JP2015018621A - Sodium battery cathode active material and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sodium battery cathode active material or the like capable of inhibiting degradation of an electrolyte.SOLUTION: There is provided a sodium battery cathode active material represented by the general formula (1) defined by NaM(AO)(PO), characterized by having on the surface a coating part containing a phosphide of M in the general formula (1). (In the general formula (1), M is at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn; A is at least one element selected from the group consisting of Al, Si, P, S, Ti, V, and W; x satisfies 4≥x≥2; y satisfies 4≥y≥1; z satisfies 4≥z≥0; w satisfies 1≥w≥0; and at least one of z and w is larger than or equal to 1.)

Description

  The present invention relates to a positive electrode active material for a sodium battery that can suppress decomposition of an electrolytic solution.

  In recent years, with the rapid spread of information-related equipment such as personal computers, video cameras, and mobile phones, and communication equipment, development of batteries that are used as power sources has been regarded as important. Also in the automobile industry, development of high-power and high-capacity batteries for electric vehicles and hybrid vehicles is underway. Among various types of batteries, lithium batteries are attracting attention because of their high energy density and output.

In a lithium battery, a lithium metal composite oxide having a layered structure such as lithium nickelate or lithium cobaltate is generally used as a positive electrode active material, and lithium ion can be occluded and released as a negative electrode active material. Carbon materials, lithium metals, lithium alloys and the like are used. Moreover, as the electrolyte interposed between the positive electrode and the negative electrode, an electrolyte solution in which a lithium salt is dissolved, a solid electrolyte containing lithium, or the like is used.
Lithium batteries are excellent in energy density and output as described above. On the other hand, the price of lithium has increased along with the expansion of demand for lithium batteries, and the amount of lithium reserves has been limited. It has become a bottleneck in the process.

  Therefore, research is being conducted on sodium batteries that use low-cost sodium, which is abundant in resource reserves, instead of lithium.

International Publication No. 2013/031331 Pamphlet

Prior to the present invention, the present inventor has developed a compound represented by Na ₄ Ni ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) and a compound represented by the following general formula (1) into a high potential in a sodium battery. It has been found as a positive electrode active material capable of operating in the region (Patent Document 1).
General formula (1)
_{Na x M y (AO 4)} z (P 2 O 7) w
(In the general formula (1), M is at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, and A is Al, Si, P, At least one selected from the group consisting of S, Ti, V and W, x satisfies 4 ≧ x ≧ 2, y satisfies 4 ≧ y ≧ 1, z satisfies 4 ≧ z ≧ 0, w Satisfies 1 ≧ w ≧ 0, and at least one of z and w is 1 or more.)
However, the above-described positive electrode active material has a problem of high activity against the decomposition of the electrolytic solution.

  This invention is made | formed in view of the said situation, and it aims at providing the positive electrode active material for sodium batteries which can suppress decomposition | disassembly of electrolyte solution, and its manufacturing method.

  As a result of diligent research to solve the above problems, the present inventor provided a coat portion containing M phosphide in the general formula (1) on the surface of the positive electrode active material, thereby decomposing the electrolytic solution. Has been found to be able to be suppressed, and the present invention has been completed.

The present invention is a sodium battery positive electrode active material represented by the following general formula (1), and has a coat portion containing M phosphide in the above general formula (1) on the surface. A positive electrode active material is provided.
General formula (1)
_{Na x M y (AO 4)} z (P 2 O 7) w
(In the general formula (1), M is at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, and A is Al, Si, P, At least one selected from the group consisting of S, Ti, V and W, x satisfies 4 ≧ x ≧ 2, y satisfies 4 ≧ y ≧ 1, z satisfies 4 ≧ z ≧ 0, w Satisfies 1 ≧ w ≧ 0, and at least one of z and w is 1 or more.)

  According to this invention, it can be set as the positive electrode active material for sodium batteries which can suppress decomposition | disassembly of electrolyte solution by having on the surface the coating part containing the phosphide of M in the said General formula (1).

In the above invention, in the above general formula (1), said M is Ni, it is preferable phosphide of said M is Ni 3 _P.

  The present invention provides a method for producing a positive electrode active material for a sodium battery for producing the above-described positive electrode active material for a sodium battery, wherein the precursor material represented by the general formula (1) is calcined together with a reducing agent, and the M The manufacturing method of the positive electrode active material for sodium batteries characterized by having the reduction | restoration process which forms the coat part containing the phosphide of this.

  According to this invention, by having the said reduction | restoration process, the above-mentioned positive electrode active material for sodium batteries can be manufactured.

  The positive electrode active material of the present invention has an effect that the decomposition of the electrolytic solution can be suppressed by having a coat portion containing the M phosphide in the general formula (1) on the surface.

It is a schematic sectional drawing which shows the example of the positive electrode active material for sodium batteries of this invention. It is the figure which looked at the crystal structure of space group Pn2 ₁ a from the a-axis direction. It is the figure which looked at the crystal structure of space group Pn2 ₁ a from the b-axis direction. It is the figure which looked at the crystal structure of space group Pn2 ₁ a from the c-axis direction. It is a schematic sectional drawing which shows an example of a sodium battery. It is process drawing which shows an example of the manufacturing method of the positive electrode active material for sodium batteries. It is an XRD pattern of the electrode (positive electrode) of an Example and a comparative example. It is a graph which shows the charging / discharging curve of the 1st cycle in the measurement cell of an Example and a comparative example. It is a graph which shows the charging / discharging curve at the time of the high current density (170 mAhg < ^-1 >, 1C rate) application in the measurement cell of an Example.

  Hereinafter, the positive electrode active material for sodium batteries of the present invention (hereinafter sometimes simply referred to as a positive electrode active material) and a method for producing the same will be described.

A. The positive electrode active material for sodium batteries The positive electrode active material for sodium batteries of the present invention will be described with reference to the drawings.
1 (a) and 1 (b) are schematic cross-sectional views showing examples of the positive electrode active material for sodium battery of the present invention. The positive electrode active material 10 of the present invention is represented by the above general formula (1), and has a coat portion 1 containing M phosphide in the above general formula (1) on the surface.
In FIG. 1A, in the positive electrode active material 10 of the present invention, an example in which the coat portion 1 and the core portion 2 on the center side of the positive electrode active material 10 from the coat portion 1 have no interface and are continuous. Show. Moreover, in FIG.1 (b), in the positive electrode active material 10 of this invention, the example in which the coat part 1 and the core part 2 have an interface is shown.

