TW201622216A - Positive electrode active material for storage device and method for producing positive electrode active material for storage device - Google Patents

Abstract

Provided is a positive electrode active substance for a storage device, the substance characterized in containing an oxide material that has an amorphous phase and in being represented by general formula Nax(Mn1-aMa)yP2Oz (M is at least one selected from the group consisting of Cr, Fe, Co, and Ni, 1.2 ≤ x &le ;2.3, 0.95 ≤ y &le ;1.6, 0 ≤ a ≤ 0.9, and 7 ≤ z ≤ 8).

Description

Positive electrode active material for electricity storage device and method for producing same

The present invention relates to a positive electrode active material for a storage battery device and a method for producing the same.

Lithium-ion secondary battery has established its position as a high-capacity and lightweight power source that is indispensable for mobile electronic terminals or electric vehicles. As its positive electrode active material, it has olivine-type crystals represented by the general formula LiFePO ₄ . The active substance is of interest. However, there is a concern about the high price of raw materials for lithium in the world, and in recent years, sodium ion secondary batteries using sodium instead of lithium have been studied.

In the redox reference potential of the lithium ion and the sodium ion, sodium is 0.3 V higher than lithium. Therefore, when the basic ion of the positive electrode active material is changed from lithium to sodium, the operating potential in the sodium-based positive electrode active material is lowered. Therefore, in order to achieve a high energy density equivalent to that of a lithium ion secondary battery, a sodium ion secondary battery is required to have a high voltage or a high capacity. For example, Non-Patent Document 1 discloses a positive electrode active material containing Na ₂ (Fe _1-y Mn _y )P ₂ O ₇ (0≦y≦1). [Prior Art Document] [Non-Patent Document]

[Non-Patent Document 1] Prabeer Barpanda et al., Solid State Ionics, 2014 (DOI: 10.1016/j.ssi.2014.03.011)

[Problems to be solved by the invention]

In the positive electrode active material containing Na ₂ (Fe _1-y Mn _y ) P ₂ O ₇ described in Non-Patent Document 1, it has been reported that the capacity is rapidly decreased as the ratio of Mn changes. Therefore, the active material has a problem that it does not have charge and discharge characteristics that can satisfy actual specifications.

An object of the present invention is to provide a positive electrode active material for an electricity storage device which is excellent in charge and discharge characteristics and a method for producing the same. [Means for solving the problem]

The positive electrode active material for a storage battery device of the present invention is characterized by having a general formula of Na _x (Mn _1-a M _a ) _y P ₂ O _z (M is at least selected from the group consisting of Cr, Fe, Co, and Ni One, and 1.2≦x≦2.3, 0.95≦y≦1.6, 0≦a≦0.9, 7≦z≦8), and contains an oxide material having an amorphous phase.

In the positive electrode active material for a storage battery device of the present invention, an amorphous phase is contained, whereby not only the sodium ion conductivity but also the distortion of the Mn-containing crystal generated by the reverse charge and discharge or the external elution of the Mn component can be suppressed. . Therefore, the positive electrode active material for a storage battery device of the present invention is excellent in charge and discharge characteristics or cycle characteristics.

The positive electrode active material for a storage battery device of the present invention preferably has a content of an amorphous phase of 1% by mass or more.

The positive electrode active material for a storage battery device of the present invention preferably further contains conductive carbon.

By including the conductive carbon in the positive electrode active material, the electron conduction path between the oxide materials can be ensured, and the charge and discharge characteristics can be improved.

The positive electrode active material for a storage battery device of the present invention preferably contains, by mass%, 80% to 99.5% of the oxide material and 0.5% to 20% of the conductive carbon.

The positive electrode active material for a storage battery device of the present invention preferably contains a triclinic crystal represented by the general formula Na ₂ MnP ₂ O ₇ .

The triclinic crystal represented by Na ₂ MnP ₂ O ₇ has a high oxidation-reduction potential due to charge and discharge, and exhibits high charge and discharge capacity and high discharge voltage when used as a positive electrode active material for an electricity storage device. .

