JP6068257B2 - Active material and secondary battery using the same - Google Patents

Description

  The present invention relates to an active material and a secondary battery using the active material.

  In recent years, secondary batteries have been used not only as mobile phones and notebook PCs, but also as electric vehicle batteries and residential / store storage batteries.

  A secondary battery is generally composed of a positive electrode, a negative electrode, and an electrolyte solution (quality). The positive electrode includes, for example, transition metal and alkali metal oxides, and the negative electrode includes, for example, carbon-based materials such as graphite and hard carbon. An alloy material, an oxide material, and an organic (non-aqueous) solvent or an aqueous solvent are used as a solvent in the electrolytic solution. Typical examples of such batteries include lithium ion batteries and lead batteries.

  Since lithium-ion batteries have a high energy density, they will not only supply storage batteries for electric vehicles and households, but also provide voltage stabilization and storage, such as wind power and solar power generation, and power stored at night during the day. It is also expected as a stationary large storage battery having a large capacity of several KW to several MW, which is used for the purpose of leveling the electric power load. However, in lithium ion batteries, rare metals such as Li, Co, and Ni are used as electrode materials, and the amount existing on the earth, the so-called Clark number, is small. If the demand for lithium ion batteries increases in the future, there is a concern that the unit price will rise further. Is done.

In recent years, sodium ion batteries that charge and discharge with Na ions have been researched and developed in place of lithium ion batteries, and complex oxides and phosphates have been studied as electrode active materials. Since Na ions have a larger ionic radius than Li ions, for example, in the case of a manganese composite oxide having a layered structure, a composite oxide with lithium is unstable as a compound, but a composite oxide with sodium (NaMnO ₂ ) Is stable as a compound and can be suitably used as an electrode active material for sodium ion batteries. On the other hand, in the case of a phosphate containing sodium, an orthorhombic marisite crystal structure is more stable than a olivine crystal structure known as LiFePO ₄ or the like in a phosphate containing lithium. However, it is generally known that there is almost no Na ion conduction path in the marisite crystal structure, and the active material is hardly studied.

Regarding a phosphate containing sodium, for example, in Patent Document 1, 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) is proposed as the positive electrode active material. Patent Document 2 proposes to use phosphoric acid-transition metal-sodium having an olivine type crystal structure as an active material of the positive electrode.

JP 2008-260666 A JP 2011-034963 A

However, a phosphate containing sodium having an orthorhombic marisite crystal structure (hereinafter sometimes simply referred to as a marisite structure or a marisite type) has a Na ion conduction path as described above. Is almost electrochemically inert (see, for example, K. Zaghib et al. J. Power Sources 2011, 196, 9612-9617). This is a structural problem due to the crystal structure, and it is difficult to use a sodium-containing phosphate having a marisite structure as an active material used for an electrode of a secondary battery.

On the other hand, a sodium-containing phosphate having an olivine-type crystal structure (hereinafter sometimes referred to simply as an olivine-type structure or an olivine-type) has a structure in which Na ions easily conduct, unlike the case of a marisite-type structure. Therefore, high ionic conductivity can be expected. However, phosphate-based active materials represented by the chemical formula LiMPO ₄ (where M is a transition metal element) are known to have low electron conductivity, and are not suitable for use as electrode active materials. It is necessary to supplement the electron conductivity by providing a conductive coating or the like on the surface of the acid salt particles. The same applies to sodium-containing phosphates, and a carbon coat or the like is used as the conductive coating. Carbon coats generally improve their electronic conductivity as they are processed at higher temperatures, but sodium-containing phosphates undergo a phase transition from an olivine-type structure to a marisite-type structure above 400 ° C. (see, for example, KT Lee et al. Chem. Mater. 2011, 23, 3593-3600), in order to perform carbon coating while maintaining the olivine type structure, it is necessary to perform heat treatment at 400 ° C. or less, and a carbon coating having sufficient electron conductivity can be obtained. There is no concern.

  The present invention has been made in view of the above problems. An active material useful as a positive electrode of a secondary battery using a sodium-containing phosphate having an olivine type crystal structure, and a secondary battery using the same The purpose is to provide.

