JP5698929B2 - Positive electrode active material, positive electrode and non-aqueous secondary battery - Google Patents

Description

  The present invention relates to a positive electrode active material, a positive electrode, and a non-aqueous secondary battery. More specifically, the present invention relates to a positive electrode active material capable of providing a non-aqueous secondary battery having excellent cycle characteristics, and a positive electrode and a non-aqueous secondary battery using the same.

As a non-aqueous secondary battery, a lithium secondary battery has been put into practical use and widely used. Further, in recent years, lithium secondary batteries are attracting attention not only as small-sized batteries for portable electronic devices but also as large-capacity devices for on-vehicle use and power storage. Therefore, demands for safety, cost, life, etc. are higher.
The lithium secondary battery has a positive electrode, a negative electrode, an electrolytic solution, a separator, and an exterior material as main components. The positive electrode includes a positive electrode active material, a conductive material, a current collector, and a binder (binder).

In general, a layered transition metal oxide typified by lithium cobaltate (LiCoO ₂ ) is used as the positive electrode active material. However, the layered transition metal oxide tends to cause oxygen desorption at a relatively low temperature of about 150 ° C. in a fully charged state, and a thermal runaway reaction of the battery can occur due to oxygen desorption. Therefore, when a battery having such a positive electrode active material is used for a portable electronic device, there is a risk that an accident such as heat generation or ignition of the battery may occur.
For this reason, a lithium-containing composite oxide, such as lithium iron phosphate (LiFePO ₄ ), which has an olivine structure that is more stable than LiCoO ₂ and has a stable structure and does not release oxygen when abnormal, is expected. Since lithium iron phosphate does not contain cobalt having a low crustal abundance, there is also an advantage that it is relatively inexpensive. In addition, lithium iron phosphate has an advantage that it is structurally more stable than the layered transition metal oxide.

  However, since lithium iron phosphate has a large electric resistance, there is a problem that a sufficient discharge capacity cannot be obtained. On the other hand, a method for improving the conductivity (reducing electrical resistance) of lithium iron phosphate by incorporating a metal element and a halogen element into lithium iron phosphate has been proposed (for example, Patent Document 1).

WO2005 / 041327

  However, in the method of Patent Document 1, sufficient cycle characteristics have not been obtained. Therefore, it has been desired to provide a positive electrode active material having excellent cycle characteristics without reducing the average potential in charge and discharge.

Thus, according to the present invention, the following general formula (1)
_{Li x M y P 1-z} Si z O 4 ... (1)
(Here, M is a combination of one or both of Fe and Mn and at least one element selected from the group consisting of Co, Ni, Zr, Sn, Al and Y, or Fe and Mn. Combination, 0 ≦ x ≦ 2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 1)
A particulate lithium-containing metal oxide as the main component represented in mainly seen containing a halogen element present on the particle surfaces of the particulate lithium-containing metal oxide,
A positive electrode active material is provided in which the lithium-containing metal oxide has a particle shape, and the halogen atom is mainly present on the surface of the particle .

Moreover, according to this invention, the positive electrode characterized by including the said positive electrode active material, a electrically conductive material, and a binder is provided.
Furthermore, according to the present invention, there is provided a nonaqueous electrolyte secondary battery comprising the positive electrode, the negative electrode, an electrolyte, and a separator.

According to the present invention, the positive electrode active material containing a lithium-containing metal oxide in which lithium iron phosphate contains a metal element and Si and a halogen element can improve conductivity and suppress volume change associated with charge and discharge. Therefore, a positive electrode and a non-aqueous secondary battery with improved cycle characteristics can be provided. Moreover, suppression of the volume change accompanying charging / discharging and suppression of the fall of the average potential of charging / discharging can be made compatible.
Moreover, when the halogen element is contained in the positive electrode active material in an amount of 0.1 to 10 atm%, the cycle characteristics are improved, the volume change associated with charge / discharge is suppressed, and the decrease in the average potential of charge / discharge is suppressed. A positive electrode and a nonaqueous secondary battery can be provided.

Furthermore, when the halogen element is contained in an amount of 5 to 40 mol% with respect to the number of moles of Si in the general formula (1), the cycle characteristics are improved, the volume change associated with charge / discharge is suppressed, and charge / discharge is suppressed. A positive electrode and a non-aqueous secondary battery in which a decrease in average potential is suppressed can be provided.
Further, when z is in the range of 0.001 to 0.05, the cycle characteristics are improved, the volume change associated with charge / discharge is suppressed, and the decrease in the average potential of charge / discharge is suppressed, and the non-aqueous two Next battery can be provided.
Further, when M contains Fe as a main component, a positive electrode and a non-aqueous secondary battery in which cycle characteristics are improved, volume change due to charge / discharge is suppressed, and a decrease in average potential of charge / discharge is suppressed. Can be provided.
Moreover, when M contains Fe as a main component and other elements other than Fe of 1 atm% or more, cycle characteristics are improved, volume change accompanying charge / discharge is suppressed, and the average potential of charge / discharge is lowered. It is possible to provide a positive electrode and a non-aqueous secondary battery in which is suppressed.

It is a schematic sectional drawing of a secondary battery.

  The present invention will be described in detail below.

(I) Positive electrode active material The positive electrode active material contains a lithium-containing metal oxide and a halogen element.
(1) Lithium-containing composite oxide The lithium-containing composite oxide is represented by the following general formula (1).
_{Li x M y P 1-z} Si z O 4 ... (1)
In the formula, M is a combination of one or both of Fe and Mn and at least one element selected from the group consisting of Co, Ni, Zr, Sn, Al, and Y, or a combination of Fe and Mn. It is. By selecting M from these combinations, physical stress (volume shrinkage expansion) of the lithium-containing composite oxide accompanying repeated charge / discharge (Li insertion / desorption) can be prevented, so that the positive electrode active life with a longer lifetime can be prevented. Can provide material.

