JP6758191B2 - Manufacturing method of positive electrode active material for power storage device and electrode sheet - Google Patents

Description

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

Lithium-ion secondary batteries have established themselves as a high-capacity, lightweight power source that is indispensable for portable electronic terminals and electric vehicles, and as their positive electrode active material, olivine-type crystals represented by the general formula LiFePO ₄ Active materials including are attracting attention. However, since lithium is concerned about problems such as soaring prices of raw materials worldwide, research on sodium-ion secondary batteries using sodium as an alternative has been conducted in recent years.

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

Prabeer Barpanda et al., Solid State Ionics, 2014 (DOI: 10.1016 / j.ssi.2014.03.011)

It has been reported that the positive electrode active material composed of Na ₂ (Fe _1-y Mn _y ) P ₂ O ₇ described in Non-Patent Document 1 undergoes a rapid decrease in volume as the proportion of Mn increases. Therefore, there is a problem that the above-mentioned active material does not have charge / discharge characteristics that can withstand actual specifications.

An object of the present invention is to provide a positive electrode active material for a power storage device having excellent charge / discharge characteristics and a method for producing the same.

At least one positive electrode active material for a power storage device has 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 of the present invention, 1. An oxide material represented by 2 ≦ x ≦ 2.3, 0.95 ≦ y ≦ 1.6, 0 ≦ a ≦ 0.9, 7 ≦ z ≦ 8) and containing 30% by mass or more of an amorphous phase. It is characterized by containing.

The positive electrode active material for a power storage device of the present invention not only improves sodium ion conductivity by containing an amorphous phase, but also causes distortion of crystals including Mn due to repeated charging and discharging, and causes Mn components. It is possible to suppress elution to the outside. Therefore, the positive electrode active material for a power storage device of the present invention is excellent in charge / discharge characteristics and cycle characteristics.

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

Since the positive electrode active material contains conductive carbon, it is possible to secure an electron conductive path between the oxide materials and improve charge / discharge characteristics.

The positive electrode active material for a power 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 in mass%.

The positive electrode active material for a power storage 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 generated by charge / discharge, and exhibits a high charge / discharge capacity and discharge voltage when used as a positive electrode active material for a power storage device. ..

Method of manufacturing an electrode sheet of the present invention have the general formula _{Na x (Mn 1-a M} a) y P 2 O z (M is Cr, Fe, transition metal elements of at least one selected from the group consisting of Co and Ni , 1.2 ≦ x ≦ 2.3, 0.95 ≦ y ≦ 1.6, 0 ≦ a ≦ 0.9, 7 ≦ z ≦ 8), conductive carbon is added to the oxide material. Then, by mixing while crushing, a positive electrode active material powder for a power storage device containing the oxide material containing an amorphous phase is obtained, and a binder is added to the positive electrode active material powder for a power storage device. It is characterized by comprising a slurrying step of adding and obtaining a slurry, and a sheeting step of forming the slurry into a sheet.

Generally, the positive electrode active material for a power storage device is used as a positive electrode by mixing and sintering with conductive carbon or an organic substance serving as a conductive carbon source. As a result, a conductive path made of conductive carbon is formed between the positive electrode active materials, and good charge / discharge characteristics can be obtained. However, the positive electrode active material represented by the general formula _{Na x (Mn 1-a M} a) y P 2 O z, the sintering was mixed with conductive carbon, conductive carbon contained in the positive electrode active material Mn It is easily oxidized by elements and released to the outside as carbon dioxide. Therefore, it is difficult to form a conductive path made of conductive carbon between the positive electrode active materials, and sufficient charge / discharge characteristics may not be obtained.

On the other hand, the energy by the production method of the present invention, the general formula _{Na x (Mn 1-a M} a) y P 2 O positive electrode active material represented by _z and conductive carbon, is mixed with pulverized Can be imparted, so that it can be composited at a relatively low temperature without being involved in sintering with conductive carbon. Therefore, oxidation of the conductive carbon by the Mn element is unlikely to occur at the time of compounding, and the conductive carbon can be dispersed in the positive electrode active material in a good state. Further, by going through the pulverization and mixing steps, the oxide materials react with each other to easily form an amorphous phase. In addition, since conductive carbon serves as a pulverization aid, it suppresses aggregation of the oxide material during pulverization and mixing, and promotes the formation of an amorphous phase.

