阅读说明：本技术 钠离子二次电池用正极活性物质 (Positive electrode active material for sodium ion secondary battery ) 是由 山内英郎 于 2018-03-23 设计创作，主要内容包括：本发明提供一种放电容量优异的Ni系的钠离子二次电池用正极活性物质。钠离子二次电池用正极活性物质的特征在于,包括通式Na<Sub>x</Sub>(Ni<Sub>1－</Sub><Sub>a</Sub>M<Sub>a</Sub>)<Sub>y</Sub>P<Sub>2</Sub>O<Sub>z</Sub>(M为选自Fe、Cr、Mn和Co中的至少一种过渡金属元素,满足0.6≤x≤4、0.3≤y≤2.7、0≤a≤0.9、6≤z＜7.5)所示的结晶。(The present invention provides a positive electrode active material for a Ni-based sodium ion secondary battery, which has excellent discharge capacity. The positive electrode active material for a sodium ion secondary battery is characterized by comprising a crystal represented by the general formula Nax (Ni 1-aMa) yP2Oz (M is at least one transition metal element selected from the group consisting of Fe, Cr, Mn and Co, and satisfies the conditions of 0.6. ltoreq. x.ltoreq.4, 0.3. ltoreq. y.ltoreq.2.7, 0. ltoreq. a.ltoreq.0.9 and 6. ltoreq. z.ltoreq.7.5).)

1. A positive electrode active material for a sodium ion secondary battery, characterized in that:

Comprises a crystal shown as a general formula Nax (Ni 1-aMa) yP2Oz, wherein M is at least one transition metal element selected from Fe, Cr, Mn and Co, and satisfies the conditions that x is more than or equal to 0.6 and less than or equal to 4, y is more than or equal to 0.3 and less than or equal to 2.7, a is more than or equal to 0 and less than or equal to 0.9, and z is more than or equal to 6 and less than or equal to 7.5.

2. the positive electrode active material for sodium-ion secondary batteries according to claim 1, characterized in that: the crystal has a structure belonging to triclinic space group P1 or P-1.

3. The positive electrode active material for sodium-ion secondary batteries according to claim 1, characterized in that: the crystals have a structure belonging to the monoclinic space group P21/c or Cm.

4. The positive electrode active material for sodium-ion secondary batteries according to claim 1, characterized in that: the crystal has a structure belonging to an orthorhombic space group Pcca.

5. A positive electrode material for a sodium ion secondary battery, characterized in that:

A positive electrode active material for a sodium-ion secondary battery, which comprises the positive electrode active material according to any one of claims 1 to 4.

6. The positive electrode material for sodium-ion secondary batteries according to claim 5, characterized in that:

Contains a sodium ion conductive solid electrolyte.

7. The positive electrode material for sodium-ion secondary batteries according to claim 6, characterized in that:

the sodium ion-conductive solid electrolyte contains beta alumina or NASICON crystals.

8. The positive electrode material for sodium-ion secondary batteries according to any one of claims 5 to 7, wherein:

The positive electrode active material for a sodium ion secondary battery comprises 30 to 100% by mass of the positive electrode active material, 0 to 70% by mass of the sodium ion conductive solid electrolyte, and 0 to 20% by mass of a conductive auxiliary agent.

9. 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 5 to 8 is used.

10. A sodium ion secondary battery characterized in that:

A positive electrode for a sodium-ion secondary battery according to claim 9.

Technical Field

The present invention relates to a positive electrode active material for a sodium ion battery used for a portable electronic device, an electric vehicle, or the like.

Background

Lithium ion secondary batteries have established an essential role as a high-capacity and lightweight power source in portable electronic terminals, electric vehicles, and the like, and as a positive electrode active material thereof, an active material containing olivine-type crystals represented by the general formula LiFePO4 has attracted attention. However, in recent years, studies have been made on sodium ion secondary batteries using Na2FeP2O7 crystal or the like, which has sodium as an alternative element to lithium, because of concerns about problems such as an increase in the raw material price of lithium in the global sense (for example, see non-patent document 1).

However, Fe-based crystals such as Na2FeP2O7 crystals have a problem that the operating voltage is as low as about 3V or less. On the other hand, Ni-based positive electrode active materials such as Na4Ni3(PO4)2(P2O7) crystal and NaNiPO4 crystal are known, and since these Ni-based positive electrode active materials have an operating voltage as high as 5V, the energy density can be improved (for example, see patent document 1).