  Further, the valence of M is not particularly limited. Specifically, Mn has a valence of 2 to 7, Fe has a valence of 2 to 4 and 6, a cobalt and nickel have a valence of 1 to 4, Zr and Ti have a valence of 2 to 4, and Sn has a valence of 2 and 4. Al, Y can be trivalent, and Nb and V can be 2-5. M may be a single valence element or a mixture of a plurality of valence elements. About Sn and Zr, it is preferable to use a tetravalent thing from a viewpoint that there is little change in a valence at the time of manufacture of a lithium containing metal oxide, and charging / discharging. Since Y and Al are only trivalent, if these are used, the change in valence can be reduced during the production of the lithium-containing metal oxide and during charge and discharge. About Fe, it is preferable to use a bivalent thing from a viewpoint of improving the insertion and detachment | desorption property of Li. In addition, when using a mixture, the valence for prescribing y in General formula (1) means an average value for convenience.

Furthermore, it is preferable that M contains Fe and / or Mn as main components. By including Fe and / or Mn, a cheaper raw material can be used for the production of the lithium-containing composite oxide. The main component means 50 atm% or more.
X is in the range of 0 ≦ x ≦ 2. Moreover, x increases / decreases with the kind of elements other than Li which comprise a lithium containing complex oxide, charge, or discharge. Preferably, the range of x is 0.8 ≦ x ≦ 1.2.
Moreover, y is in the range of 0.8 ≦ y ≦ 1.2. If it is this range, the lithium containing complex oxide which has the olivine structure which can be charged / discharged can be provided. Preferably, the range of y is 0.9 ≦ y ≦ 1.1.
Z is in the range of 0 <z ≦ 1. If it is this range, the lithium containing complex oxide which has the olivine structure which can be charged / discharged can be provided. Preferably, the range of z is 0.1 ≦ z ≦ 0.5.

As a specific example of the lithium-containing composite oxide,
_{Li x (Fe, Ni) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, Mn) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, Zr) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, Sn) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, Y) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, Ti) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, Nb) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, V) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Mn, Ni) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Mn, Zr) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Mn, Sn) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Mn, Y) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Mn, Ti) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Mn, Nb) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Mn, V) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
Etc. When M is composed of a plurality of elements, the value of each atomic% can take any value in the range of more than 0 atomic% and less than 100 atomic% with respect to the total amount of M.

From the viewpoint of use as a positive electrode active material, a particularly preferred lithium-containing composite oxide is
_{Li x (Fe, Zr) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, Sn) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, Y) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, Ti) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, Nb) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
_{Li x (Fe, V) y} P 1-z Si z O 4
(0.8 ≦ x ≦ 1.2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 0.5),
It is. In the above specific example, the ratio of Fe and / or Mn constituting M is preferably 0.75 to 0.99 atm% of the total M, and more preferably 0.85 to 0.95 atm%. preferable.

The lithium-containing composite oxide is usually used in the form of particles. The particle size of the primary particles is preferably 1 μm or less, more preferably 10 nm to 1 μm, in order to increase the efficiency of lithium ion insertion / extraction. The lower limit of the primary particle size is about 10 nm, which is realistic in view of the efficiency of insertion / desorption and the manufacturing cost. The particle size of the primary particles can be measured by direct observation using SEM and a particle size distribution measuring device using laser diffraction / scattering method.
The particle size of the secondary particles is preferably 100 μm or less, and more preferably 10 nm to 100 μm, in order to increase the efficiency of lithium ion insertion / extraction. The particle size of the secondary particles can be measured by direct observation with an SEM and a particle size distribution measuring device by a laser diffraction / scattering method.

(2) Halogen element Since the positive electrode active material contains a halogen element, cycle characteristics can be improved without lowering the average potential in charge and discharge.
Here, the inventors presume that the halogen element is mainly present on the particle surface of the lithium-containing metal oxide. For this reason, it is presumed that halogen elements are also contained in the carbon coated on the particle surface. This presumes that the halogen element imparts a catalytic action that improves the crystallinity of the carbon covering the particle surface.

Examples of the halogen element include fluorine, chlorine, bromine and the like. Among these, chlorine is preferable from the viewpoint of easy control of the amount of halogen element to be contained after firing and stability during preparation of the precursor.
The halogen element is preferably contained in the positive electrode active material in an amount of 0.1 to 10 atm%. The proportion of the halogen element in the positive electrode active material can be detected by fluorescent X-rays. In the positive electrode active material of the present invention, the structural change of the lithium-containing metal oxide due to the presence or absence of a halogen element has not been confirmed. The structural change can be observed by structural analysis using an X-ray diffractometer. The upper limit of the proportion of the halogen element is preferably an amount in which the lithium-containing metal oxide is a main component in the positive electrode active material from the viewpoint of suppressing the structural change of the lithium-containing metal oxide. Here, the main component means, for example, 50% by weight or more. The proportion of the halogen element contained in the positive electrode active material is more preferably in the range of 0.1 to 5 atm%.

  Moreover, it is preferable that the halogen element is contained in the positive electrode active material in a proportion of 5 to 40 mol% with respect to the number of moles of Si contained in the general formula (1). The upper limit of the halogen element content is a value at which the lithium-containing metal oxide can be a main component in the positive electrode active material. In the case of 5 mol% or more, the fall of the average potential of charging / discharging can be suppressed. A more desirable ratio is 10 mol% or more. Furthermore, from the viewpoint of suppressing the structural change of the lithium-containing metal oxide, the proportion of the halogen element is more preferably 20 mol% or less.