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

In the method for producing an electrode sheet of the present invention, it is preferable to use a molten solidified product as the oxide material.

According to the above configuration, it is possible to enhance the homogeneity of the positive electrode active material.

In the method for producing an electrode sheet of the present invention, it is preferable to use a crystallized product obtained by heat-treating a molten solidified product as an oxide material.
In the method for producing an electrode sheet of the present invention, it is preferable to add the conductive carbon to the oxide material and mix it while pulverizing it with a planetary ball mill in the positive electrode active material powder manufacturing step.

According to the present invention, high voltage can be achieved by containing Mn, which is a high voltage system element, capacity can be improved by containing an amorphous phase, and storage with a high energy density can obtain good cycle characteristics. A positive electrode active material for a device can be obtained.

6 is a chart showing a powder X-ray diffraction pattern of the molten solidified product obtained in Example 1. It is a chart which shows the powder X-ray diffraction pattern of the oxide material obtained in Example 1. FIG. It is a chart which shows the powder X-ray diffraction pattern of the positive electrode active material obtained in Example 1. FIG. It is a graph which shows the charge / discharge curve of the test battery produced in Example 1.

At least one positive electrode active material for a power storage device has 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 of the present invention, 1. It is represented by 2 ≦ x ≦ 2.3, 0.95 ≦ y ≦ 1.6, 0 ≦ a ≦ 0.9, 7 ≦ z ≦ 8), and contains an oxide material containing an amorphous phase. It is characterized by.

Na in the above general formula serves as a supply source of sodium ions that move between the positive electrode active material and the negative electrode active material during charging and discharging of the battery.

Mn is a component that applies a high voltage to the positive electrode active material. Specifically, a redox reaction occurs due to a change in the valence of Mn ions when sodium ions are desorbed from the positive electrode active material or occluded in the positive electrode active material as the battery is charged and discharged. Due to this redox reaction, the positive electrode active material exhibits a high redox potential.

Similar to Mn, M (at least one selected from the group consisting of Cr, Fe, Co, and Ni) also undergoes a valence change during charging and discharging of the battery, so that sodium ions are desorbed from the positive electrode active material. It has a role of increasing the redox potential of the positive electrode active material when it is stored in the positive electrode active material. Ni is preferable because it exhibits a particularly high redox potential. On the other hand, Fe is preferable because it has high structural stability in charge and discharge.

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

The reason why the range of each coefficient in the general formula Na _x (Mn _1-a M _a ) _y P _{2 Oz} is defined as described above will be described below.

x is 1.2 ≦ x ≦ 2.3, preferably 1.3 ≦ x ≦ 2.25, and more preferably 1.5 ≦ x ≦ 2.2. If x is too small, the amount of sodium ions involved in occlusion and release decreases, and the charge / discharge capacity tends to decrease. On the other hand, if x is too large, dissimilar crystals that are not involved in charging / discharging such as Na ₃ PO ₄ are likely to precipitate, so that the charging / discharging capacity tends to decrease.

y is 0.95 ≦ y ≦ 1.6, preferably 0.95 ≦ y ≦ 1.4, and more preferably 0.95 ≦ y ≦ 1.25. If y is too small, the number of transition metal elements that cause the redox reaction is reduced, and the sodium ions involved in occlusion and release are reduced, so that the charge / discharge capacity tends to decrease. On the other hand, if y is too large, dissimilar crystals that are not involved in charging / discharging such as NamnPO ₄ tend to precipitate, so that the charging / discharging capacity tends to decrease.

a is 0 ≦ a ≦ 0.9, preferably 0 ≦ a ≦ 0.5, and more preferably 0 ≦ a ≦ 0.3. The smaller a is, the higher the redox potential generated by charging / discharging is, and when used as a positive electrode active material for a power storage device, it is easy to show a high charging / discharging capacity and discharging voltage. In particular, it is preferable that a = 0.

z is 7 ≦ z ≦ 8, preferably 7 ≦ z ≦ 7.8, and more preferably 7 ≦ z ≦ 7.5. If z is too small, the valences of Mn and M become smaller than divalent, and metal tends to precipitate with charging and discharging. The precipitated metal elutes into the electrolyte and precipitates as metal dendrite on the negative electrode side, which causes an internal short circuit. On the other hand, if z is too large, the valences of Mn and M become larger than divalent, and the redox reaction associated with the charging and discharging of the battery is less likely to occur. As a result, the amount of sodium ions occluded and released is reduced, so that the capacity tends to decrease.