Disclosure of Invention

technical problem to be solved by the invention

The above-mentioned Ni-based positive electrode active material is susceptible to deoxidation from the active material in an oxidation reaction of Ni2+ → Ni3+ during initial charging, and therefore, there is a problem that a reduction reaction of Ni3+ → Ni2+ does not occur during discharging, and as a result, the discharge capacity is reduced. Further, when the above-mentioned Ni-based positive electrode active material is applied to an all-solid-state battery, it is difficult to form a Na ion conduction path between the active material and a solid electrolyte, and therefore, there is a problem that the discharge capacity is low as a result.

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

Means for solving the problems

As a result of intensive studies, the inventors of the present invention have found that the above problems can be solved by a positive electrode active material having a specific composition and containing a Ni component, and have proposed the present invention.

That is, the positive electrode active material for a sodium ion secondary battery of the present invention is characterized by comprising crystals represented by the general formula Nax (Ni 1-aMa) yP2Oz (M is at least one transition metal element selected from the group consisting of Fe, Cr, Mn and Co, and satisfies 0.6. ltoreq. x.ltoreq.4, 0.3. ltoreq. y.ltoreq.2.7, 0. ltoreq. a.ltoreq.0.9, and 6. ltoreq. z.ltoreq.7.5). According to the above crystals, since phosphoric acid forming the skeleton is mainly pyrophosphoric acid (P2O7) or metaphosphoric acid (PO3), deoxidation from the active material is difficult to occur in the oxidation reaction of Ni2+ → Ni3+ which is primarily charged. Therefore, a reduction reaction of Ni3+ → Ni2+ easily occurs at the time of discharge, and as a result, the discharge capacity easily increases. When the above-described active material is applied to an all-solid battery, a Na ion conduction path is easily formed between the active material and a solid electrolyte, and thus the discharge capacity is easily increased.

The positive electrode active material of the present invention basically has a structure composed of only crystals and containing no amorphous substance. This has the advantage that the oxidation-reduction potential associated with charge and discharge is easily maintained at a high potential, and the energy density is easily increased.

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

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

The positive electrode active material for a sodium-ion secondary battery of the present invention preferably has a structure in which crystals belong to an orthorhombic space group Pcca.

The positive electrode material for a sodium-ion secondary battery of the present invention is characterized by containing the above-described positive electrode active material for a sodium-ion secondary battery.

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

In the positive electrode material for a sodium ion secondary battery of the present invention, the sodium ion conductive solid electrolyte preferably contains beta alumina or NASICON (sodium super ion conductor) crystals.

The positive electrode material for a sodium ion secondary battery of the present invention preferably contains 30 to 100% by mass of a positive electrode active material for a sodium ion secondary battery, 0 to 70% by mass of a sodium ion conductive solid electrolyte, and 0 to 20% by mass of a conductive auxiliary agent.

The positive electrode for a sodium-ion secondary battery of the present invention is characterized by using the above-described positive electrode material for a sodium-ion secondary battery.

The sodium ion secondary battery of the present invention is characterized by having the positive electrode for a sodium ion secondary battery.

Effects of the invention

The present invention can provide a Ni-based positive electrode active material for sodium ion secondary batteries, which has excellent discharge capacity.

Detailed Description

(Positive electrode active Material for sodium ion Secondary Battery)

The positive electrode active material for a sodium ion secondary battery is characterized by comprising crystals represented by the general formula Nax (Ni 1-aMa) yP2Oz (M is at least one transition metal element selected from Fe, Cr, Mn and Co, and satisfying the conditions that x is 0.6-4, y is 0.3-2.7, a is 0-0.9 and z is 6-7.5). The reason why the crystal composition is defined in this manner will be described below.

Na serves as a supply source of sodium ions that move between the positive electrode active material and the negative electrode active material during charge and discharge. x is in the range 0.6. ltoreq. x.ltoreq.4, preferably 0.7. ltoreq. x.ltoreq.2, particularly preferably 1. ltoreq. x.ltoreq.1.9. When x is too small, Na ions involved in charge and discharge are reduced, and therefore, the discharge capacity is liable to decrease. On the other hand, when x is too large, crystals such as Na3PO4 that do not participate in charge and discharge are precipitated, and therefore, the discharge capacity is likely to decrease.

