JP2011054562A - Determination method of alkali metal-doped transition metal oxides - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a determining method of alkali metal-doped transition metal oxides that can be employed as a positive electrode active material to provide a high capacity sodium secondary battery. <P>SOLUTION: The determining method of the alkali metal-doped transition metal oxides that can be employed as the positive electrode active material for the sodium secondary battery is provided wherein: a position of an atom in a crystal structure is decided, G (x, y, z) in following formula (1) is calculated; and a material is selected whose GCRIT is small, the GCRIT being the smallest G (x, y, z) in which, in the side of a region surrounded by the G (x, y, z) isosurface containing alkali atoms, at least three alkali metal atoms are contained. Here, following formula (2) represents a Bond Valence Sum, and combinations of l<SB>0</SB>and B are Bond Valence Parameters, (x, y, z) are position coordinates of the alkaline metal atom, and l<SB>j</SB>is a distance between an electron attracting atom in the vicinity of the alkali metal atom and the alkali metal atom. The formula (1) is expressed by an absolute value formula of G (x, y, z)=[1-H (x, y, z)], and the formula (2) is expressed by H (x, y, z)=Σ<SB>j</SB>exp [(l<SB>j</SB>-l<SB>o</SB>)/B]. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for discriminating an alkali metal composite transition metal oxide used in a secondary battery using an alkali metal composite transition metal oxide as a positive electrode active material.

Sodium composite transition metal oxides have been studied as positive electrode active materials for secondary batteries. In that research, sodium composite transition metal oxide was used as the positive electrode active material to discriminate sodium composite transition metal oxide candidates that give higher capacity sodium secondary battery as the positive electrode active material of the secondary battery. It has been practiced to manufacture a secondary battery and obtain the discharge capacity by experiment.
Therefore, a method for discriminating an alkali metal composite transition metal oxide serving as a positive electrode active material that provides a high-capacity sodium secondary battery has been demanded.

  As one of methods for predicting physical properties, there is known an example in which the hole concentration is calculated using Bond Valence Sum for the purpose of improving the superconducting transition temperature Tc in the oxide superconductor (for example, Patent Documents). However, a method for discriminating an alkali metal composite transition metal oxide using Bond Valence Sum has not been known.

Japanese Patent Laid-Open No. 10-259022

  An object of the present invention is to provide a method for discriminating an alkali metal composite transition metal oxide serving as a positive electrode active material that provides a high-capacity sodium secondary battery.

  In order to solve the above problems, the present inventor has intensively studied a method for discriminating an alkali metal composite transition metal oxide serving as a positive electrode active material for a sodium secondary battery, and as a result, calculates and uses Bond Valence Sum. As a result, the present invention has been completed.

That is, the present invention provides the following <1> to <16>.
<1> A method for discriminating an alkali metal composite transition metal oxide serving as a positive electrode active material for a sodium secondary battery, wherein the position of atoms in a crystal structure is determined, and G (x, y, z) in formula (1) G is the minimum G (x, y, z) in which three or more alkali metal atoms are included on the side including the alkali metal atoms in the region surrounded by the G (x, y, z) isosurface. A discrimination method characterized by selecting a substance having a small _CRIT .
(1)
(here,
Is Bond Valence Sum, and the combination of l ₀ and B is the Bond Valence parameter. (X, y, z) is the position coordinate of the alkali metal atom, and l _j is the distance between the electron withdrawing atom near the alkali metal atom and the alkali metal atom. )
<2> The determination method according to <1>, wherein the alkali metal is sodium.
<3> The determination method according to <1> or <2>, wherein a substance having a G _CRIT of less than 0.8 is selected.
<4> A computer-readable recording medium that stores a program for executing the determination method according to any one of <1> to <3>.
<5> An apparatus equipped with a program for executing the determination method according to any one of <1> to <3>.
<6> Alkali metal composite transition metal oxide represented by formula (13), wherein G _CRIT is less than 0.8 when determined by the method according to any one of <1> to <3> Composite transition metal oxide.
A _a M _b L _c PO ₄
(13)
(Here, A represents one or more elements selected from alkali metals, M represents one or more elements selected from transition metals, and L represents one or more elements selected from transition metals other than M.) A, b and c are numerical values satisfying all of the formulas 0.5 ≦ a ≦ 1.5, 0 <b <1.5, 0 <c <1.5 and 0.5 ≦ b + c ≦ 1.5. is there.)
<7> An alkali metal composite transition metal oxide represented by the formula (13), wherein the difference between the average bond distance (d1 _AVE ) between M and O (d1 _AVE ) and the average bond distance (d2 _AVE ) between L and O (| d1 _AVE -d2 _AVE |) is greater than 0.05 Å, an alkali metal composite transition metal oxide according to <6>.
<8> The alkali metal composite transition metal oxide according to <6> or <7>, in which A contains at least sodium.
<9> The alkali metal composite transition metal oxide according to <6> or <7>, wherein A is sodium.
<10> The alkali metal composite transition metal oxide according to any one of <6> to <9>, in which M contains at least iron.
<11> The alkali metal composite transition metal oxide according to any one of <6> to <9>, wherein M is iron.
<12> A positive electrode active material for a sodium secondary battery, comprising the alkali metal composite transition metal oxide according to any one of <6> to <11>.
<13> A sodium secondary battery comprising the positive electrode active material according to <12>.
<14> The sodium secondary battery according to <13>, having a negative electrode comprising a carbonaceous material and a binder.
<15> The sodium secondary battery according to <13> or <14>, further including a separator.
<16> The sodium secondary battery according to <15>, wherein the separator is a separator having a laminated porous film in which a heat-resistant porous layer containing a heat-resistant resin and a porous film containing a thermoplastic resin are laminated.