Specific examples of the general formula _{Na x (Mn 1-a M} a) oxide material represented by _{y P 2 O z, Na 2} MnP 2 O 7, Na 2 (Mn 1-a Fe a) P 2 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. Those represented by 2 ≦ a ≦ 0.8) can be mentioned. Among them, the triclinic crystal represented by Na ₂ MnP ₂ O ₇ has a high oxidation-reduction potential generated by charging / discharging, and when used as a positive electrode active material for a power storage device, has a high charge / discharge capacity (theory). The value 97.5 mAh) and the discharge voltage (theoretical value 3.7 V) are shown.

The content of Na _x (Mn _1-a M _a ) _y P _{2 Oz} crystals in the positive electrode active material is preferably 99% by mass or less, more preferably 90% by mass or less, and 85% by mass or less. It is more preferably 80% by mass or less, and most preferably 70% by mass or less. If the content of Na ₂ MnP ₂ O ₇ crystals is too large, the amorphous phase will be reduced, and it will be difficult to obtain the effects described below.

The content of the amorphous phase in the oxide material is preferably 1% by mass or more, more preferably 10% by mass or more, further preferably 20% by mass or more, and particularly preferably 30% by mass or more. If the content of the amorphous phase is too small, the sodium ion conductivity tends to decrease. Further, with repeated charging and discharging, distortion of the crystal containing Mn is generated, and the Mn component is likely to be eluted to the outside. As a result, charge / discharge characteristics (particularly high-speed charge / discharge characteristics) and cycle characteristics are likely to deteriorate.

The content of _{Na x (Mn 1-a M} a) y P 2 O z crystal and amorphous phases in the oxide material, 10 to 60 ° in 2θ values obtained by powder X-ray diffractometry using CuKα ray It is obtained by separating the peaks into a crystalline diffraction line and an amorphous halo in the diffraction line profile of. Specifically, the integrated intensity obtained by peak-separating the broad diffraction line (amorphous halo) at 10 to 45 ° from the total scattering curve obtained by subtracting the background from the diffraction line profile is Ia10. _{~60 Na x (Mn 1-a} M a) detected in ° _y P 2 _{O z} the sum of integrated intensity of crystalline diffraction lines was determined by peak separation from crystalline Ic, crystals derived from the other crystalline If the Io the sum of integrated intensity determined from sex diffraction _{line, Na x (Mn 1-a} M a) y P 2 O z content Xg content Xc and amorphous phase of the crystals determined by: Be done.

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

As the crystallite size of the _{Na x (Mn 1-a M} a) y P 2 O z crystal is small, it is possible to improve the discharge capacity. _{Specifically,} it is preferable that the crystallite size of the _{Na x (Mn 1-a M} a) y P 2 O z crystal is 100nm or less, more preferably 60nm or less, further not more 50nm or less preferable. The lower limit is not particularly limited, but in reality, it is 1 nm or more, and further 2 nm or more. The crystallite size is determined according to Scheller's formula from the analysis result of powder X-ray diffraction using CuKα ray. Specifically, confirmation from all scattering curve obtained by subtracting the _background, in the vicinity of 2 [Theta] = 22.3 ° derived from the _{Na x (Mn 1-a M} a) y P 2 O z crystals from the diffraction line profile the diffraction line from the Bragg angle θ and the full width at half maximum beta (FWHM) obtained by peak separation _{is, Na x (Mn 1-a} M a) y P 2 O z crystallite size ε crystal is determined from the following equation ..

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

The positive electrode active material for a power storage device of the present invention preferably contains conductive carbon. As a result, it is possible to secure an electron conductive path between the oxide materials, and it is possible to improve the charge / discharge characteristics. As the conductive carbon, highly conductive carbon black such as acetylene black or Ketjen black, carbon powder such as graphite, carbon fiber or the like can be used. Of these, acetylene black, which has high electron conductivity, is preferable.

The positive electrode active material for a power storage device of the present invention preferably contains 80 to 99.5% of an oxide material and 0.5 to 20% of conductive carbon in mass%, and 85 to 98% of an oxide material and carbon. It preferably contains 2 to 15% of the material. By regulating the contents of the oxide material and the conductive carbon within the above range, it becomes easy to obtain a positive electrode active material having a high charge / discharge capacity and good cycle characteristics.