Ni and M, which are transition metals, cause oxidation-reduction reactions due to a change in valence number during charge and discharge, and thus act as driving forces for the storage and release of sodium ions. y is in the range 0.3. ltoreq. y.ltoreq.2.7, preferably 0.4. ltoreq. y.ltoreq.2, particularly preferably 0.7. ltoreq. y.ltoreq.1.3. When y is too small, the transition metal element involved in charge and discharge is reduced, and therefore, the discharge capacity is liable to decrease. On the other hand, when y is too large, NiO crystals and the like which do not participate in charge and discharge are precipitated, and thus the discharge capacity is liable to be lowered.

a is in the range of 0. ltoreq. a.ltoreq.0.9, preferably 0. ltoreq. a.ltoreq.0.5, 0. ltoreq. z.ltoreq.0.3, and particularly preferably a. ltoreq.0. The smaller a, the higher the oxidation-reduction potential, and therefore the operating voltage of the battery tends to increase.

M is at least one transition metal element selected from the group consisting of Fe, Cr, Mn and Co, and Co and Mn are preferred because of their high operating voltage. In addition, Fe is preferable because it has a high structural stability during charge and discharge, and therefore, the cycle characteristics are improved.

p2Oz has an effect of stabilizing the structure of the positive electrode active material because it forms a three-dimensional network structure. z is in the range 6. ltoreq. z < 7.5, preferably 6.3. ltoreq. z < 7.3, particularly preferably 6.7. ltoreq. z < 7. When z is too small, the amount of phosphoric acid component not involved in charge and discharge increases, and thus the discharge capacity tends to decrease. On the other hand, when z is too large, the skeleton component forming the crystal structure becomes mainly orthophosphoric acid (PO4), and therefore, deoxidation easily occurs in the oxidation-reduction reaction of Ni accompanying charge and discharge, and as a result, the discharge capacity is easily decreased.

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

The crystal represented by the general formula Nax (Ni 1-aMa) yP2Oz preferably has any one of a triclinic crystal, a monoclinic crystal and an orthorhombic crystal structure. More specifically, the crystal represented by the general formula Nax (Ni 1-aMa) yP2Oz preferably has 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, a structure belonging to the triclinic space group P1 or P-1, which has excellent structural stability, is preferable because of its excellent discharge capacity. Specific examples of the crystals include the following crystals (in parentheses, the general formula normalized so that the coefficient of P becomes 2 is shown, and the crystal structure and the theoretical capacity are shown).

Na4Ni5(PO4)2(P2O7)2(═ Na1.33ni1.67p2o7.33, monoclinic P21/c, theoretical capacity 116mAh/g)

Na3.64Ni2.18(P2O7)2(═ Na1.82Ni1.09P2O7, triclinic P-1, theoretical capacity 104mAh/g)

Na3.12Ni2.44(P2O7)2(═ Na1.56Ni1.22P2O7, triclinic P-1, theoretical capacity 116mAh/g)

Na5.6Ni4P8O28(═ Na1.4NiP2O7, triclinic P-1, theoretical capacity 103mAh/g)

Na2NiP2O7 (triclinic P-1, theoretical capacity 96mAh/g)

Na3Ni2P5O16(═ Na1.2Ni0.8P2O6.4, theoretical capacity 90mAh/g)

NaNi (PO3)3(═ na0.67ni0.67p2o6, orthorhombic Pcca, theoretical capacity 84mAh/g)

Na4Ni (PO3)6(═ Na1.33ni0.33p2o6, triclinic P-1, theoretical capacity 43mAh/g)

among them, Na3.64Ni2.18(P2O7)2, Na3.12Ni2.44(P2O7)2 and Na2NiP2O7 are preferable because of their excellent discharge capacity, and Na3.64Ni2.18(P2O7)2 is particularly preferable because of their high capacity.

The positive electrode active material of the present invention may be coated with conductive carbon or may be compounded with conductive carbon. This increases the electronic conductivity, and thus the high-rate charge/discharge characteristics are easily improved. As the conductive carbon, highly conductive carbon black such as acetylene black and ketjen black, carbon powder such as graphite, carbon fiber, and the like can be used. Among them, acetylene black having high electron conductivity is preferable.

Examples of the method for coating the positive electrode active material with conductive carbon include: a method in which the positive electrode active material and an organic compound as a conductive carbon source are mixed and then fired in an inert or reducing atmosphere to carbonize the organic compound. As the organic compound, any organic compound can be used as long as it remains as carbon during the heat treatment, and glucose, citric acid, ascorbic acid, phenol resin, a surfactant, and the like are preferably used, and a surfactant which is easily adsorbed on the surface of the positive electrode active material is particularly preferable. The surfactant may be any of a cationic surfactant, an anionic surfactant, an amphoteric surfactant, and a nonionic surfactant having excellent adsorption property to the surface of the positive electrode active material is particularly preferable.