The active material of the present invention contains at least one element selected from transition metal elements and sodium, has a olivine-type crystal structure, and at least one of the surface of the phosphate crystal and the vicinity of the surface to, Ni, characterized in that it comprises a phosphorus compound containing at least any one kind of the element group consisting of Fe and Mn.

  The secondary battery of the present invention includes a positive electrode, a negative electrode, and an electrolytic solution, and the positive electrode includes the active material described above.

  ADVANTAGE OF THE INVENTION According to this invention, the active material useful as a positive electrode of a secondary battery using a sodium containing phosphate which has an olivine type crystal structure, and a secondary battery using the same can be provided.

It is a schematic sectional drawing of the secondary battery which is one Embodiment of this invention. It is a perspective view which shows the external appearance of a secondary battery. Is a diagram showing an X-ray diffraction (XRD) measurement results of NaMnPO ₄ having synthesized olivine type crystal structure in the examples. It is a transmission electron microscope (TEM) photograph of the crystal particle surface of the active material (sample No. 5) produced in the Example.

A secondary battery according to an embodiment of the present invention will be described with reference to FIG. The secondary battery of this embodiment has an electrolyte layer 2 between the positive electrode 1 and the negative electrode 3, and these constitute a power generation element 4. Further, on the surface opposite to the side facing the electrolyte layer 2 of the positive electrode 1 and the negative electrode 3, a positive electrode side current collecting layer 5P and a negative electrode side current collecting layer 5N are provided, respectively. A secondary battery is formed by housing the power generation element 4 shown in FIG. 1 in a battery case as shown in FIG. There are various types of battery cases such as a laminate type, a cylindrical type, a coin type, and a box type, but any form may be used. 6P and 6N shown in FIG. 2 are a positive electrode terminal and a negative electrode terminal for electrically connecting the external circuit, the positive electrode 1 and the negative electrode 3, respectively, and 7 is a box type case used for a lead storage battery for automobiles and the like. is there. 8 is a safety valve for releasing hydrogen and oxygen generated by electrolysis during overcharge. As the electrolyte layer 2, a separator impregnated with an electrolytic solution is used.

  The positive electrode 1 is a phosphate containing sodium and at least one element selected from transition metal elements (hereinafter also simply referred to as sodium phosphate), and has an olivine type crystal structure Is used. Phosphate is a highly safe material because it has a tetrahedral structure centered on oxygen atoms, with phosphorus atoms at the vertices, so the bonds between oxygen atoms and phosphorus atoms are strong and it is difficult to release oxygen. Since the olivine-type sodium phosphate has a structure in which Na ions easily migrate, it is higher than the marisite structure and becomes an active material having stable ion conductivity.

In the present embodiment, at least one of the element group consisting of Fe, Ni, and Mn, at least one of the surface of the sodium phosphate crystal having an olivine structure and the vicinity of the surface, P, The metal compound containing is provided. Since sodium phosphate has low electronic conductivity, even if it has an olivine structure, it is difficult to use it as an active material in a state where the surface does not have a conductive coating. Therefore, a metal compound containing P and at least one element group consisting of Fe, Ni, and Mn on at least one of the surface of the sodium phosphate crystal having an olivine structure and the vicinity of the surface. The presence of such an electron-conducting substance compensates for the low electronic conductivity of the sodium phosphate having an olivine structure, which is an active material, and can be used as a positive electrode active material for a secondary battery. it can. Among these, transition metal phosphides such as Ni _n P, Fe _n P, and Mn _n P (n = 1 to 3) are particularly preferable because they have high electron conductivity and are electrochemically stable.

  Further, a coating layer further containing carbon may be formed on the surface of the sodium phosphate crystal having an olivine structure. This coating layer is provided on a metal compound of P and at least any one element group consisting of Fe, Ni, and Mn (hereinafter sometimes simply referred to as transition metal phosphide). Alternatively, it may be a mixture of this metal compound and carbon. Thereby, the electronic conductivity in the surface of the crystal | crystallization of the sodium-type phosphate which has an olivine type structure can further be improved. For example, the amount of transition metal phosphide formed on and near the surface of a sodium phosphate crystal having an olivine structure increases as the heat treatment temperature increases. Since the stable temperature is as low as 400 ° C. or lower, even when a sufficient transition metal phosphide cannot be formed by heat treatment at a temperature capable of maintaining the olivine structure, an active layer is formed by forming a coating layer containing carbon. Sufficient electronic conductivity can be obtained between materials.