(3) Method for Producing Positive Electrode Active Material The positive electrode active material is obtained, for example, by a step of dissolving a raw material in a solvent (hereinafter referred to as a dissolution step), a step of gelling the obtained solution (hereinafter referred to as a gelation step), At least a step of firing the obtained gel (hereinafter referred to as a firing step). If necessary, a step of adding conductive carbon particles to the solution (hereinafter referred to as conductive carbon particle addition step), a step of removing the solvent from the gel obtained in the gelation step (hereinafter referred to as drying step). Alternatively, a step of pulverizing the gel before firing (hereinafter referred to as a pulverizing step) and a step of mixing a substance that becomes a carbon source into the gel before firing (hereinafter referred to as a carbon source mixing step) may be provided. In the _{following, Li x (Fe, Zr)} y P 1-z Si z O 4 and will be described as an example, M can process in other elements are the same.

(I) Dissolution Step The lithium source, M source, phosphorus source, and Si source as raw materials are not particularly limited as long as they are substances that can be dissolved in a solvent. These substances are preferably substances that can dissolve 10 mmol or more in 100 g of a solvent. Here, the halogen element may be derived from these raw materials or may be derived from a separately added halide. Of these, it is easy to derive from the raw material.

(Lithium source)
As the lithium source, inorganic salts, hydroxides, organic acid salts, metal alkoxides and hydrates of these salts can be used. Specifically, as the inorganic salt, lithium carbonate (Li ₂ CO ₃ ) which is a salt with a weak acid (hereinafter referred to as a weak acid salt), lithium nitrate (hereinafter referred to as a strong acid salt) with a strong acid. LiNO ₃ ) and lithium chloride (LiCl) can be mentioned. Examples of organic salts include weak acetates such as lithium acetate (LiCH ₃ COO) and lithium oxalate (COOLi) ₂ . Examples of the metal alkoxide include lithium methoxide (LiOCH ₃ ), lithium ethoxide (LiOC ₂ H ₅ ), lithium-n-propoxide (LiO-n—C ₃ H ₇ ), lithium-i-propoxide (LiO). -i-C ₃ H _7), lithium -n- butoxide _{(LiO-n-C 4 H} 9), lithium -t- butoxide _{(LiO-t-C 4 H} 9), lithium -sec- butoxide (LiO-sec -C ₄ H _9), and the like. Hydrate may be sufficient about an inorganic salt and organic salt. Among these, a weak acid salt or a strong acid salt is preferable from the viewpoint of being easy to produce a uniform solution in an air atmosphere and being inexpensive, and among them, lithium acetate or lithium nitrate is preferable. In the present invention, the “homogeneous solution” refers to a state where no precipitate is observed by visual observation and the phase is not separated into two or more phases.

Hereinafter, the case where ethanol is used as a solvent will be described.
When a weak acid salt anhydride is used as the lithium source, it is preferably dissolved after dissolving the M source because of its low solubility in ethanol. When dissolving before adding the M source, it is preferable to dissolve in advance in water. Alternatively, an amount of water required to dissolve the weak acid salt anhydride may be added to ethanol. The amount of water in which the anhydride of the weak acid salt is dissolved is preferably 1 to 100 times the number of moles of Li, more preferably 4 to 20 times.

  Moreover, even when the anhydride of weak acid salt is dissolved in any order in any combination with the M source and the silicon source, a uniform solution can be obtained. After the obtained uniform solution is reacted in advance, the remaining raw materials may be added. It is preferable to react the weak acid salt anhydride with the M source in advance. By reacting the weak acid salt anhydride with the M source in advance, it is possible to suppress the formation of a precipitate when phosphoric acid is added.

  The anhydride of the weak acid salt is preferably reacted in advance with tetramethoxysilane or tetraethoxysilane, and particularly preferably reacted with tetramethoxysilane. As a mixing procedure at this time, it is preferable to dissolve the anhydride of the weak acid salt in water, add ethanol, and then add tetramethoxysilane or tetraethoxysilane. The reaction can be further promoted by heating from 30 ° C. to 60 ° C. after mixing them. The heating time is not particularly limited, but about 30 minutes to 12 hours is appropriate. By reacting the weak acid salt anhydride with the silicon source in advance, generation of impurities after firing and substitution of Fe into Li sites in the lithium composite oxide can be suppressed.

(M source)
As the M source, inorganic salts, hydroxides, organic acid salts, metal alkoxides, and hydrates of these salts can be used. Hereinafter, an iron source and a zirconium source as the M source will be described in detail.
(A) Iron source As an iron source, as an inorganic salt, iron (II) carbonate (Fe (CO ₃ )) which is a weak acid salt, iron nitrate (II) (Fe (NO ₃ ) ₂ ) which is a strong acid salt, nitric acid Mention may be made of iron (III) (Fe (NO ₃ ) ₃ ), iron (II) chloride (FeCl ₂ ) and iron (III) chloride (FeCl ₃ ). Moreover, as an organic salt, iron (II) oxalate (FeC ₂ O ₄ ), iron (III) oxalate (Fe ₂ (C ₂ O ₄ ) ₃ ), iron (II) acetate (Fe) which are weak acid salts. (CH ₃ COO) ₂ ) and iron (III) acetate (Fe (CH ₃ COO) ₃ ). A strong acid salt hydrate is preferable, and iron (III) nitrate nonahydrate is more preferable.

