JP2014192133A - Active material, and secondary battery arranged by use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an active material which is high in energy density, small in the deterioration in discharge capacity owing to the repetition of charge and discharge, and superior in cycle characteristics, and to provide a secondary battery arranged by use thereof.SOLUTION: An active material comprises a compound including Ti and P. The compound has, as a main crystal phase, a TiPOcrystal phase having a pseudo hexagonal structure. The TiPOcrystal phase having the pseudo hexagonal structure is small in volume change involved in acceptance and desorption of ions in charge of electrical conduction. Therefore, the active material like this is used for at least one of positive and negative electrodes of a secondary battery. The compound including Ti and P includes at least one crystal phase different from the TiPOcrystal phase having the pseudo hexagonal structure. A secondary battery comprises: a positive electrode; a negative electrode; and an electrolyte. In the secondary battery, at least one of the positive and negative electrodes includes the active material, and the electrolyte consists of an aqueous electrolytic solution.

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 for mobile phones and notebook PCs but also as batteries for electric vehicles, and are expected to be used for large-scale applications such as voltage stabilization for wind power and solar power generation. .

  A secondary battery is generally composed of a positive electrode, a negative electrode, and an electrolyte (quality), and the positive electrode and the negative electrode contain an active material capable of inserting or adsorbing and desorbing ions that conduct electric conduction. Yes. In the development of a secondary battery, it is particularly important to develop an active material capable of producing an electrode having a large capacity and a long life depending on charge / discharge conditions and use conditions.

Conventionally, carbon-based materials and alloy materials such as graphite and hard carbon are used as the active material for the negative electrode, and ions (Li, Na, etc.) that are responsible for electric conduction and composite materials such as Co and Ni are used as the active material for the positive electrode. Oxides have been investigated and utilized. In recent years, active materials mainly composed of low-cost Fe, Mn, Ti, P, and the like have been actively studied due to an increase in the resource risk of rare metals. Among them, TiP ₂ O ₇ has been reported to be useful as an active material even though it does not contain ions responsible for electrical conduction as constituent elements.

For example, in Patent Document 1, Li _Y Fe _X M _1-X P ₂ O ₇ (wherein M represents a transition metal that is stably present even in a tetravalent state, X is 0 ≦ X <1, Y is 0 ≦ Y An electrode active material for a non-aqueous electrolyte secondary battery comprising a compound represented by ≦ 1 has been proposed, and an example using TiP ₂ O ₇ as a positive electrode active material is shown.

On the other hand, Patent Document 2 and Non-Patent Document 1 propose using TiP ₂ O ₇ as an active material for a negative electrode of a lithium secondary battery using an aqueous electrolyte.

Japanese Patent Application Laid-Open No. 2002-246025 JP 2008-282665 A

Haibo Wang, Kelong Huang, Yuqun Zeng, Sai Yang, Liquan Chen, Electrochimica Acta, Volume 52, Issue 9, 15 February 2007, Pages 3280-3285

However, the positive electrode containing TiP ₂ O ₇ described in Patent Document 1 has a problem that the energy density is low because the potential is low. Further, as described in Patent Document 2 and Non-Patent Document 1, when TiP ₂ O ₇ is used as the negative electrode active material, there is a problem in that the capacity decreases due to repeated charge and discharge.

  The present invention has been made in view of the above problems, and provides an active material having high energy density, small reduction in discharge capacity due to repeated charge and discharge, and excellent cycle characteristics, and a secondary battery using the active material. With the goal.

The active material of the present invention is composed of a compound containing Ti and P, and includes a TiP ₂ O ₇ crystal phase having a pseudo hexagonal crystal structure as a main crystal phase constituting the compound.

  The secondary battery of the present invention includes a positive electrode, a negative electrode, and an electrolyte, and at least one of the positive electrode and the negative electrode includes the above-described active material.

  ADVANTAGE OF THE INVENTION According to this invention, the energy density is high, the fall of the discharge capacity by repetition of charging / discharging is small, and the active material excellent in cycling characteristics 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. It is an example of the X-ray-diffraction chart of the active material used for the negative electrode of an Example.