The method for producing a positive electrode active material for a storage battery device according to the present invention is characterized in that it is a group consisting of Cr, Fe, Co and Ni selected from the general formula Na _x (Mn _1-a M _a ) _y P ₂ O _z (M is selected from the group consisting of Cr, Fe, Co and Ni At least one transition metal element in the group, and an oxide material represented by 1.2≦x≦2.3, 0.95≦y≦1.6, 0≦a≦0.9, 7≦z≦8) is added with conductive carbon and is pulverized while being pulverized mixing.

In general, the positive electrode active material for a storage device is mixed with conductive carbon or an organic material as a conductive carbon source and sintered to be used as a positive electrode. Thereby, a conductive path containing conductive carbon is formed between the positive electrode active materials, and good charge and discharge characteristics can be obtained. However, when the positive electrode active material represented by the general formula Na _x (Mn _1-a M _a ) _y P ₂ O _z is mixed with the conductive carbon and sintered, the conductive carbon is easily oxidized by the Mn element contained in the positive electrode active material. It is released to the outside in the form of carbon dioxide. Therefore, it is difficult to form a conductive path containing conductive carbon between the positive electrode active materials, and sufficient charge and discharge characteristics may not be obtained.

On the other hand, according to the production method of the present invention, the positive electrode active material represented by the general formula Na _x (Mn _1-a M _a ) _y P ₂ O _z can be pulverized while being pulverized to impart energy. Therefore, it is possible to perform recombination at a relatively low temperature without accompanying sintering with conductive carbon. Therefore, when the composite is performed, it is difficult to oxidize the conductive carbon by the Mn element, and the conductive carbon can be dispersed in the positive electrode active material in a good state. Further, by the pulverization and mixing steps, the oxide materials are easily reacted with each other to form an amorphous phase. Further, since the conductive carbon is used as a pulverization aid, aggregation of the oxide material during pulverization and mixing is suppressed, and formation of an amorphous phase is promoted.

As described above, according to the production method of the present invention, the oxide material contains an amorphous phase, and conductive carbon is dispersed in a good state inside, so that a positive electrode active material excellent in charge and discharge characteristics can be obtained.

In the method for producing a positive electrode active material for a storage battery device of the present invention, it is preferred to use a molten solidified material as an oxide material.

According to the above configuration, the homogeneity of the positive electrode active material can be improved.

In the method for producing a positive electrode active material for a storage battery device of the present invention, it is preferred to use a crystal material obtained by heat-treating a molten solidified material as an oxide material. [Effects of the Invention]

According to the present invention, it is possible to obtain a positive electrode activity for a power storage device having a high energy density by increasing the capacity by containing Mn as a high voltage element, increasing the capacity by containing an amorphous phase, and obtaining good cycle characteristics. substance.

The positive electrode active material for a storage battery device of the present invention is characterized by having a general formula of Na _x (Mn _1-a M _a ) _y P ₂ O _z (M is at least selected from the group consisting of Cr, Fe, Co, and Ni One, and 1.2≦x≦2.3, 0.95≦y≦1.6, 0≦a≦0.9, 7≦z≦8), and contains an oxide material having an amorphous phase.

Na in the above formula becomes a supply source of sodium ions, and the sodium ions move between the positive electrode active material and the negative electrode active material when the battery is charged and discharged.

Mn is a component that imparts a high voltage to the positive electrode active material. Specifically, when sodium ions are separated from the positive electrode active material or absorbed in the positive electrode active material as the battery is charged and discharged, a redox reaction is generated by a change in the valence of the Mn ions. Due to this redox reaction, the positive electrode active material exhibits a high oxidation-reduction potential.

M (at least one selected from the group consisting of Cr, Fe, Co, and Ni) is also the same as Mn. When the battery is charged and discharged, the sodium ion is separated from the positive active material or absorbed into the positive electrode by the valence change. In the case of a substance, it has an effect of increasing the oxidation-reduction potential of the positive electrode active material. Ni exhibits a particularly high redox potential and is therefore preferred. On the other hand, Fe has a high structural stability during charge and discharge, and is therefore preferred.

P ₂ Oz has a three-dimensional network structure and thus has an effect of stabilizing the structure of the positive electrode active material.

