US20150004491A1 - Positive-electrode active material for non-aqueous secondary battery and method for producing the same - Google Patents

Positive-electrode active material for non-aqueous secondary battery and method for producing the same Download PDF

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US20150004491A1
US20150004491A1 US14/316,954 US201414316954A US2015004491A1 US 20150004491 A1 US20150004491 A1 US 20150004491A1 US 201414316954 A US201414316954 A US 201414316954A US 2015004491 A1 US2015004491 A1 US 2015004491A1
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positive
transition metal
electrode active
precipitate
sodium
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Hideaki YOSHIWARA
Tsutomu Yamada
Masahiro Murayama
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Nichia Corp
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Nichia Corp
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Publication of US20150004491A1 publication Critical patent/US20150004491A1/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
    • H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01D—COMPOUNDS OF ALKALI METALS, i.e. LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, OR FRANCIUM
    • C01D1/00—Oxides or hydroxides of sodium, potassium or alkali metals in general
    • C01D1/02—Oxides
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G45/00—Compounds of manganese
    • C01G45/006—Compounds containing, besides manganese, two or more other elements, with the exception of oxygen or hydrogen
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G45/00—Compounds of manganese
    • C01G45/12—Manganates manganites or permanganates
    • C01G45/1221—Manganates or manganites with a manganese oxidation state of Mn(III), Mn(IV) or mixtures thereof
    • C01G45/1228—Manganates or manganites with a manganese oxidation state of Mn(III), Mn(IV) or mixtures thereof of the type [MnO2]n-, e.g. LiMnO2, Li[MxMn1-x]O2
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G49/00—Compounds of iron
    • C01G49/0018—Mixed oxides or hydroxides
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G49/00—Compounds of iron
    • C01G49/009—Compounds containing, besides iron, two or more other elements, with the exception of oxygen or hydrogen
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G51/00—Compounds of cobalt
    • C01G51/006—Compounds containing, besides cobalt, two or more other elements, with the exception of oxygen or hydrogen
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G51/00—Compounds of cobalt
    • C01G51/40—Cobaltates
    • C01G51/42—Cobaltates containing alkali metals, e.g. LiCoO2
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G53/00—Compounds of nickel
    • C01G53/006—Compounds containing, besides nickel, two or more other elements, with the exception of oxygen or hydrogen
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G53/00—Compounds of nickel
    • C01G53/40—Nickelates
    • C01G53/42—Nickelates containing alkali metals, e.g. LiNiO2
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/054—Accumulators with insertion or intercalation of metals other than lithium, e.g. with magnesium or aluminium
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
    • C01P2002/72—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data by d-values or two theta-values, e.g. as X-ray diagram
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
    • C01P2002/76—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data by a space-group or by other symmetry indications
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/40—Electric properties
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
    • H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present invention relates to positive-electrode active materials for non-aqueous secondary batteries such as sodium ion secondary batteries, and to methods for producing the same.
  • non-aqueous secondary batteries typically lithium ion secondary batteries
  • Lithium ion secondary batteries have as high an operating voltage as about 4 V and can store large amounts of energy per unit mass. Due to these advantages, their application to large apparatuses such as electric vehicles and power storage systems has been expected.
  • lithium transition metal composite oxides such as lithium cobaltate are typically used as the positive-electrode active materials.
  • lithium and transition metals such as cobalt are rare or precious elements due to reasons such as that the reserves of these elements are unevenly distributed and also that such elements are obtained by separation as impurities from materials such as minerals.
  • sodium transition metal composite oxides having a layered structure based on abundant elements such as iron and sodium be used as the positive-electrode active materials.
  • Sodium ion secondary batteries involving such positive-electrode active materials have been also proposed.
  • Patent Literature 1 discloses composite oxides represented by NaFe 1-x M x O 2 , wherein M is a trivalent metal element and 0 ⁇ x ⁇ 0.5.
  • the disclosure describes production methods involving Na 2 O 2 and Fe 3 O 4 as examples of a sodium compound and an iron compound that are raw materials.
  • Patent Literature 2 discloses composite metal oxides with a layered rock salt structure represented by Formula Na x Fe 1-y M y O 2 , wherein M is an element such as Mn, 0.5 ⁇ x ⁇ 1 and 0 ⁇ y ⁇ 0.5. According to the disclosure, the above range of x ensures that the layered rock salt crystal structure will have high purity and more sodium ions are available for doping and dedoping. The disclosure also describes production methods involving Na 2 CO 3 , Fe 3 O 4 and MnO 2 as examples of a sodium compound, an iron compound and a manganese compound that are raw materials.
  • Patent Literature 3 discloses composite metal oxides including a P2 structure oxide and a layered oxide that are represented by Formula Na x Fe y Mn 1-y O 2 , wherein 2 ⁇ 3 ⁇ x ⁇ 1 and 0 ⁇ y ⁇ 2 ⁇ 3.
  • the disclosure describes production methods involving Na 2 CO 3 , NaHCO 3 , Na 2 O 2 , Fe 3 O 4 and MnO 2 as examples of sodium compounds, an iron compound and a manganese compound that are raw materials.
  • the composite metal oxides described in Patent Literatures 2 and 3 have an R-3m crystal structure when the sodium to transition metal ratio is 1, but the layered structure comes to take a P6 3 /mmc crystal structure when the ratio is less than 1.
  • the P6 3 /mmc structure is more resistant to breakage by the desorption and insertion of sodium ions than the R-3m structure, and this fact makes the P6 3 /mmc crystal structure advantageous in terms of charge and discharge characteristics at a high stage of charge (SOC).
  • a first embodiment is a positive-electrode active material for non-aqueous secondary battery comprising a sodium transition metal composite oxide that is represented by Formula Na x Fe 1-y M y O 2 , wherein 0.4 ⁇ x ⁇ 0.7, 0.25 ⁇ y ⁇ 1.0, and M is at least one element selected from the group consisting of manganese, cobalt and nickel and having a crystal structure substantially composed of P6 3 /mmc alone.