  According to this invention, it can be set as the positive electrode active material for sodium batteries which can suppress decomposition | disassembly of electrolyte solution by having on the surface the coating part containing the phosphide of M in the said General formula (1).

Although this reason is not necessarily clear, it is considered as follows.
It is considered that the M phosphide contained in the coat part has low activity against the decomposition of the electrolytic solution. Therefore, the positive electrode active material of the present invention has a coating portion containing the M phosphide in the general formula (1) on the surface, so that the electrolytic solution and the highly active portion of the positive electrode active material are in direct contact with each other. It is conceivable that the decomposition of the electrolytic solution (deterioration of the electrolytic solution) can be suppressed because the electrolytic solution and the coating portion are in contact with each other.

  Moreover, according to this invention, the capacity | capacitance of the sodium battery using the positive electrode active material of this invention can be increased, and charge / discharge efficiency, a capacity | capacitance maintenance factor, and a high rate charge / discharge characteristic can be made favorable.

Although this reason is not necessarily clear, it is considered as follows.
As described above, the positive electrode active material of the present invention can suppress the activity against the decomposition of the electrolytic solution by having the coat portion containing M phosphide on the surface. The M phosphide contained in the coat part is a substance having good electron conductivity. Therefore, it is conceivable that the positive electrode active material of the present invention can improve the electron conductivity by having the coating portion on the surface.
Since the positive electrode active material of the present invention has the above-described effects, it is considered that the charge / discharge efficiency, capacity retention rate, and high rate charge / discharge characteristics can be improved in the sodium battery. In addition, the positive electrode active material of the present invention is considered to have good electronic conductivity due to the presence of the coat portion, and thus it is considered that the capacity of the sodium battery can be increased.

As a method of reducing the activity of the conventional positive electrode active material for decomposing the electrolytic solution, when the conventional positive electrode active material and the electrolytic solution are not brought into direct contact, for example, a carbon coating made of carbon having high electron conductivity is used. It is also conceivable to coat the surface of the positive electrode active material. However, the positive electrode active material of the present invention is considered to be able to reduce the activity against the decomposition of the electrolytic solution even when compared with a conventional positive electrode active material surface coated with a carbon film.
As described in detail in the section of Examples to be described later, the measurement cell (sodium secondary battery) using the positive electrode containing the positive electrode active material of the present invention and carbon, the conventional positive electrode active material and carbon are used. When compared with a measurement cell using a positive electrode with an increased carbon content instead of Ni ₃ P in the sample, the initial discharge capacity and the initial charge / discharge efficiency in the measurement cell of the example are dramatically improved. The characteristics and high rate discharge characteristics were improved (FIGS. 8, 9, Table 1, and Table 2 described later). From these results, it was suggested that M phosphide may have a lower activity in the decomposition of the electrolyte than carbon.

  Hereinafter, the details of the positive electrode active material of the present invention will be described.

1. Coat part The positive electrode active material of this invention has the coat part containing the phosphide of M in the said General formula (1) on the surface. The coat portion in the present invention is present on the surface of the positive electrode active material, and the coat portion and the core portion on the center side of the positive electrode active material from the coat portion may have an interface. The coat part and the core part may have no interface and may be continuous. Moreover, when the coat part and the core part have an interface, the case where the respective components contained in the coat part and the core part are mutually diffused is included.
In the present invention, in particular, it is preferable that the coat part has a continuous part without an interface between the coat part and the core part.

(1) Component of coat part (a) M phosphide The M phosphide contained in the coat part of the present invention is appropriately determined according to the composition of the positive electrode active material. Examples of phosphides of M include, for example, Ti phosphides, V phosphides, Cr phosphides, Mn phosphides, Fe phosphides, Co phosphides, Ni phosphides, Cu phosphides and Zn. The phosphide of these can be mentioned. These M phosphides may be one kind or two or more kinds. In the present invention, Ni phosphide is particularly preferable. Examples of the phosphide of Ni include NiP, Ni ₂ P, Ni ₃ P and the like, and Ni ₃ P is particularly preferable. When the coat part contains Ni ₃ P, the coat part is satisfactorily formed on the surface of the positive electrode active material by the production method described in the section “B. Method for producing positive electrode active material for lithium secondary battery” described later. Because you can.
In addition, also when the positive electrode active material represented by the general formula (1) described above has a coating portion containing M alone in the general formula (1) on the surface, the same effects as the positive electrode active material of the present invention are exhibited. It is thought that.

The content of the M phosphide contained in the coat portion is not particularly limited, but the ratio of the M phosphide in the coat portion is preferably 60 wt% or more, particularly 70 wt% or more, and particularly preferably 80 wt% or more. .
This is because if the content of the M phosphide contained in the coat portion is too small, it may be difficult to sufficiently suppress the decomposition of the electrolytic solution.
The content of the M phosphide contained in the coat part can be measured using an energy dispersive X-ray analyzer (TEM-EDX or SEM-EDX).

In the present invention, the M phosphide contained in the coat part may be particulate.
The average particle size of the M phosphide particles is, for example, preferably in the range of 1 nm to 1000 nm, more preferably in the range of 1 nm to 100 nm, and particularly preferably in the range of 1 nm to 50 nm.
About the particle | grains of the M phosphide in a coating part, it confirms using the transmission electron microscope (TEM) or a scanning electron microscope (SEM) and the energy dispersive X-ray analyzer (EDX) attached to it, for example. be able to.
The average particle size is calculated by, for example, measuring the particle size of M phosphide particles (n ≧ 100) by observation with a transmission electron microscope (TEM) and obtaining the average. A method can be mentioned. In addition, when SEM-EDX is used, there can be mentioned a method of calculating by measuring the particle diameter of particles containing Na and N (n ≧ 100) with a compound consisting of M and P, and calculating the average.

(B) Other components The coating part in this invention may be comprised only by the M phosphide mentioned above, and may be comprised including components other than the M phosphide mentioned above. Hereinafter, such components will be described.