  According to the present invention, a secondary battery using an alkali metal composite transition metal oxide as a positive electrode active material is manufactured, and a sodium secondary battery that easily provides a high-capacity sodium secondary battery can be obtained without experimentally determining a discharge capacity. The positive electrode active material for a battery can be identified. Moreover, the positive electrode active material which gives a high capacity | capacitance sodium secondary battery can also be obtained using the discrimination method. Therefore, since the development speed can be dramatically improved, the present invention is extremely useful industrially.

It is a figure explaining Example 1 and Example 2 of this invention. It is a figure which shows the case where three sodium atoms are not contained in the side with the sodium atom of the area | region enclosed by the isosurface (G = 0.25) of G. It is a figure which shows the case where three sodium atoms are contained in the side with the sodium atom of the area | region enclosed by the isosurface (G = 0.45) of G.

The discriminating method of the present invention is a discriminating method of an alkali metal composite transition metal oxide serving as a positive electrode active material for a sodium secondary battery, wherein the position of atoms in the crystal structure is determined, and G (x, y, z) is calculated, and the minimum G (x, y, z) in which three or more alkali metal atoms are included on the side including the alkali metal atoms in the region surrounded by the G (x, y, z) isosurface ) _Is a material having a small G _CRIT .
(1)
here,
Is Bond Valence Sum, and the combination of l ₀ and B is the Bond Valence parameter. l _j is the distance between the alkali metal atom coordinate (x, y, z) and the vicinity (˜3 原子) of the electron-withdrawing atom (mainly oxygen and sometimes fluorine) and the alkali metal atom. In addition, the sum of Bond Valence is usually taken up to the nearest six at most.

Hereinafter, the present invention will be described in detail.
As mentioned above,
Is Bond Valence Sum (hereinafter sometimes referred to as BVS).

In order to calculate BVS, first, it is necessary to determine the crystal structure of the alkali metal composite transition metal oxide to be calculated.
Note that a composite oxide of an alkali metal oxide and a transition metal oxide is an alkali metal composite transition metal oxide.

  If the alkali metal composite transition metal oxide is known, the crystal structure for calculating BVS can be determined using, for example, a crystal database. As the crystal database, for example, databases such as ICSK, ICSJ, JICST, Cambridge crystal structure database (CSD), inorganic crystal structure database (ICSD), NIST crystal database (NCD), PAULING FILE database, PEARSON database can be used. , Http://www.mindat.org/, http://www.webmineral.com/, http://un2sg4.unige.ch/athena/mineral/mineral.html, http://www.meinemineraliensammlung.de You can also check from /, http://rruff.geo.arizona.edu/AMS/amcsd.php, http://database.iem.ac.ru/mincryst/index.php, but this is not a limitation.

  In addition, the structure obtained from the crystal database can be determined by optimizing the structure with a computer. Structural optimization includes quantum chemical methods, density functional methods, molecular mechanics (MM) methods, (classical) molecular dynamics (MD) methods, quantum (ab initio) molecular dynamics (MD) methods, The calculation can be performed using a Monte Carlo (MC) method, a quantum Monte Carlo (QMC) method, or the like. These calculations are, AMBER (Assisted Model Building with Energy Refinement), CHARMM (Chemistry at HARvard Molecular Mechanics), GROMACS (Groningen Machine for Chemical Simulations), NAMD, PEACH (Program for Energetic Analysis of bioCHemical molecules), Gaussian, GAMESS ( General Atomic and Molecular Electronic Structure System (ADF), MOPAC, SIESTA (Spanish Initiative for Electronic Simulation with Thousands of Atoms, VASP (Vienna Ab-initiation Simulation Package), DMol3, GULP (General Utility Lattice Program), PHASE (Nano-A) But this is not the case.

  Even when the structure of the alkali metal composite transition metal oxide is unknown, the crystal structure for calculating BVS can be determined, for example, as follows.

  For example, the structure of the crystal can be determined from analysis of measurement results such as single crystal X-ray diffraction, single crystal neutron diffraction, powder X-ray diffraction, powder neutron diffraction, and electron beam diffraction of alkali metal composite transition metal oxides.

  Even if the lattice constant, space group, etc. are determined from analysis of measurement results such as single crystal X-ray diffraction, single crystal neutron diffraction, powder X-ray diffraction, powder neutron diffraction, electron beam diffraction, etc. There are also alkali metal composite transition metal oxides that occupy (solid solution sites) and whose atomic positions are not fixed. The crystal structure of the alkali metal composite transition metal oxide with a solid solution site, for example, prepare a sufficiently large crystal structure so that the atomic position corresponding to the solid solution site satisfies the seat occupation ratio of the transition metal. This can be determined by randomly arranging atoms.

In addition, a structure in which a part of the transition metal (transition metal 1) is replaced with another transition metal (transition metal 2) in the alkali metal composite transition metal oxide (known oxide 1) having a known crystal structure is as follows. It can be determined as follows. Here, a layered rock salt type Li (Ni _1/3 Mn _1/3 Co _1/3 ) O ₂ (hereinafter referred to as oxide 1) will be described as an example, but the target alkali metal composite transition metal oxide is described. This is not the case.