Hereinafter, the reason why the range of each coefficient in the general formula Na _x (Mn _1-a M _a ) _y P ₂ O _z is defined in the above manner will be described.

x is 1.2 ≦ x ≦ 2.3, preferably ≦ x ≦ 2.25, more preferably 1.5 ≦ x ≦ 2.2. When x is too small, the amount of sodium ions involved in occlusion and release is reduced, so that the charge and discharge capacity tends to decrease. On the other hand, when x is too large, it is easy to precipitate heterogeneous crystals such as Na ₃ PO _{4 which} do not participate in charge and discharge, and thus the charge and discharge capacity tends to decrease.

y is 0.95 ≦ y ≦ 1.6, preferably 0.95 ≦ y ≦ 1.4, more preferably 0.95 ≦ y ≦ 1.25. When y is too small, the transition metal element causing the redox reaction is reduced. Therefore, the sodium ions involved in the absorption and release are reduced, so that the charge and discharge capacity tends to decrease. On the other hand, when y is too large, it is easy to precipitate heterogeneous crystals such as NaMnPO _{4 which} do not participate in charge and discharge, and thus the charge and discharge capacity tends to decrease.

a is 0 ≦ a ≦ 0.9, preferably 0 ≦ a ≦ 0.5, more preferably 0 ≦ a ≦ 0.3. The smaller the a, the higher the oxidation-reduction potential due to charge and discharge, and when used as a positive electrode active material for a storage device, it is easy to exhibit a high charge and discharge capacity and a high discharge voltage. Especially good is a=0.

z is 7≦z≦8, preferably 7≦z≦7.8, more preferably 7≦z≦7.5. When z is too small, the valence of Mn and M becomes less than 2 valence, and it is easy to precipitate a metal with charge and discharge. The precipitated metal is eluted in the electrolyte and precipitates as a metal dendritic crystal on the negative electrode side, which causes internal short-circuiting. On the other hand, when z is too large, the valence of Mn and M becomes larger than the valence, and it is difficult to cause an oxidation-reduction reaction with charge and discharge of a battery. As a result, the sodium ions occluded and released are reduced, so that the capacity tends to decrease.

Specific examples of the oxide material represented by the general formula Na _x (Mn _1-a M _a ) _y P ₂ O _z include Na ₂ MnP ₂ O ₇ and Na ₂ (Mn _1-a Fe _a )P _{2 .} Oxidation represented by O ₇ (0 < a ≦ 0.8, further 0.2 ≦ a ≦ 0.8), Na ₂ (Mn _1-a Ni _a ) P ₂ O ₇ (0 < a ≦ 0.8, further 0.2 ≦ a ≦ 0.8) Material. In addition, the triclinic crystal represented by Na ₂ MnP ₂ O ₇ has a high oxidation-reduction potential due to charge and discharge, and exhibits a high charge and discharge capacity when used as a positive electrode active material for a storage device. Theoretical value 97.5 mAh) and high discharge voltage (theoretical value 3.7 V).

The content of Na _x (Mn _1-a M _a ) _y P ₂ O _z crystal in the positive electrode active material is preferably 99% by mass or less, more preferably 90% by mass or less, still more preferably 85% by mass or less, and particularly preferably 80% by mass or less, preferably 70% by mass or less. When the content of the Na ₂ MnP ₂ O ₇ crystal is too large, the amorphous phase is reduced, and the effect described later is not easily obtained.

The content of the amorphous phase in the oxide material is preferably 1% by mass or more, more preferably 10% by mass or more, still more preferably 20% by mass or more, and particularly preferably 30% by mass or more. When the content of the amorphous phase is too small, the sodium ion conductivity is liable to lower. Further, with the reverse charge and discharge, the Mn-containing crystal is likely to be distorted, or the Mn component is easily eluted to the outside. As a result, the charge and discharge characteristics (especially the high-rate charge and discharge characteristics) or the cycle characteristics are liable to lower.

The content of the Na _x (Mn _1-a M _a ) _y P ₂ O _z crystal and the amorphous phase in the oxide material is obtained by measuring the 2θ value obtained by powder X-ray diffraction using CuKα ray. In the ray distribution of 10° to 60°, the peak is separated into a crystalline ray and an amorphous halo. Specifically, in the total scattering curve obtained by subtracting the background from the diffraction ray distribution, the integrated intensity obtained by peak-separating the wide-circle ray (amorphous halo) of 10° to 45° is set as Ia, the sum of the integrated intensities obtained by peak separation of crystalline ray-derived rays derived from Na _x (Mn _1-a M _a ) _y P ₂ O _z crystals detected at 10° to 60° is set to Ic When the total of the integrated intensities obtained by the crystallized ray from other crystals is Io, the content Xc of the crystal of Na _x (Mn _1-a M _a ) _y P ₂ O _z and the amorphous state are obtained according to the following formula. The content of the phase is Xg.