(I) Carbon component It is more preferable that the coating part in this invention contains a carbon component in addition to the M phosphide mentioned above. This is because the positive electrode active material of the present invention can have better electronic conductivity.
When the coat part contains a carbon component, the positive electrode active material of the present invention is preferably produced by the production method described in the section “B. Method for producing positive electrode active material for sodium battery” described later.
When the coating part contains a carbon component, the ratio of the carbon component to the M phosphide is not particularly limited, but the carbon component is 1 part by weight or more, especially 10 parts by weight with respect to 100 parts by weight of the M phosphide. It is preferable to be within a range of ˜1000 parts by weight, particularly within a range of 50 parts by weight to 100 parts by weight.
It is because ion conductivity may fall when there is too much content of the said carbon component.

(Ii) Other components In addition to the above-described M phosphide and carbon component, the coating portion in the present invention may contain, for example, the above-described simple substance of M. Since the simple substance of M also shows high electron conductivity, it can be used as a positive electrode active material showing good electron conductivity.

(2) Coat part Although it does not specifically limit as average thickness of the coat part in this invention, It exists in the range of 0.5 nm-100 nm, especially within the range of 1 nm-50 nm, Especially within the range of 1 nm-10 nm. Is preferred.
This is because if the average thickness of the coat portion is too thick, the ionic conductivity may be reduced, and if the average thickness of the coat portion is too thin, the catalytic action for the decomposition of the electrolyte may not be sufficiently reduced. Because. The average thickness of the coat part can be measured, for example, by observation with a transmission electron microscope (TEM) (for example, n ≧ 100).
Here, as shown in FIG. 1A, when the coat part 1 and the core part 2 do not have an interface and are continuous, the thickness of the coat part means that the content of M phosphide is 60 wt. % Refers to the thickness.

The form of the coating part is not particularly limited as long as it is present on the surface of the positive electrode active material, but it is preferable to coat the core part of the positive electrode active material. The coverage of the coat part is, for example, preferably 10% or more, especially 30% or more, particularly 50% or more.
The coat portion may cover the entire core portion. The coverage of the coat portion can be measured using, for example, a transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), or the like.

The presence of M phosphide in the positive electrode active material of the present invention is measured by, for example, an X-ray diffraction apparatus (XRD (X-ray diffraction)), and the main peak of the positive electrode active material and the peak of M phosphide are measured. It can confirm by calculating | requiring the peak intensity ratio.
For example, in the case where the positive electrode active material of the present invention is Na ₄ Ni ₃ (PO ₄ ) ₂ P ₂ O ₇ and the coat part is Ni ₃ P, a case where measurement is performed by XRD using CuKα rays will be described.
Here, since Na ₄ Ni ₃ (PO ₄ ) ₂ P ₂ O ₇ has a peak of 2θ = 34.5 °, and Ni ₃ P has a peak of 2θ = 41.8 °, the positive electrode of the present invention. The active material has a peak at a position near 2θ described above.
When a positive electrode active material of the present invention having the above-described _{configuration,} the diffraction intensity of the peak of _{Na 4 Ni 3 (PO 4)} 2θ = 34.5 ° ± 0.2 ° is a peak of the ₂ P _{2 O 7 I A} and then, if the diffraction intensity of the peak of _{Ni 3 2θ = 41.8 ° ± 0.2} ° is a peak of P was _{I B,} that I B / _{I a} is 0.001 to 0.1 Preferably, it is 0.001 or more and 0.05 or less, and more preferably 0.005 or more and 0.04 or less.
If the above value is too small, it may be difficult to suppress the decomposition of the electrolytic solution even when the positive electrode active material has a coat portion on the surface.

2. Positive electrode active material The positive electrode active material for sodium batteries of the present invention is represented by the following general formula (1).
General formula (1)
_{Na x M y (AO 4)} z (P 2 O 7) w
(In the general formula (1), M is at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, and A is Al, Si, P, At least one selected from the group consisting of S, Ti, V and W, x satisfies 4 ≧ x ≧ 2, y satisfies 4 ≧ y ≧ 1, z satisfies 4 ≧ z ≧ 0, w Satisfies 1 ≧ w ≧ 0, and at least one of z and w is 1 or more.)

As shown in Patent Document 1, prior to the present invention, the present inventor can use a compound represented by Na ₄ Ni ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) as a positive electrode active material of a sodium battery. Furthermore, it has been found that it operates in an ultrahigh potential region such as 4.6V to 4.9V. In addition, the potential range of 4.6 V to 4.9 V is a potential range in which decomposition of the electrolytic solution used in combination with the positive electrode active material can be suppressed. Therefore, by using the positive electrode active material of the present invention, the potential range is stable over a long period of time. A sodium battery exhibiting battery characteristics can be obtained. In addition, the present inventor has obtained a compound represented by Na ₄ Mn ₃ (PO ₄ ) ₂ (P ₂ O ₇ ), a compound represented by Na ₄ Co ₃ (PO ₄ ) ₂ (P ₂ O ₇ ), Na ₄ Co represented by _{(3-a) Mn a (} PO 4) compounds represented by _{2 (P 2 O 7),} Na 4 Co (3-b-c) Mn c Ni c (PO 4) 2 (P 2 O 7) Each of these compounds can also be used as a positive electrode active material for sodium batteries and has been found to operate in a high potential region exceeding 4V.
In addition, the positive electrode active material composed of the above compound can exhibit high potential operability even in a relatively low temperature range of 25 ° C.

