoroborate [abbreviation: AEImBF4], 1-allyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)imide [abbreviation: AEImTFSI], 1,3-diallyl imidazolium bromide [abbreviation: AAImBr], 1,3-diallyl imidazolium tetrafluoroborate [abbreviation: AAImBF4], and 1,3-diallyl imidazolium bis(trifluoromethanesulfonyl)imide [abbreviation: AAImTFSI].

[0074] As an electrolyte salt, examples include: PF6 － BF4 － (CF3SO2)2N － (bis(trifluoromethanesulfonylimide); commonly known as TFSI), CF3SO3 － (commonly known as TFS), (C2F5SO2)2N － (bis(pentafluoroethanesulfonamide; commonly known as BETI), ClO4 － AsF6 － SbF6 － bis(oxalato)boronic acid (B(C2O4)2) － Commonly known as BOB), difluoro(trifluoro-2-oxo-2-trifluoro-methylpropionate (2-)-0,0) boric acid (BF2OCOOC(CF3)3) － Sodium salts of electrolytes, such as HHIB (H2O2), are commonly referred to as B. These electrolyte salts can be used alone or in mixtures of two or more. PF6 is particularly preferred due to its low cost. － BF4 － Sodium salts. The concentration of electrolyte salts is usually adjusted appropriately within the range of 0.5 to 3 M.

[0075] In addition, non-aqueous electrolytes may also contain additives such as vinylene carbonate (VC), vinyl acetate (VA), vinyl butyrate, vinyl hexanoate, vinyl crotonate, and catechol carbonate. These additives have the function of forming a protective film on the surface of the active material. The concentration of the additives is preferably 0.1 to 3 parts by mass relative to 100 parts by mass of the non-aqueous electrolyte, and particularly preferably 0.5 to 1 part by mass.

[0076] The aforementioned solid electrolytes can be used as solid electrolytes. Since solid electrolytes have a wider potential window than aqueous and non-aqueous electrolytes, and produce almost no gas during decomposition, they can improve the safety of sodium-ion secondary batteries.

[0077] In the case of sodium-ion secondary batteries using aqueous or non-aqueous electrolytes, a separator is preferably provided between the electrodes. The separator is made of an insulating material, and specifically, porous membranes or nonwoven fabrics made of polymers such as polyolefins, cellulose, polyethylene terephthalate, and vinylon, glass nonwoven fabrics containing fibrous glass, glass cloth woven from fibrous glass, and membrane glass can be used.

[0078] Example

[0079] The present invention will now be described in detail based on embodiments, but the present invention is not limited to any of the following embodiments.

[0080] Tables 1 and 2 represent the examples (No. 1-4, 6-8) and the comparative example (No. 5).

[0081] [Table 1]

[0082]

[0083] [Table 2]

[0084]

[0085] Electrolyte-based sodium-ion secondary batteries

[0086] (1) Preparation of positive electrode active material

[0087] Sodium carbonate, sodium metaphosphate, nickel oxide, and orthophosphoric acid were weighed according to the compositions listed in Tables 1 (Nos. 1-5) to prepare a raw material batch. The raw material batch was mixed in ethanol using a planetary ball mill and then dried at 100°C. The dried raw material batch was degassed by pre-calcining at 900°C for 6 hours in an electric furnace. The pre-calcined raw material batch was then subjected to a process at 500 kgf / cm³. 2The material is pressurized and molded, then fired at 800°C for 12 hours in an atmospheric atmosphere to undergo a solid-phase reaction. The resulting solid-phase reactant is then subjected to a 12-hour period of use. ZrO2 granules were pulverized in a ball mill and then subjected to air classification to obtain an average particle size D. 50 It is a 2μm powdery solid reactant.

[0088] Relative to 100 parts by mass of the powdered solid reactant obtained above, 21.4 parts by mass (equivalent to 12 parts by mass of carbon equivalent) of polyethylene oxide nonylphenyl ether (HLB value: 13.3, mass-average molecular weight: 660) as a carbon source and 10 parts by mass of ethanol as a nonionic surfactant were thoroughly mixed, and then dried at 100°C for about 1 hour. Then, the nonionic surfactant was carbonized by calcination at 650°C for 1 hour under a nitrogen atmosphere to obtain a positive electrode active material powder with a carbon-coated surface.

[0089] The crystal structure of the obtained positive electrode active material powder was identified by powder X-ray diffraction and Rietveld analysis.