  The carbon contained in the coating layer is preferably mainly composed of electrochemically stable acetylene black, ketjen black, graphite, graphite and the like. The coating layer containing carbon does not need to cover the entire surface of the sodium phosphate crystal having an olivine structure, and may cover, for example, 1% or more of the surface. Thus, when 1% or more of the surface of the sodium phosphate crystal having an olivine structure is covered with a coating layer containing carbon, when this is used as a positive electrode active material, The electron conductivity in can be further improved.

  The average thickness of the coating layer containing carbon is preferably in the range of 1 to 100 nm. When the thickness is greater than 100 nm, the ionic conductivity between the active material and the electrolyte is reduced. When the thickness is less than 1 nm, the effect of improving the electronic conductivity between the active materials by the coating layer containing carbon cannot be sufficiently obtained. There is a case.

  In addition, it can be confirmed by the impedance measurement method such as the Nyquist plot that the substance having electron conductivity exists on the surface of the sodium phosphate having the olivine structure or in the vicinity of the surface. Moreover, the kind of substance which exists in the surface of sodium-type phosphate which has an olivine type structure, or the surface vicinity can be confirmed by energy dispersive X-ray spectroscopy (EDS). The coating layer containing carbon can be confirmed by a transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDS). Moreover, what is necessary is just to confirm the thickness of the coating layer containing carbon using a transmission electron microscope (TEM). For example, a cross-sectional photograph of a sodium phosphate having an olivine structure in the cross section of the electrode is taken with a TEM, a coating layer containing carbon is discriminated by EDS, and its thickness is measured. As the average thickness of the coating layer containing carbon, for example, the thickness may be measured at five locations and the average value may be calculated.

  The composition of the active material can be confirmed by ICP emission spectral analysis or the like. The crystal structure of the active material can be confirmed by identifying the crystal phase from the diffraction profile obtained by X-ray diffraction (XRD) measurement.

  A method for producing such an active material will be described. Active materials such as complex oxides and phosphates used for electrodes of secondary batteries can be usually synthesized by a solid phase method, but sodium phosphates having an olivine type structure as described above are When heat treatment is performed at 400 ° C. or higher, the phase transition to a thermodynamically stable marisite structure cannot be made by a general solid phase reaction.

Accordingly, when preparing a sodium phosphate having an olivine structure, Na ions are obtained after electrochemically desorbing Li ions from a lithium phosphate in which the olivine structure is thermodynamically stable. (See, for example, K. Zaghib et al. J. Power Sources 2011, 196, 9612-9617) or two-step synthesis via intermediates (eg, KT
Lee et al. Chem. Mater. 2011, 23, 3593-3600).

  Here, as an example, a method for synthesizing a sodium phosphate having an olivine structure via an intermediate will be described. First, using triammonium phosphate, manganese (II) sulfate pentahydrate, iron (II) sulfate heptahydrate, nickel (II) sulfate hexahydrate and polyethylene glycol 400 as raw materials, these are desired. Then, the mixture is reacted at a low temperature of 40 to 60 ° C. to synthesize an intermediate ammonium phosphate compound. Next, the obtained ammonium phosphate compound and sodium acetate are mixed and subjected to a heat treatment at 80 to 100 ° C., whereby a sodium phosphate crystal having an olivine structure can be obtained.

  The obtained sodium phosphate crystals having an olivine type structure may be adjusted in particle size by pulverization by a technique such as a ball mill or a bead mill, if necessary. When adjusting the particle size, the average particle size of the sodium phosphate powder having an olivine structure can be set to 0.5 to 1.0 μm, for example, so that high output can be handled. The average particle size of the powder can be confirmed by, for example, particle size distribution measurement by a diffraction scattering method.

  In order to form a metal compound containing P and at least one of element groups consisting of Ni, Fe and Mn on the surface of the obtained sodium phosphate crystal having an olivine structure, for example, A method such as CVD, sputtering, or vapor deposition may be used.