The general formula _{(1) Na x M y (} AO 4) z (P 2 O 7) compounds represented by _w, said _{Na 4 Ni} 3 _(PO 4) similar to _{2 (P 2 O 7)} or the like, sodium batteries As a positive electrode active material, it can operate in a high potential region. The reason is considered as follows.
That is, in the general formula (1), M is an electrochemically active divalent or higher-valent transition metal and has an ionic radius close to Ni or Ni.
Moreover, in General formula (1), A is easy to take a tetrahedral structure like P or P similarly. Here, the tetrahedral structure is a structure in which one A covalently bonded to these four oxygen atoms is contained in a tetrahedral void having four oxygen atoms as apexes.
For (AO ₄ ) and (P ₂ O ₇ ) that are polyanion parts, at least one of z representing the composition ratio of (AO ₄ ) in the positive electrode active material and w representing the composition ratio of (P ₂ O ₇ ) Is 1 or more, the positive electrode active material obtained is considered to operate in a high potential region due to the inductive effect on the MO bond by at least one of (AO ₄ ) and (P ₂ O ₇ ). The inductive effect means that the A—O bond constituting (AO ₄ ) and the P—O bond constituting (P ₂ O ₇ ) are highly covalent, so that the M—O bond electrons are converted into A—O bonds and P— As a result of being pulled to the -O bond side, the covalent bond between M-O is lowered, and the energy gap of the mixed orbitals is reduced. As a result, the redox level of M is lowered and the energy difference from sodium is increased. That is, the redox potential of sodium increases.

Hereinafter, the configuration of the positive electrode active material of the present invention will be described in detail.
In the positive electrode active material of the present invention, the M may be at least one metal species selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. In this state, it is preferably divalent. This is because when M is a metal species that is divalent in a state before charging, it can operate at a high potential by being in a highly oxidized state of trivalent or higher during charging.

Of the M, at least one selected from the group consisting of Mn, Co, and Ni is particularly preferable. This is because Mn, Co, and Ni are divalent in a state before charging, and Mn and Co can form a crystal structure similar to Ni. In the Patent Document 1, in the general formula _{(1) Na x M y (} AO 4) z (P 2 O 7) w, the M is, is either Ni, Mn or Co, other configurations When the values of (x, y, z, and w, and A) are the same, it was confirmed to have the same crystal structure.
These Mn, Co, and Ni are partially selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, and the above M (that is, Mn, Co, and Ni). It may be substituted with at least one different from at least one selected from Ni).

In the general formula (1), when M is Ni, a positive electrode active material having high electron conductivity can be obtained. This is because, when the redox element, that is, the element that transmits and receives electrons is Ni, the Ni ion oxide has a general olivine type crystal structure due to the desorption of Na ions during charging. Whereas the number changes from divalent to trivalent, in the positive electrode active material of the present invention, the valence of Ni ions is larger than the valence of 2 to 3 (for example, Na ₄ Ni ₃ (PO ₄ ) ₂ In the case of (P ₂ O ₇ ), it is considered that the number of electrons changes to about 3.3 and more electrons move. Ni may be substituted with at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Cu, and Zn.

  Further, in the above general formula (1), when M is Mn, the reversibility and stability of the crystal structure at the time of charging / discharging is higher and the operating potential is relatively lower than when M = Ni. A positive electrode active material can be obtained. Since the operating potential is relatively low, decomposition degradation of the electrolytic solution can be further suppressed. As described above, when M is Mn, higher cycle characteristics can be exhibited by improving the reversibility and stability of the crystal structure and suppressing deterioration of the electrolytic solution as compared with the case of M = Ni. A part of Mn may be substituted with at least one selected from the group consisting of Ti, V, Cr, Fe, Co, Ni, Cu and Zn.

Further, in the above general formula (1), when M is Co, the reversibility and stability of the crystal structure at the time of charge / discharge is higher and the operating potential is relatively lower than when M = Ni. A positive electrode active material can be obtained. Since the operating potential is relatively low, decomposition degradation of the electrolytic solution can be further suppressed. In addition, when M is Co, an effect of improving reversibility and stability of the crystal structure and suppressing deterioration of the electrolytic solution is added, and the positive electrode active material can exhibit a large reversible capacity. As described above, when M is Co, excellent cycle characteristics and capacity characteristics can be exhibited as compared with the case of M = Ni.
A part of Co may be substituted with at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Ni, Cu and Zn.

When a part of Co is substituted with Mn when M is Co in the general formula (1), more excellent capacity characteristics can be exhibited as compared with the case where M is only Co. This is considered to be because by replacing a part of the Co ²⁺ site with Mn ²⁺ , the substituted Mn ²⁺ can compensate for not only Mn ^{2 + / 3 +} but also Mn ^{3 + / 4 +} . A part of Co and Mn may be substituted with at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn.

Further, when a part of Co is substituted with Mn and Ni when M is Co in the general formula (1), the M is compared with a case where Co is partially substituted with Mn. Higher working potential. This is because the substituted Mn ²⁺ can compensate for not only Mn ^{2 + / 3 +} but also Mn ^{3 + / 4 +} , and Ni in which charge compensation (Ni ²⁺ → Ni ³⁺ ) proceeds in a higher potential region compared to Co. This is considered to replace Co. A part of Co, Mn and Ni may be substituted with at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn.

In the above invention, among the above-mentioned, in particular, in the above general formula (1), the above M is preferably Ni. In this case, the phosphide of M contained in the above-mentioned coat portion is Ni ₃ P. Preferably there is.

  In the positive electrode active material of the present invention, A may be at least one selected from the group consisting of Al, Si, P, S, Ti, V and W, but selected from the group consisting of Si, P and S. It is preferable that it is at least one kind. This is because Si, P, and S are particularly easy to form a tetrahedral structure, and Si and S can form a crystal structure similar to P. Among these, A is preferably P. In addition, a part of these Si, P and S is selected from the group consisting of Al, Si, P, S, Ti, V and W. At least one selected from the above A (ie, Si, P and S) It may be substituted with at least one species different from the species.

In the general formula (1), x satisfies 4 ≧ x ≧ 2, y satisfies 4 ≧ y ≧ 1, z satisfies 4 ≧ z ≧ 0, w satisfies 1 ≧ w ≧ 0, z and w At least one of these may be 1 or more.
If z and w are 1 or more, polyanion portion, and AO ₄ tetrahedra, to include a _P 2 _{O 7,} which share the AO ₄ tetrahedra and one oxygen, a high inductive effect on M-O bond As a result, a positive electrode active material having a higher potential is obtained, which is preferable.

