JP2016100058A - Negative electrode active material for sodium ion battery, negative electrode for sodium ion battery, and sodium ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide negative electrode active material capable of manufacturing a sodium ion battery having high safety property and satisfactory battery performance.SOLUTION: A sodium ion battery having high safety property and satisfactory battery performance by using transition metal oxide having a layered crystal structure consisting of a transition metal layer including tetravalent Ti, and mixing simple orthorhombic lattice and body center orthorhombic lattice.SELECTED DRAWING: None

Description

  The present invention has a layered crystal structure composed of a transition metal layer containing tetravalent Ti, and uses a transition metal oxide in which a simple orthorhombic lattice and a body-centered orthorhombic lattice are mixed. , A sodium ion battery negative electrode, and a sodium ion battery.

Lithium secondary batteries are high energy density secondary batteries that have already been put into practical use as small power sources for mobile phones, notebook computers and the like. Lithium secondary batteries have already been put into practical use as compact power sources for electric vehicles, hybrid vehicles, and the like. In addition, since lithium secondary batteries can be used as large-scale power sources such as power sources for automobiles such as electric vehicles and hybrid vehicles, and distributed power storage power sources, the demand is increasing.
However, since a rare metal element such as lithium is used in the lithium secondary battery, there is a concern about unstable supply of the rare metal element when the demand for the lithium secondary battery increases.

Therefore, recently, attention has been focused on sodium batteries using sodium, which is more abundant than lithium, and research on materials such as a positive electrode active material and a negative electrode active material used in sodium batteries is also underway.
For example, Patent Document 1 describes an example in which Na ₂ Ti ₃ O ₇ having a zigzag layered structure is used as a negative electrode material for a sodium ion battery.
Patent Document 2 describes that Y ₂ Ti ₂ O ₅ S ₂ having a Ruddleden-Popper structure is used as an active material for a sodium ion battery, and describes that precipitation of metal Na can be suppressed. ing.
Further, Non-Patent Document 1, it is described that utilize _Na x TiO ₂ type O3 type structure (0.7 <x <1) as an active material for a sodium ion battery.
Furthermore, Non-Patent Document 2 describes that a lepidocrotite-type titanate belonging to the space group Pmmm is used as a negative electrode of a sodium ion battery.

International Publication No. 2013/004957 JP 2013-062121 A

J. et al. Inclusion Phenomena, 1, 45-51 (1983) Chem. Mater. , 26, 2502-2512 (2014)

Carbon materials such as hard carbon are known as typical negative electrode active materials for sodium ion batteries. The carbon material such as hard carbon can be expected to improve the cycle characteristics. However, in the carbon material, metal Na is likely to precipitate, and there is room for improvement in safety. On the other hand, for example, composite metal oxides such as Na ₂ Ti ₃ O ₇ described in Patent Document 1 can be used as the negative electrode active material, but these materials are irreversible caused by the first charge / discharge. Large capacity,
At present, sufficient performance as a secondary battery cannot be obtained.
That is, this invention makes it a subject to provide the negative electrode active material which can manufacture a sodium ion battery provided with high safety | security and favorable battery performance.

As a result of intensive studies to solve the above problems, the inventors of the present invention contain a transition metal oxide having a layered crystal structure composed of a transition metal layer containing tetravalent Ti, A negative electrode active material having a mixed phase of a body-centered rhombic lattice is a very suitable material for the negative electrode active material of a sodium ion battery, and by using this, a sodium ion battery having high safety and good battery performance can be obtained. The present invention has been completed by finding that it can be produced.
That is, the present invention is as follows.

(1) A simple orthorhombic lattice and a body-centered orthorhombic lattice comprising a transition metal oxide having a compositional formula represented by the following formula (I) and having a layered crystal structure composed of a transition metal layer containing tetravalent Ti A negative electrode active material for a sodium ion battery having a mixed phase of
_{A x Ti 2-y M y} O 4 ··· (I)
(In formula (I), 0 <x <1, A is at least one element selected from H, Li, Na, K, Rb and Cs, and M is a vacancy, Li, Mg, Mn, (At least one selected from Fe, Co, Ni, Cu and Zn, which can replace Ti constituting the transition metal layer, and 0 <y <2.)
(2) In the diffraction line by powder X-ray diffraction, the intensity of the 010 diffraction line attributed to the simple orthorhombic lattice is less than half of the intensity of the 020 diffraction line attributed to the body-centered orthorhombic lattice. The negative electrode active material for sodium ion batteries containing the transition metal oxide as described in (1).
(3) A negative electrode for a sodium ion battery, comprising the negative electrode active material for a sodium ion battery according to (1) or (2).
(4) A sodium ion battery comprising the negative electrode for sodium ion battery according to (3).