The oxide 1 is an oxide in which a part of Co in LiCoO ₂ is substituted with Ni and Mn, and belongs to the same space group as LiCoO ₂ . Ni and Mn are replaced with Co seats (3b seats) (here, Ni occupying a part of the 3a seats will be ignored). Therefore, using a crystal structure of LiCoO _2, a structure having a size including six locations corresponding to 3b-seat atomic positions is prepared. Two Ni atoms, Mn atoms, and two Co atoms are randomly arranged in a place corresponding to six 3b seats. Li atoms are arranged at locations corresponding to six 3a seats, and O atoms are arranged at locations corresponding to twelve 6c seats. Finally, lattice constants and atomic positions can be determined from computer structure optimization.

  Further, even if the composition of the alkali metal composite transition metal oxide to be discriminated (ratio of the number of atoms constituting the compound) does not exactly match the composition of the structure for calculating BVS, it can be discriminated. it can. Specifically, when the ratio of the number of atoms of two transition metals contained in the alkali metal composite transition metal oxide is 80%: 20%, in the structure for calculating BVS, the ratio is 75%: 25 % Or 85%: 15%.

  Further, the alkali metal composite transition metal oxide to be discriminated may be a single phase or a mixture containing two or more phases. In the case of a mixture containing two or more phases, the phase having the largest weight ratio (phase 1) may be discriminated. However, the smaller the sum of the weight ratios of all phases other than phase 1, the higher the discrimination accuracy. Become.

Once the crystal structure is determined and the lattice space in the range to be calculated is determined, the BVS can be calculated by the following procedure, for example.
Since there are n oxygen atoms in the crystal structure, names 1 to n (labels) are attached to each of the n oxygen atoms.

Let the coordinate of the i-th oxygen atom be XO (i).
The lattice space (superlattice) of the range to be calculated is divided into a pieces in the X direction, b pieces in the Y direction, c pieces in the Z direction, and divided into a total of a × b × c lattices, and the ix, iy, and iz th Let the coordinates of the grid points be XL (ix, iy, iz). For example, a = b = c = about 100. Hereinafter, (ix, iy, iz) is not displayed and may be described as “XL”.

  For all XL (ix, iy, iz), a maximum of 6 oxygens (labeled 6 oxygen atoms as i1, i2, i, i2, i, i) in the order of the distance ABS (XL-XO (i)) to XO (i). i3, i4, i5, i6), and the distance (R1, R2, R3, R4, R5, R6) from XL is examined. At that time, oxygen whose distance from XL is larger than 2.8 cm is excluded.

When obtaining G (x, y, z) of the equation (1) for the coordinates (x, y, z), the value actually obtained for the coordinates (x, y, z) = (ix, iy, iz) is F ( ix, iy, iz), and F (ix, iy, iz) is calculated according to equation (2).


(2)

Here, the combination of the numerical values of l ₀ and B is the Bond Valence parameter (see RE Bresse and M. O'Keeffe, Acta Crystallogr., Sect. B, 47, 192 (1991)).
The values of l ₀ and B are, for example, below http://www.ccp14.ac.uk/
The parameters are listed in ccp / web-mirrors / i_d_brown / bond_valence_param / bvparm2006.cif and can be obtained. ABS indicates an absolute value, and Σ ^j _m indicates a sum for all XL.

For example, l ₀ = 1.5766 mm and B = 0.475 mm are used.

  At the stage where equation (2) is calculated, the calculation of BVS at each lattice point is completed.

Next, the isosurface is written in increments of about 0.01 from F (ix, iy, iz) = 0.00 to F (ix, iy, iz) = 0.50. At that time, the number of alkali metal atoms contained in the _isosurface is examined, and the minimum value F _CRIT of F (ix, iy, iz) for containing three or more alkali metal atoms is examined. The F _CRIT obtained here is regarded as G _CRIT .
The isosurface is a surface connecting points where F (ix, iy, iz) has the same value, and the space surrounded by the curved surface is within the isosurface.

Next, the calculation target substance and crystal structure are changed, and the F _CRIT is calculated. The smaller the value, the higher the discharge capacity of the secondary battery using the alkali metal composite transition metal oxide as the positive electrode active material. Expect big.

Examples of the alkali metal composite transition metal oxide to be subjected to the prediction method of the present invention include composite oxides containing a transition metal element of Group 3 to Group 11 of the periodic table and an alkali metal element. Specific examples of the transition metal elements of Groups 3 to 10 of the periodic table include Ni, Co, Mn, Fe, Ti, Zr, Hf, V, Nb, Ta, Cr, Sc, Y, Cu, Ag, and Mo is mentioned. As the alkali metal element, K and Na are preferable, and Na is more preferable. The alkali metal composite transition metal oxide of the present invention is more preferably a sodium composite transition metal oxide containing one or more of Ni, Co, Mn, and Fe.
An oxide of an alkali metal composite transition metal oxide means a compound to which oxygen is bonded, and includes not only a metal oxide but also a sulfate, phosphate, nitrate, and silicate.

Examples of the alkali metal composite transition metal oxide to be subjected to the prediction method of the present invention include NaFe _x Ma _(1-x) PO ₄ , NaMn _x Ma _(1-x) PO ₄ , and Na ₂ Mn _x Ma _{(1 -x) SiO 4, Na 2 Fe} x Ma (1-x) SiO 4 (Ma represents Ni, Co, Mo, one or more selected from Cu and _{Ti), NaMbPO 4, Na 2} MbSiO 4 (Mb Represents one or more selected from Ni, Mn, Co, Mo, Cu, Fe, and Ti.). Note that x represents a number greater than 0 and less than 1.

As the alkali metal composite transition metal oxide discriminated by the discriminating method of the present invention, the alkali metal composite transition metal oxide whose G _CRIT is judged to be smaller than 0.8 is a positive electrode that provides a high-capacity sodium secondary battery. Since it is an alkali metal composite transition metal oxide used as an active material, it is preferable.