In the present invention, specific examples of particularly preferable positive electrode active materials include compounds represented by Na ₄ Ni ₃ (PO ₄ ) ₂ (P ₂ O ₇ ). Na ₄ Ni ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) contains Ni as a redox element and has (PO ₄ ) and (P ₂ O ₇ ) as a polyanion part. In addition to such high electronic conductivity, it has high potential operability due to a high inductive effect.
Furthermore, Na ₄ Ni ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) has a crystal structure belonging to the space group Pn2 ₁ a. FIGS. 2 to 4 show the crystal structure (Na ₄ Ni ₃ (PO ₄ ) ₂ (P ₂ O ₇ )) belonging to the space group Pn2 ₁ a as seen from the a-axis direction (FIG. 2), the b-axis direction. The figure seen from (FIG. 3) and the figure seen from the c-axis direction (FIG. 4) are shown. 2 to 4 show the crystal structure belonging to the space group Pn2 ₁ a by taking Na ₄ Ni ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) as an example, By replacing Ni with the other metal species of M (for example, Co or Mn), the crystal structure of another positive electrode active material having a crystal structure belonging to the space group Pn2 ₁ a is shown.
As can be seen from FIGS. 2 to 4, in the crystal structure belonging to the space group Pn2 ₁ a, all Na ions in the crystal structure are arranged in any of the a-axis, b-axis, and c-axis directions. Na ion mobility is very high. That is, the crystal structure belonging to the space group Pn2 ₁ a is very advantageous for the conduction of Na ions, and insertion / extraction of Na ions proceeds smoothly.
For the above reasons, the positive electrode active material of the present invention preferably has a crystal structure belonging to the space group Pn2 ₁ a.
The crystal structure belonging to the space group Pna2 ₁ is also preferable because it has the same Na ion arrangement as that of the crystal structure belonging to the space group Pn2 ₁ a.

When the positive electrode active material has a crystal structure belonging to the space group Pn2 ₁ a or the space group Pna2 ₁ described above, the portion having the crystal structure is preferably present in the core portion of the positive electrode active material.

In the present invention, specific examples of particularly preferable positive electrode active materials include compounds represented by the general formula Na ₄ Mn ₃ (PO ₄ ) ₂ (P ₂ O ₇ ), and a general formula Na ₄ Co ₃ (PO _4). ) ₂ (compound represented by _P 2 _{O 7),} compounds represented by the general formula _{Na 4 Co (3-a)} Mn a (PO 4) 2 (P 2 O 7), and the general formula _Na 4 _{Co (3-} compounds represented by the _{b-c) Mn b Ni c} (PO 4) 2 (P 2 O 7) and the like. Each of these compounds has a crystal structure belonging to the space group Pn2 ₁ a shown in FIGS.

As described above, Na ₄ Mn ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) containing Mn as a redox element (M mentioned above) has improved reversibility and stability of the crystal structure and suppressed deterioration of the electrolyte solution. High cycle characteristics can be expressed.

Further, as described above, the general formula Na ₄ Co ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) containing Co as a redox element (M mentioned above) is improved in reversibility and stability of the crystal structure, electrolyte solution By suppressing deterioration of the resin and increasing the reversible capacity, excellent cycle characteristics and capacity characteristics can be exhibited.

Also includes Co as a redox element (the M), a part of the Co is replaced by _{Mn, Na 4 Co (3-} a) Mn a (PO 4) 2 (P 2 O 7) is already As described above, by charge compensation with Mn, more excellent capacity characteristics can be exhibited as compared with Na ₄ Co ₃ (PO ₄ ) ₂ (P ₂ O ₇ ).
In the general formula Na ₄ Co _(3-a) Mn _a (PO ₄ ) ₂ (P ₂ O ₇ ), a representing the substitution amount of Mn may be a number less than 3, but 0.01 ≦ a ≦ It is preferably within the range of 0.8, particularly preferably within the range of 0.3 ≦ a ≦ 0.8, and more preferably a = 0.6.

Also, Na ₄ Co _(3-bc) Mn _b Ni _c (PO ₄ ) ₂ (P ₂ ), which contains Co as a redox element (M above), and a part of the Co is substituted with Mn and Ni. As described above, O ₇ ) is Na ₄ Co _(3-a) Mn _a (PO ₄ ) ₂ (P ₂ ) by the charge compensation effect in the high potential region by Ni in addition to the charge compensation effect by Mn. Compared to O ₇ ), a higher working potential can be developed.
In the general formula Na ₄ Co _(3-bc) Mn _b Ni _c (PO ₄ ) ₂ (P ₂ O ₇ ), b representing the amount of Mn substitution and c representing the amount of Ni substitution are the sum (b + c) ) May be a number less than 3, but preferably within the range of 0.01 ≦ b ≦ 1.0 and 0.01 ≦ c ≦ 1.0, particularly 0.3 ≦ b ≦ 1.0. And it is preferable that it is the range of 0.3 <= c <= 1.0.

The shape of the positive electrode active material of the present invention may be, for example, particulate or layered. When the positive electrode active material is in the form of particles, the average particle diameter of the positive electrode active material of the present invention is, for example, in the range of 10 nm to 5 μm, in particular in the range of 50 nm to 3 μm, particularly in the range of 100 nm to 2 μm. Preferably there is.
If the average particle size is too large, the ionic conductivity may decrease, and if the average particle size is too small, the reactivity with the electrolyte that increases the specific surface area is improved, so the discharge capacity maintenance rate may decrease. It is because there is sex.
In addition, as a measuring method of the average particle size, for example, by measuring the particle size of the positive electrode active material (n ≧ 100) by SEM (scanning electron microscope) observation, and calculating the average, A method of measuring the average particle diameter (d ₅₀ ) of the positive electrode active material by particle size distribution measurement can be mentioned.

3. Method for Producing Positive Electrode Active Material The method for producing the positive electrode active material of the present invention is not particularly limited as long as the above-described positive electrode active material can be produced. For example, “B. The manufacturing method described in the section can be suitably used.
Further, as another manufacturing method, a precursor material of the positive electrode active material represented by the general formula (1) is prepared, and a phosphide of M is formed on the surface of the precursor material by using a vapor deposition method such as a sputtering method. The method of forming the coat part containing can be mentioned. By this method, the positive electrode active material 10 as shown in FIG. 1B can be manufactured. The precursor material of the positive electrode active material will be described in the section “B. Method for producing positive electrode active material for sodium battery” described later.