  According to the present invention, it is possible to manufacture a sodium ion battery that is excellent in safety and has a very small initial irreversible capacity. Further, since Ti is tetravalent, a sodium ion battery that is relatively stable even in air, can be easily handled and stored, and is excellent in productivity can be manufactured.

2 is an XRD measurement result of Compound 2 prepared in Example 1. 3 is an XRD measurement result of Compound 3 prepared in Example 1. It is an XRD measurement result of the electrode produced in Example 1. It is a XRD measurement result of the electrode produced in Example 2. It is a XRD measurement result of the electrode produced in Example 3. It is an XRD measurement result of the electrode produced in Example 4. It is an XRD measurement result of the electrode produced in Comparative Example 1.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

<Negative electrode active material>
The negative electrode active material of the present invention contains a transition metal oxide having a layered crystal structure composed of a transition metal layer containing tetravalent Ti, and has a mixed phase of a simple orthorhombic lattice and a body-centered orthorhombic lattice. Features.
It has been reported that a composite metal oxide containing Na and Ti can be used as a negative electrode active material in a sodium ion battery, but the irreversible capacity generated by the first charge / discharge is very large in these metal oxides. At present, sufficient performance as a secondary battery cannot be obtained. The present inventors have prepared a negative electrode active material containing a transition metal oxide having a layered crystal structure composed of a transition metal layer containing tetravalent Ti and having a mixed phase of a simple rhomboid and a body-centered rhombic lattice. It has been found that the irreversible capacity can be remarkably suppressed by using it as a negative electrode active material of a sodium ion battery. It is presumed that this is because the structure has a stable layered structure, so that the structural change accompanying charging / discharging is small, and excessive side reactions accompanying cracking of the active material are reduced. Moreover, since such a transition metal oxide can be operated at a higher potential than a carbon material, there is an advantage that metal Na is not easily deposited. Further, since Ti is tetravalent, it is relatively stable even in the air, and handling and storage become easy, and as a result, the productivity of the battery can be improved. That is, the negative electrode active material containing a transition metal oxide having a layered crystal structure composed of a transition metal layer containing tetravalent Ti of the present invention and having a mixed phase of a simple orthorhombic lattice and a body-centered orthorhombic lattice, This is a very suitable material for the negative electrode active material for sodium ion batteries.
Note that “having a layered crystal structure” and “having a transition metal layer containing tetravalent Ti” means that the transition metal atoms and oxygen atoms (counter anions) are arranged in a planar shape, respectively. This means that the transition metal layer contains at least tetravalent Ti atoms.
The simple orthorhombic lattice and the body-centered orthorhombic lattice are one type of 14 types of Bravey lattices. Specific space groups of the simple orthorhombic lattice include P222, P222 ₁ , P2 ₁ 22, P2 ₁ 2 ₁ 2, P2 ₁ 2 ₁ 2 ₁ , Pmm2, Pmc2 ₁ , Pcc2, Pma2, Pca2 ₁ , Pnc2, Pmn2 ₁ , Pba2, Pna2 ₁ , Pnn2, Pmmm, Pnnn, Pccm, Pban, Pmma, Pnna, Pmna, Pcca, Pbam, Pccn, Pbcm, Pnnm, Pmmn, Pbcn, Pbca, Pnma. Specific space groups of the body-centered rhombic lattice include C222 ₁ , C222, Cmm2, Cmc2 ₁ , Ccc2, Cmcm, Cmca, Cmmm, Cccm, Cmma, and Ccca.
Note that whether or not the negative electrode active material of the present invention has a mixed phase of a simple orthorhombic lattice and a body-centered orthorhombic lattice can be confirmed by XRD measurement. Specifically, it can confirm by the method as described in an Example.

The negative electrode active material of the present invention contains a transition metal oxide having a layered crystal structure composed of a transition metal layer containing tetravalent Ti, and has a mixed phase of a simple rhombic lattice and a body-centered rhombic lattice, The transition metal oxide can be represented by the following formula (I).
_{A x Ti 2-y M y} O 4 ··· (I)
(In formula (I), 0 <x <1, A is at least one element selected from H, Li, Na, K, Rb and Cs, and M is a vacancy, Li, Mg, Mn, (At least one selected from Fe, Co, Ni, Cu and Zn, which can replace Ti constituting the transition metal layer, and 0 <y <2.)
If it is a metal oxide represented by Formula (I), since rare metals, such as Y and Nd, are not used, the cost of a battery can be held down.
The specific numerical values of x and y in the formula (I) are not particularly limited as long as the above conditions are satisfied, but x is preferably 0.3 or more (0.3 ≦ x), more preferably 0.5. This is the case (0.5 ≦ x). y is preferably 1.0 or less (y ≦ 1.0), more preferably 0.5 or less (y ≦ 0.5).
When x and y in the formula (I) are within the above ranges, a sodium ion battery having good battery performance can be produced.
M is at least one selected from vacancies, Li, Mg, Mn, Fe, Co, Ni, Cu and Zn, and at least one selected from vacancies, Li, Mg, Cu and Zn. preferable.
Furthermore, A is at least one element selected from H, Li, Na, K, Rb, and Cs, and preferably one or more elements selected from H, Na, and K.
Note that M is “which can replace Ti constituting the transition metal layer”, but this is because even when a part of the Ti layer arranged on the plane is replaced by M, the layered crystal structure Is a metal oxide that can be stably maintained. That is, for example, when a part of Ti is replaced with M, an element whose crystal structure becomes unstable and the layered crystal structure is broken cannot be M.

