JP2008260666A - Active material for sodium secondary battery, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide maricite type NaMn<SB>x</SB>M<SB>(1-x)</SB>PO<SB>4</SB>comprising elements abundant as resources and having a high operating voltage, and a method for producing the same; to provide an active material for a sodium secondary battery containing the same, and the battery. <P>SOLUTION: This active material for a sodium secondary battery contains the maricite type NaMn<SB>x</SB>M<SB>(1-x)</SB>PO<SB>4</SB>, where M is any of Fe, Co and Ni; and x satisfies 0≤x≤1. The battery uses the active material. Maricite type NaMnPO<SB>4</SB>as an example of the maricite type compounds has a symmetry of a Pnmb space group and lattice constants a, b and c are: 6.86 Å≤a≤7.00 Å; 8.98 Å≤b≤9.15 Å; and 5.04 Å≤c≤5.14 Å, respectively. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

TECHNICAL FIELD The present invention relates to an active material for a sodium secondary battery and a method for producing the same, and more specifically, marisite-type NaMn _x M _(1-x) PO ₄ and a method for producing the same, and a sodium secondary battery including the same. The present invention relates to an active material and a battery.

As secondary batteries for portable electronic devices and the like, nonaqueous electrolyte lithium ion secondary batteries have been put into practical use and are widely used. Lithium ions are most advantageous from the viewpoint of energy density because they have the lowest redox potential and the smallest ionic radius among solids present on the earth.

However, since lithium has a problem that it has a small reserve amount as a resource and is expensive, research and development on secondary batteries using sodium ions, which are abundant elements as resources, are being carried out. Sodium has a low redox potential next to lithium, and thus can be expected to have a high electromotive force. Since sodium is abundant on the earth, it is a preferable element from the viewpoint of cost.
Under such circumstances, NaNiO ₂ , which is a complex oxide with abundant nickel as a resource, has been proposed as a positive electrode active material for non-aqueous electrolyte secondary batteries (see, for example, Non-Patent Document 1). However, a nonaqueous electrolyte secondary battery using NaNiO ₂ as a positive electrode active material has a problem that the operating voltage is low (see, for example, Patent Document 1).

JP 2003-151549 A Delmas et al., “Revue de Chimie Minerale” (France), Gautier-Villars, 1982, Vol. 19, p. 343

In order to intercalate sodium ions having an ionic radius about 1.3 times that of lithium ions into the crystal structure, the apex has a larger gap than the layered compound used as the positive electrode active material for lithium ion secondary batteries. Although a compound having a shared structure and a layered compound having a two-dimensional diffusion layer are required, a positive electrode active material for sodium secondary batteries that is practically usable and does not use rare elements has not been obtained at present.

The present invention has been made in view of such circumstances, consist abundant element as a resource, maricite type _{NaMn x M (1-x)} PO 4 and a method of manufacturing the same having a high operating voltage, and maricite type NaMn _x M _(1-x) active material for sodium secondary batteries containing PO _4, a positive electrode, and to provide a battery for the purpose.

The mariticite type NaMnPO _{4 according} to the first invention that meets the above object has the symmetry of the space group Pnmb, and the lattice constants a, b, and c are respectively 6.86Å ≦ a ≦ 7.00Å. 8.98 Å ≦ b ≦ 9.15 Å, 5.04 Å ≦ c ≦ 5.14 Å.
NaMnPO ₄ having a space group Pnam symmetry is known as a naturally occurring NaMnPO ₄ , but NaMnPO ₄ having a marisite crystal structure has not been known. The marisite-type NaMnPO _{4 according to the} invention has a novel crystal structure as NaMnPO ₄ .

In the method for producing the marisite-type NaMnPO _{4 according} to the second invention, the Na source, the Mn source, and the phosphoric acid source are mixed in water, the water is evaporated, and the resulting mixture is heated at 600 to 1000 ° C. in the atmosphere. It is characterized by heating.

The active material for a sodium secondary battery according to the third invention is a marisite type NaMn _x M _(1-x) PO ₄ (wherein M is any one of Fe, Co and Ni, and x is 0 ≦ x ≦ 1).
The sodium secondary battery according to the fourth invention uses the active material for sodium secondary battery according to the third invention.