[0090] (2) Fabrication of the positive electrode for electrolyte-based batteries

[0091] Relative to the positive electrode active material powder obtained above, acetylene black (Super C65 manufactured by Timcal Corporation) as the conductive additive and polyvinylidene fluoride as the binder were weighed in a ratio of positive electrode active material powder: conductive additive: binder = 90:5:5 (mass ratio), dispersed in N-methylpyrrolidone, and then thoroughly stirred using a rotation / revolution mixer to make it into a slurry to obtain the positive electrode material.

[0092] Next, using a scraper with a 125 μm gap, the obtained positive electrode material was coated onto a 20 μm thick aluminum foil, which served as the positive electrode current collector. After vacuum drying in a dryer at 70°C, the foil was extruded between a pair of rotating rollers to obtain an electrode sheet. Using an electrode stamping machine, the electrode sheet was stamped to a diameter of 11 mm and dried at 150°C under reduced pressure for 8 hours to obtain a spherical positive electrode.

[0093] (3) Fabrication of the test battery

[0094] The positive electrode obtained above was placed on the lower cover of a coin cell with the aluminum foil side facing down. A 16 mm diameter polypropylene porous membrane (dried under reduced pressure at 70°C for 8 hours), metallic sodium as the counter electrode, and the upper cover of the coin cell were then stacked on top to create a test cell. A 1M NaPF6 solution with an EC:DEC ratio of 1:1 (EC = ethylene carbonate, DEC = diethyl carbonate) was used as the electrolyte. Furthermore, the assembly of the test cell was carried out in an environment with a dew point temperature below -70°C.

[0095] (4) Charge and discharge test

[0096] A constant-current (CC) charge was performed from the open-circuit voltage to 5.2V at 30°C, and the charge amount per unit mass of the positive electrode active material (initial charge capacity) was calculated. Then, a constant-current (CC) discharge was performed from 5.2V to 2V, and the charge amount discharged per unit mass of the positive electrode active material (initial discharge capacity) was calculated. Furthermore, the charge rate was set to 0.1C. The results are shown in Table 1.

[0097] As shown in Table 1, in Examples No.1 to 4, the discharge capacity was 32 to 38 mAh / g. In contrast, in Comparative Example No.5, the discharge capacity was worse, at 21 mAh / g.

[0098] All-solid sodium-ion secondary battery

[0099] (1) Preparation of positive electrode active material

[0100] For items No. 1 to 3 and 5 in Table 1, the positive electrode active material powder prepared above is used. For items No. 6 to 8 in Table 2, appropriate raw material mixtures are prepared by selecting and weighing sodium carbonate, sodium metaphosphate, nickel oxide, manganese oxide, iron oxide, cobalt oxide, and orthophosphoric acid in a manner that results in the composition described in Table 2. Otherwise, the positive electrode active material powder is prepared in the same manner as for items No. 1 to 3 and 5.

[0101] (2) Preparation of sodium ion conductive solid electrolyte

[0102] The composition formula Na 1.6 Li 0.34 Al 10.66 O 17 The blocky Li₂O-stabilized β” alumina (manufactured by Ionotec) was dry-milled to a thickness of 0.2 mm to obtain a solid electrolyte sheet. Separately, the blocky Li₂O-stabilized β” alumina was pulverized using a planetary ball mill and passed through a 10 μm sieve to prepare a separate solid electrolyte powder (average particle size D). 50 =1.5μm).

[0103] (3) Fabrication of the test battery

[0104] The obtained positive electrode active material powder, solid electrolyte powder, and acetylene black (Timcal Super C65) as a conductive additive were weighed separately in a ratio of 72:25:3 and mixed for 30 minutes using an agate mortar and pestle. 15 parts by weight of polypropylene carbonate were added to 100 parts by weight of the resulting mixed powder, followed by 30 parts by weight of N-methylpyrrolidone. The mixture was thoroughly stirred using a rotary / revolutionary mixer to form a slurry.

[0105] The obtained slurry is divided into sections with an area of ​​1 cm². 2 A 70 μm thick coating is applied to one surface of the solid electrolyte sheet obtained above and dried at 70°C for 3 hours. Then, it is pre-fired in nitrogen at 350°C for 1 hour. Subsequently, a sintered body (positive electrode layer) of positive electrode material is formed on the surface of the solid electrolyte sheet by firing in Ar at 650°C for 10 minutes and 50 MPa using a hot isostatic pressing apparatus.