  A preferred embodiment of the negative electrode active material of the present invention has a mixed phase of a simple orthorhombic lattice and a body-centered orthorhombic lattice, and in a diffraction line by powder X-ray diffraction, a 010 diffraction line belonging to the simple orthorhombic lattice The intensity is less than half of the intensity of the 020 diffraction line attributed to the body-centered rhombic lattice. The intensity of the 010 diffraction line attributed to the simple orthorhombic lattice is more preferably ¼ or less of the intensity of the 020 diffraction line attributed to the body-centered orthorhombic lattice, and further preferably / or less. preferable. As the intensity of the 010 diffraction line attributed to the simple orthorhombic lattice is smaller than the intensity of the 020 diffraction line attributed to the body-centered orthorhombic lattice, the proportion of the body-centered orthorhombic lattice in the negative electrode active material increases. This is preferable because electrochemical characteristics are improved.

(Average primary particle size)
The average primary particle diameter of the negative electrode active material of the present invention is not particularly limited, but the lower limit is usually 0.1 μm or more, preferably 0.2 μm or more, and the upper limit is usually 20 μm or less, preferably 10 μm or less. is there. If the average primary particle diameter exceeds the above upper limit, the specific surface area may decrease, so that there is a high possibility that battery characteristics such as rate characteristics and output characteristics will decrease. If the lower limit is not reached, there are cases where the crystal is underdeveloped and the reversibility of charge / discharge is poor.
In addition, the average primary particle diameter in this invention is an average diameter observed with the scanning electron microscope (SEM), and the primary particle diameter of about 10-30 negative electrode active materials using a SEM image of 30,000 times. It can be calculated as an average value.

(Median diameter (secondary particles))
The median diameter (50% cumulative diameter (D ₅₀ )) of the secondary particles of the negative electrode active material of the present invention is not particularly limited, but is usually 2 μm or more, preferably 2.5 μm or more, more preferably 4 μm or more, and usually 50 μm. Below, it is preferably 30 μm or less, more preferably 20 μm or less. If the median diameter is less than this lower limit, there may be a problem in applicability at the time of forming the negative electrode active material layer, and if it exceeds the upper limit, battery performance may be deteriorated.

(BET specific surface area)
The BET specific surface area of the negative electrode active material of the present invention is not particularly limited, but is usually 0.1 m ² / g or more, preferably 0.2 m ² / g or more, more preferably 0.3 m ² / g or more, usually 10 m. ² / g or less, preferably 5 m ² / g or less, more preferably 3 m ² / g or less, still more preferably 2 m ² / g or less, and most preferably 1 m ² / g or less. If the BET specific surface area is smaller than this range, the battery performance is likely to be lowered, and if it is larger, the bulk density is difficult to increase, and there is a possibility that a problem is likely to occur in the coating property at the time of forming the negative electrode active material layer.

(Tap density)
The tap density of the negative electrode active material of the present invention is not particularly limited, but is usually 0.8 g / cc or more, preferably 1 g / cc or more, more preferably 1.4 g / cc or more, and further preferably 1.5 g / cc or more. In general, it is 3.0 g / cc or less, preferably 2.8 g / cc or less, more preferably 2.7 g / cc or less. While it is preferable for the tap density to exceed this upper limit for improving powder filling properties and electrode density, the specific surface area may be too low, and battery performance may be reduced. If the tap density is below this lower limit, there is a possibility that the powder filling property and the negative electrode preparation may be adversely affected.
The tap density in the present invention is determined as the powder packing density when 5 to 10 g of the transition metal oxide powder used as the negative electrode active material is placed in a 10 ml graduated cylinder and tapped 200 times with a stroke of 20 mm.