The mixing ratio of the positive electrode active material to the conductive carbon is preferably 80 to 99.5: 0.5 to 20, and more preferably 85 to 98: 2 to 15 in terms of mass ratio. When the content of the conductive carbon is too small, the electron conductivity tends to be poor. On the other hand, when the content of the conductive carbon is too large, the content of the positive electrode active material is relatively decreased, and thus the discharge capacity tends to be decreased.

When the surface of the positive electrode active material is coated with conductive carbon, the thickness of the conductive carbon coating is preferably 1 to 100nm, and particularly preferably 5 to 80 nm. When the thickness of the conductive carbon coating is too small, the conductive carbon coating disappears during charge and discharge, and battery characteristics are likely to be deteriorated. On the other hand, when the thickness of the conductive carbon coating is too large, a decrease in discharge capacity, a voltage drop, and the like are likely to occur.

In the Raman spectroscopy of the positive electrode active material for a sodium ion secondary battery of the present invention, the ratio (D/G) of the peak intensity D of 1300 to 1400 cm-1 to the peak intensity G of 1550 to 1650 cm-1 is preferably 1 or less, and particularly preferably 0.8 or less, and the ratio (F/G) of the peak intensity F to the peak intensity G of 800 to 1100 cm-1 is preferably 0.5 or less, and particularly preferably 0.1 or less. When these peak intensity ratios satisfy the above ranges, the electron conductivity of the positive electrode active material tends to be increased.

The shape of the positive electrode active material for a sodium ion secondary battery is not particularly limited, and when it is in a powder form, it is preferable because sites for storing and releasing sodium ions increase. In this case, the average particle diameter is preferably 0.1 to 20 μm, 0.3 to 15 μm, 0.5 to 10 μm, and particularly preferably 0.6 to 5 μm. The maximum particle size is preferably 150 μm or less, 100 μm or less, 75 μm or less, and particularly preferably 55 μm or less. If the average particle diameter or the maximum particle diameter is too large, the sites where sodium ions are stored and released during charge and discharge are reduced, and the discharge capacity tends to decrease. On the other hand, if the average particle size is too small, the dispersion state of the powder is deteriorated during pasting, and it tends to be difficult to manufacture a uniform electrode.

Here, the average particle diameter and the maximum particle diameter represent the median particle diameter D50 (50% volume cumulative diameter) and D99 (99% volume cumulative diameter) of the primary particles, respectively, and refer to values measured by a laser diffraction particle size distribution measuring apparatus.

The positive electrode active material for a sodium ion secondary battery of the present invention can be produced by, for example, a solid-phase reaction method. If necessary, the obtained positive electrode active material may be added with conductive carbon and mixed while being pulverized to impart conductivity. Examples of the method of mixing while pulverizing include a method using a general pulverizer such as a mortar, a grinder (ら い か い), a ball mill, an attritor, a vibration ball mill, a satellite ball mill, a planetary ball mill, a jet mill, a bead mill, or the like. Among them, a planetary ball mill is preferably used. The planetary ball mill can efficiently generate very high impact energy by revolution of the base plate while rotating the ball mill pot. Therefore, the conductive carbon can be uniformly dispersed in the positive electrode active material, and the electron conductivity can be improved.

Alternatively, the positive electrode active material may be coated with conductive carbon by mixing the positive electrode active material with an organic compound as a conductive carbon source and then firing the mixture in an inert or reducing atmosphere to carbonize the organic compound.

(Positive electrode Material for sodium ion Secondary Battery)

The positive electrode active material for a sodium ion secondary battery of the present invention can be mixed with a conductive additive, a binder, and the like to obtain a positive electrode material for a sodium ion secondary battery.

Examples of the conductive aid include highly conductive carbon black such as acetylene black and ketjen black, and powdery or fibrous conductive carbon such as graphite. Among them, acetylene black, which can improve conductivity by adding a small amount, is preferable.