Specifically, an alkali metal composite transition metal oxide represented by the formula (11) or (12) is more preferable.

A _a M _b PO ₄
(11)
(Here, A represents one or more elements selected from alkali metals, M represents one or more elements selected from transition metals, 0.5 ≦ a ≦ 1.5, 0.5 ≦ b ≦ 1.5.)

A _p M _q SiO ₄
(12)
(Here, A represents one or more elements selected from alkali metals, M represents one or more elements selected from transition metals, and 1.5 ≦ p ≦ 2.5, 0.5 ≦ q ≦ 1.5.)

An alkali metal composite transition metal oxide represented by the formula (13) or (14) is also more preferable.

A _a M _b L _c PO ₄
(13)
(Here, A represents one or more elements selected from alkali metals, M represents one or more elements selected from transition metals, and L represents one or more elements selected from transition metals other than M.) A, b and c are numerical values satisfying all of the formulas 0.5 ≦ a ≦ 1.5, 0 <b <1.5, 0 <c <1.5 and 0.5 ≦ b + c ≦ 1.5 .)

A _p M _q L _r SiO ₄
(14)
(Here, A represents one or more elements selected from alkali metals, M represents one or more elements selected from transition metals, and L represents one or more elements selected from transition metals other than M.) P, q and r are numerical values satisfying all of the formulas 1.5 ≦ p ≦ 2.5, 0 <q <1.5, 0 <r <1.5 and 0.5 ≦ q + r ≦ 1.5. is there.)

  Among the alkali metal composite transition metal oxides represented by the above formula (11), (12), (13) or (14), the alkali metal composite transition metal oxide represented by the formula (13) or (14) Preferably, an alkali metal composite transition metal oxide represented by the formula (13) is even more preferable.

G _CRIT can be _reduced by, for example, changing the lattice constant of the crystal or changing the atomic position in the crystal. The change in lattice constant and atomic position in the crystal is realized by substituting a part of M1 with an element (M2) having a different bond distance to oxygen in an alkali metal composite transition metal oxide containing transition metal M1. There is a case. The difference (| d (M2-O) -d (M2-O)) between the bond distance (d (M1-O)) of M1 with oxygen and the bond distance (d (M2-O)) of oxygen with M2 ) Gives changes to lattice constants and atomic positions in the crystal. In general, the larger the | d (M2-O) -d (M2-O) |, the greater the effect of such a structural change, and | d (M2-O) -d (M2-O) | If it is larger than .05 mm, the effect of changing the structure appears.

As an alkali metal composite transition metal oxide discriminated by the discriminating method of the present invention, G _CRIT is judged to be smaller than 0.8, and in formula (13), the average bond distance M of M, L and oxygen is An alkali metal composite transition metal oxide in which | d1 _AVE −d2 _AVE | which is the difference between the average bond distance d1 _AVE of O and the average bond distance d2 _AVE of L and O is larger than 0.05Å is preferable. Here, the average bond distance between the transition metal M (L) and oxygen is a distance between the oxygen contained in the sphere and M (L), which forms a sphere having a radius of 2.8 mm centered on the transition metal M (L). Is the average.

The average bond distance in the crystal can be determined by, for example, EXAFS (Extended X-ray Absorption Fine Structure) analysis of XAFS (X-ray Absorption Fine Structure) measurement. Further, for example, it can be obtained from an analysis using an atom pair correlation function of X-ray diffraction or neutron powder diffraction measurement (Pair Distribution Function).

  Even if it is not obtained experimentally, for example, a bond distance obtained from a crystal structure optimized by a computer can be used.

  In the above formula (11), (12), (13) or (14), when A contains at least sodium, it is an alkali metal composite transition metal oxide serving as a positive electrode active material that provides a high-capacity sodium secondary battery. Therefore, the case where A is sodium is more preferable. Moreover, the case where M contains iron at least is preferable, and the case where M is iron is more preferable.

The alkali metal composite transition metal oxide of the present invention can be produced by firing a mixture of metal-containing compounds having a composition that can be converted into the alkali metal composite transition metal oxide of the present invention by firing. Specifically, the metal-containing compound containing the corresponding metal element can be produced by weighing and mixing so as to have a predetermined composition, and then firing the resulting mixture. For example, an alkali metal composite transition metal oxide having an atomic ratio represented by Na: Mn: Ni: P = 1: 0.9: 0.1: 1, which is one of preferable compounds, is Na ₂ CO _3. , Each raw material compound of MnO ₂ and NiO and H ₃ PO ₄ are weighed so that the molar ratio of Na: Mn: Ni: P is 1: 0.9: 0.1: 1, and they are mixed, It can manufacture by baking the obtained mixture.

  Examples of the metal-containing compound that can be used to produce the alkali metal composite transition metal oxide of the present invention include oxides and compounds that can be converted to oxides when decomposed and / or oxidized at high temperatures, such as hydroxylated compounds. Products, carbonates, nitrates, halides, and oxalates can be used.

  For the mixing of the metal-containing compounds, industrially used apparatuses such as a ball mill, a V-type mixer, and a stirrer can be used. The mixing at this time may be either dry mixing or wet mixing. Further, a mixture of metal-containing compounds having a predetermined composition may be obtained by a crystallization method.

  The composite metal oxide of the present invention is obtained by holding and firing the mixture of the above metal-containing compounds in a temperature range of 600 ° C. to 1600 ° C. for 0.5 hours to 100 hours, for example.