(Method for producing negative electrode active material)
The negative electrode active material of the present invention contains a transition metal oxide having a specific composition and a layered crystal structure composed of a transition metal layer containing tetravalent Ti, and includes a simple orthorhombic lattice and a body-centered orthorhombic lattice. If it has a mixed phase of, the manufacturing method will not be specifically limited.
For example, a raw material to be a K source, a raw material to be a Li source, and a raw material to be a Ti source are mixed so as to have a target composition ratio, and this is fired in an air atmosphere (baking treatment), and then K ^{+ It} can be produced by ion exchange (ion exchange treatment) and drying (dry treatment) with ⁺ , and other transitions containing tetravalent Ti by appropriately adjusting the composition ratio of Ti, Li, Na, K, etc. A transition metal oxide containing a transition metal oxide having a layered crystal structure composed of a metal layer and having a mixed phase of a simple orthorhombic lattice and a body-centered orthorhombic lattice can be produced. In addition, it is possible to add a step of subjecting the obtained compound to atmospheric release or water washing treatment.
Hereinafter, the manufacturing method will be described more specifically, but it is needless to say that the manufacturing method is not limited to these methods.

The kind of raw material used for producing the negative electrode active material of the present invention is not particularly limited. For example, an oxide, hydroxide, oxyhydroxide, bicarbonate, carbonate, nitrate containing a metal element to be introduced , Sulfates, halides, organic acid salts such as oxalates, and metal-containing compounds such as alkoxides.
For example, examples of the Ti-containing compound serving as the Ti source include anatase TiO ₂ , rutile TiO ₂ , and Ti (OC ₂ H ₅ ) _4. Anatase TiO ₂ is preferable from the viewpoints of handling and reactivity. .
As the Li-containing compound serving as the Li source, Li ₂ CO ₃ and LiOH are preferable.
Examples of the K-containing compound serving as the K source include K ₂ CO ₃ , KOH, KHCO _{3 and the} like, and K ₂ CO ₃ is preferable from the viewpoint of handleability.

In order to adjust the negative electrode active material of the present invention to a target composition / crystal structure, a Ti-containing compound and a compound containing a metal element to be introduced are weighed so as to have a target ratio and then mixed.
For example, when producing K _0.9 Li _0.3 Ti _1.7 O ₄ , the molar ratio of K ₂ CO ₃ , Li ₂ CO ₃ and TiO ₂ is 0.45: 0.15: 1.7. And mixing after weighing.
In addition, for mixing Ti-containing compounds and the like, industrially usual apparatuses such as a ball mill, a V-type mixer, a stirrer, and a dyno mill can be used. The mixing at this time may be either dry mixing or wet mixing. Moreover, you may obtain the mixture of the metal containing compound of a predetermined composition by the crystallization method. Furthermore, you may obtain the mixture with a potassium compound, after obtaining the composite metal carbonate or hydroxide of a predetermined composition by a coprecipitation method.
Specific mixing conditions are not particularly limited. For example, when a wet ball mill is used, the rotation speed is usually 10 to 1000 rpm, and the mixing time is 1 to 24 hours.

(Baking process)
The firing conditions for producing the transition metal oxide used as the negative electrode active material of the present invention should be appropriately set according to the composition of the transition metal oxide, etc., but the usual firing temperature is 700 to 1000 ° C., The firing time is 2 to 24 hours. A firing temperature of 750 ° C. or higher is preferable for suppressing generation of excessive stacking faults, and a firing temperature of 950 ° C. or lower is preferable for reducing primary particle size. Further, if the firing time is 6 hours or more, it is preferable for obtaining a uniform chemical composition of single particles, and if the firing time is 24 hours or less, it is preferable from the viewpoint of production cost.

  Examples of the atmosphere during firing include an inert atmosphere such as nitrogen and argon: an oxidizing atmosphere such as air, oxygen, oxygen-containing nitrogen, and oxygen-containing argon: and hydrogen-containing nitrogen containing 0.1 to 10% by volume of hydrogen. Any reducing atmosphere such as hydrogen-containing argon containing 0.1 to 10% by volume of hydrogen may be used. In order to fire in a strong reducing atmosphere, an appropriate amount of carbon may be contained in a mixture of metal-containing compounds and fired. In order to obtain Ti in a tetravalent state, the atmosphere during firing is preferably an oxidizing atmosphere such as air. When firing in a reducing atmosphere, it is preferable to subsequently perform firing in an air atmosphere or release to the atmosphere.

  When a compound that can be decomposed and / or oxidized at a high temperature, such as a hydroxide, carbonate, nitrate, sulfate, halide, or oxalate, is used as a raw material metal-containing compound, before performing the above firing, The metal-containing compound may be calcined in the temperature range of 200 to 500 ° C. to obtain an oxide or crystal water may be removed. The atmosphere in which the calcination is performed may be an inert gas atmosphere, an oxidizing atmosphere, or a reducing atmosphere. Moreover, you may grind | pulverize and use the calcined material after calcining.