  • the positive-electrode active material is satisfactory both in charge-discharge capacity and cycle characteristics and in other properties.
  • a second embodiment is a method for producing a positive-electrode active material for non-aqueous secondary battery comprising a sodium transition metal composite oxide represented by Formula Na x Fe 1-y M y O 2 , wherein 0.4 ⁇ x ⁇ 0.7, 0.25 ⁇ y ⁇ 1.0, and M is at least one element selected from the group consisting of manganese, cobalt and nickel, comprising a precipitate formation step of obtaining a precipitate of a transition metal composite compound from a transition metal ion-containing aqueous solution, a heat treatment step of heat treating the precipitate from the precipitate formation step to obtain a transition metal composite oxide precursor, a mixing step of mixing the precursor from the heat treatment step with at least a sodium compound to obtain a raw material mixture, and a calcination step of calcining the raw material mixture from the mixing step to obtain a calcined product.
  • a precipitate formation step of obtaining a precipitate of a transition metal composite compound from a transition metal ion-containing aqueous solution
  • a third embodiment is a method for producing a positive-electrode active material for non-aqueous secondary battery comprising a sodium transition metal composite oxide represented by Formula Na x Fe 1-y M y O 2 , wherein 0.4 ⁇ x ⁇ 0.7, 0.25 ⁇ y ⁇ 1.0, and M is at least one element selected from the group consisting of manganese, cobalt and nickel, comprising a precipitate formation step of obtaining a precipitate of a transition metal composite compound other than a hydroxide from a transition metal ion-containing aqueous solution, a mixing step of mixing the precipitate from the precipitate formation step with at least a sodium compound to obtain a raw material mixture, and a calcination step of calcining the raw material mixture from the mixing step to obtain a calcined product.
  • a precipitate formation step of obtaining a precipitate of a transition metal composite compound other than a hydroxide from a transition metal ion-containing aqueous solution
  • a mixing step of mixing the precipitate from
  • FIG. 1 is an example of XRD spectra showing data of positive-electrode active materials of the embodiment and comparative positive-electrode active materials.
  • FIG. 2 illustrates an example of moisture absorption data of positive-electrode active materials of the embodiment and comparative positive-electrode active materials.
  • FIG. 3 illustrates an example of cycle characteristics of non-aqueous secondary batteries involving positive-electrode active materials of the embodiment and comparative positive-electrode active materials in the positive-electrodes.
  • Patent Literatures 2 and 3 mention that a mixture of metal-containing compounds with a prescribed chemical composition may be obtained by a crystallization method, the specifications do not teach any specific conditions in the production methods or whatsoever.
  • the methods disclosed in Patent Literatures 1 to 3 involve the calcination of a raw material mixture produced by a so-called dry process.
  • sodium transition metal composite oxides obtained by such methods contain subphases which have crystal structures different from the desired structure.
  • the target crystal structure is P6 3 /mmc
  • subphases such as R-3m structure, Pnma structure and Fd-3m structure occur. The occurrence of such subphases is probably ascribed to the elements in the raw material mixture being mixed with insufficient uniformity.
  • transition metal elements may be effected to sufficient uniformity by a so-called coprecipitation process in which transition metal ions are precipitated by pH control or with an agent such as a complexing agent.
  • counter ions for the transition metal ions for example, nitrate ions in the case of an aqueous transition metal nitrate salt solution, are often incorporated into the crystal or the precipitate aggregate during precipitation. Such counter ions that have been incorporated can adversely affect the performance of non-aqueous secondary batteries. Thus, the best performance of sodium ion secondary batteries cannot be achieved by the simple application of a coprecipitation process.
  • An object of the invention is to provide methods for producing positive-electrode active materials for non-aqueous secondary battery which have a high purity of P6 3 /mmc structure and allow non-aqueous sodium secondary batteries such as sodium ion secondary batteries to fully exhibit their performance.
  • Another object of the invention is to provide positive-electrode active materials for non-aqueous sodium secondary batteries which are satisfactory both in charge-discharge capacity and cycle characteristics and in other properties.
  • a sodium transition metal composite oxide which is substantially composed of P6 3 /mmc alone and has a very low content of counter ions may be obtained by a process in which a transition metal composite compound as a precursor is prepared by a coprecipitation method including specific steps, the precursor is then mixed with other raw material compounds, and the mixture is calcined.
  • the scope of the present invention includes the following aspects.
  • a first aspect of a method for producing a positive-electrode active material for non-aqueous secondary battery of the invention resides in a method for producing, the positive-electrode active material for non-aqueous secondary battery including a sodium transition metal composite oxide represented by Formula Na x Fe 1-y M y O 2 , wherein 0.4 ⁇ x ⁇ 0.7, 0.25 ⁇ y ⁇ 1.0, and M is at least one element selected from the group consisting of manganese, cobalt and nickel, the method comprising a precipitate formation step of obtaining a precipitate of a transition metal composite compound from a transition metal ion-containing aqueous solution, a heat treatment step of heat treating the precipitate from the precipitate formation step to obtain a transition metal composite oxide precursor, a mixing step of mixing the precursor from the heat treatment step with at least a sodium compound to obtain a raw material mixture, and a calcination step of calcining the raw material mixture from the mixing step to obtain a calcined product.
  • a precipitate formation step
  • a second aspect of a method for producing a positive-electrode active material for non-aqueous secondary battery of the invention resides in a method for producing the positive-electrode active material for non-aqueous secondary battery including a sodium transition metal composite oxide represented by Formula Na x Fe 1-y M y O 2 , wherein 0.4 ⁇ x ⁇ 0.7, 0.25 ⁇ y ⁇ 1.0, and M is at least one element selected from the group consisting of manganese, cobalt and nickel, the method comprising a precipitate formation step of obtaining a precipitate of a transition metal composite compound other than a hydroxide from a transition metal ion-containing aqueous solution, a mixing step of mixing the precipitate from the precipitate formation step with at least a sodium compound to obtain a raw material mixture, and a calcination step of calcining the raw material mixture from the mixing step to obtain a calcined product.