  The alkali metal composite transition metal oxide of the present invention can be used as an electrode active material either alone or after being subjected to a surface treatment such as coating. The electrode active material of the present invention comprises the alkali metal composite transition metal oxide of the present invention. If the electrode which has the electrode active material of this invention is used as a positive electrode of a sodium secondary battery, the sodium secondary battery obtained will have a large discharge capacity when it repeats charging / discharging compared with the past. In addition, according to the present invention, the internal resistance of the obtained sodium secondary battery can be reduced, and the overvoltage during charging and discharging can be reduced. If the overvoltage at the time of charging / discharging can be made small, the large current discharge characteristic of a secondary battery can be improved more. In addition, the stability of the battery when the secondary battery is overcharged can be improved.

  The electrode of the present invention contains the electrode active material of the present invention. The electrode of the present invention can be produced by supporting an electrode mixture containing the electrode active material of the present invention, a conductive material and a binder on an electrode current collector. Hereinafter, the manufacturing method will be described by taking a sodium secondary battery as an example.

  Examples of the conductive material include carbon materials such as natural graphite, artificial graphite, cokes, and carbon black. Examples of the binder include thermoplastic resins. Specifically, polyvinylidene fluoride (hereinafter sometimes referred to as “PVDF”), polytetrafluoroethylene, tetrafluoroethylene, hexafluoropropylene, and fluoride. Fluorine resins such as vinylidene copolymers, propylene hexafluoride / vinylidene fluoride copolymers, tetrafluoroethylene / perfluorovinyl ether copolymers; and polyolefin resins such as polyethylene and polypropylene . As the electrode current collector, Al, Ni, stainless steel or the like can be used.

  As a method of supporting the electrode mixture on the electrode current collector, it is fixed by press molding or pasting using an organic solvent, coating on the electrode current collector, drying and pressing. A method is mentioned. In the case of forming a paste, a slurry made of an electrode active material, a conductive material, a binder, and an organic solvent is prepared. Examples of organic solvents 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 N-methyl- Examples include aprotic polar solvents such as 2-pyrrolidone. Examples of the method of coating the electrode mixture on the 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.

  The sodium secondary battery of the present invention has the electrode of the present invention as a positive electrode. The sodium secondary battery of the present invention, for example, obtains an electrode group by laminating and winding the electrode (positive electrode) of the present invention, a separator and a negative electrode in which a negative electrode mixture is supported on a negative electrode current collector in this order, The electrode group is housed in a battery case, and can be manufactured by impregnating the electrode group with an electrolytic solution containing an electrolyte and an organic solvent.

  Here, as the shape of the electrode group, for example, a cross section when the electrode group is cut in a direction perpendicular to the winding axis is a circle, an ellipse, an ellipse, a rectangle, a rectangle with rounded corners, or the like. Can be mentioned. In addition, examples of the shape of the battery include a paper shape, a coin shape, a cylindrical shape, and a square shape.

  As a negative electrode that can be used in the sodium secondary battery of the present invention, a negative electrode mixture containing a negative electrode active material is supported on a negative electrode current collector, and sodium ions such as sodium metal or sodium alloy can be occluded / desorbed. An electrode can be used. Examples of the negative electrode active material include carbon materials such as natural graphite, artificial graphite, coke, carbon black, pyrolytic carbon, carbon fiber, and organic polymer compound fired body capable of absorbing and desorbing sodium ions. . The negative electrode used in the sodium secondary battery of the present invention is preferably a negative electrode containing a carbonaceous material and a binder.

  Further, as the negative electrode active material, chalcogen compounds such as oxides and sulfides that can occlude and desorb sodium ions at a lower potential than the positive electrode can also be used.

  The negative electrode mixture may contain a binder and a conductive material as necessary. Therefore, the negative electrode of the sodium secondary battery of the present invention may contain a mixture of a negative electrode active material and a binder. Examples of the binder include a thermoplastic resin, and specific examples include PVDF, thermoplastic polyimide, carboxymethyl cellulose, polyethylene, and polypropylene.

  Examples of the material for the negative electrode current collector include metals such as Cu, Ni, and stainless steel. Cu is preferable because it is difficult to form an alloy with sodium and it is easy to process into a thin film. The method of supporting the negative electrode mixture on the negative electrode current collector is the same as the above case. The method of pressure molding, pasting with a solvent, etc., coating on the negative electrode current collector, and pressing after drying For example, a method of fixing by, for example.

  Examples of the separator that can be used in the sodium secondary battery of the present invention include porous films, nonwoven fabrics, woven fabrics, and the like made of materials such as polyolefin resins such as polyethylene and polypropylene, fluororesins, and nitrogen-containing aromatic polymers. A material having a form can be used. Moreover, it is good also as a single layer or laminated separator which used 2 or more types of these materials. Examples of the separator include separators described in JP 2000-30686 A, JP 10-324758 A, and the like. The thickness of the separator is preferably as thin as possible as long as the mechanical strength is maintained in that the volume energy density of the battery is increased and the internal resistance is reduced. In general, the thickness of the separator is preferably about 5 to 200 μm, more preferably about 5 to 40 μm.