The binder is a component added to bind the materials constituting the positive electrode material to each other and prevent the positive electrode active material from being detached due to a volume change accompanying charge and discharge. Specific examples of the binder include: thermoplastic linear polymers such as Polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluorine-based rubber, and styrene-butadiene rubber (SBR); thermosetting resins such as thermosetting polyimide, polyamideimide, polyamide, phenol resin, epoxy resin, urea resin, melamine resin, unsaturated polyester resin, and polyurethane; cellulose derivatives such as carboxymethyl cellulose (including carboxymethyl cellulose salts such as sodium carboxymethyl cellulose, the same applies hereinafter), hydroxypropyl methyl cellulose, hydroxypropyl cellulose, hydroxyethyl cellulose, ethyl cellulose, and hydroxymethyl cellulose; water-soluble polymers such as polyvinyl alcohol, polyacrylamide, polyvinylpyrrolidone and copolymers thereof. Among them, from the viewpoint of excellent adhesiveness, a thermosetting resin, a cellulose derivative, and a water-soluble polymer are preferable, and thermosetting polyimide or carboxymethyl cellulose which is widely used industrially is more preferable. Particularly, carboxymethyl cellulose, which is inexpensive and requires no organic solvent when producing an electrode forming paste, is most preferable. These binders may be used alone or in combination of two or more.

The positive electrode active material for a sodium ion secondary battery of the present invention has a relatively high operating potential (for example, 5V) with charge and discharge, and therefore is suitable for a solid sodium ion secondary battery using a solid electrolyte that is not decomposed with charge and discharge. When the positive electrode active material for a sodium ion secondary battery of the present invention is used in a solid sodium ion secondary battery, it is preferable to add a sodium ion conductive solid electrolyte as a constituent component of the positive electrode material for a sodium ion secondary battery. The sodium ion conductive solid electrolyte is a component responsible for sodium ion conduction between the positive electrode and the negative electrode in the all-solid secondary battery. When the sodium ion conductive solid electrolyte is β alumina or NASICON crystal, sodium ion conductivity is excellent, and therefore it is preferable. Beta-alumina exists in two crystal forms of beta-alumina (theoretical composition formula: Na2O & 11Al2O3) and beta-alumina (theoretical composition formula: Na2O & 5.3Al2O 3). Since β "alumina is a metastable substance, β" alumina containing Li2O or MgO as a stabilizer is generally used. Since β "alumina has a higher sodium ion conductivity than β alumina, it is preferable to use β" alumina alone or a mixture of β "alumina and β alumina, and it is more preferable to use Li 2O-stabilized β" alumina (na1.6li0.34al10.66o17) or MgO-stabilized β "alumina ((al10.32mg0.68o16) (na1.68o)).

The NASICON crystal is preferably Na3Zr2Si2PO12, Na3.2zr1.3si2.2p0.8o10.5, Na3zr1.6ti0.4si2po12, Na3Hf2Si2PO12, Na3.4zr0.9hf1.4al0.6si1.2p1.8o12, Na3zr1.7nb0.24si2po12, Na3.6ti0.2y0.8si2.8o9, na3zr1.12y0.12si2po12, Na5YSi4O12, Na3.12zr1.88y0.12si2po12, Na 3.6z0.13yb1.67si0.1p2.9o12 or the like, and particularly, Na 3.2zr1.1y1ypo0.1sq12sq2p2.9sodium is preferable for its ion conductivity.

The sodium ion conductive solid electrolyte has an average particle diameter D50 of 0.3 to 25 μm, preferably 0.5 to 20 μm, and particularly preferably 1.2 to 15 μm. When the average particle diameter D50 of the sodium ion conductive solid electrolyte is too small, it is difficult to uniformly mix the sodium ion conductive solid electrolyte with the positive electrode active material, and the ion conductivity is liable to decrease due to moisture absorption or carbonation. As a result, the internal resistance increases, and the charge-discharge voltage and discharge capacity tend to decrease. On the other hand, when the average particle diameter D50 of the sodium ion-conductive solid electrolyte is too large, softening and flowing of the positive electrode active material is significantly inhibited during sintering for forming the positive electrode layer, and therefore, the smoothness of the obtained positive electrode layer tends to deteriorate, and the mechanical strength tends to decrease, or the internal resistance tends to increase.