  • a precipitate formation step of obtaining a precipitate of a transition metal composite compound other than a hydro
  • a positive-electrode active material for non-aqueous secondary battery of the invention includes a sodium transition metal composite oxide that is represented by Formula Na x Fe 1-y M y O 2 , wherein 0.4 ⁇ x ⁇ 0.7, 0.25 ⁇ y ⁇ 1.0, and M is at least one element selected from the group consisting of manganese, cobalt and nickel and has a crystal structure substantially composed of P6 3 /mmc alone.
  • the inventive methods for the production of positive-electrode active materials for non-aqueous secondary battery can produce sodium transition metal composite oxides which are substantially composed of a P6 3 /mmc structure alone and have a sufficiently low content of counter ions contained in the oxide.
  • the inventive positive-electrode active material for non-aqueous secondary battery allows a battery to achieve advantageous charge-discharge capacity and cycle characteristics due to the P6 3 /mmc structure and also to perform well in other battery characteristics.
  • step encompasses not only an independent step but also a step in which the anticipated effect of this step is achieved, even if the step cannot be clearly distinguished from another step.
  • the content of each ingredient of the composition denotes the total amount of the plural materials included in the composition.
  • non-aqueous secondary battery positive-electrode active materials and methods for producing the active materials according to the present invention will be described in detail with reference to embodiments and examples.
  • positive-electrode active materials for non-aqueous secondary battery hereinafter, also written simply as “positive-electrode active materials”, according to the present invention will be discussed in detail.
  • the positive-electrode active material for non-aqueous secondary battery of the invention includes a sodium transition metal composite oxide that is represented by Formula Na x Fe 1-y M y O 2 , wherein 0.4 ⁇ x ⁇ 0.7, 0.25 ⁇ y ⁇ 1.0, and M is at least one element selected from the group consisting of manganese, cobalt and nickel and has a crystal structure substantially composed of P6 3 /mmc alone.
  • the presence of a P6 3 /mmc structure in the crystal structure may be identified based on, for example, a powder X-ray diffractometry (XRD) spectrum.
  • XRD powder X-ray diffractometry
  • the phrase “substantially composed of P6 3 /mmc alone” means that the crystal structure may include other subphases in addition to the P6 3 /mmc structure as long as the advantageous effects of the invention are achieved.
  • the content of the P6 3 /mmc structure is not less than 95%, and preferably not less than 98% of the crystal structure.
  • the chemical composition of the sodium transition metal composite oxide that is the main component is represented by the above formula.
  • the main crystal structure of the sodium transition metal composite oxide is P6 3 /mmc and the breakage of the crystal structure by the desorption and insertion of sodium ions is prevented.
  • the crystal structure starts to shift from the P6 3 /mmc if the value of x is outside the above range.
  • the crystal takes an R-3m structure when x is around 1.
  • a preferred range is 0.5 ⁇ x ⁇ 0.7. While conventional sodium transition metal composite oxides with a P6 3 /mmc structure inevitably contain subphases having other crystal structures, the sodium transition metal composite oxide obtained by the producing methods of the invention is substantially composed of a P6 3 /mmc structure alone.
  • y By limiting y to 0.25 ⁇ y ⁇ 1.0, the obtainable P6 3 /mmc sodium transition metal composite oxide attains good crystallinity. It should be noted that any value of y outside this range causes a decrease in crystallinity and/or the occurrence of subphases. A preferred range is 0.25 ⁇ y ⁇ 0.75.
  • the letter M is at least one element selected from the group consisting of manganese, cobalt and nickel having a similar ion radius to iron.
  • the use of these elements M advantageously makes it easy to obtain the desired crystal structure.
  • M is manganese, the target material with a stable crystal structure may be obtained easily.
  • the positive-electrode active material of the invention sometimes contains a trace amount of oxo acid ions.
  • the content of oxo acid ions is preferably not more than 0.3 wt %, more preferably not more than 0.1 wt %, further preferably not more than 0.05 wt %, and particularly preferably below the detection limit or around the detection limit (about 300 ppm).
  • the oxo acid ions include sulfate ions and nitrate ions.
  • the crystal structure of the positive-electrode active material of the invention shows a specific characteristic in powder X-ray diffractometry.
  • the peak intensity assigned to the (110) plane has a high ratio to the peak intensity assigned to the (016) plane, hereinafter, also written as “(110)/(016)”
  • the crystal structure achieves higher strength and becomes more resistant to breakage by the desorption and insertion of sodium ions, resulting in improved cycle characteristics.
  • the peak intensity ratio is preferably not less than 0.30.
  • a peak intensity ratio higher than 2.00 indicates a possibility of the presence of crystal phases other than the P6 3 /mmc phase.
  • the peak intensity ratio is preferably not more than 2.00, more preferably 0.35 to 1.00, and particularly preferably 0.40 to 0.80.
  • the (110) plane shows a diffraction peak in the range where 2 ⁇ is 61.2° to 61.7°
  • the (016) plane shows a diffraction peak in the range where 2 ⁇ is 63.3° to 63.8°.
  • the integral widths of the (016) plane peak and the (110) plane peak serve as an indicator of how well the sodium transition metal composite oxide has been crystallized. Smaller widths are more preferable.
  • the integral width is preferably not more than 1.00°, and more preferably not more than 0.50° for the (016) plane peak, and is preferably not more than 0.30°, and more preferably not more than 0.25° for the (110) plane peak. A realistic value of the integral width is 0.10° or above for both peaks.
  • the content of the sodium transition metal composite oxide represented by the above formula is not particularly limited.