Hereinafter, the case where ethanol is used as a solvent will be described.
Even if the strong acid salt hydrate is dissolved in any order in any combination of a lithium source, a zirconium source and a silicon source, a uniform solution can be obtained. After the obtained uniform solution is reacted in advance, the remaining raw materials may be added. It is preferable to add the strong acid salt hydrate to the solvent prior to phosphoric acid. Since the formation of impurities after calcination can be suppressed by reacting only the strong acid salt hydrate in advance, the strong acid salt hydrate is precipitated after dissolving only the strong acid salt hydrate in ethanol. You may make it react previously by applying heat to such an extent that a thing does not arise.

(B) Zirconium source As the zirconium source, inorganic zirconium salts, organic acid salts, metal alkoxides, and hydrates of these salts can be used. The zirconium source, inorganic salts, zirconium chloride (ZrCl ₄₎ zirconium halide, bromide zirconium (ZrBr _4), zirconium iodide (ZrI _4), an oxy zirconium salts, zirconium oxychloride (ZrOCl ₂₎ And zirconium oxynitrate (ZrO (NO ₃ ) ₂ ). Examples of the metal alkoxide include zirconium methoxide (Zr (OCH ₃ ) ₄ ), zirconium ethoxide (Zr (OC ₂ H ₅ ) ₄ ), zirconium-n-propoxide (Zr (On-C ₃ H _7). ) _4), zirconium -i- propoxide (Zr (O-i-C 3 H 7) 4), zirconium -n- butoxide (Zr (O-n-C 4 H 8) 4), zirconium -t- butoxide (Zr (O-t-C 4 H 8) 4), zirconium -sec- butoxide (Zr (O-t-C 4 H 8) 4) , and the like. Zirconium halides are preferable, and among them, zirconium chloride is preferable.

  Zirconium halide can obtain a uniform solution even if it is dissolved in any order in any combination of a lithium source, an iron source, and a silicon source. Zirconium halide is preferably reacted in advance with an iron source comprising a strong acid salt hydrate. By previously reacting zirconium halide with an iron source composed of a strong acid salt hydrate, it is possible to suppress the formation of impurities such as zirconia and zirconium phosphate after firing. The zirconium halide is preferably reacted in advance with tetramethoxysilane or tetraethoxysilane, particularly preferably with tetramethoxysilane. By reacting the zirconium halide with the silicon source in advance, generation of impurities after firing and substitution of Fe with Li sites in the lithium composite oxide can be suppressed.

(Phosphorus source)
Examples of the phosphorus source substance include phosphoric acid (H ₃ PO ₄ ), ammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), and the like. . Among these, phosphoric acid is preferable.

Hereinafter, a method for dissolving a phosphorus source will be described in the case where ethanol is used as a solvent.
Phosphoric acid needs to be added after at least a lithium source, an iron source and a zirconium source are dissolved. This is because when phosphoric acid is mixed with lithium weak acid anhydride or zirconium halide, a precipitate is formed. When adding phosphoric acid, phosphoric acid may be added in excess. By adding phosphoric acid in excess, generation of impurities after firing and substitution of Fe with Li sites in the lithium composite oxide can be suppressed. When phosphoric acid is added excessively, it can be added excessively in the range of 5 to 20% by weight, more preferably in the range of 5 to 15% by weight with respect to the stoichiometric ratio of phosphoric acid.

(Silicon source)
As the silicon source material, silicon metal alkoxide can be used. Specific examples include tetraethoxysilane (Si (OC ₂ H ₅ ) ₄ ), tetramethoxysilane (Si (OCH ₃ ) ₄ ), methyltriethoxysilane (CH ₃ Si (OC ₂ H ₅ ) ₃ ), methyltri Various types such as methoxysilane (CH ₃ Si (OCH ₃ ) ₃ ), ethyl methoxysilane (C ₂ H ₅ Si (OCH ₃ ) ₃ ), ethyltriethoxysilane (C ₂ H ₅ Si (OC ₂ H ₅ ) ₃ ), etc. The silicon alkoxide can be mentioned. Tetraethoxysilane or tetramethoxysilane is preferable.

Hereinafter, a method for dissolving a silicon source will be described in the case where ethanol is used as a solvent.
Silicon alkoxide can obtain a uniform solution even if it is dissolved in an arbitrary order in any combination of a lithium source, an iron source and a zirconium source. Water may be added to promote the reaction of silicon alkoxide. The amount of water to be added is preferably 1 to 100 times, more preferably 2 to 20 times the number of moles of silicon. By adding water, hydrolysis proceeds and the reaction can be accelerated. Silicon alkoxide can be pre-reacted with phosphoric acid. When using tetraethoxysilane, it is preferable to make it react at 40-80 degreeC, and it is more preferable to make it react at 50-80 degreeC. When using tetramethoxysilane, it is preferable to make it react at 20-60 degreeC. When tetramethoxysilane is reacted with an anhydride of a weak acid salt serving as a lithium source, it is preferable that (number of moles of Li as a lithium source / number of moles of Si as a silicon source) ≧ 2.

  As the solvent, at least one alcohol selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, and n-butanol can be used. Ethanol is preferable. In addition, in order to dissolve the raw material with low solubility in alcohol, a mixed solvent with water may be used as necessary. The amount of the solvent is not particularly limited as long as all raw materials can be dissolved. However, in consideration of the solvent recovery cost, the amount of the solvent is in the range of 1 to 100 times the molar ratio, more preferably in the range of 2 to 15 times the molar ratio with respect to the total moles of all raw materials.