In addition, a metal compound containing P can be formed on the surface of the crystal or in the vicinity of the surface by using an element constituting sodium phosphate. For example, the obtained sodium phosphate is heat-treated in a non-oxidizing atmosphere at a temperature of 400 ° C. or lower to form sodium phosphate on or near the surface of crystal grains of sodium phosphate. As a result, a metal compound containing P is formed by a decomposition reaction originating from the element to be formed. Further, during the synthesis of the sodium phosphate, the heat treatment may be performed in a non-oxidizing atmosphere. In this case, a metal compound containing P more easily on the surface of the sodium phosphate crystal or in the vicinity thereof. Can be formed.

In addition, the metal compound which is not contained as a structural element of sodium-type phosphate can also be formed with the same method. For example, when forming a metal compound of Ni and P on the surface of the crystal grains of NaMnPO ₄ is, Ni compounds on the surface of the crystal grains of NaMnPO ₄ (e.g. Ni _{(NO 3) 2,} etc.) depositing a non-oxidizing Heat treatment may be performed in an atmosphere.

  When a coating layer containing carbon is further formed on the surface of a sodium phosphate crystal having an olivine structure, a phosphate powder in which a metal compound is formed on the crystal surface, PVA, a phenol resin, etc. It can be formed by mixing a carbon source with a known method such as a ball mill and heat-treating it at a temperature of 400 ° C. or lower in a non-oxidizing atmosphere. Moreover, the coating layer containing a metal compound and carbon is obtained by mixing a powder of a phosphate not forming a metal compound on the crystal surface with a carbon source and heat-treating it in a non-oxidizing atmosphere at a temperature of 400 ° C. or lower. Can be formed simultaneously.

  An electrode is produced using the sodium-type phosphate provided with the coating layer containing the metal compound which has electronic conductivity, and carbon obtained in this way. For example, 70% by mass of sodium phosphate as an active material, 25% by mass of acetylene black as a conductive additive, 5% by mass of polytetrafluoroethylene (PTFE) as a binder, and water as a solvent are added to form a slurry. Is made. The produced slurry is applied onto a Ti foil, for example, by a known molding method such as a doctor blade method or a roll press, and the solvent is dried, so that the active material, conductive auxiliary agent, and binder (binder) of this embodiment are used. Can be produced.

  In addition to polytetrafluoroethylene, a binder selected according to the application, such as polyvinylidene fluoride (PVDF), fluorine rubber, styrene butadiene rubber, polyimide resin (PI), polyamide resin, polyamideimide resin, etc. is used. it can. Also, the conductive auxiliary agent is ketjen black, carbon nanotube, graphite, hard carbon, metal (Ni, gold, platinum, etc.) powder instead of acetylene black, inorganic conductive oxide (indium tin oxide (ITO) glass, Any material may be used as long as it is chemically stable and exhibits electron conductivity in the operating voltage range, such as tin oxide.

  Alternatively, the obtained sodium phosphate powder may be used to form an electrode by forming a green compact by a molding method such as extrusion molding or a roll compaction method.

  The average particle size of the active material particles in the electrode may be adjusted by selecting an appropriate range according to the use conditions such as the voltage range and temperature of the secondary battery using the active material. Preferably there is. If the average particle size of the active material particles is too small, it can cope with high output, but the contact area with the electrolytic solution becomes large, so that deterioration may easily occur depending on use conditions. Also, the tape molding itself becomes difficult. On the other hand, if the particles are too large, protrusions may be formed in the tape molding, and there is a concern that the output response of the battery is lowered. The average particle diameter of the active material particles in the electrode is determined by, for example, distinguishing the active material particles in the cross section of the electrode using a scanning electron microscope (SEM) and wavelength dispersive X-ray analysis (WDS), and taking a photograph. It can be obtained by analyzing and calculating.

As the electrolyte layer 2, a separator impregnated with an electrolytic solution is used. The separator impregnated with the electrolytic solution is required to pass ions and prevent positive and negative electrodes from being short-circuited. Specifically, a polyolefin fibrous nonwoven fabric, a polyolefin microporous film, a glass filter, a ceramic porous material, or the like can be used. Here, examples of the polyolefin include polyethylene and polypropylene, and a separator generally used for a secondary battery such as a lithium ion battery is applicable.