(Ion exchange treatment)
The ion exchange treatment is a step of exchanging alkali metal ions present between the transition metal layers in the transition metal oxide after calcination with Na ⁺ . Specifically, for example, the transition metal oxide obtained after the firing is placed in an aqueous NaCl solution and held, and finally washed with water to obtain ion-exchanged powder.
In order to promote ion exchange, the NaCl aqueous solution may be heated. Usually, the temperature of aqueous solution is 60 to 100 degreeC. If the temperature is lower than 60 ° C., the effect of promoting ion exchange is small, and if it is higher than 100 ° C., the evaporation of moisture becomes remarkable, which is not preferable.

The ion exchange may be performed in a molten salt such as sodium nitrate. When using a molten salt, it is necessary to raise the temperature to the melting point of the molten salt, which is usually higher than 100 ° C., and since ion exchange is promoted from the aqueous solution, the time for ion exchange treatment can be shortened. However, since it is usually necessary to use a considerably excessive molten salt with respect to the required amount of Na ⁺ in the ion exchange treatment in the molten salt, an ion exchange treatment method suitable for the purpose is applied. Is preferred.

(Drying process)
The drying process is a process of drying the powder obtained by the ion exchange process. Specifically, for example, the negative electrode active material is obtained by holding at a temperature of about 50 to 300 ° C. for about 1 to 24 hours. The atmosphere during drying may be a vacuum or an inert atmosphere or an air atmosphere. The apparatus used for drying is not particularly limited, and examples thereof include a hot air type box dryer, a tunnel dryer, a rotary dryer, a spray dryer, a conductive drum dryer and an infrared dryer.

  Further, it may be preferable to adjust the particle size by subjecting the transition metal oxide used as the negative electrode active material obtained as described above to pulverization, classification, or the like using a ball mill or a jet mill as necessary. . Moreover, you may perform baking twice or more. Further, a surface treatment such as coating the particle surface of the transition metal oxide with an inorganic substance containing W, Mo, Zr, Si, Y, B or the like may be performed. The transition metal oxide preferably has a crystal structure that is not a tunnel structure.

<Anode active material for sodium ion battery / Anode for sodium ion battery>
The negative electrode active material of the present invention is a material particularly suitable as a negative electrode for sodium ion batteries.
The negative electrode of the present invention is not particularly limited as long as it contains the negative electrode active material of the present invention. Usually, a negative electrode active material layer containing a negative electrode active material, a conductive agent, a binder and the like is used as a current collector. It is formed on the surface.

  As long as the negative electrode active material of the present invention includes the above-described negative electrode active material, the negative electrode active material may include other known negative electrode active materials as long as the effects of the present invention are not impaired. Examples thereof include carbon materials such as natural graphite, artificial graphite, coke, hard carbon, carbon black, pyrolytic carbon, carbon fiber, and organic polymer compound fired body that can occlude and desorb sodium ions. The shape of the carbon material may be any of a flake shape such as natural graphite, a spherical shape such as mesocarbon microbeads, a fibrous shape such as graphitized carbon fiber, or an aggregate of fine powder. Here, the carbon material may play a role as a conductive agent.

  Although content of the negative electrode active material in a negative electrode active material layer is not specifically limited, It is preferable that it is 80-95 mass%.

  Examples of the binder include polyvinylidene fluoride (hereinafter sometimes referred to as PVDF), polytetrafluoroethylene (hereinafter sometimes referred to as PTFE), tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride copolymer. Examples thereof include a copolymer, a hexafluoropropylene / vinylidene fluoride copolymer, and a tetrafluoroethylene / perfluorovinyl ether copolymer. These may be used alone or in combination of two or more. Other examples of the binder include, for example, starch, methylcellulose, carboxymethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, carboxymethylhydroxyethylcellulose, nitrocellulose, polyacrylic acid, sodium polyacrylate, polyacrylonitrile and the like. Examples include polysaccharides and derivatives thereof. Examples of usable binders include inorganic fine particles such as colloidal silica. Although content of the binder in a negative electrode active material layer is not specifically limited, It is preferable that it is 5-10 mass%.

  Examples of the current collector include foil, mesh, expanded grid (expanded metal), punched metal, and the like using a conductive material such as nickel, aluminum, copper, and stainless steel (SUS). The mesh opening, wire diameter, number of meshes, etc. are not particularly limited, and conventionally known ones can be used. The general thickness of the current collector is 5 to 30 μm. However, a current collector having a thickness outside this range may be used.

  The size of the current collector is determined according to the intended use of the battery. If a large electrode used for a large battery is manufactured, a current collector having a large area is used. If a small electrode is produced, a current collector with a small area is used.