  The separator preferably has a porous film containing a thermoplastic resin. In a secondary battery, the separator is placed between the positive electrode and the negative electrode, and when an abnormal current flows in the battery due to a short circuit between the positive electrode and the negative electrode, the current is interrupted and an excessive current flows. It preferably plays the role of blocking (shuts down). Here, the shutdown is performed by closing the micropores of the porous film in the separator when the normal use temperature is exceeded. And even if the temperature of the separator rises to a certain high temperature after shutting down, the separator is preferably maintained in the shut-down state without being broken by the temperature, in other words, preferably has high heat resistance. . As such a separator, a porous film having a heat-resistant material such as a laminated film in which a heat-resistant porous layer and a porous film are laminated, preferably a heat-resistant porous layer containing a heat-resistant resin and a porous film containing a thermoplastic resin Can be mentioned, and by using a porous film having such a heat-resistant material as a separator, it is possible to further prevent thermal breakage of the secondary battery of the present invention. Here, the heat-resistant porous layer may be laminated on both surfaces of the porous film.

  Examples of the heat-resistant resin contained in the heat-resistant porous layer include polyamide, polyimide, polyamideimide, polycarbonate, polyacetal, polysulfone, polyphenylene sulfide, polyether ketone, aromatic polyester, polyether sulfone, and polyetherimide. From the viewpoint of further improving heat resistance, polyamide, polyimide, polyamideimide, polyethersulfone, and polyetherimide are preferable, and polyamide, polyimide, and polyamideimide are more preferable.

In the electrolyte solution that can be used in the sodium secondary battery of the present invention, electrolytes include NaClO ₄ , NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ , lower Examples thereof include aliphatic carboxylic acid sodium salt and NaAlCl _4, and a mixture of two or more of these may be used. Among these, it is preferable to use those containing at least one selected from the group consisting of NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ and NaN (SO ₂ CF ₃ ) ₂ containing fluorine.

  In the electrolyte solution that can be used in the sodium secondary battery of the present invention, examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4-trifluoromethyl. Carbonates such as -1,3-dioxolan-2-one and 1,2-di (methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2 , 3,3-tetrafluoropropyldifluoromethyl ether, ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; esters such as methyl formate, methyl acetate and γ-butyrolactone; Nitriles such as tolyl and butyronitrile; amides such as N, N-dimethylformamide and N, N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfolane, dimethyl sulfoxide and 1,3-propane sultone Sulfur-containing compounds such as those described above; or those obtained by further introducing a fluorine substituent into the above organic solvent can be used. Usually, two or more of these are mixed and used as the organic solvent.

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.

Example 1
(Positive electrode active material 1: Synthesis of Na (Fe _0.8 Cu _0.2 ) PO ₄ )
Sodium carbonate (Na ₂ CO ₃ ), iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), copper oxide (CuO) and diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ) After weighing in an amount such that the molar ratio of Na): iron (Fe): copper (Cu): phosphorus (P) was 1: 0.8: 0.2: 1, the mixture was mixed in an agate mortar for 20 minutes. The obtained unfired sample was put into an alumina crucible, and pre-baked for 10 hours in an electric furnace at 450 ° C. while aeration of nitrogen gas at a flow rate of 2 liters / minute.

The sample after this preliminary calcination was pulverized in an agate mortar for 20 minutes. Thereafter, while firing again at a flow rate of 2 liters / minute, main firing was performed in an electric furnace at 800 ° C. for 24 hours, and pulverization was performed with a ball mill to synthesize positive electrode active material 1.

(Example 2)
(Distinction of positive electrode active material 1)
The crystal structure of the positive electrode active material 1 was unknown. In the case of the active material 1 in which a part of Fe atoms of NaFePO ₄ is substituted with Cu atoms, since Cu is considered to be dissolved in the Fe site, the position of the atoms in the crystal structure was determined by the following procedure.

The crystal structure of NaFePO ₄ was searched from the ICSD database, and the structure (structure 1, FIG. 1) of the crystal structure of ID85671 (Table 1) was prepared. The unit cell contains 4 Fe, but a crystal structure (Structure 2, FIG. 1) in which an Fe atom at a fractional coordinate (0.5, 0.5, 0.5) is replaced with a Cu atom. ) Was made. The structure optimization calculation by the first principle calculation was performed with the structure as the initial structure. The structure optimization was calculated as the space group P1 using version 4.6 of the Vienna Ab-initio Simulation Package (VASP). The following parameters were adopted in the calculation.

Space group: PNMA
Lattice constants: a = 8.99Å, b = 6.862Å, c = 5.047Å
α = β = γ = 90 °

In the calculation of structural optimization, atomic coordinates, cell volume, and cell shape were all moved simultaneously. When the force acting on all the atoms became smaller than 0.02 eV / Å, the optimum structure was obtained. The energy cut-off was 500 eV, the wave number space mesh was 0.5 / G, GGA-PBE PAW, and Magnetism were calculated with Spin-polarized.
This gave structure 3 (details are given in Table 2).

Lattice constants: a = 9.0264 Å, b = 6.950 ｃ, c = 5.121 Å
α = 93.4 °, β = 90.3 °, γ = 90.2 °
Space group: P1

For structure 3, G (x, y, z) was calculated according to the following method.
A structure (structure 4) having a size of 2a × 2b × 4c with the structure 3 as a unit cell was produced. First, 256 oxygen coordinates contained therein were extracted. Here, these 256 oxygen atoms were labeled up to 256 in the order of 1, 2, and 3, and the coordinates of the j-th oxygen atom were XO (j). Tables 3 to 9 show the coordinates XO (j) of 256 oxygen atoms as fractional coordinates when an orthogonal lattice of x = 18.038Å, y = 13.749Å and z = 20.183Å is a unit cell. Indicated. In subsequent calculations, only the relative distance is required, and therefore, the fractional coordinate (0, 0, 0) is used as the relative coordinate origin (0, 0, 0). Therefore, the subsequent calculations do not depend on the origin setting here.