  As the electrolytic solution, an aqueous electrolytic solution is preferably used. Since the aqueous electrolyte has a higher ionic conductivity than the organic electrolyte, better charge / discharge characteristics can be expected. In addition, in the case of a secondary battery using an aqueous electrolyte, unlike an organic electrolyte using an organic solvent, there is an advantage that there is no danger of ignition and excellent safety.

In the case of a secondary battery using an aqueous electrolyte, the positive electrode active material needs to be able to be charged at a potential lower than 3.9 V, which is a potential at which water is electrolyzed. This is because the potential at which water is electrolyzed, that is, the potential at which oxygen is generated when the pH is 7, is +0.82 V (from the Nernst equation) with respect to the standard hydrogen electrode, which is the redox potential of Na. This is because it is 3.5 V when converted to the potential difference of Na / Na ⁺ and is about 3.9 V even if the voltage is taken into consideration.

In the Na-based phosphate represented by the chemical formula NaMPO ₄ (where M is a transition metal element), Mn and Fe were used as the transition metal element M as those that can be charged at a base potential lower than 3.9 V. Things. In particular, when Mn is used as M, the charging potential is close to the electrolysis potential of water and a high voltage is obtained, which is preferable. When Co, Ni, or the like is used as M, the chargeable potential is 4.2 V or higher, which is higher than the electrolysis potential of water, that is, cannot be charged at a potential in a range where water is not electrolyzed. It cannot be used as an active material used for a sodium ion battery using an electrolytic solution.

As the aqueous electrolyte, for example, a 1-2 mol / L sodium sulfate aqueous solution may be used. As the electrolyte, in addition to sodium sulfate, sodium nitrate, and sodium chloride, lithium sulfate, lithium nitrate, lithium chloride, and the like may be used, and any sodium salt and lithium salt that dissolves in water can be used. . Thus, the active material of this embodiment can be used as a secondary battery excellent in safety by using it as a positive electrode for an aqueous secondary battery using an aqueous solution as an electrolyte.

As the active material used for the negative electrode 3, for example, activated carbon, NaTi ₂ (PO ₄ ) ₃ , LiTi ₂ (PO ₄ ) ₃ , Ti ₂ P ₂ O ₇ or the like can be used. In particular, a secondary battery having a high energy density can be obtained by using any one of NaTi ₂ (PO ₄ ) ₃ and LiTi ₂ (PO ₄ ) ₃ .

  For the positive current collecting layer 5P, a material having no corrosion and no generation of oxygen at the potential of the positive electrode may be used. Examples of such a material include nickel, titanium, stainless steel, gold, platinum, and a graphite sheet. Among these, titanium and nickel are preferable because they are readily available and are relatively easily available in terms of cost. In particular, titanium is preferable because it is excellent in corrosion resistance even at a noble potential.

  A material that does not cause side reactions such as alloying with Na at the potential of the negative electrode may be used for the negative current collecting layer 5N. Examples of such materials include metal materials and alloys including nickel, titanium, stainless steel and the like, carbonaceous materials such as graphite, hard carbon, and glassy carbon. In particular, it is preferable to use titanium or nickel from the viewpoint of high conductivity and relatively low cost.

The positive electrode side current collecting layer 5P and the negative electrode side current collecting layer 5N may use a metal foil or mesh made of these metal materials, or an inorganic conductive oxide such as a metal material, a carbonaceous material, ITO glass or tin oxide. A conductive ink or the like using a material material as a filler may be applied to the electrode material surface and dried.

  In addition, when using metal foil or a mesh, it is preferable that the thickness shall be 5-40 micrometers. Moreover, when using metal foil, you may use what roughened the surface of metal foil in order to improve the adhesive force with electrode material. In this case, the surface roughness of the metal foil is preferably 0.5 to 2 μm in terms of arithmetic average roughness (Ra). The surface roughness of the metal foil is measured using a surface roughness meter such as a stylus type or a light interference type, a laser microscope, an atomic force microscope (AFM), or the like. When using a stylus-type surface roughness meter that is generally used, based on JIS B0601, for example, on the condition that the stylus tip diameter is 2 μm, the measurement length is 4.8 mm, and the cutoff value is 0.8 mm Just measure.