The marisite-type NaMnPO _{4 according to} claim 1 has a marisite-type crystal structure, can be manufactured using abundant Na and Mn as resources, and has a high operating voltage and a high capacity. It can be used as an active material.

The method for producing the marisite-type NaMnPO _{4 according to} claim 2, wherein NaMnPO ₄ having a marisite-type crystal structure that can be used as an active material for a sodium secondary battery having a high operating voltage and a high capacity is obtained in a stoichiometric manner. It can manufacture by carrying out wet synthesis using the raw material of a composition ratio.

The active material for a sodium secondary battery according to claim 3 can provide a positive electrode for a sodium secondary battery having excellent electrode characteristics.
The sodium secondary battery according to claim 4 can provide a sodium secondary battery having an operating voltage and capacity comparable to that of a lithium secondary battery using sodium that is less expensive than lithium.

The marisite-type NaMnPO _{4 according} to the first embodiment of the present invention has the symmetry of the space group Pnmb, and the lattice constants a, b, and c are 6.86Å ≦ a ≦ 7.00Å, 8 .98 Å ≦ b ≦ 9.15 Å, 5.04 ≦≦ c ≦ 5.14 Å. The lattice constants a, b, and c are determined using any known method such as the Rietveld method using powder XRD (X-ray diffraction).

Marisite-type NaMnPO ₄ weighs sodium carbonate (Na ₂ CO ₃ ) as a Na source, Mn powder as a Mn source, and diphosphorus pentoxide (P ₂ O ₅ ) as a phosphate source in a stoichiometric ratio. Then, after reacting in a solvent or dispersion medium such as water, the solvent or dispersion medium is evaporated and heated in the atmosphere.
The reaction time is preferably about 24 to 72 hours at room temperature, and is preferably carried out with stirring by any known means such as a magnetic stirrer or a ball mill.
Water is evaporated by a known means such as vacuum distillation using a rotary evaporator or the like, a dry spray method or the like.

The heating temperature is 600 to 1000 ° C, preferably 600 to 800 ° C, more preferably 600 to 700 ° C. In order to make the reaction proceed completely, the heating temperature needs to be 600 ° C. or higher. On the other hand, when the temperature exceeds 1000 ° C., the purity of NaMnPO ₄ is lowered by the by-product, which is not preferable.

The sodium secondary battery according to the second embodiment of the present invention has a marisite crystal structure, and a compound represented by the general formula NaMn _x M _(1-x) PO ₄ is an active material (positive electrode active). Substances). In the formula, M is any one of Fe, Co, and Ni, or any combination of two or more, and x is 0 ≦ x ≦ 1.
Among the general formula _{NaMn x M (1-x)} PO 4 compound represented by, (in the general formula _{NaMn x M (1-x)} PO 4, x = 1) NaMnPO 4 Synthesis of By the above wet method However, the synthesis of other compounds may be carried out by the wet method described above, and includes Na source, M source (Mn source, Fe source, Ni source, Co source, and mixtures thereof), and phosphate source. May be mixed by a stoichiometric ratio, and may be performed by a conventional solid phase method in which baking is performed without adding water.

For example, marisite-type NaFePO ₄ can be synthesized by weighing Na source, Fe source, and phosphate source in a stoichiometric ratio and then heating in a reducing atmosphere. The source of Na, NaOH, NaCl, NaBr, but may be any Na salt such as _Na 2 CO _3, may also act as a melting material with low melting point _Na 2 CO ₃ are preferred.
As the Fe source, Fe ₂ O ₃ , Fe ₃ O ₄ , metallic iron, Fe (CH ₃ COO) ₂ , FeC ₂ O _{4 and the} like can be used.
As the phosphoric acid source, P ₂ O ₅ , (NH ₄ ) ₂ HPO ₄ , NH ₄ H ₂ PO ₄ or the like can be used.

A nitrogen or argon atmosphere is usually used as the reducing atmosphere.
The heating method is not particularly limited, and any heating means such as induction heating or microwave heating can be used. The heating temperature is, for example, 300 to 600 ° C., and the heating time is, for example, 3 to 10 hours.