The shape of the positive electrode active material for the power storage device is not particularly limited, but it is preferably in the form of powder. In that case, the average particle size of the positive electrode active material for the power storage device is preferably 0.1 to 20 μm, 0.3 to 15 μm, 0.5 to 10 μm, and particularly preferably 0.6 to 5 μm. The maximum particle size is preferably 150 μm or less, 100 μm or less, 75 μm or less, and particularly preferably 55 μm or less. If the average particle size or the maximum particle size is too large, it becomes difficult to occlude and release sodium ions during charging / discharging, so that the charging / discharging capacity tends to decrease. On the other hand, if the average particle size is too small, the powder is inferior in the dispersed state when it is made into a paste, and it tends to be difficult to produce a uniform electrode.

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

Next, a method for producing the positive electrode active material for a power storage device of the present invention will be described. The positive electrode active material for an electricity storage device of the present invention, for example, the general formula _{Na x (Mn 1-a M} a) y P 2 O z (M is Cr, Fe, at least one selected from the group consisting of Co and Ni Conductivity with respect to the transition metal element, the oxide material represented by 1.2 ≦ x ≦ 2.3, 0.95 ≦ y ≦ 1.6, 0 ≦ a ≦ 0.9, 7 ≦ z ≦ 8) It can be produced by adding carbon and mixing while pulverizing.

As the oxide material represented by the general formula Na _x (Mn _1-a M _a ) _y P _{2 Oz} , a melt-solidified product of the raw material powder (oxide, etc.) of each component, a solid-phase reaction product, or the like is used. Can be done. In particular, when a molten solidified product is used as the oxide material, it is preferable because a positive electrode active material having excellent homogeneity can be easily obtained.

The molten solidified product can be produced as follows. First, the general formula _{Na x (Mn 1-a M} a) y P 2 O z (M is Cr, Fe, Mn, of at least one transition metal element selected from the group consisting of Co and Ni) to a composition of Prepare the raw material powder and obtain the raw material batch. Next, the obtained raw material batch is melted. The melting temperature may be appropriately adjusted so that the raw material batch is uniformly melted. Specifically, the melting temperature is preferably 800 ° C. or higher, more preferably 900 ° C. or higher. The upper limit is not particularly limited, but if the melting temperature is too high, energy loss and evaporation of the sodium component will occur. Therefore, the temperature is preferably 1500 ° C. or lower, and more preferably 1400 ° C. or lower.

A molten solidified product is obtained by molding the obtained molten product. The molding method is not particularly limited, and for example, the melt may be poured between a pair of cooling rolls and molded into a film shape while quenching, or the melt may be poured into a mold and molded into an ingot shape. It doesn't matter. The melt-solidified product may be an amorphous substance, a crystalline substance, or a mixture of a crystalline phase and an amorphous phase.

The molten solidified product made of an amorphous material may be crystallized by heat treatment at a predetermined temperature for a predetermined time (crystallized glass). The heat treatment is performed, for example, in an electric furnace in which the temperature can be controlled. The heat treatment temperature is preferably equal to or higher than the glass transition temperature of the amorphous material, and more preferably equal to or higher than the crystallization temperature. Specifically, it is preferably 350 ° C. or higher, and more preferably 400 ° C. or higher. The heat treatment time is appropriately adjusted so that the crystallization of the amorphous substance proceeds sufficiently. Specifically, it is preferably 20 to 300 minutes, and more preferably 30 to 240 minutes.

The heat treatment of the amorphous body may be carried out in any of an air atmosphere, an inert atmosphere and a reducing atmosphere, but it is particularly preferable to carry out the heat treatment in the reducing atmosphere, whereby Mn and M in the molten solidified product are 2 Can be valued. Examples of the reducing atmosphere include a hydrogen atmosphere and 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, and the content of the reducing gas at that time is preferably 2% by volume or more.

As a method of mixing the oxide material and conductive carbon while crushing, a general crusher such as a mortar, a mortar, a ball mill, an attritor, a vibrating ball mill, a satellite ball mill, a planetary ball mill, a jet mill, or a bead mill is used. The method to be used can be mentioned. Of these, it is preferable to use a planetary ball mill. In the planetary ball mill, the base rotates while the pot rotates, and it is possible to efficiently generate extremely high impact energy, and it is possible to uniformly disperse conductive carbon in the oxide material. Not only that, an amorphous phase is likely to be formed in the oxide 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 non-aqueous solvent, or an ionic liquid. It can also be used in an all-solid-state sodium-ion secondary battery using a solid electrolyte.