4). Sodium battery The positive electrode active material of this invention is normally used for the positive electrode active material of a sodium battery.
Hereinafter, a sodium battery in which the positive electrode active material of the present invention is used will be described with reference to the drawings.
FIG. 5 is a schematic cross-sectional view showing an example of a sodium battery. As shown in FIG. 5, the sodium battery 20 usually has a structure in which the electrolyte layer 23 is interposed between the negative electrode 21 and the positive electrode 22. The negative electrode 21 includes a negative electrode active material layer 24 containing a negative electrode active material, and a negative electrode current collector 25 that collects current from the negative electrode active material layer 24. The positive electrode 22 includes a positive electrode active material layer 26 containing a positive electrode active material and a positive electrode current collector 27 that collects current from the positive electrode active material layer 26. The sodium battery 20 usually has a battery case 28 that houses the negative electrode 21, the positive electrode active material layer 22, and the electrolyte layer 23.
Each configuration will be described below.

  The negative electrode contains a negative electrode active material capable of releasing and capturing sodium ions. The negative electrode usually has a negative electrode active material layer containing at least a negative electrode active material, and further includes a negative electrode current collector that collects current from the negative electrode active material layer as necessary.

Examples of the negative electrode active material include hard carbon, sodium metal, tin, and the like.
The negative electrode active material layer may contain only the negative electrode active material, but may contain a binder, a conductive material, an electrolyte and the like in addition to the negative electrode active material. For example, when the negative electrode active material has a plate shape, a foil shape, or the like, a negative electrode layer containing only the negative electrode active material can be obtained. On the other hand, when the negative electrode active material is in a powder form, a negative electrode layer containing a binder in addition to the negative electrode active material can be obtained.
Examples of the binder include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), and the like. Examples of the conductive material include carbon materials such as carbon black, activated carbon, carbon carbon fiber (eg, carbon nanotube, carbon nanofiber, etc.), graphite, and the like.

  The positive electrode contains the positive electrode active material of the present invention as a positive electrode active material capable of releasing and incorporating sodium ions. The positive electrode usually has a positive electrode active material layer containing at least a positive electrode active material, and further includes a positive electrode current collector that collects the positive electrode active material layer as necessary.

  Similar to the negative electrode active material layer, the positive electrode active material layer may contain only the positive electrode active material, but contains a conductive material, a binder, an electrolyte, an electrode catalyst, etc. in addition to the positive electrode active material. You may do. About the electroconductive material and binder in a positive electrode active material, since the material similar to a negative electrode active material layer can be used, description here is abbreviate | omitted.

  The negative electrode active material layer and the positive electrode active material layer can be prepared by, for example, slurry containing each material by any coating method such as a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, and a screen printing method. The electrode active material layer can be formed by coating, drying, and rolling as necessary.

The positive electrode current collector and the negative electrode current collector are not particularly limited in material, structure, and shape as long as the materials have desired electronic conductivity and do not cause an alloying reaction with sodium ions in the battery environment. There is no limitation.
Examples of the material for the positive electrode current collector include metal materials such as stainless steel, nickel, aluminum, iron, titanium, and copper, carbon materials such as carbon fiber and carbon paper, and high electron conductive ceramic materials such as titanium nitride. It is done. The battery case may have a function as a positive electrode current collector.
Examples of the material for the negative electrode current collector include copper, stainless steel, nickel, and aluminum. The battery case may have a function as a negative electrode current collector.
Examples of the shape of the positive electrode current collector and the negative electrode current collector include a plate shape, a foil shape, and a mesh shape, and a mesh shape is preferable.

The electrolyte layer contains at least an electrolyte that enables conduction of sodium ions between the positive electrode and the negative electrode.
The electrolyte only needs to have sodium ion conductivity, and examples thereof include an electrolyte, a gel electrolyte obtained by gelling the electrolyte using a polymer, a solid electrolyte, and the like.
Examples of the electrolytic solution having sodium ion conductivity include an electrolytic solution in which a sodium salt is dissolved in an aqueous solvent or a non-aqueous solvent.

  The non-aqueous solvent is not particularly limited, and examples thereof include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), and fluoroethylene carbonate (FEC), cyclic esters such as γ-butyrolactone (GBL), and dimethyl carbonate. Examples thereof include chain carbonates such as (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). These nonaqueous solvents may be used alone or in combination of two or more. Further, a nitrile compound in which a CN group is bonded to the end of the chain saturated hydrocarbon compound may be used by mixing with a non-aqueous solvent. By adding a nitrile compound to a non-aqueous solvent electrolyte, a stable non-aqueous solvent electrolyte that does not decompose can be obtained even in a high potential region where the positive electrode active material for a sodium battery of the present invention operates. it can.

The sodium salt is not particularly limited, for _{example, NaPF 6, NaBF 4, NaClO} 4, NaCF 3 SO 3, (CF 3 SO 2) 2 NNa, NaN (FSO 2), NaC (CF 3 SO 2) 3 , etc. Is mentioned. These sodium salts may be used individually by 1 type, and may be used in combination of 2 or more type. NaPF ₆ which is stable even in a high potential region is particularly preferable.
In the non-aqueous electrolyte, the concentration of sodium salt is not particularly limited.

  The non-aqueous electrolyte can be used after adding a polymer to gel. Examples of the gelation method of the nonaqueous electrolyte include, for example, a polymer such as polyethylene oxide (PEO), polyacrylonitrile (PAN), polyvinylidene fluoride (PVdF), or polymethyl methacrylate (PMMA). The method of adding is mentioned.

  When using an electrolytic solution as the electrolyte, it is possible to ensure insulation between the positive electrode and the negative electrode by disposing a separator that is an insulating porous body between the positive electrode and the negative electrode and impregnating the separator with the electrolytic solution. it can. Examples of the separator include porous membranes such as polyethylene porous membrane and polypropylene porous membrane; and nonwoven fabrics such as resin nonwoven fabric and glass fiber nonwoven fabric.