[0106] When confirming the powder X-ray diffraction pattern of the materials constituting the positive electrode layer, diffraction lines originating from crystals, as described in Tables 1 and 2, were identified. Furthermore, crystalline diffraction lines originating from each of the solid electrolyte powders used were identified in all positive electrode layers.

[0107] Next, using a sputtering apparatus (SANYU Electronics Co., Ltd. SC-701AT), a current collector with a thickness of 300 nm, consisting of gold electrodes, was formed on the surface of the positive electrode layer. Then, in an argon atmosphere with a dew point below -70°C, metallic sodium, which would serve as the counter electrode, was pressed onto the surface of the solid electrolyte layer opposite to the surface where the positive electrode layer was formed. The resulting laminate was placed on the lower cover of a coin cell, and then the upper cover was placed on top to fabricate a CR2032 type experimental battery.

[0108] (4) Charge and discharge test

[0109] For the prepared test battery, a constant current (CC) charge was performed at 60°C from the open-circuit voltage to 5.2V, and the amount of charge per unit mass of positive electrode active material was calculated (initial charge capacity). Then, a constant current (CC) discharge was performed from 5.2V to 2V, and the amount of charge released per unit mass of positive electrode active material was calculated (initial discharge capacity). Furthermore, in this experiment, the C-rate was set to 0.01C. The results are shown in Tables 1 and 2.

[0110] As shown in Tables 1 and 2, the discharge capacity of Examples No. 1 to 3 and 6 to 8 is 28 to 51 mAh / g, while the discharge capacity of Comparative Example No. 5 is as low as 13 mAh / g.

[0111] Industrial availability

[0112] The positive electrode active material for sodium-ion secondary batteries of the present invention is suitable for use in sodium-ion secondary batteries used in portable electronic devices, electric vehicles, power tools, backup emergency power supplies, etc.

Claims

1. A positive electrode active material for sodium-ion secondary batteries, characterized in that: Including general formula Na x (Ni 1－a M a ) y P2O z The crystal shown is wherein M is at least one transition metal element selected from Cr, Mn and Co, and satisfies 0.6≤x≤1.9, 0.3≤y≤2.7, 0≤a≤0.9, 6≤z<7.5, x / y=1.35~2, and the positive electrode active material for the sodium-ion secondary battery does not contain amorphous material.

2. A positive electrode active material for sodium-ion secondary batteries, characterized in that: Including general formula Na x (Ni 1－a M a ) y P2O z The crystal shown is wherein M is at least one transition metal element selected from Cr, Mn and Co, and satisfies 0.6≤x≤4, 0.3≤y≤2.7, 0.1≤a≤0.9, 6≤z<7.5, x / y=1.35~2, and the positive electrode active material for the sodium-ion secondary battery does not contain amorphous material.

3. The sodium-ion secondary battery positive electrode active material as described in claim 1 or 2, characterized in that: The crystal has a structure belonging to the triclinic space group P1 or P-1.

4. The sodium-ion secondary battery positive electrode active material as described in claim 1 or 2, characterized in that: The crystal has a structure belonging to the monoclinic space group P21 / c or Cm.

5. The sodium-ion secondary battery positive electrode active material as described in claim 1 or 2, characterized in that: The crystal has a structure belonging to the orthorhombic space group Pcca.

6. A positive electrode material for sodium-ion secondary batteries, characterized in that: The active material for a sodium-ion secondary battery as described in any one of claims 1 to 5 is contained in the active material for the positive electrode.

7. The positive electrode material for sodium-ion secondary batteries as described in claim 6, characterized in that: It contains a sodium ion-conductive solid electrolyte.

8. The positive electrode material for sodium-ion secondary batteries as described in claim 7, characterized in that: The sodium ion-conducting solid electrolyte contains β-alumina or NASICON crystals.

9. The positive electrode material for sodium-ion secondary batteries as described in claim 7 or 8, characterized in that: It contains, by mass%, 30-100% of the positive electrode active material for sodium-ion secondary batteries, 0-70% of the sodium-ion conductive solid electrolyte, and 0-20% of the conductive additive.

10. A positive electrode for a sodium-ion secondary battery, characterized in that: The positive electrode material for sodium-ion secondary batteries according to any one of claims 6 to 9.

11. A sodium-ion secondary battery, characterized in that: A positive electrode for a sodium-ion secondary battery as described in claim 10.

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