The structure 4 was cut into a 100 × 100 × 100 orthogonal lattice, and the coordinates of the ix, iy, and iz-th lattice points were XL (ix, iy, iz).
Next, in each XL (ix, iy, iz), a maximum of six oxygen atoms (labels j1, j2, j3, j4, j5, j6) are searched in the order of the closest distance ABS to the XL (XL-XO). , XL distances (R1, R2, R3, R4, R5, R6) were examined. At that time, oxygen atoms whose distance to XL was larger than 2.8 cm were excluded.

F (ix, iy, iz) was calculated according to equation (2).
(2)

The combination of l ₀ and B is a parameter called Bond Valence parameter, and can be obtained from ccp / web-mirrors / i_d_brown / bond_valence_param / bvparm2006.cif below http://www.ccp14.ac.uk/ it can.
Although there are several parameters for monovalent sodium atom, the value of F was calculated according to the equation (2) using l ₀ = 1.5766７ and B = 0.475Å among them.

Next, the isosurface of F (ix, iy, iz) is changed from F (ix, iy, iz) = 0.00 to F (ix, iy, iz) = 0.50 in steps of 0.01. Visualization started. At this time, the coordinates of the sodium atom of the structure 4 were made visible on the isosurface display of F (ix, iy, iz). For visualization, MicroAVS4.0 was used.
On the isosurface of F (ix, iy, iz) = 0.00, 3 or more sodium atoms were not included on the side including the sodium atoms in the region surrounded by the isosurface. Therefore, when an isosurface was drawn while increasing F (ix, iy, iz) from 0.01 to 0.01, even if F (ix, iy, iz) = 0.25 Three or more sodium atoms were not contained on the side containing the sodium atoms in the region surrounded by the surface (FIG. 2). However, when the isosurface of F (ix, iy, iz) = 0.40 is displayed, the side including the sodium atom in the region surrounded by the isosurface of F (ix, iy, iz) = 0.47 3 contained 3 sodium atoms (FIG. 3). When an isosurface of F (ix, iy, iz) is drawn while increasing by 0.01 to F (ix, iy, iz) = 0.50, an F (ix, iy, iz) isosurface is obtained. Three sodium atoms were contained on the side of the enclosed region containing sodium atoms.

For structure 3, the average bond distance d (Fe—O) _AVE of Fe and O atoms is 2.2 Å, the average bond distance d (Cu—O) _AVE of Cu and O atoms is 2.3 、 ２, and | d (Fe—O) _AVE −d (Cu—O) _AVE |> 0.05%.
Therefore, it was determined that Na (FeCu) PO ₄ is suitable as a positive electrode active material for an alkali metal secondary battery and gives a high capacity alkali metal secondary battery.

(Example 3)
(Preparation of positive electrode including positive electrode active material 1)
Positive electrode active material 1, acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive material, and PVDF (manufactured by Kureha Co., Ltd., Poly Vinylide Polyfluoride) as a binder, positive electrode active material 1: conductive material: binder = 80: Each was weighed to have a composition of 15: 5 (weight ratio). Thereafter, the positive electrode active material and acetylene black are first thoroughly mixed in an agate mortar, and an appropriate amount of N-methyl-2-pyrrolidone (NMP: manufactured by Tokyo Chemical Industry Co., Ltd.) is added to this mixture, followed by further addition of PVDF and uniform. Were mixed to form a slurry. The obtained slurry is applied to an aluminum foil having a thickness of 40 μm, which is a current collector, with a thickness of 100 μm using an applicator, and this is put into a dryer and sufficiently dried while removing NMP. As a result, an electrode sheet was obtained. This electrode sheet was punched to a diameter of 1.5 cm with an electrode punching machine, and then sufficiently pressed with a hand press to obtain a positive electrode sheet.

(Production of test battery)
A positive electrode sheet is placed in a dent on the lower side of a coin cell (manufactured by Hosen Co., Ltd.) with the aluminum foil facing down, and 1M NaClO ₄ / PC (propylene carbonate) as an electrolyte and a polypropylene porous film as a separator A test battery was prepared by combining (thickness 20 μm) and metallic sodium (manufactured by Aldrich) as a negative electrode. The test battery was assembled in a glove box in an argon atmosphere.

(Charge / discharge test)
Using the positive electrode active material 1 obtained above as a positive electrode active material for a sodium secondary battery, a test battery was prepared and a constant current charge / discharge test was performed.

Charging was performed by CC (Constant Current) at a 0.05 C rate (rate of complete charging in 20 hours) from rest potential to 4.2 V. As for the discharge, CC discharge was performed at a 0.05 C rate and cut off at a voltage of 1.5 V. As a result, the charge capacity in the second cycle was 83 mAh / g, the discharge capacity was 74 mAh / g, and the subsequent cycles were also maintained.
Therefore, it was found that Na (Fe _0.8 Cu _0.2 ) PO ₄ is suitable as a positive electrode active material for sodium secondary batteries, has a high capacity, and provides a sodium secondary battery excellent in cycle characteristics.

Example 4
(Determination of NaFePO ₄ )
Using structure 1 as the initial structure, structure optimization calculation by first principle calculation was performed under the same conditions as in Example 2, and structure 5 was obtained. Using this structure as a unit cell, a structure 6 having a size of 2a × 2b × 4c was obtained. For structure 6, calculation was performed in the same procedure as in Example 2 to obtain F (ix, iy, iz). With respect to the obtained F (ix, iy, iz), when an isosurface is drawn while increasing from 0.00 to 0.01, when F (ix, iy, iz) <0.81, F (ix, iy, iz) ix, iy, iz) In the region surrounded by the isosurface, no more than 3 sodium atoms were contained on the side containing sodium atoms. When an isosurface of F (ix, iy, iz) = 0.81 was drawn, three sodium atoms were included on the side of the region surrounded by the isosurface with the sodium atoms.
Therefore, it was determined that the compound of structure 5 is not suitable as a positive electrode active material for sodium secondary batteries and does not give a high-capacity sodium secondary battery.