(Dissolution method)
In the dissolving step, a precipitate may be generated depending on the order of dissolving the raw materials, and a uniform solution may not be obtained. Therefore, it is preferable to appropriately adjust the order in which the raw materials are dissolved. For example, when phosphoric acid is mixed with a zirconium source, a precipitate is generated, but when iron ions are present, the zirconium ions are stabilized and the formation of the precipitate is suppressed. Therefore, it is preferable to dissolve the phosphorus source in a solvent in which at least the iron source and the zirconium source are dissolved. The silicon source may be dissolved before the phosphorus source is dissolved, or may be dissolved after the phosphorus source is dissolved.
The order in which the raw materials are dissolved refers to the order of the raw materials to be charged when the raw materials are sequentially added to the solvent. A solution in which a plurality of raw materials are dissolved in a solvent is prepared in advance and the solutions are mixed. In some cases, it refers to the order of mixing.

  The order of preparing the solvent in which the iron source and the zirconium source are dissolved is not particularly limited as long as the zirconium ions can be stabilized by the iron ions. As a method of stabilizing zirconium ions with iron ions, after dissolving a strong acid salt hydrate of iron in a solvent, a method of dissolving a zirconium halide, or after dissolving a zirconium halide in a solvent, And a method of dissolving an iron strong acid salt hydrate and a zirconium halide simultaneously in a solvent. In addition, the order of melt | dissolution of an iron source and a zirconium source is not specifically limited, Either may melt | dissolve first or may melt | dissolve both simultaneously.

In addition, when a salt anhydride such as lithium acetate is used as the lithium source, it does not dissolve unless water is contained in the solvent. Therefore, when an anhydrous salt is used as the lithium source, it is preferable to dissolve the iron salt hydrate and zirconium salt hydrate after dissolving them in a solvent.
When dissolving a raw material in a solvent, you may heat so that it may become room temperature or more. As heating temperature, it is the range of the boiling point of the solvent to be used from room temperature, Preferably it is 30-80 degreeC, More preferably, it is 30-60 degreeC. In the present invention, room temperature means a range of 20 ± 10 ° C.
In the above description of the dissolution step, an example in which iron and zirconium are used as the element M has been described. However, the element M is included in the general formula (1), and all the raw materials can be uniformly dissolved in the solvent. If it is, it will not specifically limit.

(Ii) Conductive carbon particle addition step In this step, the conductive carbon particles may be added and dispersed in the solution prepared in the dissolution step. By dispersing in the solution, the conductive carbon particles are uniformly dispersed in the gel, and the conductivity of the lithium-containing composite oxide is improved.
Use carbon black such as acetylene black, channel black, ketjen black, natural graphite, artificial graphite, needle coke, carbon fiber, vapor grown carbon fiber, carbon nanotube, carbon nan fiber, etc. as conductive carbon particles Can do. Acetylene black and vapor grown carbon fiber are preferred. Two or more of these conductive carbon particles may be used. The addition amount of the conductive carbon particles is 1 to 40% by weight with respect to the predicted product. If it is in this range, sufficient conductivity and capacity can be secured. The average particle size of the conductive carbon particles is 50 nm to 100 μm.

(Iii) Gelation step In this step, the solution obtained in the dissolution step is gelled. Gelation is a group of aggregates in which raw material elements such as Li, Fe, Zr, P and Si are bonded through oxygen atoms, and these aggregates are formed as fine particles having a particle diameter of several to several tens of nm in the gel. The inventors believe that the precipitation is carried out by increasing the viscosity of the solution.
Gelation may be performed in a state where the solution is left standing or in a state where the solution is stirred. Moreover, in order to accelerate | stimulate gelatinization, you may heat to the temperature more than room temperature. The heating temperature is in the range from room temperature to the boiling point of the solvent used, preferably 30 ° C to 80 ° C, more preferably 40 ° C to 60 ° C. The heating time is preferably 10 minutes to 48 hours, more preferably 30 minutes to 24 hours.

(Iv) Drying step In this step, the remaining solvent is removed from the gel. As a method for removing the solvent, a method of standing at room temperature, a method of removing the solvent by heating to 30 to 80 ° C., a gel is placed in a chamber using a rotary pump, etc., and the solvent is removed by reducing the pressure. A method or the like can be used. Alternatively, the solvent may be removed by the same method as described above after performing solvent exchange with a solvent having higher volatility than the solvent used at the time of preparing the solution or a solvent having a different surface tension. Examples of the solvent used for solvent exchange include toluene, benzene, hexane, tetrahydrofuran, isopropanol, and mixed solvents thereof. The solvent can also be removed by immersing the gel obtained in this step in carbon dioxide in a supercritical state and extracting the solvent. These removed solvents are preferably recovered and reused from an industrial viewpoint.

(V) Grinding step The size of the secondary particles may be controlled by mechanically grinding the obtained gel. The pulverization method is not particularly limited, and examples thereof include a method of heating, cooling and controlling the atmosphere as necessary.
Examples of the pulverization method include, but are not limited to, a planetary ball mill, a ball mill, a bead mill, a vibration mill, a pin mill, an atomizer, a homogenizer, a rotor mill, a roller mill, a hammer mill, and a jet mill.
In this step, conductive carbon particles may be further added and pulverized. The effect of suppressing the aggregation of the primary particles and suppressing the coarsening of the secondary particles can be further improved.

(Vi) Carbon source mixing step Sugars, fats and synthetic resin materials may be mixed with the crushed gel. These compounds are carbonized during firing, thereby forming a carbon coating on the surface of the positive electrode material, and improving the conductivity of the positive electrode material. As the saccharide, sucrose, fructose and the like can be used. As the synthetic resin material, polyethers such as polyethylene glycol and polypropylene glycol, polyvinyl alcohol, polyacrylamide, carboxymethyl cellulose, polyvinyl acetate, and the like can be used as the polyether.