  • the content may be 80 mass % or more, and preferably 95 mass % or more. It is more preferable that the positive-electrode active material is substantially composed of the sodium transition metal composite oxide represented by the above formula alone.
  • the term “substantially” means that the positive-electrode active material may include compounds other than the sodium transition metal composite oxide represented by the above formula as long as the advantageous effects of the invention are achieved.
  • the positive-electrode active materials for non-aqueous secondary battery of the invention may be preferably produced by any of the following production methods which advantageously allow for efficient production.
  • inventive methods for producing positive-electrode active materials for non-aqueous electrolyte secondary battery may be performed largely in two embodiments.
  • the first embodiment is a method for producing a positive-electrode active material for non-aqueous secondary battery including a sodium transition metal composite oxide represented by Formula Na x Fe 1-y M y O 2 , wherein 0.4 ⁇ x ⁇ 0.7, 0.25 ⁇ y ⁇ 1.0, and M is at least one element selected from the group consisting of manganese, cobalt and nickel, and is characterized by including a precipitate formation step of obtaining a precipitate of a transition metal composite compound from a transition metal ion-containing aqueous solution, a heat treatment step of heat treating the precipitate from the precipitate formation step to obtain a transition metal composite oxide precursor, a mixing step of mixing the precursor from the heat treatment step with at least a sodium compound to obtain a raw material mixture, and a calcination step of calcining the raw material mixture from the mixing step to obtain a calcined product.
  • a precipitate formation step of obtaining a precipitate of a transition metal composite compound from a transition metal ion-containing aque
  • the sodium transition metal composite oxide that is the main component in the target positive-electrode active material is represented by the above formula. Details are as mentioned in the description of the positive-electrode active materials for non-aqueous secondary battery according to the invention.
  • a precipitate of a transition metal composite compound is obtained from a transition metal ion-containing aqueous solution.
  • the precipitate formation step is preferably a step in which a basic compound such as sodium hydroxide is added to the transition metal ion-containing aqueous solution to adjust the pH and to obtain a precipitate of a poorly soluble transition metal composite compound.
  • the transition metal ion-containing aqueous solution may be appropriately prepared by, for example, dissolving transition metal compounds such as chlorides, sulfate salts and nitrate salts into an acid or pure water, or by dissolving transition metals into an acid. Any appropriate acids such as hydrochloric acid, nitric acid and sulfuric acid may be selected in accordance with the solutes.
  • the transition metal ion-containing aqueous solution that is obtained is preferably an aqueous sulfate salt solution.
  • an aqueous sulfate salt solution is used, the heat treatment step described later has a particular importance.
  • the transition metal ion-containing aqueous solution contains at least iron ions, and further contains ions of at least one transition metal selected from the group consisting of manganese, cobalt and nickel (hereinafter, also written as specific transition metal ions).
  • the ratio of the content of iron ions to the content of specific transition metal ions may be selected appropriately in accordance with the chemical composition of the target transition metal composite compound.
  • Examples of the poorly soluble transition metal composite compounds include hydroxides, carbonate salts and oxalate salts. From the viewpoint of handling, hydroxides are preferable.
  • the transition metal composite compound obtained in the precipitation step is heat treated to form a transition metal composite oxide precursor.
  • This step removes the counter ions (for example, sulfate ions when the transition metal ion-containing aqueous solution is an aqueous sulfate salt solution) from the transition metal composite compound, resulting in a transition metal composite oxide precursor which contains less impurities and has been crystallized to a degree.
  • the counter ions remaining in the precursor cause a decrease in the crystallinity of the final sodium transition metal composite oxide.
  • the counter ions are sulfate ions, the importance of this step further increases because the sulfate ions will remain even in the final sodium transition metal composite oxide to possibly adversely affect moisture absorption properties and various battery characteristics.
  • the temperature of the heat treatment requires careful control because the treatment at an excessively low temperature results in insufficient crystallization and insufficient counter ion removal while too high temperatures cause sintering to proceed excessively and undesired phases to occur.
  • the heat treatment temperature is preferably in the range of 600° C. to 1000° C., although the tendencies of heat treatment vary slightly depending on the chemical composition.
  • the heating temperature is more preferably 800° C. to 950° C. because the unity or uniformity of the crystal structure is markedly increased.
  • the heat treatment is performed for at least a certain time period because too short a treatment time does not allow the reaction to complete.
  • the heat treatment time may be extended to any extent without a problem. However, performing the heat treatment for an overly long time only protracts the step and is thus not necessary.
  • the heat treatment time is preferably 0.5 hours to 50 hours, and more preferably 3 hours to 24 hours.
  • the heat treatment step may be performed in any atmosphere without limitation, but is preferably carried out in an oxidizing atmosphere.
  • the oxidizing atmospheres include air atmosphere and oxygen-containing atmospheres.
  • the precursor is mixed with at least a sodium compound to give a raw material mixture.
  • the sodium compounds may be any compounds which can be decomposed into oxides at high temperatures, with examples including sodium carbonate, sodium hydroxide, sodium oxide, sodium peroxide, sodium chloride, sodium nitrate and sodium sulfate.
  • sodium oxide and sodium peroxide are preferable due to their high reactivity in the calcination step, but this characteristic also requires careful attention in the mixing step such as the need of handling the compound in an inert atmosphere such as nitrogen or argon.
  • Sodium carbonate is advantageously easy to handle in the mixing step.
  • the sodium compounds such as sodium nitrate should be handled sufficiently carefully although the counter ions in these compounds are removed more easily in the subsequent calcination step compared to the counter ions contained in the precursor. From these viewpoints, sodium compounds other than strong acid salts are preferable such as sodium carbonate, sodium hydroxide, sodium oxide and sodium peroxide.
  • the mixing ratio of the precursor to the sodium compound in the raw material mixture may be selected appropriately in accordance with the chemical composition of the target sodium transition metal composite oxide.