In addition, NiCl ₂ , NiO, metal Ni, or the like can be used as the Ni source, and CoCl ₂ , CoO, metal Co, or the like can be used as the Co source. Alternatively, a mixed oxide such as Fe ₂ NiO ₄ or CoFe ₂ O ₄ may be used.
When a metal element M other than Mn and Fe described above is used alone or mixed with other elements, the preferred heating temperature is uniquely determined because it varies depending on the composition and properties of the raw materials used. In general, however, the temperature is appropriately selected within a temperature range higher than about 200 ° C. at which the solid as a raw material melts and lower than about 600 ° C. at which the influence of side reactions becomes significant.
Similarly, a preferable reaction time is generally appropriately within a time range of about 1 hour or more necessary for the reaction to sufficiently proceed and about 10 hours or less to prevent the crystal growth from proceeding excessively. Selected.

The active material thus obtained may be used usually in a powder form, and the average particle size may be about 50 nm to 10 μm. The average particle diameter is a value measured by, for example, a laser diffraction particle size distribution measuring apparatus. In addition, as long as a predetermined charging / discharging characteristic is acquired as a positive electrode active material, the said positive electrode active material may be used independently, but the mixture with the other positive electrode active material known conventionally may be sufficient.

(1) Production of positive electrode For production of the positive electrode, the above active material may be used as it is, but in order to improve electrical conductivity, a known conductive material and a composite may be formed. Examples of the conductor include carbon-based materials such as acetylene black, carbon, graphite, natural graphite, artificial graphite, and needle coke, and acetylene black is preferable.
The composite is formed by mixing the positive electrode active material and the conductor using a planetary ball mill, stone mill, etc., and coating the surface of the active material with a conductor (carbon coating) and then heating (carbothermal treatment). .
The carbon coating treatment is performed by mixing a powdery active material and a carbon-based material as a conductor at room temperature for 8 to 48 hours, preferably 20 to 30 hours.
The heating temperature in the carbothermal treatment is difficult to determine uniquely because it varies depending on the properties and composition of the active material and conductor used. However, the preferable heating temperature is, for example, 400 to 700 ° C., and the preferable heating time. Is, for example, 1 to 5 hours.
In addition, in order to prevent wear and tear by carbon combustion, heating is performed in a reducing atmosphere.

The carbon content can be controlled by the amount of carbon source added. If the carbon content is low, the effect of improving the conductivity will not appear, so 2% or more is preferable. Moreover, since the quantity of a positive electrode active material will reduce when carbon content increases too much, it is preferable that it is 30% or less.

The positive electrode active material powder thus obtained was mixed with a known binder (polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, ethylene propylene diene polymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluorine rubber, After mixing with vinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, etc.), the obtained mixed powder may be pressure-formed on a support made of stainless steel or filled into a metal container. Alternatively, for example, the mixed powder is mixed with an organic solvent (N-methylpyrrolidone, toluene, cyclohexane, dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, methyl acrylate, diethyltriamine, N, N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran. The positive electrode can also be produced by a method such as applying a slurry obtained by mixing with a metal substrate such as aluminum, nickel, stainless steel, or copper.
The thickness of the positive electrode is usually about 1 to 1000 μm, preferably about 10 to 300 μm. If it is too thick, the conductivity tends to decrease, and if it is too thin, the capacity tends to decrease. The positive electrode obtained by coating and drying may be consolidated by a roller press or the like in order to increase the packing density of the active material.

(2) Production of negative electrode Production of the negative electrode may be carried out by a known method. As the negative electrode active material, any material capable of inserting and extracting sodium can be used.
For example, when a powdery substance such as a carbon material is used as the negative electrode active material, the negative electrode active material can be produced in the same manner as the above-described method (1) used for the production of the positive electrode. That is, a powdered negative electrode active material is mixed with a known binder as necessary, and further with the aforementioned known conductive material as necessary, and then the mixed powder is formed into a sheet shape, which is made of stainless steel, copper, etc. What is necessary is just to crimp | bond to the electrical conductor network (current collector). For example, it can also produce by apply | coating the slurry obtained by mixing the said mixed powder with a well-known organic solvent on metal substrates, such as copper.
When tin (Sn) or germanium (Ge) is used alone as the negative electrode active material, it is deposited on the current collector (for example, to a thickness of 0.2 to 5 μm) using a vapor deposition method such as sputtering. Can be made.
Alternatively, metallic sodium may be used as the negative electrode.