the composition of the positive electrode material is preferably selected as appropriate according to the type of electrolyte used. For example, in a sodium ion secondary battery using an aqueous or nonaqueous liquid electrolyte, the positive electrode active material is preferably contained in an amount of 70 to 95% by mass, the conductive auxiliary agent is preferably contained in an amount of 1 to 15% by mass, and the binder is preferably contained in an amount of 3 to 15% by mass, and the positive electrode active material is more preferably contained in an amount of 80 to 95% by mass, the conductive auxiliary agent is more preferably contained in an amount of 2 to 10% by mass, and the binder is more preferably contained in an amount of 3 to 10% by mass. When the content of the positive electrode active material is too small, the discharge capacity of the sodium ion secondary battery is liable to be reduced, and when it is too large, the contents of the conductive assistant and the binder are relatively reduced, so that the electron conductivity and the cycle characteristics are liable to be reduced. When the content of the conductive additive is too small, the electron conductivity is deteriorated, while when it is too large, the adhesion between the constituent components of the positive electrode material is lowered to increase the internal resistance, and thus the charge-discharge voltage and the discharge capacity tend to be lowered. When the content of the binder is too small, the adhesion between the constituent materials of the positive electrode material is reduced, and the cycle characteristics are easily reduced, while when too large, the electron conductivity is reduced, and therefore, the rapid charge-discharge characteristics are easily reduced.

In the case of a solid sodium ion secondary battery using a sodium ion conductive solid electrolyte as an electrolyte, the solid sodium ion secondary battery preferably contains 30 to 100% by mass of a positive electrode active material, 0 to 70% by mass of a solid electrolyte, and 0 to 20% by mass of a conductive additive, more preferably contains 34.5 to 94.5% by mass of a positive electrode active material, 5 to 65% by mass of a solid electrolyte, and 0.5 to 15% by mass of a conductive additive, and further preferably contains 40 to 92% by mass of a positive electrode active material, 7 to 50% by mass of a solid electrolyte, and 1 to 10% by mass of a conductive additive. When the content of the positive electrode active material is too small, the discharge capacity of the sodium ion secondary battery is liable to decrease. When the content of the conductive aid or the solid electrolyte is too large, the adhesion between the constituent components of the positive electrode material is lowered to increase the internal resistance, and thus the charge-discharge voltage and the discharge capacity tend to be lowered.

The components of the positive electrode material can be mixed using a mixer such as a rotary mixer or a drum mixer, a mortar, a mill (ら い か い), a ball mill, an attritor, a vibration ball mill, a satellite ball mill, a planetary ball mill, a jet mill, a bead mill, or other conventional pulverizer. In particular, by using a planetary ball mill, the constituent materials can be uniformly dispersed.

The positive electrode material for a sodium ion secondary battery of the present invention can be used as a positive electrode for a sodium ion secondary battery by coating the current collector made of a metal foil of aluminum, copper, gold, or the like and drying the current collector. Alternatively, the positive electrode material for a sodium ion secondary battery of the present invention may be formed into a sheet shape, and then the current collector including the metal coating may be formed by sputtering, plating, or the like.

(sodium ion secondary battery)

The sodium ion secondary battery of the present invention has a negative electrode as a counter electrode and an electrolyte in addition to the positive electrode for a sodium ion secondary battery described above.

The negative electrode contains a negative electrode active material capable of storing and releasing sodium ions in association with charge and discharge. Examples of the negative electrode active material include metallic materials such as metallic Na, metallic Sn, metallic Bi, metallic Zn, Sn — Cu alloy, and Bi — Cu alloy, carbon materials such as hard carbon, and oxide materials containing Ti and/or Nb as an element. Among these, an oxide material containing Ti and/or Nb as an element is preferable because it has high safety and is abundant in resources. Particularly, an oxide material containing a crystal phase represented by Na4TiO (PO4)2 or Na5Ti (PO4)3 having an oxidation-reduction potential of 1.5V (vs. Na/Na +) or less associated with charge and discharge is preferably used. In this case, the operating voltage of the sodium ion secondary battery increases, and the precipitation of metallic Na dendrites during repeated charge and discharge can be suppressed.

As the electrolyte, an aqueous electrolyte, a nonaqueous electrolyte, a solid electrolyte, or the like can be used. Since the potential window of the nonaqueous electrolyte or the solid electrolyte is wide, gas generated by decomposition of the electrolyte during charge and discharge hardly occurs, and the safety of the sodium ion secondary battery can be improved. Among them, a nonflammable solid electrolyte is preferable.

the aqueous electrolyte contains a water-soluble electrolyte salt. Examples of the electrolyte salt include NaNO3, Na2SO4, NaOH, NaCl, and CH3 COONa. These electrolyte salts may be used alone or in combination of two or more. The concentration of the electrolyte salt is appropriately adjusted usually within a range from 0.1M to the saturation concentration.