  The secondary battery of the present embodiment has been described above, but the present invention is not limited to the present embodiment, and can be applied to various modifications without departing from the present invention.

  Hereinafter, the secondary battery of this invention is demonstrated in detail based on an Example. First, an olivine-type sodium phosphate containing a transition metal, which becomes a positive electrode active material, was synthesized by a two-step synthesis method via an intermediate. As raw materials, triammonium phosphate trihydrate (manufactured by Kanto Chemical), manganese (II) sulfate pentahydrate (manufactured by Kanto Chemical), iron (II) sulfate heptahydrate (manufactured by Kanto Chemical), polyethylene glycol 400 (manufactured by Wako Pure Chemical Industries) and sodium acetate trihydrate (manufactured by Kanto Chemical) were used.

  First, triammonium phosphate trihydrate, manganese sulfate (II) pentahydrate and iron (II) sulfate heptahydrate, and 1: 1 equivalent ratio of phosphoric acid to the transition metal shown in Table 1 Further, 40% by mass of polyethylene glycol 400 was added to triammonium phosphate trihydrate and mixed in a mortar. The mixture was allowed to stand at 40 ° C. for 48 hours, then washed with water and dried to obtain an intermediate powder. The obtained powder was confirmed to be a hydrate of an ammonium phosphate compound by X-ray diffraction (XRD) measurement using CuKα rays and elemental analysis (ICP emission spectroscopic analysis).

Next, the obtained intermediate powder and sodium acetate trihydrate were mixed so that phosphoric acid and sodium were in a 1: 3 equivalent ratio and mixed in a mortar, and then at 100 ° C. for 48 hours. Let stand and react. The obtained massive solid was washed with water and dried, and then heat-treated at 400 ° C. for 1 hour in a nitrogen atmosphere to obtain sodium phosphate. The obtained sodium phosphate confirmed the crystal structure by X-ray diffraction (XRD) measurement using CuKα rays, and the results are shown in Table 1. As an example, the X-ray diffraction (XRD) measurement result of the synthesized olivine type sodium manganese phosphate (NaMnPO ₄ ) is shown in FIG.

The treatment for forming a metal compound containing Ni or Fe and P on or near the surface of the obtained sodium phosphate crystal particles was performed as follows. First, the obtained sodium phosphate was pulverized in a planetary mill for 2 hours using IPA as a solvent and ZrO ₂ balls as pulverization media. The ground sodium phosphate powder is mixed with Ni (NO ₃ ) ₂ or FeCl ₃ (both manufactured by Kanto Chemical) dissolved in ethanol, and ultrasonic dispersion of the mixed solution is performed using an ultrasonic cleaning device. The suspension was made for a minute. At this time, the mass ratio of Ni (NO ₃ ) ₂ or FeCl ₃ was adjusted to 0.08 to 0.12 with respect to the sodium phosphate. This suspension was dried at 90 ° C. for 1 hour to obtain a sodium phosphate powder in which Ni (NO ₃ ) ₂ or FeCl ₃ was added to the surface of the crystal particles.

The obtained powder was heat-treated at 400 ° C. (temperature increase rate: 300 ° C./h) for 1 hour in a nitrogen atmosphere containing 12% H ₂ . As a result of observing the heat-treated powder using a transmission electron microscope (TEM), it was confirmed that fine particles were adhered to the surface of the crystal particles. As an example, sample no. A transmission electron microscope (TEM) photograph of No. 5 is shown in FIG.

In addition, as a result of elemental analysis by energy dispersive X-ray spectroscopy (EDS) at the measurement points # 1 to # 5 illustrated in FIG. 4, fine particles (# 2, # 4) existing on the crystal particle surface are Since the detected amount of oxygen was smaller than that in the regions (# 1, # 3), it is considered that a metal compound was formed. Note that a large amount of Zr was detected from the slightly larger particles (# 5) seen on the surface of the crystal particles, which is considered to originate from the ZrO ₂ balls used during pulverization. Table 2 shows whether or not fine particles are present on the surface of the crystal grains of each sample and the constituent elements.