Xc=[Ic/(Ic+Ia+Io)]×100 (% by mass) Xg=100-[100×(Ic+Io)/(Ic+Ia+Io)] (% by mass)

The smaller the crystallite size of the Na _x (Mn _1-a M _a ) _y P ₂ O _z crystal, the higher the discharge capacity can be. Specifically, the crystallite size of the Na _x (Mn _1-a M _a ) _y P ₂ O _z crystal is preferably 100 nm or less, more preferably 60 nm or less, and still more preferably 50 nm or less. The lower limit is not particularly limited, but is actually 1 nm or more, and further 2 nm or more. The crystallite size can be determined from the analysis results of powder X-ray diffraction using CuKα rays and according to the Scherrer formula. Specifically, it is confirmed that the total scattering curve obtained by subtracting the background from the diffraction ray distribution is a ray around the 2θ=22.3° of the Na _x (Mn _1-a M _a ) _y P ₂ O _z crystal. The peak separation is based on the obtained full width at half maximum (FWHM) and black angle θ, and the crystallite size ε of the crystal of Na _x (Mn _1-a M _a ) _y P ₂ O _{z is} obtained according to the following formula.

ε=Kλ/βicosθ (Xiele constant K=0.85, X-ray wavelength λ=1.541)

The positive electrode active material for a storage battery device of the present invention preferably contains conductive carbon. Thereby, the electron conduction path between the oxide materials can be ensured, and the charge and discharge characteristics can be improved. As the conductive carbon, highly conductive carbon black such as acetylene black or ketjen black, carbon powder such as graphite, or carbon fiber can be used. Among them, acetylene black having high electron conductivity is preferred.

The positive electrode active material for an electricity storage device of the present invention preferably contains 80% to 99.5% of the oxide material and 0.5% to 20% of the conductive carbon, and preferably 85% to 98% of the oxide material, by mass%. Carbon materials are 2% to 15%. By limiting the content of the oxide material and the conductive carbon to the above range, it is easy to obtain a positive electrode active material having a high charge and discharge capacity and good cycle characteristics.

The shape of the positive electrode active material for a storage battery device is not particularly limited, but is preferably a powder. In this case, the average particle diameter of the positive electrode active material for a storage device is 0.1 μm to 20 μm, 0.3 μm to 15 μm, 0.5 μm to 10 μm, and more preferably 0.6 μm to 5 μm. Further, the maximum particle diameter is 150 μm or less, 100 μm or less, or 75 μm or less, and particularly preferably 55 μm or less. When the average particle diameter or the maximum particle diameter is too large, sodium ions are hard to be absorbed and released during charge and discharge, and thus the charge and discharge capacity tends to decrease. On the other hand, when the average particle diameter is too small, the dispersion state of the powder is poor when it is in a paste state, and it tends to be difficult to produce a uniform electrode.

Here, the average particle diameter and the maximum particle diameter respectively represent the median diameter of the primary particles, that is, D50 (50% volume cumulative particle diameter) and D99 (99% volume cumulative particle diameter), and are laser diffraction type particle size distribution. The value measured by the measuring device.

Next, a method of producing a positive electrode active material for a storage battery device of the present invention will be described. The positive electrode active material for an electricity storage device of the present invention can be produced, for example, by a method consisting of a general formula of Na _x (Mn _1-a M _a ) _y P ₂ O _z (M is selected from the group consisting of Cr, Fe, Co, and Ni). At least one transition metal element in the group, and an oxide material represented by 1.2≦x≦2.3, 0.95≦y≦1.6, 0≦a≦0.9, 7≦z≦8) is added with conductive carbon while being pulverized mixing.

As the oxide material represented by the general formula Na _x (Mn _1-a M _a ) _y P ₂ O _z , a melt-solidified material or a solid phase reactant of a raw material powder (oxide or the like) of each constituent component can be used. In particular, when a molten solidified material is used as the oxide material, it is easy to obtain a positive electrode active material having excellent homogeneity, which is preferable.