When an aqueous electrolyte is used, the redox potential of the positive electrode active material for a sodium ion secondary battery of the present invention may be within the potential window of water.

The nonaqueous electrolyte contains an organic solvent and/or an ionic liquid as a nonaqueous solvent, and an electrolyte salt dissolved in the nonaqueous solvent. Examples of the organic solvent to be used as the nonaqueous solvent include, but are not particularly limited to, 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), ethyl methyl carbonate (MEC), and dipropyl carbonate (DPC). These nonaqueous solvents may be used alone or in combination of two or more. Among these, propylene carbonate having excellent low-temperature characteristics is preferable.

The ionic liquid is not particularly limited as long as it can dissolve the electrolyte salt used, and specific examples thereof include: n, N-trimethyl-N-propylammonium bis (trifluoromethanesulfonyl) imide [ abbreviation: TMPA-TFSI ], N-methyl-N-propylpiperidinebis (trifluoromethanesulfonyl) imide [ abbreviation: PP 13-TFSI ], N-methyl-N-propylpyrrolidine bis (trifluoromethanesulfonyl) imide [ abbreviation: P13-TFSI ], N-methyl-N-butylpyrrolidine bis (trifluoromethanesulfonyl) imide [ abbreviation: aliphatic quaternary ammonium salts such as P14-TFSI; 1-methyl-3-ethylimidazolium tetrafluoroborate [ abbreviation: EMIBF4], 1-methyl-3-ethylimidazolium bis (trifluoromethanesulfonyl) imide [ abbreviation: EMITFSI), 1-allyl-3-ethylimidazole bromide salt [ abbreviation: AEImBr ], 1-allyl-3-ethylimidazolium tetrafluoroborate [ abbreviation: AEImBF4], 1-allyl-3-ethylimidazolium bis (trifluoromethanesulfonyl) imide [ abbreviation: AEImTFSI ], 1, 3-diallylimidazolium bromide salt [ abbreviation: AAImBr ], 1, 3-diallylimidazolium tetrafluoroborate [ abbreviation: AAImBF4], 1, 3-diallylimidazolium bis (trifluoromethanesulfonyl) imide [ abbreviation: and alkylimidazolium quaternary salts such as AAImTFSI.

As the electrolyte salt, there can be mentioned: PF 6-, BF 4-, (CF3SO2) 2N- (bistrifluoromethanesulfonylimide; commonly known as TFSI), CF3SO 3- (commonly known as TFS), (C2F5SO2) 2N- (bistrifluoroethanesulfonamide; commonly known as BETI), ClO 4-, AsF 6-, SbF 6-, bisoxalatoboric acid (B (C2O4) 2-; commonly known as BOB), difluoro (trifluoro-2-oxo-2-trifluoro-methylpropionate (2-) -0, 0) boric acid (BF2OCOOC (CF3) 3-, commonly known as B (HHIB)), and the like. These electrolyte salts may be used alone or in combination of two or more. Particularly preferred is the sodium salt of PF 6-or BF 4-which is inexpensive. The concentration of the electrolyte salt is usually adjusted within a range of 0.5 to 3M.

The nonaqueous electrolyte may contain additives such as Vinylene Carbonate (VC), Vinyl Acetate (VA), vinyl butyrate, vinyl hexanoate, vinyl crotonate, and catechol carbonate. These additives have an effect of forming a protective film on the surface of the active material. The concentration of the additive is preferably 0.1 to 3 parts by mass, and particularly preferably 0.5 to 1 part by mass, per 100 parts by mass of the nonaqueous electrolyte.

As the solid electrolyte, the above-described solid electrolyte can be used. Since the potential window of the solid electrolyte is wider than that of the aqueous electrolyte or the nonaqueous electrolyte, gas generated by decomposition is hardly generated, and the safety of the sodium ion secondary battery can be improved.

In the case of an electrolyte-based sodium ion secondary battery using an aqueous electrolyte or a nonaqueous electrolyte, a separator is preferably provided between electrodes. The separator is made of a material having an insulating property, and specifically, a porous film or nonwoven fabric made of a polymer such as polyolefin, cellulose, polyethylene terephthalate, vinylon, or the like, a glass nonwoven fabric containing fibrous glass, a glass cloth woven with fibrous glass, film-like glass, or the like can be used.