(3) Electrolytic solution The electrolytic solution usually contains an electrolyte and a solvent. The solvent of the electrolytic solution is not particularly limited as long as it is non-aqueous, can dissolve a sufficient amount of electrolyte, and does not undergo electrolysis at the operating voltage. For example, carbonates, ethers, ketones, sulfolane System compounds, lactones, nitriles, chlorinated hydrocarbons, ethers, amines, esters, amides, phosphate ester compounds, and the like can be used. Typical examples of these include 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, ethylene carbonate, vinylene carbonate, methyl formate, dimethyl sulfoxide, propylene carbonate, acetonitrile, γ -Butyrolactone, dimethylformamide, dimethyl carbonate, diethyl carbonate, sulfolane, ethyl methyl carbonate, 1,4-dioxane, 4-methyl-2-pentanone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether , Sulfolane, methyl sulfolane, propionitrile, benzonitrile, butyronitrile, valeronitrile, 1,2-dichloroethane, trimethyl phosphate, triethyl phosphate, etc. That. These can be used alone or in combination of two or more.

As the electrolytic solution, one having a lipophilic counter ion is preferable from the viewpoint of solubility in the electrolytic solution. For example, NaClO ₄ , NaPF ₆ , NaBF ₄ , CF ₃ SO ₃ Na, NaAsF ₆ , NaB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Na, CF ₃ SO ₃ Na, NaN (SO ₂ CF ₃ ) ₂ , NaN (SO ₂ C ₂ F ₅ ) ₂ , NaC (SO ₂ CF ₃ ) ₃ , NaN (SO ₃ CF _{3 2} ) etc. can be used. If soluble in the electrolyte, NaCl, NaBr, or the like can be used.

(4) Manufacture of a sodium secondary battery In manufacturing a sodium secondary battery, conventionally known various materials can be used for elements such as a separator, a battery case, and other structural materials, and there is no particular limitation. For example, when using a separator between the positive electrode and the negative electrode, a microporous polymer film is used, such as nylon, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polypropylene, polyethylene, polybutene, etc. Those made of a polyolefin polymer are used. Polyolefin polymers are preferred in view of the chemical and electrochemical stability of the separator, and are preferably made of polyethylene in view of the self-occluding temperature, which is one of the purposes of the battery separator.

In the case of a polyethylene separator, ultrahigh molecular weight polyethylene is preferable from the viewpoint of maintaining high-temperature shape, and the lower limit of the molecular weight is preferably 500,000, more preferably 1,000,000, and most preferably 1.5 million. On the other hand, the upper limit of the molecular weight is preferably 5 million, more preferably 4 million, and most preferably 3 million. This is because if the molecular weight is too large, the pores of the separator may not close when heated because the fluidity is too low. What is necessary is just to assemble a battery according to a well-known method using these battery elements. In this case, the shape of the battery is not particularly limited, and various shapes and sizes such as a cylindrical shape, a square shape, and a coin shape can be appropriately employed.

Next, examples carried out for confirming the effects of the present invention will be described.
Example 1 Preparation of Active Material for Sodium Secondary Battery (1) Synthesis of Marisite Type NaFePO ₄ and Preparation of Positive Electrode Active Material Containing It (NaFePO ₄ / C) First, to synthesize marisite type NaFePO ₄ , the starting material is a sodium carbonate _(Na 2 CO _3, manufactured by Wako pure Chemical, 98%), iron oxalate dihydrate _{(FeC 2 O 4 · 2H 2} O, manufactured by Kanto Kagaku, 98.5%), phosphoric acid Ammonium dihydrogen (NH ₄ H ₂ PO ₄ , Wako Pure Chemicals, 99%) was weighed in a stoichiometric ratio in a glove box, mixed well using an agate mortar, and then 400 ° C. in an Ar atmosphere. Heat treatment was performed for 5 hours. Then, after sufficiently removing the sample again with an agate mortar, acetylene black was added so that the weight ratio of the sample and acetylene black was 70:25 in an Ar atmosphere, and the planetary mill was maintained while maintaining the Ar atmosphere. Carbon coating treatment was performed at 200 rpm for 24 hours. Thereafter, carbothermal treatment was performed in an Ar atmosphere at 550 ° C. for 5 hours to prepare NaFePO ₄ / C.