  A secondary battery according to an embodiment of the present invention will be described with reference to FIGS. 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 tube type, a coin type, and a box type, but any form may be used. 6N and 6P shown in FIG. 2 are a negative electrode terminal and a positive electrode terminal for electrically connecting the external circuit, the negative electrode 3 and the positive electrode 1, respectively, and 7 is a laminate film.

In the present embodiment, a compound containing Ti and P is used as the active material of the negative electrode 3. The compound containing Ti and P includes a TiP ₂ O ₇ crystal phase having a pseudo hexagonal crystal structure as a main crystal phase. The main crystal phase is a crystal having a diffraction peak having a diffraction intensity of 50% or more with respect to this Io, where Io is the diffraction intensity of the diffraction peak having the maximum intensity in the X-ray diffraction profile of the compound. It shall refer to the phase.

_TiP 2 _{O 7} crystal phase having a pseudo-hexagonal structure is not only a theoretical capacity of about 120 mAh / g as with _TiP 2 _{O 7} crystal phase of cubic, insertion of ions and responsible for electrical conduction Since the volume change accompanying desorption is small, excellent characteristics are exhibited even in the charge / discharge cycle.

Furthermore, in a TiP ₂ O ₇ crystal phase having a pseudo hexagonal crystal structure (hereinafter sometimes referred to as pseudo hexagonal TiP ₂ O ₇ or simply pseudo hexagonal crystal), −0.5 V with respect to the standard hydrogen electrode potential. The charge / discharge potential is 0.1 to 0.2 V higher than a cubic TiP ₂ O ₇ crystal phase having a potential of about (hereinafter, sometimes referred to as cubic TiP ₂ O ₇ or simply cubic). This is a value sufficiently higher than the hydrogen generation potential (about −1.0 V) in the negative electrode of the battery using the aqueous electrolyte, and therefore, in the battery using the aqueous electrolyte, pseudo-hexagonal as an active material of the negative electrode. By using crystalline TiP ₂ O ₇ , it is possible to reduce the risk of hydrogen generation. Further, when pseudo hexagonal TiP ₂ O ₇ is used as the positive electrode active material, a higher energy density can be obtained.

Furthermore, since pseudo hexagonal TiP ₂ O ₇ has not only ion conductivity but also high electron conductivity, the use of this as an active material can reduce the internal resistance as a secondary battery.
Since cubic TiP ₂ O ₇ and LiTi ₂ (PO ₄ ) ₃ have low electron conductivity, when these are used as an active material, it is necessary to form a conductive film on the surface of the active material. There is concern that irreversible capacity will be generated. On the other hand, when pseudo hexagonal TiP ₂ O ₇ having high electron conductivity is used as an active material, excellent performance can be expressed without forming a film on the surface of the active material.

The presence of pseudo hexagonal TiP ₂ O _{7 in the} active material can be confirmed by X-ray diffraction (XRD) measurement. For example, when measured using CuKα rays, in the X-ray diffraction profile of pseudo-hexagonal TiP ₂ O ₇ , the diffraction peak having the maximum diffraction intensity in the range of 2θ to 22.8 to 23.4 ° (hereinafter referred to as pseudo-hexagonal TiP ₂ O ₇ ). A main peak of hexagonal TiP ₂ O ₇ ). This indicates that the crystal structure has a surface spacing of 3.8 to 3.9 mm.

The compound containing Ti and P in the present embodiment may contain a crystal phase different from the pseudo hexagonal TiP ₂ O ₇ described above, and is a compound containing one or more of such crystal phases. Also good. Examples of such a crystal phase include cubic TiP ₂ O ₇ , Ti ₅ P ₄ O ₂₀ , Ti ₅ P ₆ O ₂₅ , LiTi ₂ (PO ₄ ) ₃ , and NaTi ₂ (PO ₄ ) _3. . In particular, it is preferable to include a crystal phase whose constituent element is an ion responsible for electrical conduction, such as LiTi ₂ (PO ₄ ) ₃ or NaTi ₂ (PO ₄ ) ₃ , because the capacity can be increased.