The molten solidified product can be produced by the following method. First, as shown by the general formula Na _x (Mn _1-a M _a ) _y P ₂ O _z (M is at least one transition metal element selected from the group consisting of Cr, Fe, Mn, Co, and Ni) A raw material powder is prepared in a compositional manner to obtain a raw material batch. Then, the obtained raw material batch is melted. The melting temperature can be appropriately adjusted to uniformly melt the raw material batch. Specifically, the melting temperature is preferably 800 ° C or higher, more preferably 900 ° C or higher. The upper limit is not particularly limited. However, if the melting temperature is too high, energy loss and evaporation of the sodium component are caused. Therefore, the upper limit is preferably 1,500 ° C or lower, more preferably 1400 ° C or lower.

The melted product is obtained by molding the obtained melt. The molding method is not particularly limited. For example, the melt stream may be formed between the pair of cooling rolls, and may be formed into a film shape by quenching, or the molten material may be discharged to a mold to form a block. The molten solidified material may be either an amorphous body or a crystalline body, or may be a mixture of a crystalline phase and an amorphous phase.

Further, the molten solidified product containing the amorphous body may be crystallized (crystallized glass) by heat treatment for a predetermined time at a predetermined temperature. The heat treatment is carried out, for example, in an electric furnace capable of temperature control. The heat treatment temperature is preferably at least the glass transition temperature of the amorphous body, more preferably at a crystallization temperature or higher. Specifically, it is preferably 350 ° C or higher, more preferably 400 ° C or higher. The heat treatment time can be appropriately adjusted to sufficiently crystallize the amorphous body. Specifically, it is preferably from 20 minutes to 300 minutes, more preferably from 30 minutes to 240 minutes.

Further, the heat treatment of the amorphous body can be carried out in any of an atmosphere, an inert atmosphere, and a reducing atmosphere, but it is particularly preferably carried out in a reducing atmosphere, whereby Mn and M in the molten solidified body can be made. 2 price. The reducing atmosphere may, for example, be a hydrogen atmosphere or the like. Alternatively, a mixed gas containing a reducing gas such as hydrogen in an inert gas such as nitrogen or argon may be used. In this case, the content of the reducing gas is preferably 2% by volume or more.

As a method of mixing the oxide material and the conductive carbon while pulverizing, a mortar, a crusher, a ball mill, a mill, a vibrating ball mill, a satellite ball mill, a planetary ball mill, a jet mill, and a bead are used. A method of a conventional pulverizer such as a mill. Among them, a planetary ball mill is preferably used. In the planetary ball mill, while the tank rotates and rotates, the stage rotates and rotates, so that high impact energy can be generated with high efficiency, and not only the conductive carbon can be uniformly dispersed in the oxide material, but also easily oxidized. An amorphous phase is formed in the material.

The positive electrode active material of the present invention can be used in a sodium ion secondary battery using an electrolytic solution such as an aqueous solvent, a nonaqueous solvent or an ionic liquid. Moreover, it can also be used in an all-solid sodium ion secondary battery using a solid electrolyte. [Examples]

Hereinafter, the present invention will be described in detail based on examples. Further, the present invention is not limited by the following examples.

(Example 1) (a) In the melting step, sodium hydrogen phosphate (NaH ₂ PO ₄ ) and manganese oxide (Mn ₃ O ₄ ) were used as raw materials, and it was 33.3% of Na ₂ O and 33.3% of MnO ₂ in terms of mol%. The raw material powder was blended in a manner of 33.3% of P ₂ O ₅ and melted at 1050 ° C for 15 minutes in the atmosphere. Thereafter, the molten solidified material is obtained by flowing the melted material into an iron plate and quenching. The molten solidified product was pulverized by a planetary ball mill P7 manufactured by Fritch Co., Ltd. to obtain a powdery molten solidified product. The powder X-ray diffraction pattern of the obtained molten solidified product was confirmed, and no crystallinity was observed, and it was confirmed that it was an amorphous body (FIG. 1).

The obtained molten solidified product was calcined at 463 ° C for 3 hours in an argon atmosphere containing 5 vol% of H ₂ to obtain an oxide material. The powder X-ray diffraction pattern of the oxide material was confirmed, and it was found that Na ₂ MnP ₂ O ₇ crystals (the triclinic space group P1) were precipitated ( FIG. 2 ).