(2) Synthesis of Marisite-type NaMnPO ₄ and Preparation of Cathode Active Material Containing It (NaMnPO ₄ / C) In order to synthesize NaMnPO ₄ , sodium carbonate as a starting material, manganese powder (Mn, manufactured by Wako Pure Chemical Industries, Ltd., 98%) and diphosphorus pentoxide (P ₂ O ₅ , Wako Pure Chemical Industries, 98%) were weighed in a stoichiometric ratio in a glove box, and excess pure water was added in a fume hood. Then, it stirred for 2 days using the magnetic stirrer and made it react. Then, in order to make it react completely, it stirred at 200 rpm for 24 hours in the planetary mill in the 250 ml container (made by Ito Seisakusho) made from zirconia. As balls for pulverization, 1.5 mmφ (40 g), 3.0 mmφ (60 g), 10.0 mmφ (five) balls made of zirconia were mixed and used. The solution obtained by stirring with a planetary mill was dried by a vacuum evaporator. Thereafter, the agate mortar was sufficiently covered, and heat treatment was performed at 350 to 700 ° C. for 1 hour in the air. The obtained powder sample was weighed with acetylene black at a weight ratio of 70:25, and subjected to carbon coating treatment in a planetary mill by dry method at 200 rpm for 24 hours. Thereafter, the carbon composite sample obtained was subjected to carbothermal treatment at 550 ° C. for 1 hour in an Ar atmosphere to prepare NaMnPO ₄ / C.

(3) A powder X-ray diffractometer (RINT2100HLR / PC manufactured by Rigaku) was used to identify NaFePO ₄ and NaMnPO ₄ prepared by XRD measurement and SEM observation (1) and (2) of the positive electrode active material. The measurement was performed at 10 ° to 80 ° at 2 ° / min. In addition, FE-SEM (JSM-6340F manufactured by JEOL Ltd.) measurement was performed to observe the morphology of NaFePO ₄ / C and NaMnPO ₄ / C prepared in (1) and (2).

(4) XRD measurement and SEM observation of synthetic sample FIG. 1 shows the XRD profile of NaFePO ₄ . Almost all diffraction peaks were consistent with marisite-type NaFePO ₄ (ICDD 29-1216, space group Pnmb). Although the peak observed around 25-30 ° is not described in the data recorded in ICDD 29-1216, it can be confirmed in marisite type NaCoPO ₄ (ICDD 32-1070), and has the same structure as NaCoPO _4. I can judge. In any case, it was possible to synthesize marisite-type NaFePO ₄ which can be said to be almost a single phase. Moreover, the lattice constants of the obtained samples are a = 6.880 (0) ＝, b = 8.980 (8) Å, c = 5.039 (4) Å, and the literature value a = 6. Good agreement was shown for 867 Å, b = 8.989 Å, and c = 5.049 Å.

Next, FIG. 2 shows an XRD profile of NaMnPO ₄ baked at 600 ° C. for 1 hour. Regarding NaMnPO _4, there was no report of marisite type NaMnPO ₄ , and it was not possible to strictly identify it. However, since it was very similar to the profile obtained by shifting the diffraction pattern of NaFePO ₄ (ICDD 29-1216) to the low angle side, it was judged that marisite-type NaMnPO ₄ could be synthesized.

FIG. 3 shows XRD profiles of NaMnPO ₄ baked at 350 to 700 ° C., respectively. The main peak of marisite-type NaMnPO ₄ began to appear at around 35 ° from the 450 ° C. fired sample, but an impurity peak was also observed. On the other hand, in the sample fired at 600 ° C. or higher, almost single-phase marisite-type NaMnPO ₄ could be obtained. The olivine type LiMnPO ₄ could be synthesized by a low temperature firing of 350 ° C. by a wet process using the same planetary mill, but the marisite type NaMnPO ₄ could be synthesized in a single phase at 350 ° C. even if the same process was used. Thus, it was found that firing at 600 ° C. or higher is necessary.