(Vii) Firing step In this step, the obtained gel is fired to obtain a lithium-containing composite oxide. Firing is preferably performed in a temperature range of 400 to 700 ° C., more preferably 400 to 600 ° C., for example, for 1 to 24 hours. As an atmosphere during firing, an inert atmosphere (an atmosphere such as argon, nitrogen, or vacuum) or a reducing atmosphere (an atmosphere such as a hydrogen-containing inert gas or carbon monoxide) can be used. In order to perform uniform firing, the gel may be stirred, and if a toxic gas such as NO _x , SO _x , or chlorine is generated during firing, a removal device may be provided.

(Vii) Other steps The obtained lithium-containing composite oxide may be adjusted to a desired particle size by subjecting it to a pulverization step and / or a classification step, if necessary.
The content of the halogen element can be changed by changing the material used for producing the precursor. By synthesizing compounds such as fluoride, chloride and bromide as raw materials, the halogen content can be increased. In addition, the content can be adjusted by controlling the atmosphere during firing to an atmosphere containing a halogen gas.

(3) Use The obtained lithium containing complex oxide can be used for the positive electrode active material of a nonaqueous electrolyte secondary battery. The positive electrode active material, in addition to the above-mentioned lithium-containing complex _{oxide, LiCoO 2, LiNiO 2, LiFeO} 2, LiMnO 2, LiMn 2 O 4, Li 2 MnO 3, LiCoPO 4, LiNiPO 4, LiMnPO 4, LiFePO 4 , etc. Other oxides may be included.

(II) Positive electrode The positive electrode active material can be used for a positive electrode for a non-aqueous secondary battery. The positive electrode includes the positive electrode active material, a conductive material, and a binder. Hereinafter, each constituent material will be described.
The positive electrode can be produced using a known method. For example, a positive electrode active material, a conductive material, and a binder can be kneaded and dispersed using an organic solvent to obtain a paste, and the paste can be applied to a current collector. Note that when the obtained positive electrode active material has sufficiently high conductivity, the conductive material is not necessarily added.
As binders, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, ethylene propylene diene polymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, styrene Butadiene rubber or the like can be used. If necessary, a thickener such as carboxymethylcellulose can also be used.

As the conductive material, acetylene black, natural graphite, artificial graphite, needle coke, or the like can be used.
As the current collector, foamed (porous) metal having continuous pores, metal formed in a honeycomb shape, sintered metal, expanded metal, non-woven fabric, plate, perforated plate, foil, and the like can be used.
As the organic solvent, N-methyl-2-pyrrolidone, toluene, cyclohexane, dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, methyl acrylate, diethyltriamine, N, N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc. are used. be able to. When a water-soluble binder is used, water can be used as a solvent.
The thickness of the positive electrode is preferably about 0.01 to 20 mm. If it is too thick, the conductivity is lowered, and if it is too thin, the capacity per unit area is lowered. The positive electrode obtained by coating and drying may be compacted by a roller press or the like in order to increase the packing density of the active material.

(III) Non-aqueous secondary battery The non-aqueous secondary battery includes the positive electrode, the negative electrode, the electrolyte, and the separator.
(I) Negative electrode The negative electrode can be produced by a known method. For example, a negative electrode active material, a binder, and a conductive material are mixed, and the obtained mixed powder is formed into a sheet shape. The obtained molded body can be produced by pressure bonding to a current collector, for example, a stainless steel or copper mesh current collector. Moreover, it can produce using the method using the paste which was demonstrated in the column of a positive electrode. In that case, a negative electrode active material, a conductive material, and a binder can be kneaded and dispersed using an organic solvent to obtain a paste, and the paste can be applied to a current collector.

A known material can be used as the negative electrode active material. In order to constitute a high energy density battery, it is preferable that the potential at which lithium is inserted / desorbed is close to the deposition / dissolution potential of metallic lithium. A typical example is a carbon material such as natural or artificial graphite in the form of particles (scale-like, lump-like, fibrous, whisker-like, spherical, pulverized particles, etc.).
Examples of the artificial graphite include graphite obtained by graphitizing mesocarbon microbeads, mesophase pitch powder, isotropic pitch powder, and the like. Also, graphite particles having amorphous carbon attached to the surface can be used. Among these, natural graphite is preferable because it is inexpensive and close to the redox potential of lithium and can constitute a high energy density battery.
Further, lithium transition metal oxide, lithium transition metal nitride, transition metal oxide, silicon oxide, and the like can be used as the negative electrode active material. Among these, Li ₄ Ti ₅ O ₁₂ is preferable because of high potential flatness and small volume change due to charge and discharge.

(Ii) Electrolyte As the electrolyte, for example, an organic electrolyte, a gel electrolyte, a polymer solid electrolyte, an inorganic solid electrolyte, a molten salt, or the like can be used.
Examples of the organic solvent constituting the organic electrolyte include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), and butylene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate, and dipropyl carbonate. Chain carbonates such as γ-butyrolactone (GBL), lactones such as γ-valerolactone, furans such as tetrahydrofuran and 2-methyltetrahydrofuran, diethyl ether, 1,2-dimethoxyethane, 1,2-diethoxy Examples include ethers such as ethane, ethoxymethoxyethane, dioxane, dimethyl sulfoxide, sulfolane, methyl sulfolane, acetonitrile, methyl formate, methyl acetate, and the like. Can.
Moreover, since cyclic carbonates such as PC, EC and butylene carbonate are high-boiling solvents, they are suitable as solvents to be mixed with GBL.

Examples of the electrolyte salt constituting the organic electrolyte include lithium borofluoride (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium trifluoroacetate (LiCF ₃ COO) ), Lithium salts such as lithium bis (trifluoromethanesulfone) imide (LiN (CF ₃ SO ₂ ) ₂ ), and one or more of these can be used in combination. The salt concentration of the electrolytic solution is preferably 0.5 to 3 mol / L.