The ratio of quasi-hexagonal TiP ₂ O _{7 in} the active material indicates the diffraction intensity of the diffraction peak having the maximum diffraction intensity (hereinafter sometimes referred to as the main peak of the active material) in the X-ray diffraction profile of the active material. When the diffraction intensity of the main peak of pseudo hexagonal TiP ₂ O ₇ that appears in the range of 2θ of 22.8 to 23.4 ° in the X-ray diffraction measurement using the Cukα ray is Ip, It is preferable that the intensity ratio Ip / Io with Ip is 0.7 or more. When the ratio of the pseudo hexagonal TiP ₂ O ₇ in the active material is such a ratio that shows such an intensity ratio, the effect of improving charge / discharge cycle characteristics by the pseudo hexagonal TiP ₂ O ₇ becomes more prominent, The electron conductivity and ionic conductivity of the pseudo hexagonal TiP ₂ O ₇ can be fully utilized. Note that the main peak of the active material and the main peak of pseudo-hexagonal TiP ₂ O ₇ may coincide with each other. In addition, since the interplanar spacing of the crystals constituting the active material varies slightly due to charge and discharge, all X-ray diffraction measurements in this specification are performed in a state where ions responsible for electrical conduction are desorbed.

  In the present embodiment, the negative electrode 3 may contain an active material other than the compound containing Ti and P described above. Moreover, you may contain Al, Zr, Mg, Mn, Fe, V, Co, Cr etc. in the ratio of 0.5 mass% or less as an inevitable impurity on a process.

Such an active material can be produced by various known methods such as a solid phase reaction method and a hydrothermal synthesis method. For example, a mixed raw material powder containing Ti and P having a predetermined composition and a precursor containing Ti and P obtained by a coprecipitation method are heated in the atmosphere at a temperature in the range of 300 to 600 ° C. Thus, a target active material can be synthesized. Note that when the heat treatment is performed at a temperature lower than 300 ° C., a large amount of unreacted substances may remain. Moreover, when performed at a temperature higher than 600 ° C., cubic TiP ₂ O ₇ may be easily formed.

  The obtained active material 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 powder of the active material may be adjusted to an appropriate range according to the use condition of the secondary battery using the active material and the method for producing the electrode, for example, in the range of 0.1 to 50 μm. It is sufficient to select and adjust an appropriate range according to the purpose. The average particle size of the powder can be confirmed by, for example, particle size distribution measurement by a diffraction scattering method.

An electrode is produced using the obtained active material. For example, 80% by mass of the obtained active material, 10% by mass of acetylene black as a conductive assistant, 10% by mass of polyvinylidene fluoride as a binder, and 15% by mass of NMP (N-methylpyrrolidone) as a solvent are added. To prepare a slurry. The prepared slurry is applied to a metal foil such as Al, stainless steel, Ni, Cu, etc. by a well-known sheet forming method such as a doctor blade method, and the solvent is dried, so that pseudo hexagonal TiP ₂ O ₇ is a main crystal. An electrode containing an active material contained as a phase, a conductive additive, and a binder can be produced.

  In addition to polyvinylidene fluoride, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride, fluorine rubber, styrene butadiene rubber, polyimide resin (PI), polyamide resin, polyamideimide resin, etc. Can be used. In addition, the conductive auxiliary agent is ketjen black, carbon nanotube, graphite, hard carbon, metal (aluminum, 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 conductivity in the operating voltage range, such as tin oxide.

  The obtained active material 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. Alternatively, the active material powder may be fired and used as a sintered body that does not contain a conductive additive or a binder.

  The average particle size of the active material particles in the electrode can be adjusted by selecting an appropriate range from a range of 0.1 to 50 μm, for example, depending on the use conditions such as the voltage range and temperature of the secondary battery using the active material. Good. For example, when used in a secondary battery application that requires high output, the average particle size of the active material particles is preferably in a relatively small range of 0.5 to 1.0 μm.