  • the raw material mixture may further contain additives such as sintering auxiliaries (fluxes) in accordance with the purpose.
  • the mixing step may involve using any of known mixers such as ball mills, twin-cylinder mixers and stirrers.
  • the raw material mixture from the mixing step is calcined to give a calcined product.
  • Any of known calcination means may be selected appropriately in accordance with the purpose.
  • the raw material mixture may be compacted before the calcination or may be charged into a crucible directly.
  • calcination furnaces examples include batch furnaces, tunnel furnaces and rotary kilns.
  • the calcination temperature needs to be controlled carefully because too low calcination temperatures cause undesired subphases to occur and too high calcination temperatures cause sintering to proceed excessively.
  • Calcination temperatures of 700° C. to 1100° C. advantageously ensure that a calcined product with a P6 3 /mmc phase may be obtained.
  • the calcination temperature is more preferably 800° C. to 1000° C.
  • the calcination step may be performed in any atmosphere without limitation, but is preferably carried out in an oxidizing atmosphere.
  • the oxidizing atmospheres include air atmosphere and oxygen-containing atmospheres.
  • the calcined product obtained above may be subjected to treatments such as pulverization, washing and sieving as required, thereby obtaining a positive-electrode active material for non-aqueous secondary battery including the target sodium transition metal composite oxide.
  • the second embodiment is a method for producing a positive-electrode active material for non-aqueous secondary battery including a sodium transition metal composite oxide represented by Formula Na x Fe 1-y M y O 2 (wherein 0.4 ⁇ x ⁇ 0.7, 0.25 ⁇ y ⁇ 1.0, and M is at least one element selected from the group consisting of manganese, cobalt and nickel), and is characterized by including a precipitate formation step of obtaining a precipitate based on (namely, containing as a main component) a transition metal composite compound other than a hydroxide from a transition metal ion-containing aqueous solution, a mixing step of mixing the precipitate from the precipitate formation step with at least a sodium compound to obtain a raw material mixture, and a calcination step of calcining the raw material mixture from the mixing step to obtain a calcined product.
  • a precipitate formation step of obtaining a precipitate based on (namely, containing as a main component) a transition metal composite compound other than a hydrox
  • the chemical composition is similar to that described in the first embodiment.
  • a precipitate based on a transition metal composite compound other than a hydroxide is obtained from a transition metal ion-containing aqueous solution.
  • the precipitate formation step is preferably a step in which a basic compound such as sodium hydroxide is added to the transition metal ion-containing aqueous solution to adjust the pH, and a specific precipitating agent is added to obtain a precipitate of a poorly soluble transition metal composite compound. This precipitate is based on a transition metal composite compound derived from the precipitating agent, and contains no or little hydroxide.
  • the content of hydroxide ions in the precipitate is preferably about 3 mol % or less, and particularly preferably 1 mol % or less relative to 100 mol % of the anions derived from the precipitating agent.
  • the transition metal ion-containing aqueous solution may be appropriately prepared by, for example, dissolving transition metal compounds such as chlorides, sulfate salts and nitrate salts into an acid or pure water, or by dissolving transition metals into an acid. Any appropriate acids such as hydrochloric acid, nitric acid and sulfuric acid may be selected in accordance with the solutes. In view of factors such as loads to the facility and the environment, availability and easiness in handling, the transition metal ion-containing aqueous solution that is obtained is preferably an aqueous sulfate salt solution.
  • the transition metal ion-containing aqueous solution contains at least iron ions, and further contains at least one type of specific transition metal ions selected from the group consisting of manganese, cobalt and nickel.
  • the ratio of the content of iron ions to the content of specific transition metal ions may be selected appropriately in accordance with the chemical composition of the target transition metal composite compound.
  • Examples of the poorly soluble transition metal composite compounds include carbonate salts and oxalate salts. Carbonate salts are preferable from the viewpoint of the relation with the precipitating agent as will be described below and also from the viewpoint of the performance of the positive-electrode active material.
  • a precipitate obtained by pH adjustment alone is based on a hydroxide, hereinafter, also written as hydroxide precipitate for convenience.
  • a hydroxide precipitate formed sometimes contains the counter ions present in the transition metal ion-containing aqueous solution, for example, sulfate ions in the case of an aqueous sulfate salt solution.
  • the remaining of the counter ions can cause a decrease in the crystallinity of the final sodium transition metal composite oxide.
  • the counter ions are sulfate ions, the sulfate ions will remain even in the final sodium transition metal composite oxide to possibly adversely affect moisture absorption properties and various battery characteristics.
  • a precipitating agent is used in combination with the pH adjustment to form a precipitate that is based on a transition metal composite compound other than a hydroxide. In this manner, the precipitate is prevented from containing an excessively large amount of undesired counter ions.
  • the precipitating agents include carbon dioxide, water-soluble carbonate salts, oxalic acid and water-soluble oxalate salts. From viewpoints such as easiness in handling and costs, carbon dioxide and water-soluble carbonate salts are preferable, and carbon dioxide is particularly preferable.
  • the transition metal composite compound from the precipitate formation step may be subjected to a heat treatment to form a transition metal composite oxide. Even in the case where a slight amount of a hydroxide precipitate has been formed in the precipitate formation step, this heat treatment removes sufficiently the counter ions from the precipitate of the transition metal composite compound.
  • the heat treatment temperature may be appropriately 600° C. to 1000° C. At such temperatures, the counter ions may be removed to a sufficient extent without causing excessive sintering.
  • the heat treatment step refer to the description of the heat treatment step in the first embodiment.
  • the precipitate, or the transition metal composite oxide from the heat treatment step is mixed with at least a sodium compound to give a raw material mixture.
  • the sodium compounds may be any compounds which can be decomposed into oxides at high temperatures, with examples including sodium carbonate, sodium hydroxide, sodium oxide, sodium peroxide, sodium chloride, sodium nitrate and sodium sulfate.