The treatment for forming a coating layer containing carbon on the surface of the obtained sodium phosphate crystal particles was performed as follows. First, polyvinyl alcohol or citric acid was used as a carbon source for the coating layer containing carbon. The carbon source was blended so that the amount of carbon contained in the coating layer after the heat treatment was 3% by mass with respect to the sodium phosphate, and an appropriate amount of distilled water was added and mixed in a mortar while heating. The mixed powder was heat-treated at 400 ° C. (heating rate: 300 ° C./h) for 1 hour in a nitrogen atmosphere containing 12% H ₂ . Whether or not carbon is present on the surface of the produced powder crystal particles was confirmed by a transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDS). The results are shown in Table 2.

  Note that the formation of the metal compound and the formation of the coating layer containing carbon may be simultaneously performed by a single heat treatment.

  Using the prepared powder, a sodium press is mixed and mixed so that 70% by mass of phosphine, 25% by mass of acetylene black as a conductive auxiliary agent, and 5% by mass of polytetrafluoroethylene as a binder. Was formed into a sheet shape having a thickness of 200 μm, and then processed into a disk shape having a diameter of 15 mm to obtain a positive electrode. The polytetrafluoroethylene used was previously dispersed in water at a solid content concentration of 60% by mass.

  The negative electrode was produced by the following method. 80% by mass of activated carbon, 15% by mass of acetylene black as a conductive additive, and 5% by mass of polytetrafluoroethylene as a binder were mixed in a mortar, passed through a mesh, and then a disk having a diameter of 15 mm and a thickness of 3 mm by press molding. A negative electrode was prepared. The same polytetrafluoroethylene as that for the positive electrode was used.

Ti and Ni meshes were pressure bonded to the prepared positive and negative electrodes, respectively, to form current collecting layers. Using these, a positive electrode, a separator, and a negative electrode were arranged in this order inside the battery evaluation cell and assembled, and an electrolytic solution was injected. A nylon mesh was used as the separator, and a 1 mol / L aqueous solution in which sodium sulfate was dissolved in pure water was used as the electrolyte. An Ag / AgCl electrode was used as the reference electrode.

  The charge / discharge test of the manufactured cell was performed under the following conditions.

Charge / discharge potential: Upper limit 0.89V, lower limit -0.41V (vs. Ag / AgCl)
Charging / discharging rate: 0.05C (constant current charging / discharging)
Measurement temperature: 25 ° C

  Sample No. In Nos. 2 to 8, a sodium-based phosphate having an olivine structure is used as a positive electrode active material, and the surface of the active material is provided with a metal compound containing any of Ni, Fe and Mn and P. In this case, the secondary battery could be charged and discharged. On the other hand, sample no. No. 1 could not be charged / discharged.

A negative electrode was prepared in the same manner as the above positive electrode using NaTi ₂ (PO ₄ ) ₃ as the negative electrode active material. When a charge / discharge test was performed in combination with the positive electrode used in No. 5, the initial discharge capacity was 22 mAh / g with a power generation element having a small volume compared to the case where activated carbon was used as the negative electrode active material.

DESCRIPTION OF SYMBOLS 1 .... Positive electrode 2 .... Electrolyte layer 3 .... Negative electrode 4 .... Electric power generation element 5N ... Negative electrode side current collecting layer 5P ... Positive electrode side current collecting layer 6N ... Negative electrode terminal 6P ... Positive terminal 7 ... Case 8 ... Safety valve

Claims (6)

A phosphate containing at least one element selected from transition metal elements and sodium and having an olivine type crystal structure;
On at least one surface and near surface of the crystals of the phosphate, active material, characterized in that and a phosphide containing at least any one kind of the element group consisting of Ni, Fe and Mn.   The active material according to claim 1, wherein the phosphate contains manganese.   The active material according to claim 1, further comprising a coating layer containing carbon on at least a part of a surface of the phosphate crystal.   A secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte solution, wherein the positive electrode includes the active material according to claim 1.   The secondary battery according to claim 4, wherein the electrolytic solution is an aqueous electrolytic solution. The secondary battery according to claim 4, wherein the negative electrode contains at least one of NaTi ₂ (PO ₄ ) ₃ and LiTi ₂ (PO ₄ ) ₃ as an active material.