As a method for producing a negative electrode, first, a transition metal compound used as a negative electrode active material, a conductive agent, a binder, and an organic solvent are mixed to prepare a negative electrode active material slurry. Examples of organic solvents that can be used here include amines such as N, N-dimethylaminopropylamine and diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketones such as methyl ethyl ketone; esters such as methyl acetate; dimethylacetamide And aprotic polar solvents such as N-methyl-2-pyrrolidone.
In addition, as described in the Examples, the negative electrode active material of the present invention can be used for electrode preparation even in a slurry using water as a solvent.

Next, the negative electrode active material slurry is coated on the negative electrode current collector, and dried and pressed to fix the slurry. Here, examples of the method of coating the negative electrode active material slurry on the negative electrode current collector include a slit die coating method, a screen coating method, a curtain coating method, a knife coating method, a gravure coating method, and an electrostatic spray method. be able to.

  As a method for forming the negative electrode active material layer on the negative electrode current collector, in addition to the above method, a mixture of a transition metal oxide, a conductive agent, and a binder used as the negative electrode active material is formed on the negative electrode current collector. It is also possible to use a method of installing in a pressure molding.

<Sodium ion battery>
The sodium ion battery provided with the negative electrode of the present invention is also an embodiment of the present invention (hereinafter may be abbreviated as “sodium ion battery of the present invention”). The sodium ion battery of the present invention is not particularly limited as long as it includes the above-described negative electrode, but generally includes a positive electrode capable of inserting and extracting sodium ions and an electrolyte in addition to the negative electrode.

[Positive electrode]
The positive electrode includes a current collector and a positive electrode active material layer formed on the surface of the current collector. The positive electrode active material layer includes a positive electrode active material, a conductive agent, and a binder.

Although the kind of positive electrode active material will not be specifically limited if it can be used for a sodium ion battery, Specifically, a layered active material, a spinel type active material, an oxo acid salt active material etc. can be mentioned. For _{example, NaFeO 2, NaNiO 2, NaCoO} 2, NaMnO 2, NaVO 2, Na (Ni X Mn 1-X) O 2 (0 <X <1), Na (Fe X Mn 1-X) O 2 (0 < X <1), NaVPO ₄ F, Na ₂ FePO ₄ F, Na ₃ V ₂ (PO ₄ ) _{3 and the} like.

  Although content of the positive electrode active material in a positive electrode active material layer is not specifically limited, It is preferable that it is 80-95 mass%.

  As the conductive agent, the binder, and the like in the positive electrode, and the method for forming the positive electrode active material layer on the current collector, the same ones and methods used when manufacturing the negative electrode can be employed.

[Electrolytes]
The electrolyte is not particularly limited, and either a general electrolytic solution or a solid electrolyte can be used. Examples of the electrolyte include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, 1,2-di ( Carbonates such as methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, 2-methyl Ethers such as tetrahydrofuran; esters such as methyl formate, methyl acetate and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; N, N-dimethylformamide, N, N Amides such as dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propane sultone; or a fluorine substituent further introduced into the above organic solvent Things can be used. Usually, two or more of these are mixed and used as the organic solvent.

Among the above electrolytic solutions, it is preferable to employ a non-aqueous solvent consisting essentially of a saturated cyclic carbonate (excluding the sole use of ethylene carbonate) or a mixed solvent of a saturated cyclic carbonate and a chain carbonate. In particular, when any one of these non-aqueous solvents is employed and hard carbon is included in the negative electrode active material layer, the sodium ion secondary battery has excellent charge / discharge efficiency and charge / discharge characteristics. You may use the known additive currently used for the lithium ion battery as needed. In particular, fluoroethylene carbonate (FEC) is preferred as an additive.

  Here, “substantially” means a non-aqueous solvent composed only of a saturated cyclic carbonate (excluding the single use of ethylene carbonate), a non-aqueous solvent composed of a mixed solvent of a saturated cyclic carbonate and a chain carbonate. In the range which does not affect the performance of the sodium ion secondary battery such as the charge / discharge characteristics, it means that the solvent includes the solvent contained in the non-aqueous solvent used in the present invention.

  Among the saturated cyclic carbonates, use of propylene carbonate is preferable. Of the mixed solvents, it is preferable to use a mixed solvent of ethylene carbonate and diethyl carbonate or a mixed solvent of ethylene carbonate and propylene carbonate.

  As an electrolyte, an electrolyte salt that can be used when an electrolytic solution is employed is not particularly limited, and an electrolyte salt that is generally used for sodium secondary batteries can be used.