As a battery case that accommodates the negative electrode, the electrolyte layer, and the positive electrode, for example, a battery case having a general shape such as a coin shape, a flat plate shape, a cylindrical shape, or a laminate shape can be used.
In the case of a battery having a structure in which a laminated body arranged in the order of a positive electrode, an electrolyte layer, and a negative electrode is repeatedly stacked, from the viewpoint of safety, a separator made of an insulating material between the positive electrode and the negative electrode Can be provided. Examples of such a separator include porous membranes such as polyethylene porous membrane and polypropylene porous membrane; and nonwoven fabrics such as resin nonwoven fabric and glass fiber nonwoven fabric.
In addition, the current collector of each electrode can be provided with a terminal serving as a connection portion with the outside.

  The sodium battery may be a primary battery or a secondary battery, but is preferably a secondary battery.

B. Manufacturing method of positive electrode active material for sodium battery The manufacturing method of the positive electrode active material for sodium batteries of this invention is demonstrated using figures.
FIG. 6 is a process diagram showing an example of a method for producing a positive electrode active material for a sodium battery according to the present invention. The method for producing a positive electrode active material for a sodium battery according to the present invention is a method for producing the above-described positive electrode active material 10 as shown in FIG. 6, wherein the precursor material represented by the general formula (1) is used as a reducing agent. It is characterized by having a reduction process which forms coat part 1 containing M phosphide by baking together.

  According to this invention, by having the said reduction | restoration process, the above-mentioned positive electrode active material for sodium batteries can be manufactured.

  Hereinafter, the manufacturing method of the positive electrode active material of this invention is demonstrated.

1. Reduction step The reduction step in the present invention is a step of forming a coat portion by baking the precursor substance represented by the general formula (1) together with a reducing agent.
In this step, by firing the precursor material together with a reducing agent, M present on the surface of the precursor material can be reduced to a phosphide of M, and a coat portion can be formed.

(1) Precursor substance The precursor substance used for this invention is represented by General formula (1) mentioned above. The precursor material has the same configuration as the positive electrode active material described in the above-mentioned section “A. Positive electrode active material for sodium battery” except that it does not have a coating portion. Moreover, such a precursor substance is the same as the positive electrode active material for sodium batteries in Patent Document 1.

The method for forming the precursor material is not particularly limited. For example, at least a Na-containing compound, an M-containing compound containing M, an A-containing compound containing A, and a raw material mixture containing a P-containing compound, Temporary firing step for firing within the range of 150 ° C. to 500 ° C. in the air atmosphere, and main firing for firing the temporary fired product obtained within the range of 500 ° C. to 800 ° C. under the air atmosphere after the temporary firing. And a forming method including a process can be suitably used.
The details of the method for forming the precursor material described above can be the same as the contents described in the item of the method for producing the positive electrode active material for sodium batteries described in Patent Document 1, and therefore the description here is as follows. Omitted.

(2) Reducing agent The reducing agent used in the present invention is not particularly limited as long as M present on the surface of the precursor material can be reduced, and may be an organic reducing agent or an inorganic reducing agent. However, an organic reducing agent is more preferable. This is because a coating part containing a carbon component can be formed by firing an organic reducing agent together with the precursor substance and carbonizing the organic substance.

Examples of organic reducing agents include alcohols such as methanol, ethanol and isopropyl alcohol, or ascorbic acid, ethylene glycol, glutathione, organic acids (citric acid, malic acid, tartaric acid, etc.), reducing sugars (glucose, galactose, Mannose, fructose, sucrose, maltose, raffinose, stachyose, etc.) and sugar alcohols (sorbitol, etc.).
On the other hand, examples of the inorganic reducing agent include carbon monoxide and carbon materials.

The ratio (mol ratio) between the precursor substance and the reducing agent is not particularly limited as long as M present on the surface of the precursor substance can be reduced to generate a phosphide of M.
For example, when the reducing agent is an organic reducing agent, the ratio (mol ratio) between the precursor substance and the reducing agent ranges from 10 mmol to 50 mol of carbon contained in the organic reducing agent with respect to 1 mol of M. Among them, it is particularly preferable that it is in the range of 100 mmol to 50 mol, particularly in the range of 300 mmol to 20 mol.
For example, when the organic reducing agent is a disaccharide such as sucrose, the ratio (weight ratio) between the precursor substance and the reducing agent is 1 mg to 1 g with respect to 1 g of the weight of the precursor substance. In particular, it is preferable to be within the range of 10 mg to 1 g, particularly within the range of 10 mg to 0.5 g.
This is because if the ratio of the reducing agent to the precursor material is small, it may be difficult to form a coat portion. If the ratio of the reducing agent is large, the composition of the positive electrode active material of the present invention is controlled. This may be difficult.

(3) Reduction step In this step, the atmosphere for firing the precursor material and the reducing agent described above is usually performed in an inert gas atmosphere.

  The firing temperature of the precursor material and the reducing agent in this step is not particularly limited, but is, for example, in the range of 450 ° C. to 800 ° C., particularly in the range of 500 ° C. to 750 ° C., particularly in the range of 550 ° C. to 700 ° C. It is preferable to be within. This is because if the firing temperature is low, the reduction reaction may not proceed sufficiently. Moreover, it is because the positive electrode active material obtained may change in quality when the said baking temperature is high.

2. Other Steps The method for producing a positive electrode active material of the present invention can be added by appropriately selecting necessary steps other than the above-described reduction step. For example, it may include a step of forming the precursor material described above.

3. Positive electrode active material The positive electrode active material produced by the method for producing a positive electrode active material of the present invention has been described in the above-mentioned section “A. Positive electrode active material for sodium battery”, and thus description thereof is omitted here.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.

[Example]
(Synthesis of precursor materials)
Na ₄ P ₂ O ₇ (Na-containing compound and P-containing compound), (CH ₃ COO) ₂ Ni (Ni-containing compound), and NH ₄ H ₂ PO ₄ (P-containing compound) were used as starting materials. These starting materials were weighed so as to be Na: Ni: P = 4: 3: 4 (mol ratio) and dissolved in an aqueous nitric acid solution (acidic solution) together with glycolic acid (gelling agent) at 80 ° C. Stir.
The obtained gel was collected and baked at 700 ° C. for 50 hours in an air atmosphere to obtain a precursor material powder.