(Iii) Separator As the separator, a known material such as a porous material or a nonwoven fabric can be used. As a material for the separator, a material that does not dissolve or swell in the organic solvent in the electrolytic solution is preferable. Specific examples include polyester polymers, polyolefin polymers (for example, polyethylene and polypropylene), ether polymers, and glass fibers.

(Iv) Other members Various other known materials can be used for other members such as a battery container, and there is no particular limitation.

(V) Manufacturing method of secondary battery The secondary battery includes a laminate including, for example, a positive electrode, a negative electrode, and a separator sandwiched between them. The laminate may have, for example, a strip-like planar shape. In the case of producing a cylindrical or flat battery, the laminate may be wound to form a wound body.
One or more of the laminates are inserted into the battery container. Usually, the positive electrode and the negative electrode are connected to the external conductive terminal of the battery. Thereafter, the battery container is sealed to block the positive electrode, the negative electrode, and the separator from the outside air.

  In the case of a cylindrical battery, the sealing method is generally a method in which a lid having a resin packing is fitted into the opening of the battery container and the battery container and the lid are caulked. In the case of a square battery, a method of attaching a lid called a metallic sealing plate to the opening and performing welding can be used. In addition to these methods, a method of sealing with a binder and a method of fixing with a bolt via a gasket can also be used. Furthermore, a method of sealing with a laminate film in which a thermoplastic resin is attached to a metal foil can also be used. An opening for electrolyte injection may be provided at the time of sealing. When using an organic electrolyte, the organic electrolyte is injected from the opening, and then the opening is sealed. Gas generated by energization before sealing may be removed.

EXAMPLES Hereinafter, although this invention is demonstrated further in detail using an Example, this invention is not limited to a following example.
Example 1
<I. Dissolution process>
LiCH ₃ COO which is a lithium source is 0.9899 g, and Li: Fe: Zr: P: Si = 1: 0.85: 0.15: 0.7: 0.3 (molar ratio). Raw materials (iron source, zirconium source, phosphorus source, silicon source) were weighed. Next, it was dissolved in a solvent in the order of iron source, lithium source, zirconium source, silicon source, and phosphorus source as follows.

The mixture was stirred until Fe (NO ₃ ) ₃ .9H ₂ O as an iron source was completely dissolved in 30 times the molar amount of ethanol with respect to the molar amount of Li. After confirming complete dissolution, LiCH ₃ COO as the lithium source, ZrCl ₄ as the zirconium source, and Si (OC ₂ H ₅ ) ₄ as the silicon source were sequentially dissolved to prepare a uniform solution. Finally, H ₃ PO ₄ (85% by weight) as a phosphorus source was stirred until a homogeneous solution was obtained. The resulting uniform solution was stirred for 2 hours at room temperature with a stirrer.

<Ii. Gelation process>
The uniform solution stirred at room temperature for 1 hour was stored in a thermostatic bath at 60 ° C. for 24 hours to perform gelation. At the time of gelation, the container was covered to suppress evaporation of the solvent.
<Iii. Drying process>
After the gelation step, the container was opened and left in a constant temperature bath at 60 ° C. for 12 hours to volatilize the solvent contained in the gel to obtain a precursor.
<Iv. Grinding process>
The obtained precursor was pulverized in a mortar.

<V. Carbon source mixing process>
A carbon source dissolved in water was added to the ground precursor. Sucrose was used as the carbon source. The amount added was 15% by weight based on the weight of the precursor. The precursor to which sucrose was added was dried and pulverized in a mortar.
<Vi. Firing step>
The precursor obtained by the pulverization step was calcined at 550 ° C. for 12 hours. Firing was performed by first evacuating the furnace, then flowing nitrogen, and heating at a heating rate of 200 ° C./h. The cooling rate was furnace cooling. The obtained sample is designated as A1.

Example 2
Sample A2 was prepared in the same procedure as in Example 1 except that the raw material ratio was Li: Fe: Zr: P: Si = 1: 0.875: 0.125: 0.75: 0.25 (molar ratio). Obtained.
Example 3
Sample A3 was prepared in the same procedure as in Example 1 except that the raw material ratio was Li: Fe: Zr: P: Si = 1: 0.9: 0.1: 0.8: 0.2 (molar ratio). Obtained.
Example 4
Sample A4 was prepared in the same procedure as in Example 1 except that the raw material ratio was Li: Fe: Zr: P: Si = 1: 0.925: 0.075: 0.85: 0.15 (molar ratio). Obtained.
Example 5
Sample A5 was prepared in the same procedure as in Example 1 except that the raw material ratio was Li: Fe: Zr: P: Si = 1: 0.95: 0.05: 0.9: 0.1 (molar ratio). Obtained.

Comparative Example 1
Sample B1 was obtained in the same procedure as in Example 1 except that the raw material ratio was Li: Fe: P = 1: 1: 1 (molar ratio).

(result)
(1) Composition analysis The obtained composite oxide was subjected to composition analysis using a wavelength dispersion type fluorescent X-ray apparatus (XRF-1800 manufactured by Shimadzu Corporation). The FP method was used to obtain quantitative values for composition analysis. The results obtained for A1 to A5 and B1 are shown in Table 1.

(2) Measurement of volume change rate A battery was prepared for each sample by the following method.
About 1 g of each of A1 to A5 and B1 was weighed and ground in an agate mortar. About 10% by weight of acetylene black (trade name: “DENKA BLACK”, manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive material, and about 10% by weight of polyvinylidene fluoride resin as a binder, based on the positive electrode active material Powder (Kureha KF polymer) was mixed. The mixture was dispersed in N-methyl-2-pyrrolidone as a solvent to obtain a slurry. The slurry was applied to both surfaces of an aluminum foil having a thickness of 20 μm by a doctor blade method. The coating amount was about 5 mg / cm ² . After the coating film was dried, pressing was performed to obtain a positive electrode.