Examples of the electrolyte salt generally used in sodium secondary batteries include NaClO ₄ , NaPF ₆ , NaBF ₄ , CF ₃ SO ₃ Na, NaAsF ₆ , NaB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Na, Examples thereof include NaN (SO ₂ CF ₃ ) ₂ , NaN (SO ₂ C ₂ F ₅ ) ₂ , NaC (SO ₂ CF ₃ ) ₃ , NaN (SO ₃ CF ₃ ) _{2 and the} like. Moreover, you may use Li salt other than the said Na salt as electrolyte salt. In addition, 1 type may be used among the said electrolytes, and may be used in combination of 2 or more type.

  The concentration of the electrolyte salt in the electrolytic solution is not particularly limited, but the concentration of the electrolyte salt is preferably 3 to 0.1 mol / L, and more preferably 1.5 to 0.5 mol / L. .

As the solid electrolyte, for example, an organic solid electrolyte such as a polyethylene oxide polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain and a polyoxyalkylene chain can be used. Moreover, what is called a gel type which hold | maintained the nonaqueous electrolyte solution in the high molecular compound can also be used. _{Further, Na 2 S-SiS 2,} Na 2 S-GeS 2, NaTi 2 (PO 4) 3, NaFe 2 (PO 4) 3, Na 2 (SO 4) 3, Fe 2 (SO 4) 2 (PO 4 ), Fe ₂ (MoO ₄ ) ₃ or other inorganic solid electrolytes may be used.

[Structure of sodium secondary battery]
The structure of the sodium secondary battery of the present invention is not particularly limited, and when distinguished by form and structure, any conventionally known form such as a stacked (flat) battery, a wound (cylindrical) battery, etc.・ Applicable to structures. Further, when viewed in terms of electrical connection form (battery structure) in the sodium ion secondary battery, it can be applied to both (internal parallel connection type) batteries and bipolar (internal series connection type) batteries. .

  Hereinafter, the present invention will be described in detail with reference to examples. In addition, this invention is not limited to the Example shown below at all.

Example 1
K ₂ CO ₃ , Li ₂ CO ₃ , TiO ₂ (anatase) as raw materials at a molar ratio of 0.45:
After weighing to 0.15: 1.7, it was mixed at 600 rpm for 3 hours using a wet ball mill. After pellet-molding the obtained mixture, K _0.9 Li _0.3 Ti _1.7 O ₄ (compound 1) having a lipidocrosite structure was synthesized by firing at 900 ° C. for 12 hours in an air atmosphere. .
Was added Compound 1 to NaCl aqueous solution 5 mol / L, 3 days on a hot stirrer set at 60 ° C., stirring is carried out for, for the ^{K +} Compound 1 was ion-exchanged to Na ^+. After filtration, the powder dried at room temperature (Compound 2) was measured by a powder X-ray diffraction method. CuKα was used as the radiation source, and the scanning axis was θ / 2θ. As a result of XRD analysis, it was confirmed to have a layered structure belonging to the space group Pmmm. The XRD measurement result of Compound 2 is shown in FIG.
As a result of XRD analysis of the powder (Compound 3) after drying Compound 2 for 1 day with a 200 ° C. vacuum dryer, it was confirmed that it had a layered structure belonging to the space group Cmcm. The XRD measurement result of Compound 3 is shown in FIG.

A compound 3 is used, and compound 3: CMC (carboxymethyl cellulose): AB (acetylene black) = 80: 5: 10 (mass ratio) is mixed and slurried using pure water to obtain a current collector. The electrode was produced by applying to Al. The XRD measurement result of the electrode dried at 200 ° C. is shown in FIG. The unmarked diffraction line belongs to the space group Cmcm, and the diffraction line indicated by the down arrow belongs to the space group Pmmm, confirming that the electrode contains a negative electrode active material having a mixed phase of a body-centered rhombic lattice and a simple rhombic lattice. It was done. Moreover, the 010 diffraction line attributed to Pmmm was hardly observed, and it was also confirmed that it was less than half the intensity of the 020 diffraction line attributed to Cmcm. A battery using 1M NaPF ₆ / PC (propylene carbonate) as the electrolytic solution and Na metal as the counter electrode was produced.
About the produced battery, the charge / discharge test was implemented in the voltage range of 0.1V-2V with the current density of 10 mA / g.
The initial discharge capacity, the Coulomb efficiency, and the discharge capacity after 15 cycles (cyc) were 117.6 mAh / g, 72.1%, and 113.5 mAh / g, respectively.

(Example 2)
A battery was prepared in the same manner as in Example 1 except that the drying temperature after electrode preparation was 120 ° C. The XRD measurement result of the electrode dried at 120 ° C. is shown in FIG. The unmarked diffraction line belongs to the space group Cmcm, and the diffraction line indicated by the down arrow belongs to the space group Pmmm, confirming that the electrode contains a negative electrode active material having a mixed phase of a body-centered rhombic lattice and a simple rhombic lattice. It was done. Moreover, although the 010 diffraction line attributed to Pmmm was observed, it was also confirmed that it was less than half the intensity of the 020 diffraction line attributed to Cmcm. As a result of the charge / discharge test, the initial discharge capacity, the Coulomb efficiency, and the discharge capacity after 15 cycles were 109.6 mAh / g, 60.7%, and 104.9 mAh / g, respectively.