(b) The pulverization and mixing step The oxide material obtained above and the acetylene black as the conductive carbon (electrochemical made by Electrochemical Industry Co., Ltd.) were weighed in an amount of 90% by mass of the oxide material and 10% by weight of acetylene black. Black (DENKA BLACK), and invested in the planetary ball mill P7 manufactured by Fritch. The cathode active material for a storage battery device was obtained by pulverizing and mixing at 800 rpm for 30 minutes in an air atmosphere. The powder X-ray diffraction pattern of the obtained positive electrode active material was confirmed to contain an amorphous phase (Fig. 3).

Further, data analysis of the diffraction distribution was carried out using JADE Ver. 6.0 manufactured by Materials Data Inc. as an analytical quantitative software. First, after subtracting the diffraction distribution of the background from the diffraction distribution in the range of 10° to 60° to obtain the diffraction distribution, the content of the amorphous phase and the crystal content of Na ₂ MnP ₂ O ₇ are determined by the method described above. The crystallite size of the Na ₂ MnP ₂ O ₇ crystal. The results are shown in Table 1.

(c) Preparation of a sodium ion secondary battery For the positive electrode active material obtained in the above manner, polyvinylidene fluoride was used as a binder, and the positive electrode active material: binder = 95:5 (mass ratio) was weighed. After dispersing it in N-methylpyrrolidone, it is fully stirred by a rotation-revolution mixer, and it is slurry-formed.

Then, using a doctor blade having a gap of 50 μm, the obtained slurry was applied onto an aluminum foil having a thickness of 20 μm as a positive electrode current collector, dried at 70 ° C in a dryer, and passed through a pair of rotating rolls. The pressure was applied at 1 t/cm ² to obtain an electrode sheet. The electrode sheet was punched into a diameter of 11 mm by an electrode punch, and dried at 160 ° C for 6 hours to obtain a circular working electrode.

Then, the obtained working electrode was placed on the lower lid of the coin battery with the aluminum foil facing downward, and the glass filter dried at 200 ° C for 8 hours was laminated thereon, and the glass filter was dried at 60 ° C for 8 hours. Press-dried 16 mm diameter separator film comprising a polypropylene porous film (Celgard #2400 manufactured by Hoechst Celanese) and metal sodium as a counter electrode battery. As the electrolytic solution, a 1 M NaPF ₆ solution/EC: DEC = 1:1 (EC = ethyl carbonate, DEC = diethyl carbonate) was used. Further, the test cell was assembled in an argon atmosphere having a dew condensation temperature of -70 ° C or less and an oxygen concentration of less than 0.2 ppm.

(d) Charge and discharge test The charge and discharge test was carried out as follows. CC (constant current) charging (sodium ion release from the positive electrode active material) was carried out at 30 ° C from the open circuit voltage (OCV) to 4.5 V, and the amount of charge (charge capacity) charged per unit mass of the positive electrode active material was determined. Then, CC discharge (sodium ion occluded in the positive electrode active material) was performed at 4.5 V until 2 V, and the amount of electricity (primary discharge capacity) released per unit mass of the positive electrode active material was determined. Thereafter, CC charging and discharging were repeatedly performed at 2 V to 4.5 V to obtain a charge and discharge capacity. Furthermore, the C rate of charging and discharging was performed at 0.1 C (=0.07 mA). Further, as the discharge capacity retention ratio, the ratio of the discharge capacity at the 50th cycle during the reverse charge/discharge to the initial discharge capacity was determined. The results are shown in Table 1. In addition, the charge and discharge curve of Example 1 is shown in FIG.

(Comparative Example 1) Sodium hydrogencarbonate (NaHCO ₃ ), manganese oxalate (MnC ₂ O ₄ ), and diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ) were used as raw materials, and Na ₂ O 33.3 was expressed in terms of mol%. The raw material powder was prepared in such a manner that the composition of %, MnO ₂ 33.3%, and P ₂ O ₅ 33.3% was used to prepare a raw material batch. Ethanol was added so that the solid content concentration of the raw material batch was 30% by mass, and the mixture was wet-pulverized and mixed at 500 rpm for 1 hour using the planetary ball mill used in Example 1.