Further, FIG. 4 shows an XRD profile of NaMnPO ₄ / C obtained by subjecting NaMnPO ₄ to carbon coating and carbothermal treatment. Even after the carbothermal treatment, the marisite-type NaMnPO ₄ could be maintained, and a new impurity phase could not be confirmed.

Figure 5 NaFePO ₄ / C sample was calcined at 550 ° C. (a), the FIG. 5 (b) to NaMnPO ₄ samples were calcined at 600 ℃, NaMnPO was carbonitrile thermal treatment at 550 ° C. Figure 5 (c) ₄ / The SEM photograph of C sample is shown, respectively.
The primary particle diameter of NaFePO ₄ / C was about 100 to 500 nm. On the other hand, the primary particle diameter of NaMnPO ₄ was 0.2 to 1.0 μm, and a slight particle growth was confirmed as compared with the particle diameter of LiMnPO ₄ synthesized by the same method. This is caused by firing at a high temperature of 600 ° C. Furthermore, the primary particle diameter of NaMnPO ₄ / C after carbothermal treatment was as very small as about 50 nm.

Example 2: Preparation of coin cell and charge / discharge measurement (1) Preparation of positive electrode The positive electrode active material (NaFePO ₄ / C and NaMnPO ₄ / C) prepared in Example 1 and a binder (PTFE, Polyflon (registered by Daikin)) (Trademark) Tfe F-201L) was weighed at a weight ratio of 95: 5, and was removed for 15 seconds using an agate mortar. Furthermore, after extending | stretching to uniform thickness with an agate mortar, it punched as a disk-shaped pellet using the cork borer of diameter 10mm. At this time, the pellet was adjusted to have a thickness of 250 μm and a weight of about 30 mg.

(2) Production of Coin Cell A sodium foil having a diameter of 1.5 mm and a thickness of 0.15 mm was used as the negative electrode. As the separator, a porous polyethylene sheet having a diameter of 22 mm and a thickness of 0.02 mm was used. Moreover, 1M NaClO ₄ propylene carbonate solution (Toyama Pharmaceutical Co., Ltd.) was used for the non-electrolytic solution. These components were incorporated into a positive electrode vessel having a stainless steel negative electrode lid and a gasket inside, and as shown in FIG. 6, a coin-type cell having a thickness of 2 mm and a diameter of 32 mm (2032 type) was produced.

(3) Evaluation of Charge / Discharge Characteristics of Coin Cell FIG. 7 shows a charge / discharge profile at a pseudo open circuit voltage (Quasi-Open-Circuit-Voltage: QOCV) of a coin cell produced using NaFePO ₄ as a positive electrode active material. This time, in order to measure the pseudo-open circuit, a theoretical capacity of 154 mAh / g is divided into 40 cycles, 0.025Na is charged (discharged) in one cycle, and the method is paused for the same time as the charge (discharge). Using. FIG. 8 shows a charge / discharge profile at a closed circuit voltage (CCV) of the coin cell. The measurement was performed at a voltage range of 1.5 to 4.0 V and a current density of 0.1 mA / cm ² .
First, in QOCV measurement, about 60% of the theoretical amount of Na could be inserted and desorbed. However, it can be seen that the overvoltage in each cycle is very large, the polarization is large, and Na ions hardly move. On the other hand, in the CCV measurement, since charging could not be sufficiently performed up to 4.0 V, only insertion and desorption of Na of about 20% of the theoretical amount could be performed.
Standard electrodes of olivine-type LiFePO _{4 having} an average operating potential (3.4 V vs. Li / Li ⁺ ), Li (−3.045 V vs. SHE), and Na (−2.714 V vs. SHE), although having different crystal structures Considering the potential difference, the average working potential of marisite-type NaFePO ₄ can be expected to be 3.0 V (vs. Na ^/ Na ⁺ ). Looking at these charge / discharge profiles, it can be seen that it has become gentle once around 3.0 V, but it is never a clear plateau.