(Example 3)
The electrode was prepared in the same manner as in Example 1 except that the compound 1 was mixed with NaNO ₃ so that the molar ratio was Na: K = 30: 1 and kept at 320 ° C. for 4 hours to perform ion exchange. Produced. The XRD measurement result of the electrode dried at 200 ° C. is shown in FIG. The unmarked diffraction line belongs to the space group Cmcm, and the diffraction line indicated by the down arrow belongs to the space group Pmmm, confirming that the electrode contains a negative electrode active material having a mixed phase of a body-centered rhombic lattice and a simple rhombic lattice. It was done. Moreover, the 010 diffraction line attributed to Pmmm was hardly observed, and it was also confirmed that it was less than half the intensity of the 020 diffraction line attributed to Cmcm. As a result of the charge / discharge test, the initial discharge capacity, the Coulomb efficiency, and the discharge capacity after 15 cycles were 97.8 mAh / g, 71.0%, and 97.8 mAh / g, respectively.

Example 4
An electrode was produced in the same manner as in Example 3 except that the electrode drying temperature was 150 ° C. 15
The XRD measurement result of the electrode dried at 0 ° C. is shown in FIG.
The unmarked diffraction line belongs to the space group Cmcm, and the diffraction line indicated by the down arrow belongs to the space group Pmmm, confirming that the electrode contains a negative electrode active material having a mixed phase of a body-centered rhombic lattice and a simple rhombic lattice. It was done. Moreover, although the 010 diffraction line attributed to Pmmm was observed, it was also confirmed that it was less than half the intensity of the 020 diffraction line attributed to Cmcm. As a result of the charge / discharge test, the initial discharge capacity, the Coulomb efficiency, and the discharge capacity after 15 cycles were 95.1 mAh / g, 68.3%, and 95.2 mAh / g, respectively.

(Comparative Example 1)
The electrode was prepared in the same manner as in Example 1 except that the electrode was dried at 80 ° C., allowed to stand at room temperature for 5 hours, and stored in a vacuum dryer at 25 ° C. for 12 hours. Produced. FIG. 7 shows the XRD measurement results of the electrode of Comparative Example 1. From the XRD measurement results, it was confirmed that the electrode contained a negative electrode active material having a simple orthorhombic lattice single phase belonging to the space group Pmmm. As a result of the charge / discharge test, the initial discharge capacity, the Coulomb efficiency, and the discharge capacity after 15 cycles were 93.2 mAh / g, 44.8%, and 96.6 mAh / g, respectively.

  The use of the sodium ion battery provided with the negative electrode using the negative electrode active material of the present invention is not particularly limited, and can be used for various known uses. Specific examples include notebook computers, pen input computers, mobile computers, electronic book players, mobile phones, mobile faxes, mobile copy, mobile printers, headphone stereos, video movies, LCD TVs, handy cleaners, portable CDs, minidiscs, and transceivers. , Electronic notebook, calculator, memory card, portable tape recorder, radio, backup power supply, stationary power supply, motor, lighting equipment, toys, game equipment, clock, strobe, camera, pacemaker, power tool, power source for bicycles and motorcycles, Examples thereof include an automobile power source, an orbital vehicle power source, and a satellite power source.

Claims (4)

A mixed phase of a simple orthorhombic lattice and a body-centered orthorhombic lattice, comprising a transition metal oxide having a compositional formula represented by the following formula (I) and having a layered crystal structure comprising a transition metal layer containing tetravalent Ti A negative electrode active material for sodium ion batteries.
_{A x Ti 2-y M y} O 4 ··· (I)
(In formula (I), 0 <x <1, A is at least one element selected from H, Li, Na, K, Rb and Cs, and M is a vacancy, Li, Mg, Mn, (At least one selected from Fe, Co, Ni, Cu and Zn, which can replace Ti constituting the transition metal layer, and 0 <y <2.)   The intensity of a 010 diffraction line attributed to a simple orthorhombic lattice in a diffraction line by powder X-ray diffraction is less than half of the intensity of a 020 diffraction line attributed to a body-centered orthorhombic lattice. The negative electrode active material for sodium ion batteries according to 1.   A sodium ion battery negative electrode comprising the sodium ion battery negative electrode active material according to claim 1.   A sodium ion battery comprising the negative electrode for a sodium ion battery according to claim 3.

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