  • sodium oxide and sodium peroxide are preferable due to their high reactivity in the calcination step, but this characteristic also requires careful attention in the mixing step such as the need of handling the compound in an inert atmosphere such as nitrogen or argon.
  • Sodium carbonate is advantageously easy to handle in the mixing step.
  • the sodium compounds such as sodium nitrate should be handled sufficiently carefully although the counter ions in these compounds are removed more easily in the subsequent calcination step compared to the counter ions contained in the precipitate. From these viewpoints, sodium compounds other than strong acid salts are preferable such as sodium carbonate, sodium hydroxide, sodium oxide and sodium peroxide.
  • the mixing ratio of the precipitate to the sodium compound in the raw material mixture may be selected appropriately in accordance with the chemical composition of the target sodium transition metal composite oxide.
  • the raw material mixture may further contain additives such as sintering auxiliaries (fluxes) in accordance with the purpose.
  • the mixing step may involve using any of known mixers such as ball mills, twin-cylinder mixers and stirrers.
  • the calcination step is performed in accordance with the first embodiment.
  • non-aqueous secondary batteries are manufactured with the positive-electrode active materials for non-aqueous secondary battery of the invention.
  • the positive-electrode active material for non-aqueous secondary battery may be mixed with known components such as a conductive material and a binder to give a positive-electrode mixture, which is then applied to a known positive-electrode collector to form a positive-electrode active material layer.
  • a positive-electrode for non-aqueous secondary battery may be obtained.
  • Examples of the conductive materials include natural graphite, artificial graphite and acetylene black.
  • Examples of the binders include polyvinylidene fluoride, polytetrafluoroethylene and polyamide acrylic resin.
  • Examples of the materials of positive-electrode collectors include aluminum, nickel and stainless steel.
  • the positive-electrode active material layer may be formed by dispersing the positive-electrode mixture in a solvent, applying the dispersion to the positive-electrode collector, and drying and pressing the wet coating, or may be formed by directly providing the positive-electrode mixture on the positive-electrode collector by pressure forming
  • a non-aqueous secondary battery may be obtained using the positive-electrode for non-aqueous secondary battery obtained above and other known components such as a negative electrode for non-aqueous secondary battery, a non-aqueous electrolytic solution or a solid electrolyte, and a separator.
  • the negative electrode for non-aqueous secondary battery may be obtained by applying a known negative electrode active material for non-aqueous secondary battery on a known negative electrode collector to form a negative electrode active material layer.
  • the negative electrode active materials include metallic sodium, sodium alloys and materials capable of doping and dedoping of sodium ions.
  • Exemplary materials capable of doping and dedoping of sodium ions include carbonaceous materials, chalcogen compounds such as oxides and sulfides capable of sodium ion doping and dedoping at lower potential than the positive-electrode, and borate salts.
  • the negative electrode active material may form a negative electrode mixture with a thermoplastic resin as a binder.
  • the thermoplastic resins include polyvinylidene fluoride, polyethylene and polypropylene.
  • the negative electrode active material layer may be formed by directly providing the negative electrode active material or the negative electrode mixture on the negative electrode collector by pressure forming, or may be formed by dispersing the negative electrode active material optionally together with other components in a solvent, applying the dispersion on the negative electrode collector, and drying and pressing the wet coating.
  • the solvent of the electrolytic solution may be an organic solvent.
  • organic solvent examples thereof include dimethoxyethane, diethoxyethane, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl formate, ⁇ -butyrolactone, 2-methyltetrahydrofuran, dimethylsulfoxide and sulfolane.
  • electrolytes examples include sodium salts such as sodium perchlorate, sodium tetrafluoroborate, sodium hexafluorophosphate and sodium trifluoromethanesulfonate.
  • the electrolytic solution may be gelled by the addition of an agent such as a gelling agent. Further, the electrolytic solution may be absorbed into an absorbent polymer. The sodium ion concentration in the electrolytic solution may be adjusted appropriately in accordance with the purpose.
  • a solid electrolyte for example, a polymer compound having a polyethylene oxide backbone, or a polymer compound containing at least one of polyorganosiloxane chains and polyoxyalkylene chains may be used. Further, solid electrolytes including inorganic compounds may be used.
  • separators examples include porous films such as of polyethylene and polypropylene.
  • the shapes of the non-aqueous secondary batteries are not particularly limited and may be determined in accordance with any known configurations and appropriate modifications of such known shapes.
  • a reaction field consisting of an aqueous transition metal sulfate salt solution was prepared.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped to the reaction field, thereby obtaining a precipitate of a transition metal composite hydroxide represented by the composition formula (Fe 0.5 Mn 0.5 )(OH) 2 .
  • transition metal precursor represented by the composition formula (Fe 0.5 Mn 0.5 ) 2 O 3 , hereinafter, also written as “transition metal precursor”.
  • the raw material mixture was calcined in air at 900° C. to give a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.5 Mn 0.5 )O 2 .
  • a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.5 Mn 0.5 )O 2 was obtained in the same manner as in Example 1, except that the heat treatment temperature in the heat treatment step was changed to 650° C.
  • a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.5 Mn 0.5 )O 2 was obtained in the same manner as in Example 1, except that the heat treatment step in Example 1 was omitted.
  • a positive-electrode active material represented by the composition formula Na 0.3 (Fe 0.5 Mn 0.5 )O 2 was obtained in the same manner as in Example 1, except that the mixing step involved 0.15 mol of sodium carbonate.
  • a positive-electrode active material represented by the composition formula Na(Fe 0.5 Mn 0.5 )O 2 was obtained in the same manner as in Example 1, except that the mixing step involved 0.50 mol of sodium carbonate.
  • a reaction field consisting of an aqueous transition metal sulfate salt solution was prepared.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped to the reaction field, thereby obtaining a precipitate of a transition metal composite hydroxide represented by the composition formula (Fe 0.2 Mn 0.8 )(OH) 2 .