  The average particle size of the active material particles in the electrode can be controlled by adjusting the particle size of the active material powder when the electrode is formed by sheet molding or green compact. When forming an electrode, it can be performed by adjusting the particle size of the active material powder and adjusting the firing temperature. 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.

  Examples of the active material used for the positive electrode 1 include lithium cobalt composite oxide, lithium manganese composite oxide, manganese dioxide, lithium nickel composite oxide, lithium nickel cobalt composite oxide, lithium vanadium composite oxide, vanadium oxide, and sodium. Examples include cobalt composite oxide, sodium manganese composite oxide, manganese dioxide, sodium nickel composite oxide, sodium nickel iron composite oxide, sodium iron composite oxide, sodium chromium composite oxide, and the like. The positive electrode 1 is obtained by producing an electrode by the above-described manufacturing method using these active material particles.

In the present embodiment, a compound containing pseudo hexagonal TiP ₂ O ₇ as a main crystal phase is used for the negative electrode 3 as a negative electrode active material, but this can also be used for the positive electrode 1 as a positive electrode active material. When this compound is used for the positive electrode 1, the negative electrode active material used for the negative electrode 3 can occlude / release ions that are responsible for electrical conduction, and the occlusion / release of these ions is lower than that of the positive electrode active material used for the positive electrode 1. A negative electrode active material that can be performed at a low potential, for example, a carbonaceous material such as lithium metal or sodium metal, artificial and natural graphite, non-graphitizable carbon, and graphitizable low-temperature calcined carbon may be used.

As the electrolyte layer 2, any one of an aqueous electrolyte, a non-aqueous electrolyte such as an organic electrolyte or an ionic liquid impregnated in a separator, a polymer solid electrolyte, an inorganic solid electrolyte, or a molten salt may be used. it can.

  The separator impregnated with the aqueous electrolyte solution or the non-aqueous electrolyte solution is required to pass ions and prevent the 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 aqueous electrolyte, for example, an aqueous solution of 1 to 2 mol / L lithium sulfate, lithium nitrate, sodium sulfate, sodium nitrate, or the like can be used. Such an aqueous electrolyte solution can change the electrolysis potential of water by adjusting the pH, and thus can change the charging potential of the secondary battery.

The organic electrolyte is composed of an organic solvent and an electrolyte salt, and an additive for the purpose of forming a film on the electrode surface, preventing overcharge, imparting flame retardancy, or the like may be added as necessary. As the organic solvent, those having a high dielectric constant, low viscosity and low vapor pressure are preferably used. Examples of such materials include ethylene carbonate (EC), propylene carbonate, butylene carbonate, and γ-butyrolactone. , Sulfolane, 1,2-dimethoxyethane, 1,3-dimethoxypropane, dimethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, or a mixture of two or more thereof. It is done. As the electrolyte salt, for example LiBF4, LiPF6, LiClO4, LiCF3SO3, LiAsF6, LiN (CF3SO2) 2, LiN (C2F5SO2) and the lithium salt of _2, etc., sodium perchlorate (NaClO _4), tetrafluoride sodium borate (NaBF ₄ ), sodium salts such as sodium hexafluorophosphate (NaPF ₆ ), NaN (FSO ₂ ) ₂ , NaN (CF ₃ SO ₂ ) ₂ , and NaN (C ₂ F ₅ SO ₂ ) ₂ . Among these, NaN (SO ₂ F) ₂ , NaN (CF ₃ SO ₂ ) _2, and NaN (C ₂ F ₅ SO ₂ ) ₂ are mixed with other alkali metal salts and used in an environment at a certain temperature or higher. Therefore, it can also be used as a molten salt.

  When a polymer solid electrolyte or an inorganic solid electrolyte is used as the electrolyte layer 2, the thickness can be reduced to, for example, 10 μm or less, and further 3 μm or less, and more active materials can be used compared to a secondary battery having the same volume. Since packing can be performed, the capacity can be increased, and as a result, the energy density can be improved. However, in order to prevent a short circuit, the solid electrolyte needs to have a necessary minimum thickness that does not cause a dielectric breakdown or a short circuit due to a pinhole.