(Synthesis of positive electrode active material)
1.19 g of the obtained powder and 0.166 g of sucrose were weighed and mixed. The obtained mixture was baked at 700 ° C. for 5 hours under an Ar atmosphere to obtain a positive electrode active material.

(Preparation of positive electrode)
As the positive electrode material, the above-described positive electrode active material, carbon (conductive aid), and PVdF (binder) were used. These positive electrode materials were weighed so that the positive electrode active material: carbon (conducting aid): PVdF (binder) = 87: 8: 5 (weight ratio), and N-methyl-2-pyrodoline (NMP) ) (Dispersant) to form a slurry. The slurry was applied on an aluminum foil (current collector), dried and rolled to obtain a positive electrode.

[Comparative example]
(Synthesis of positive electrode active material Na ₄ Ni ₃ (PO ₄ ) ₂ P ₂ O ₇ )
Na ₄ P ₂ O ₇ (Na-containing compound and P-containing compound), (CH ₃ COO) ₂ Ni (Ni-containing compound), and NH ₄ H ₂ PO ₄ (P-containing compound) were used as starting materials. These starting materials were weighed so as to be Na: Ni: P = 4: 3: 4 (mol ratio) and dissolved in an aqueous nitric acid solution (acidic solution) together with glycolic acid (gelling agent) at 80 ° C. Stir.
The obtained gel was collected and fired at 700 ° C. for 50 hours in an air atmosphere to obtain a positive electrode active material.

(Preparation of positive electrode)
As the positive electrode material, the above-described positive electrode active material, carbon (conductive aid), and PVdF (binder) were used. The positive electrode material was weighed so that the positive electrode active material: carbon (conducting aid): PVdF (binder) = 75: 20: 5 (weight ratio), and N-methyl-2-pyrodoline (NMP) A slurry was formed by mixing with (dispersant). The obtained slurry was applied on an aluminum foil (current collector), dried and rolled to obtain a positive electrode.

[Evaluation]
(Crystal structure analysis)
The positive electrodes of Examples and Comparative Examples were analyzed with an X-ray analyzer (XRD). The results are shown in FIG. FIG. 7A is an XRD pattern in the range of 5 ° to 95 ° in the positive electrode (electrode), and FIG. 7B is in the range of 41.6 ° to 41.9 ° in FIG. It is an enlarged view.
Peaks attributed to the Al foil are observed around 65.1 ° and 78.3 °. When the peak positions were shifted in the measurement, the comparative examples were corrected to match those peak positions in the examples and then compared with the examples.
As shown in FIG. 7B, in the example, a peak attributed to the (231) plane of the reaction product Ni ₃ P with sucrose was observed.

(Measurement of charge / discharge characteristics)
<Production of measurement cell>
A coin-type measurement cell was produced using the positive electrode, Na metal (negative electrode), non-aqueous solvent electrolyte, and separator.
To the non-aqueous solvent electrolyte, sodium salt (NaPF ₆ ) is added to a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at 1: 1 (volume ratio), and the sodium salt concentration Was prepared so as to be 1.0 part by weight dm ⁻³ .
The separator used was a microporous membrane in which a polypropylene (PP) porous membrane, a polyethylene (PE) porous membrane, and a polypropylene (PP) porous membrane were laminated in this order.

<Measurement of charge / discharge characteristics>
In the charge / discharge test, the upper limit was 5.1 V, the lower limit was 3.0 V, and the current density was 34 mAg ⁻¹ (0.2 C) at 50 ° C. for 50 cycles.
Moreover, 1 C was defined as the current density which can charge / discharge a theoretical capacity of 170 mAhg ⁻¹ in 1 hour.
Moreover, the high rate discharge characteristic at the time of a high current density (170 mAhg < ^-1 >, 1C rate) application to the measurement cell of an Example was measured.

FIG. 8 is a graph showing a charge / discharge curve in the first cycle of the measurement cells of the example and the comparative example. As shown in FIG. 8, the initial discharge capacity (discharge capacity at the first cycle) and the initial charge / discharge efficiency (charge / discharge efficiency at the first cycle) are dramatically increased by the effect of Ni ₃ P generated by the reduction action with sucrose. Improved.
Also, the cycle characteristics (discharge capacity retention after 50 cycles) were improved in the measurement cell of the example as compared with the comparative example.

FIG. 9 is a graph showing a charge / discharge curve when a high current density (170 mAhg ⁻¹ , 1C rate) is applied to the measurement cell of the example.
In order to be applied as an in-vehicle power source, high rate discharge characteristics are also required in order to meet strict input / output requirements. In the example, a high voltage was maintained even at the time of evaluation at the 1C rate, and a capacity of about 90% was obtained at the low rate (0.2C) even when compared with the charge / discharge. The above results are shown in Tables 1 and 2.

  The high rate discharge characteristics in Table 2 are values determined as high rate discharge characteristics (%) = (discharge capacity at 1 C) / (discharge capacity at 0.2 C) × 100.

DESCRIPTION OF SYMBOLS 1 ... Coat part 10 ... Positive electrode active material for sodium batteries

Claims (3)

A positive electrode active material for a sodium battery represented by the following general formula (1):
A positive electrode active material for a sodium battery, having a coat portion containing a phosphide of M in the general formula (1) on the surface.
General formula (1)
_{Na x M y (AO 4)} z (P 2 O 7) w
(In the general formula (1), M is at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, and A is Al, Si, P, At least one selected from the group consisting of S, Ti, V and W, x satisfies 4 ≧ x ≧ 2, y satisfies 4 ≧ y ≧ 1, z satisfies 4 ≧ z ≧ 0, w Satisfies 1 ≧ w ≧ 0, and at least one of z and w is 1 or more.) 2. The positive electrode active material for a sodium battery according to claim 1, wherein, in the general formula (1), M is Ni, and the phosphide of M is Ni ₃ P. ₃ . It is a manufacturing method of the positive electrode active material for sodium batteries which manufactures the positive electrode active material for sodium batteries of Claim 1 or Claim 2, Comprising:
A method for producing a positive electrode active material for a sodium battery, comprising a reduction step of forming a coat portion containing the M phosphide by firing the precursor material represented by the general formula (1) together with a reducing agent. .