Next, FIG. 9 shows a QOCV charge / discharge profile of a coin cell manufactured using NaMnPO ₄ as a positive electrode active material. FIG. 10 shows a charge / discharge profile at CCV of the coin cell. The measurement was performed at a voltage range of 1.5 to 4.0 V and a current density of 0.1 mA / cm ² . In the QOCV measurement, since charging was performed to a potential sufficiently higher than the expected average operation potential of NaMnPO ₄ of 3.7 V (vs. Na / Na ⁺ ), more than 70% of Na of the theoretical amount more than NaFePO ₄ was inserted and It was possible to desorb.
Further, it concluded although it is possible to NaFePO ₄ many Na insertion and elimination than by QOCV measurement, which, for synthetic methods are different, is simply positive electrode material NaMnPO ₄ is superior NaFePO ₄ Cannot be attached.

On the other hand, in the CCV measurement, only about 4.0 V and 0.3 V have a potential margin, and only about 20% of the theoretical amount of Na could be inserted and desorbed. A flat part like a gentle plateau was confirmed at around 7V. From the results of FIGS. 7 to 10, it is considered that the charge and discharge reaction of these marisite type positive electrodes NaFePO ₄ and NaMnPO ₄ is caused by the redox reaction of Fe ²⁺ / Fe ³⁺ and Mn ²⁺ / Mn ³⁺ , respectively.

11 and 12 show the crystal structure of marisite-type NaFePO ₄ viewed from each direction and the crystal structure of olivine-type LiFePO ₄ viewed from each direction, respectively. In these figures, Na ions and Li ions are respectively shown by the largest circles, but it is understood that both are composed of FeO ₆ octahedron and PO ₄ tetrahedron. As shown in FIG. 12, in the olivine-type LiFePO ₄ , a one-dimensional diffusion path can be confirmed in the crystal structure. However, in the marisite-type NaFePO ₄ , O ions are present as in the reverse spinel structure. Since Na ions and Na ions are alternately arranged, there is no one-dimensional diffusion path. Probably curved and Na ion diffusion occurs, which is also considered to be a cause of large polarization.

NaFePO is an explanatory diagram showing an XRD profile of _4. 600 ° C., is an explanatory diagram showing an XRD profile of NaMnPO ₄ was baked for one hour. 350 to 700 is an explanatory diagram showing an XRD profile of the fired NaMnPO ₄ at ° C.. The NaMnPO ₄ is an explanatory diagram showing an XRD profile of NaMnPO ₄ / C was subjected to carbon coating treatment and carbo thermal processing. (A) SEM photograph of NaFePO ₄ / C calcined at 550 ° C., (b) SEM photograph of NaMnPO ₄ calcined at 600 ° C., (c) NaMnPO ₄ / C subjected to carbothermal treatment at 550 ° C. It is a SEM photograph. It is the schematic which shows the structure of the coin cell produced for evaluation of the active material for sodium secondary batteries. Pseudo open circuit voltage of the coin cells were prepared NaFePO ₄ as the positive electrode active material: is an explanatory view showing a charge-discharge profile at (Quasi-Open-Circuit-Voltage QOCV). It is explanatory drawing which shows the charging / discharging profile in the closed circuit voltage (Closed-Circuit-Voltage: CCV) of the coin cell. The NaMnPO ₄ is an explanatory diagram showing charge-discharge profile at QOCV coin cells prepared as the positive electrode active material. It is explanatory drawing which shows the charging / discharging profile in CCV of the coin cell. Is an explanatory view showing the crystal structure of maricite type NaFePO ₄ as viewed from each direction. It is an explanatory view showing the crystal structure of olivine-type LiFePO ₄ as viewed from each direction.

Claims (4)

It has symmetry of the space group Pnmb, and the lattice constants a, b, and c are 6.86Å ≦ a ≦ 7.00Å, 8.98Å ≦ b ≦ 9.15Å, 5.04Å ≦ c ≦ 5.14Å, respectively. Marisite-type NaMnPO ₄ . A method for producing marisite-type NaMnPO ₄ , which comprises mixing a Na source, a Mn source, and a phosphoric acid source in water, evaporating water, and heating the resulting mixture at 600 to 1000 ° C. in the atmosphere. Sodium secondary characterized in that it contains marisite-type NaMn _x M _(1-x) PO ₄ (wherein M is any of Fe, Co and Ni, and x is 0 ≦ x ≦ 1). Battery active material. A sodium secondary battery comprising the active material for a sodium secondary battery according to claim 3.