Moreover, it is preferable to use the active material in this embodiment for the secondary battery (sodium secondary battery) which charges / discharges especially by exchanging sodium ion. A sodium secondary battery tends to have a lower capacity than a lithium secondary battery. However, by using a compound containing pseudo-hexagonal TiP ₂ O ₇ as an active material, the cycle characteristics are improved and the capacity is improved. can get.

  A material having corrosion resistance that does not cause a reaction such as dissolution at the potential of the positive electrode may be used for the positive current collecting layer 5P. Examples of such materials include metal materials and alloys including aluminum, tantalum, niobium, titanium, gold, platinum, etc., carbonaceous materials such as graphite, hard carbon, and glassy carbon, inorganic materials such as ITO glass and tin oxide. A conductive oxide material or the like can be used. Among these, aluminum, gold, and platinum are preferable because they are excellent in corrosion resistance and easily available. Aluminum is particularly preferable because it is passivated by forming an oxide film on the surface and excellent in corrosion resistance even at a high potential.

  A material that does not cause side reactions such as alloying with Li or 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 copper, nickel, brass, zinc, aluminum, stainless steel, tungsten, gold, platinum, carbonaceous materials such as graphite, hard carbon, and glassy carbon, ITO glass. An inorganic conductive oxide material such as tin oxide can be used. In particular, it is preferable to use aluminum or nickel from the viewpoint of high conductivity and relatively low cost. Aluminum, in particular, has high conductivity and is relatively inexpensive, like copper and nickel, and cannot be used because it forms an alloy with lithium, but is inactive with sodium. Also, it can be used as a current collector.

  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. Moreover, what vapor-deposited metals, such as platinum, aluminum, and titanium, on the electrode material surface may be used.

  In addition, when using metal foil or a mesh, it is preferable that the thickness shall be 5-20 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, an active material of the present invention and a secondary battery using the active material will be described in detail based on examples. First, anatase TiO ₂ (manufactured by Toho Titanium) and (NH ₄ ) ₂ HPO ₄ (diammonium hydrogen phosphate) are blended so that the ratio of Ti and P is 1: 2, as raw materials for the active material, Slurry with isopropyl alcohol (IPA) as a solvent was mixed with a ball mill using ZrO ₂ balls for 20 hours. After drying the mixed slurry, heat treatment was performed in the air to synthesize an active material. The heat treatment was performed under the conditions shown in Table 1.

The synthesized active material was measured by X-ray diffraction (XRD) using CuKα rays, and the diffraction pattern was analyzed to identify the crystal phase contained in the active material. Pseudo hexagonal TiP appearing in the range of 22.8 to 23.4 °, where Io is the diffraction intensity of the diffraction peak having the maximum intensity in the X-ray diffraction profile of the obtained active material (main peak of the active material) The diffraction intensity of the diffraction peak of ₂ O ₇ (a pseudo hexagonal main peak) was defined as Ip. Table 1 shows the main crystal phase contained in the active material and the intensity ratio Ip / Io of the X-ray diffraction peak. The main peak of the active material in this example is TiP ₂ O identified by JCPDS No. 00-038-1468.
₇ is the main peak of the crystalline phase of ( _{7) and} the main peak of pseudo hexagonal TiP ₂ O ₇ , and the main peaks of the active materials A, B and C are pseudo hexagonal TiP. _{Since this} was the main peak of ₂ O ₇ , these Ip / Io were set to 1.

  A negative electrode was produced using the synthesized active material. 80% by mass of the synthesized active material powder, 10% by mass of acetylene black as a conductive assistant, 10% by mass of polyvinylidene fluoride as a binder, and 15% by mass of NMP (N-methylpyrrolidone) as a solvent were mixed to prepare a slurry. Produced. This slurry was applied onto a metal foil serving as a negative electrode side current collecting layer by a doctor blade method, and the solvent was dried to form a negative electrode having a thickness of 50 μm.