LiPF ₆ was dissolved in a mixed solution of ethylene carbonate and diethyl carbonate having a volume ratio of 1: 2 so as to be 1 mol / l to obtain an electrolyte. About 30 ml of electrolyte was injected into a 50 ml beaker. A battery (beaker cell) was fabricated by immersing the 2 cm × 2 cm positive electrode, metallic lithium as a reference electrode, and metallic lithium as a counter electrode in a beaker.
The battery was charged for the first time in an environment of 25 ° C. The charging current was 0.1 mA, and the charging was terminated when the battery potential reached 4V. After charging was completed, discharging was performed at 0.1 mA, and discharging was terminated when the battery potential reached 2.0V.

The battery was further charged to 4 V with a current of 0.1 mA to make Li detached, and then the positive electrode was taken out. The positive electrode was subjected to powder X-ray diffraction measurement to determine the lattice constant after lithium desorption.
The volume change rate was obtained from the lattice constant before charging and the lattice constant after charging. The volume change rate was obtained by the following formula.
Volume change rate (%) = (V ₀ −V ₁ ) / V ₀ × 100
(In the formula, V ₀ is the volume when Li is present, and V ₁ is the volume when Li is removed). Table 1 shows the results obtained in A1 to A5 and B1.

(3) Evaluation of capacity retention)
<Production of battery>
Natural graphite powder was used as the negative electrode active material. About 10% by weight of polyvinylidene fluoride resin powder was mixed with this negative electrode active material as a binder. This mixture was dissolved in N-methyl-2-pyrrolidone to obtain a slurry. The slurry was applied to both sides of a 20 μm thick copper foil and dried, and then the coating film was pressed to prepare a negative electrode.
The positive electrode produced in each Example and the comparative example and the above-mentioned negative electrode were each cut out to a size of 30 mm × 30 mm. As the current introduction terminal of the battery, an aluminum tab having a width of 3 mm and a length of 50 mm was welded to the positive electrode, and a copper tab having a width of 3 mm and a length of 50 mm was welded to the negative electrode.

A laminated body was obtained by sandwiching a separator made of porous polyethylene (2500 manufactured by Polypore) between a tabbed positive electrode and a negative electrode. The laminate is sandwiched between two laminated foils made of a thermoplastic resin and a periphery of the laminated film is sealed by thermal welding, leaving an opening for injecting the electrolyte. The exterior was obtained.
LiPF ₆ was dissolved in a mixed solution of ethylene carbonate and diethyl carbonate having a volume ratio of 1: 1 so as to be 1 mol / l to obtain an electrolyte. About 30 ml of electrolyte was injected into the battery from the opening. Thereafter, the opening was sealed to obtain a secondary battery.
A schematic cross-sectional view of the obtained secondary battery is shown in FIG. In FIG. 1, 1 is a positive electrode, 2 is a negative electrode, 3 is a separator, 4 is a positive electrode and a negative electrode tab, and 5 is a laminate.

<Evaluation of capacity retention>
The secondary battery was charged for the first time in an environment of 25 ° C. The charging current was 0.2 mA, and the charging was terminated when the battery potential reached 4V. After charging was completed, discharging was performed at 0.2 mA, and when the battery potential reached 2.0 V, discharging was terminated, and the initial capacity of the battery was obtained. Furthermore, charging / discharging was repeated at a current of 0.2 mA, the discharge capacity at the 300th time was measured, and the capacity retention rate was obtained by the following formula.
Capacity retention ratio = 300th discharge capacity / initial discharge capacity Table 2 shows the capacity retention ratio and the average discharge potential during charging and discharging.

From Tables 1 and 2, the following can be understood.
From Samples A1 to A5 and B1, it can be seen that the capacity retention rate (cycle characteristics) can be remarkably improved when the positive electrode active material contains chlorine.
From Samples A1 to A5, it can be seen that as the proportion of Zr increases, the average discharge potential tends to decrease and the capacity retention tends to increase.
From Samples A1 to A5, it can be seen that as the proportion of P increases, the average discharge potential tends to increase and the capacity retention tends to decrease.

Claims (8)

The following general formula (1)
_{Li x M y P 1-z} Si z O 4 ... (1)
(Here, M is a combination of one or both of Fe and Mn and at least one element selected from the group consisting of Co, Ni, Zr, Sn, Al and Y, or Fe and Mn. Combination, 0 ≦ x ≦ 2, 0.8 ≦ y ≦ 1.2, 0 <z ≦ 1)
Includes a particulate lithium-containing metal oxide as the main component, and c androgenic element represented in,
The positive electrode active material before Symbol halogen element, characterized in that predominantly present on the surface of the particles of the lithium-containing metal oxide.   The positive electrode active material according to claim 1, wherein the halogen element is contained in an amount of 0.1 to 10 atm% in the positive electrode active material.   3. The positive electrode active material according to claim 1, wherein the halogen element is contained in an amount of 5 to 40 mol% with respect to the number of moles of Si contained in the general formula (1).   The positive electrode active material according to claim 1, wherein z is in a range of 0.001 to 0.05.   The positive electrode active material according to claim 1, wherein M contains Fe as a main component.   The positive electrode active material according to claim 1, wherein the M includes Fe as a main component and other elements other than Fe of 1 atm% or more.   A positive electrode comprising the positive electrode active material according to claim 1, a conductive material, and a binder.   A nonaqueous secondary battery comprising the positive electrode according to claim 7, a negative electrode, an electrolyte, and a separator.

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