After the ethanol was removed by an evaporator, the powder was pressed at 30 MPa to prepare a powder compact, and calcination was carried out for 12 hours at 600 ° C in an argon atmosphere containing 5 vol% of H ₂ . The obtained sample was subjected to dry pulverization at 500 rpm for 1 hour by the planetary ball mill, whereby a positive electrode active material was obtained. The powder X-ray diffraction distribution of the obtained positive electrode active material was confirmed, and no amorphous phase was observed, and only the ray originating from the Na ₂ MnP ₂ O ₇ crystal was found. The radiation distribution around, determined using the method already described the content of amorphous phase, Na ₂ MnP ₂ O ₇ crystal content, Na ₂ MnP ₂ O ₇ crystals crystallite size. The results are shown in Table 1.

The obtained positive electrode active material, polyvinylidene fluoride as a binder, and highly conductive carbon (Super C65 manufactured by Timcal Co., Ltd.) as a conductive auxiliary agent were weighed so that the positive electrode active material: binder: The conductive auxiliary agent = 75:5:20 (mass ratio), and after being dispersed in N-methylpyrrolidone, it was sufficiently stirred by a spin-rotation mixer to be slurried. Using the obtained slurry, a test battery was produced in the same manner as in Example 1, and a charge and discharge test was performed. The results are shown in Table 1.

[Table 1] Table 1

As shown in Table 1, since the positive electrode active material of Example 1 contained an amorphous phase, the initial discharge capacity was as high as 93 mAh/g, and the discharge capacity retention ratio was also high, which was 82%. On the other hand, since the positive electrode active material of Comparative Example 1 did not contain an amorphous phase, the initial discharge capacity was as low as 16 mAh/g. Further, in Table 1, the amorphous content and the Na ₂ MnP ₂ O ₇ crystal content were both % by mass. [Industrial availability]

The positive electrode active material for a storage battery device of the present invention is suitably used as a positive electrode active material for a sodium ion secondary battery used in an electric automobile, an electric tool, an emergency backup power source, or the like.

no

Fig. 1 is a view showing a powder X-ray diffraction pattern of a molten solid obtained in Example 1. 2 is a view showing a powder X-ray diffraction pattern of the oxide material obtained in Example 1. FIG. 3 is a view showing a powder X-ray diffraction pattern of the positive electrode active material obtained in Example 1. FIG. 4 is a graph showing a charge and discharge curve of a test battery produced in Example 1. FIG.

Claims (10)

A positive electrode active material for an electricity storage device, characterized by: a general formula of Na _x (Mn _1-a M _a ) _y P ₂ O _z (M is at least one selected from the group consisting of Cr, Fe, Co, and Ni And 1.2≦x≦2.3, 0.95≦y≦1.6, 0≦a≦0.9, 7≦z≦8), and contain an oxide material having an amorphous phase. The positive electrode active material for a storage battery device according to the first aspect of the invention, wherein the content of the amorphous phase in the oxide material is 1% by mass or more. The positive electrode active material for a storage battery device according to claim 1 or 2, further comprising conductive carbon. The positive electrode active material for a storage battery device according to claim 3, wherein the oxide material contains 80% to 99.5% of the oxide material and 0.5% to 20% of the conductive carbon. The positive electrode active material for a storage battery device according to the first or second aspect of the invention, which contains a triclinic crystal represented by the general formula Na ₂ MnP ₂ O ₇ . The positive electrode active material for a storage battery device according to claim 3, which contains a triclinic crystal represented by the general formula Na ₂ MnP ₂ O ₇ . The positive electrode active material for a storage battery device according to claim 4, which contains a triclinic crystal represented by the general formula Na ₂ MnP ₂ O ₇ . A method for producing a positive electrode active material for an electricity storage device, characterized by: a group consisting of a compound of Na _x (Mn _1-a M _a ) _y P ₂ O _z (M is selected from the group consisting of Cr, Fe, Co, and Ni At least one of the transition metal elements, and the oxide material represented by 1.2≦x≦2.3, 0.95≦y≦1.6, 0≦a≦0.9, 7≦z≦8) is added with conductive carbon and mixed while being pulverized. The method for producing a positive electrode active material for a storage battery device according to claim 8, wherein a molten solidified material is used as the oxide material. The method for producing a positive electrode active material for a storage battery device according to claim 9, wherein the crystal material obtained by subjecting the molten solidified material to heat treatment is used as the oxide material.