(Example 5)
(Discrimination 2 of positive electrode active material 1)
The crystal structure of the positive electrode active material 1 was unknown. In the case of the active material 1 in which a part of Fe atoms of NaFePO ₄ is substituted with Cu atoms, the positions of atoms in the crystal structure different from that in Example 2 are determined on the assumption that Cu is dissolved in the Na site.
The crystal structure of NaFePO ₄ was searched from the ICSD database, and the structure (structure 1, FIG. 1) of the crystal structure of ID85671 (Table 1) was prepared. The unit cell contains four Na atoms, but a crystal structure was formed in which Na atoms at fractional coordinates (0.1504, 0.25, 0.5296) were replaced with Cu atoms. The structure optimization calculation by the first principle calculation was performed with the structure as the initial structure. For structural optimization, the same calculation as in Example 2 was performed for the space group P1 using version 4.6 of the Vienna Ab-initio Simulation Package (VASP).
In the calculation of structural optimization, atomic coordinates, cell volume, and cell shape were all moved simultaneously. When the force acting on all the atoms became smaller than 0.02 eV / Å, the optimum structure was obtained. The energy cut-off was 500 eV, the wave number space mesh was 0.5 / G, GGA-PBE PAW, and Magnetism were calculated with Spin-polarized.
This resulted in structure 7.

For structure 7, G (x, y, z) was calculated according to the following method.
Using structure 7 as a unit cell, structure 8 having a size of 2a × 2b × 4c was obtained. For structure 8, calculation was performed in the same procedure as in Example 2 to obtain F (ix, iy, iz).
When the isosurface display is performed for the obtained F (ix, iy, iz), there are three on the side containing the sodium atom in the region surrounded by the isosurface of F (ix, iy, iz) = 0.50. Of sodium atoms.
For structure 8, the average bond distance d (Fe—O) _AVE of Fe and O atoms is 2.11 Å, the average bond distance d (Cu—O) _AVE of Cu and O is 2.23 、, | d ( Fe-O) _AVE- d (Cu-O) _AVE |> 0.05%.
Therefore, it was determined that Na (FeCu) PO ₄ is suitable as a positive electrode active material for an alkali metal secondary battery and gives a high capacity alkali metal secondary battery.

Claims (16)

A method for discriminating an alkali metal composite transition metal oxide serving as a positive electrode active material for a sodium secondary battery, wherein the position of an atom in a crystal structure is determined, and G (x, y, z) in equation (1) is calculated. G _CRIT which is the minimum G (x, y, z) containing three or more alkali metal atoms on the side containing the alkali metal atoms in the region surrounded by the G (x, y, z) _isosurface is small. A discrimination method characterized by selecting a substance.
(1)
(here,
Is Bond Valence Sum, and the combination of l ₀ and B is the Bond Valence parameter. (X, y, z) is the position coordinate of the alkali metal atom, and l _j is the distance between the electron withdrawing atom near the alkali metal atom and the alkali metal atom. ) The discrimination method according to claim 1, wherein a substance having a G _CRIT of less than 0.8 is selected.   The discrimination method according to claim 1 or 2, wherein the alkali metal is sodium.   A recording medium readable by a computer and storing a program for executing the determination method according to claim 1.   The apparatus which mounts the program for performing the determination method in any one of Claims 1-3. An alkali metal composite transition metal oxide represented by the formula (13), wherein when determined by the method according to claim 1, an alkali metal composite transition metal oxide having a G _CRIT of less than 0.8 .
A _a M _b L _c PO ₄
(13)
(Here, A represents one or more elements selected from alkali metals, M represents one or more elements selected from transition metals, and L represents one or more elements selected from transition metals other than M.) A, b and c are numerical values satisfying all of the formulas 0.5 ≦ a ≦ 1.5, 0 <b <1.5, 0 <c <1.5 and 0.5 ≦ b + c ≦ 1.5. is there.) An alkali metal composite transition metal oxide represented by the formula (13), wherein the difference between the average bond distance (d1 _AVE ) between M and O (d1 _AVE ) and the average bond distance (d2 _AVE ) between L and O (| d1 _AVE −d2) The alkali metal composite transition metal oxide according to claim 6, wherein _AVE |) is greater than 0.05.   The alkali metal complex transition metal oxide according to claim 6 or 7, wherein A contains at least sodium.   The alkali metal complex transition metal oxide according to claim 6 or 7, wherein A is sodium.   The alkali metal composite transition metal oxide according to any one of claims 6 to 9, wherein M contains at least iron.   M is iron, The alkali metal complex transition metal oxide in any one of Claims 6-9.   The positive electrode active material for sodium secondary batteries containing the alkali metal composite transition metal oxide in any one of Claims 6-11.   A sodium secondary battery comprising the positive electrode active material according to claim 12.   The sodium secondary battery according to claim 13, comprising a negative electrode containing a carbonaceous material and a binder.   The sodium secondary battery according to claim 13 or 14, further comprising a separator.   The sodium secondary battery according to claim 15, wherein the separator is a separator having a laminated porous film in which a heat-resistant porous layer containing a heat-resistant resin and a porous film containing a thermoplastic resin are laminated.