As the positive electrode, a commercially available LiMn ₂ O ₄ powder (manufactured by Toda Kogyo) or a synthesized Na _0.7 MnO _2.05 powder was used as an active material. A slurry was prepared by mixing 80% by mass of these active material powders, 10% by mass of acetylene black as a conductive additive, 10% by mass of polyvinylidene fluoride as a binder, and 15% by mass of NMP (N-methylpyrrolidone) as a solvent. And it apply | coated with the doctor blade method on the metal foil used as a positive electrode side current collection layer, and the positive electrode with a thickness of 50 micrometers was formed by drying a solvent.

  The positive electrode and the negative electrode obtained were cut into a 10 × 10 cm square shape together with the metal foil as the current collecting layer, and the same as the end of the surface on the side where the electrode of the metal foil as the current collecting layer was not formed A metal foil made of the above material was attached as a positive electrode terminal or a negative electrode terminal by spot welding.

Between the produced positive electrode and negative electrode, a separator made of polypropylene / polyethylene containing an electrolytic solution was placed, housed in a bag-like aluminum laminate film as an exterior body, and the electrolytic solution was injected. As the electrolytic solution, LiClO ₄ or NaN (CF ₃ SO ₂ ) ₂ (also referred to as NaTFSI) dissolved in 1 mol / L in propylene carbonate (PC), which is an organic solvent, or Li ₂ SO ₄ or Na ₂ SO ₄ An aqueous solution in which 2 mol / L was dissolved was used. Table 2 shows the active material used for the positive electrode and the negative electrode, the material of the current collecting layer, and the type of the electrolytic solution.

  After injecting the electrolytic solution, the opening of the outer package was sealed by thermal welding in a state where the ends of the positive electrode terminal and the negative electrode terminal were exposed from the opening of the outer package, thereby obtaining a secondary battery.

  The charge / discharge characteristics of the produced secondary battery were evaluated under the condition of 0.2C. Table 2 shows the initial discharge capacity and the discharge capacity retention rate after a 100-cycle charge / discharge test with one discharge-charge cycle.

As shown in Table 1 and Table 2, cycle characteristics are improved by using an active material containing pseudo hexagonal TiP ₂ O ₇ as one of the main crystal phases, that is, Ip / Io is 0.5 or more. The capacity maintenance rate after the 100-cycle charge / discharge test was greatly improved. In particular, when Ip / Io was 0.7 or more, the capacity retention rate showed excellent characteristics exceeding 80% (see Sample Nos. 1-4, 7-10, 13-16, 19-22). .

Further, when the sodium secondary battery was formed, the initial discharge capacity was greatly improved by including pseudo hexagonal TiP ₂ O ₇ as the main crystal phase.

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 ... Laminate film

Claims (7)

A compound containing Ti and P,
An active material, wherein the compound contains a TiP ₂ O ₇ crystal phase having a pseudo hexagonal crystal structure as a main crystal phase. The TiP ₂ O ₇ crystal phase having the quasi-hexagonal structure has a maximum diffraction intensity in a range of 2θ of 22.8 to 23.4 ° in X-ray diffraction measurement using Cukα rays. The active material according to claim 1. 3. The active material according to claim 1, wherein the compound containing Ti and P contains at least one crystal phase different from the TiP ₂ O ₇ crystal phase having the pseudo hexagonal crystal structure. In the X-ray diffraction profile of the active material obtained by X-ray diffraction measurement using Cukα rays, the diffraction intensity Io of the diffraction peak having the maximum intensity and 2θ appearing in the range of 22.8 to 23.4 ° The intensity ratio Ip / Io of the diffraction intensity Ip of the diffraction peak of the TiP ₂ O ₇ crystal phase having a pseudo hexagonal crystal structure is 0.7 or more, or any one of claims 1 to 3, Active material.   A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein at least one of the positive electrode and the negative electrode includes the active material according to any one of claims 1 to 4.   The secondary battery according to claim 5, wherein the electrolyte is an aqueous electrolyte.   The secondary battery according to claim 5, wherein charging and discharging are performed by exchanging sodium ions between the positive electrode and the negative electrode.

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