  • Example 2 The precipitate was treated in the same manner as in Example 1 to give a positive-electrode active material represented by the composition formula Na 0.2 (Fe 0.2 Mn 0.8 )O 2 .
  • a reaction field consisting of an aqueous transition metal sulfate salt solution was prepared.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped to the reaction field, thereby obtaining a precipitate of a transition metal composite hydroxide represented by the composition formula (Fe 0.8 Mn 0.2 )(OH) 2 .
  • Example 2 The precipitate was treated in the same manner as in Example 1 to give a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.8 Mn 0.2 )O 2 .
  • Manganese sulfate was dissolved in pure water to form a reaction field consisting of an aqueous manganese sulfate solution.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped to the reaction field, thereby obtaining a precipitate of manganese hydroxide represented by the composition formula Mn(OH) 2 .
  • the precipitate was treated in the same manner as in Example 1 to give a positive-electrode active material represented by the composition formula Na 0.7 MnO 2 .
  • a reaction field consisting of an aqueous transition metal sulfate salt solution was prepared.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped to the reaction field, thereby obtaining a precipitate of a transition metal composite hydroxide represented by the composition formula (Fe 0.5 Co 0.5 )(OH) 2 .
  • the precipitate was treated in the same manner as in Example 1 to give a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.5 Co 0.5 )O 2 .
  • a reaction field consisting of an aqueous transition metal sulfate salt solution was prepared.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped to the reaction field, thereby obtaining a precipitate of a transition metal composite hydroxide represented by the composition formula (Fe 0.5 Ni 0.5 )(OH) 2 .
  • the precipitate was treated in the same manner as in Example 1 to give a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.5 Ni 0.5 )O 2 .
  • a reaction field consisting of an aqueous transition metal sulfate salt solution was prepared.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped and simultaneously a prescribed amount of CO 2 gas was blown to the reaction field, thereby obtaining a precipitate of a transition metal composite carbonate salt represented by the composition formula (Fe 0.5 Mn 0.5 )CO 3 .
  • the precipitate was separated from the reaction field and was dried into a powder.
  • a raw material mixture was obtained by mixing 0.35 mol of sodium carbonate and 1.0 mol of the precipitate of the transition metal composite carbonate salt.
  • the raw material mixture was calcined in air at 900° C. to give a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.5 Mn 0.5 )O 2 .
  • a positive-electrode active material represented by the composition formula Na 0.3 (Fe 0.5 Mn 0.5 )O 2 was obtained in the same manner as in Example 6, except that the mixing step involved 0.15 mol of sodium carbonate.
  • a positive-electrode active material represented by the composition formula Na(Fe 0.5 Mn 0.5 )O 2 was obtained in the same manner as in Example 6, except that the mixing step involved 0.50 mol of sodium carbonate.
  • a reaction field consisting of an aqueous transition metal sulfate salt solution was prepared.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped and simultaneously a prescribed amount of CO 2 gas was blown to the reaction field, thereby obtaining a precipitate of a transition metal composite carbonate salt represented by the composition formula (Fe 0.2 Mn 0.8 )CO 3 .
  • the precipitate was separated from the reaction field and was dried into a powder.
  • the precipitate was treated in the same manner as in Example 6 to give a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.2 Mn 0.8 )O 2 .
  • a reaction field consisting of an aqueous transition metal sulfate salt solution was prepared.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped and simultaneously a prescribed amount of CO 2 gas was blown to the reaction field, thereby obtaining a precipitate of a transition metal composite carbonate salt represented by the composition formula (Fe 0.8 Mn 0.2 )CO 3 .
  • the precipitate was separated from the reaction field and was dried into a powder.
  • the precipitate was treated in the same manner as in Example 6 to give a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.8 Mn 0.2 )O 2 .
  • Manganese sulfate was dissolved in pure water to form a reaction field consisting of an aqueous manganese sulfate solution.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped and simultaneously a prescribed amount of CO 2 gas was blown to the reaction field, thereby obtaining a precipitate of manganese carbonate represented by the composition formula MnCO 3 .
  • the precipitate was separated from the reaction field and was dried into a powder.
  • the precipitate was treated in the same manner as in Example 6 to give a positive-electrode active material represented by the composition formula Na 0.7 MnO 2 .
  • a reaction field consisting of an aqueous transition metal sulfate salt solution was prepared.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped and simultaneously a prescribed amount of CO 2 gas was blown to the reaction field, thereby obtaining a precipitate of a transition metal composite carbonate salt represented by the composition formula (Fe 0.5 Co 0.5 )CO 3 .
  • the precipitate was separated from the reaction field and was dried into a powder.
  • the precipitate was treated in the same manner as in Example 6 to give a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.5 Co 0.5 )O 2 .
  • a reaction field consisting of an aqueous transition metal sulfate salt solution was prepared.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • a prescribed amount of an aqueous sodium hydroxide solution was dropped and simultaneously a prescribed amount of CO 2 gas was blown to the reaction field, thereby obtaining a precipitate of a transition metal composite carbonate salt represented by the composition formula (Fe 0.5 Ni 0.5 )CO 3 .
  • the precipitate was separated from the reaction field and was dried into a powder.
  • the precipitate was treated in the same manner as in Example 6 to give a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.5 Ni 0.5 )O 2 .
  • a precipitate of a transition metal composite carbonate salt represented by the composition formula (Fe 0.5 Mn 0.5 )CO 3 was obtained in the same manner as in Example 6. The precipitate was separated from the reaction field and was dried into a powder.
  • the transition metal composite carbonate salt was heat treated in air at 900° C. for 12 hours to give a transition metal composite oxide represented by the composition formula (Fe 0.5 Mn 0.5 ) 2 O 3 .
  • the raw material mixture was calcined in air at 900° C. to give a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.5 Mn 0.5 )O 2 .
  • a reaction field consisting of an aqueous transition metal sulfate salt solution was prepared.