Hereinafter, the present invention will be described in detail based on Examples. The present invention is not limited to the following examples.

(Example 1)
(A) Melting step Using sodium hydrogen phosphate (NaH ₂ PO ₄ ) and manganese oxide (Mn ₃ O ₄ ) as raw materials, in mol%, Na ₂ O 33.3%, Mn O ₂ 33.3%, P ₂ O. _The raw material powder was prepared so as to have a composition of ₅ 33.3%, and melted in an air atmosphere at 1050 ° C. for 15 minutes. Then, it was poured onto an iron plate and rapidly cooled to obtain a molten solidified product. This molten solidified product was pulverized with a planetary ball mill P7 manufactured by Fritch, Inc. to obtain a powdered molten solidified product. When the powder X-ray diffraction pattern of the obtained molten solidified product was confirmed, no crystalline diffraction line was confirmed and it was confirmed that it was an amorphous substance (FIG. 1).

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

(B) Grinding / Mixing Step The oxide material obtained above and acetylene black (Denka Black, manufactured by Denki Kagaku Kogyo Co., Ltd.) as conductive carbon are used in mass% of 90% of the oxide material and 10% of acetylene black. It was weighed at a ratio and put into P7 manufactured by Fritch, a planetary ball mill. A positive electrode active material for a power storage device was obtained by pulverizing and mixing at 800 rpm for 30 minutes in an air atmosphere. When the powder X-ray diffraction pattern of the obtained positive electrode active material was confirmed, it was confirmed that it contained an amorphous phase (FIG. 3).

Furthermore, as analysis / quantification software, Materials Data Inc. Made by JADE Ver. Data analysis of the diffraction line profile was performed using 6.0. First, the diffraction profile of the background is subtracted from the diffraction line profile in the range of 10 to 60 ° to obtain a diffraction profile, and then the amorphous phase content and the Na ₂ MnP ₂ O ₇ crystal content are obtained by the method described above. , Na ₂ MnP ₂ O ₇ crystal crystallite size was determined. The results are shown in Table 1.

(C) Preparation of Sodium Ion Secondary Battery The positive electrode active material obtained above is weighed using polyvinylidene fluoride as a binder so that the positive electrode active material: binder = 95: 5 (mass ratio). After dispersing these in N-methylpyrrolidone, the mixture was sufficiently stirred with a rotation / revolution mixer to form a slurry.

Next, using a doctor blade with a gap of 50 μm, the obtained slurry is coated on an aluminum foil having a thickness of 20 μm, which is a positive electrode current collector, dried at 70 ° C. in a dryer, and then between a pair of rotating rollers. An electrode sheet was obtained by passing through and pressing at 1 t / cm ² . The electrode sheet was punched to a diameter of 11 mm with an electrode punching machine and dried at 160 ° C. for 6 hours to obtain a circular working electrode.

Next, the obtained working electrode was placed on the lower lid of the coin cell with the aluminum foil surface facing down, and the glass filter was dried at 200 ° C. for 8 hours, and the pressure was reduced at 60 ° C. for 8 hours. A test battery was prepared by laminating a separator made of a dry polypropylene porous membrane having a diameter of 16 mm (Celguard # 2400 manufactured by Hoechst Ceramics) and metallic sodium as a counter electrode. As the electrolytic solution, 1M NaPF ₆ solution / EC: DEC = 1: 1 (EC = ethylene carbonate DEC = diethyl carbonate) was used. The test battery was assembled in an argon atmosphere environment with a dew point temperature of −70 ° C. or lower and an oxygen concentration of less than 0.2 ppm.

(D) Charge / discharge test The charge / discharge test was performed as follows. CC (constant current) charging (release of sodium ions from the positive electrode active material) is performed from the open circuit voltage (OCV) to 4.5 V at 30 ° C., and the amount of electricity charged in the unit mass of the positive electrode active material (charging capacity). Asked. Next, CC discharge (sodium ion storage in the positive electrode active material) was performed from 4.5 V to 2 V, and the amount of electricity discharged (initial discharge capacity) in the unit mass of the positive electrode active material was determined. After that, CC charge / discharge was repeatedly performed at 2 V to 4.5 V to determine the charge / discharge capacity. The C rate for charging and discharging was 0.1 C (= 0.07 mA). Further, as the discharge capacity retention rate, the ratio of the discharge capacity in the 50th cycle to the initial discharge capacity when repeatedly charged and discharged was obtained. The results are shown in Table 1. Note that FIG. 4 shows the charge / discharge curve of Example 1.