  • the liquid temperature of the reaction field was adjusted to 50° C.
  • prescribed amounts of an aqueous sodium hydroxide solution and an aqueous oxalic acid solution were dropped to the reaction field, thereby obtaining a precipitate of a transition metal composite oxalate salt represented by the composition formula (Fe 0.5 Mn 0.5 )(COO) 2 .
  • the precipitate was separated from the reaction field and was dried into a powder.
  • the raw material mixture was calcined in air at 900° C. to give a positive-electrode active material represented by the composition formula Na 0.7 (Fe 0.5 Mn 0.5 )O 2 .
  • the resultant raw material mixture was calcined in air at 900° C. for 12 hours to give a positive-electrode active material represented by the composition formula Na 0.2 (Fe 0.5 Mn 0.5 )O 2 .
  • XRD spectra of the positive-electrode active materials were obtained.
  • the measurement was performed under conditions of a tube current of 40 mA and a tube voltage of 40 kV.
  • the positive-electrode active material weighing 20 g was placed onto a tray and was allowed to stand in a thermostatic chamber in which the dew point was 20° C. After 24 hours and 72 hours, the positive-electrode active material was heated at 200° C. The weight change between before and after the heating was calculated to obtain the moisture absorption rate of the positive-electrode active material.
  • Sample batteries were manufactured using the positive-electrode active materials of Examples 1 to 11 and Comparative Examples 1 to 10, and battery characteristics were evaluated.
  • NMP N-methyl-2-pyrrolidone
  • 90 parts by weight of the positive-electrode composition 5.0 parts by weight of acetylene black and 5.0 parts by weight of PVDF (polyvinylidene fluoride) were dispersed to give a positive-electrode slurry.
  • the positive-electrode slurry was applied to an aluminum foil as a collector, dried, and compression formed with a roll press machine. The positive-electrodes were cut to a prescribed size.
  • NMP N-methyl-2-pyrrolidone
  • hard carbon 95 parts by weight of hard carbon and 5.0 parts by weight of PVDF (polyvinylidene fluoride) were dispersed to give a negative electrode slurry.
  • PVDF polyvinylidene fluoride
  • the negative electrode slurry was applied to an aluminum foil as a collector, dried, and compression formed with a roll press machine. The negative electrodes were cut to a prescribed size.
  • EC ethylene carbonate
  • DEC diethyl carbonate
  • Lead electrodes were attached to the respective collectors of the positive-electrode and the negative electrode, and the electrodes were dried in vacuum at 120° C.
  • a porous polyethylene separator was provided between the positive-electrode and the negative electrode, and the unit was placed into a laminate pack in the form of a bag.
  • the package was dried in vacuum at 60° C. to remove water that had been adsorbed onto the components.
  • the non-aqueous electrolytic solution was poured into the laminate pack, and the pack was sealed. In this manner, laminate-type samples of non-aqueous electrolyte secondary batteries were obtained.
  • a weak current was applied to the sample battery to perform aging, and thereby the positive-electrode and the negative electrode were allowed to sufficiently conform to the electrolyte.
  • the battery was charged by constant current-constant voltage charging with a full charge voltage of 4.4 V and a charging rate of 0.2 C (1 C: a current density at which a fully charged battery is fully discharged in 1 hour) (charging finish conditions: 0.008 C).
  • the capacity obtained was the charge capacity.
  • the battery was discharged at a constant current with a discharging voltage of 1.7 V and a discharging rate of 0.2 C.
  • the capacity obtained was the discharge capacity.
  • the ratio of the discharge capacity to the charge capacity was calculated to determine the charging-discharging efficiency.
  • the constant current-constant voltage charging and the constant current discharging described in the charge and discharge characteristics were repeated fifty times, and changes in discharge capacity were studied.
  • the ratio of the discharge capacity in the n-th discharging to the discharge capacity in the first discharging was obtained as the capacity retention Ps (n) after n cycles.
  • Examples 1 to 11 and Comparative Examples 1 to 10 the production conditions, the properties of the positive-electrode active materials, and the battery characteristics for Examples in which M was manganese are described in Tables 1, 2 and 3, respectively, and the production conditions, the properties of the positive-electrode active materials, and the battery characteristics for Examples in which M was cobalt or nickel are described in Tables 4, 5 and 6, respectively.
  • the XRD spectra of the positive-electrode active materials are shown in FIG. 1
  • the moisture absorption properties of the positive-electrode active materials are shown in FIG. 2
  • the cycle characteristics of the secondary batteries are shown in FIG. 3 .
  • Omitting the heat treatment step in the first embodiment allows a large amount of sulfate ions to remain and hence results in high moisture absorption rate as illustrated in Comparative Example 1.
  • the amount of residual sulfate ions is sufficiently decreased and the moisture absorption rate is lowered by performing the heat treatment as demonstrated in Example 1 or by obtaining the positive-electrode active material in accordance with the second embodiment as illustrated in Example 6.
  • the non-aqueous electrolyte secondary batteries using the positive-electrode active materials of Examples 1 and 6 outperformed the battery based on Comparative Example 1 in terms of cycle characteristics. This result is probably attributed to the difference in crystallinity between Examples 1 and 6 and Comparative Example 1.
  • the non-aqueous electrolyte secondary batteries using the positive-electrode active materials of Examples 1 and 6 achieved higher charging-discharging efficiency than the battery based on Comparative Example 10. This result is probably ascribed to the presence or absence of subphases other than the P6 3 /mmc structure.
  • the positive-electrode active materials of the invention have good charge and discharge characteristics and good cycle characteristics, and are substantially free from residual counter ions.
  • the inventive positive-electrode active materials may be suitably used in non-aqueous sodium secondary batteries.
  • the inventive methods for the producing of positive-electrode active materials can produce positive-electrode active materials which are satisfactory both in charge and discharge characteristics and cycle characteristics and in other properties.

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