(Comparative Example 1)
Sodium bicarbonate (NaHCO _3), manganese oxalate _{(MnC 2 O} 4), the diammonium phosphate _{((NH 4) 2 HPO 4} ) as a raw material, in _{mol%, Na 2 O 33.3%,} MnO 2 33 Raw material powders were mixed so as to have a composition of .3% and P ₂ O ₅ 33.3% 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 wet pulverization and mixing was carried out at 500 rpm for 1 hour using the planetary ball mill used in Example 1.

After removing ethanol with an evaporator, to prepare a green compact by pressing at 30 MPa, 600 ° C., 12 hours, sintering was carried out in an Ar gas atmosphere containing H ₂ 5% by volume. The obtained sample was dry-pulverized at 500 rpm for 1 hour using the above planetary ball mill to obtain a positive electrode active material. When the powder X-ray diffraction profile of the obtained positive electrode active material was confirmed, no amorphous phase was confirmed, and only diffraction lines derived from Na ₂ MnP ₂ O ₇ crystals were confirmed. From the diffraction line profile, the content of the amorphous phase by the method described _above, Na 2 _MnP 2 _{O 7} crystal content _was determined crystallite size of Na 2 _MnP 2 _{O 7} crystals. The results are shown in Table 1.

The obtained positive electrode active material contains polyvinylidene fluoride as a binder, highly conductive carbon (Super C65 manufactured by Timcal) as a conductive auxiliary agent, and positive electrode active material: binder: conductive auxiliary agent = 75: 5: 20 (mass ratio). These were weighed so as to be, dispersed in N-methylpyrrolidone, and then sufficiently stirred with a rotating / revolving mixer to form a slurry. Using the obtained slurry, a test battery was prepared in the same manner as in Example 1, and a charge / discharge test was performed. The results are shown in 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 rate was also as high as 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. In Table 1, the amorphous content and the Na ₂ MnP ₂ O ₇ crystal content are both mass%.

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

Claims (8)

At least one of the general formula _{Na x (Mn 1-a M} a) y P 2 O z (M selected from the group consisting of Cr, Fe, Co and Ni, 1.2 ≦ x ≦ 2.3,0.95 For a power storage device represented by ≦ y ≦ 1.6, 0 ≦ a ≦ 0.9, 7 ≦ z ≦ 8) and containing an oxide material containing an amorphous phase in an amount of 30% by mass or more. Positive electrode active material. The positive electrode active material for a power storage device according to claim 1, further comprising conductive carbon. The positive electrode active material for a power storage device according to claim 2, wherein the oxide material contains 80 to 99.5% and the conductive carbon contains 0.5 to 20% in mass%. The positive electrode active material for a power storage device according to any one of claims 1 to 3, wherein the triclinic crystal represented by the general formula Na ₂ MnP ₂ O ₇ is contained. General formula Na _x (Mn _1-a M _a ) _y P _{2 Oz} (M is at least one transition metal element selected from the group consisting of Cr, Fe, Co and Ni, 1.2 ≦ x ≦ 2.3 , 0.95 ≦ y ≦ 1.6, 0 ≦ a ≦ 0.9, 7 ≦ z ≦ 8), by adding conductive carbon to the oxide material and mixing while pulverizing. A step of producing a positive electrode active material powder for obtaining a positive electrode active material powder for a power storage device containing the oxide material containing an amorphous phase ,
A slurrying step of adding a binder to the positive electrode active material powder for a power storage device to obtain a slurry, and
Sheet making step of making the slurry into a sheet,
A method for manufacturing an electrode sheet, which comprises. The method for producing an electrode sheet according to claim 5, wherein a molten solidified product is used as the oxide material. The method for producing an electrode sheet according to claim 6, wherein a crystallized product obtained by subjecting the molten solidified product to heat treatment is used as the oxide material. The invention according to any one of claims 5 to 7, wherein in the positive electrode active material powder manufacturing step, the conductive carbon is added to the oxide material and mixed while being pulverized by a planetary ball mill. Method of manufacturing electrode sheet.

Images