JP2013047162A - Methods for production of silicic acid compound, positive electrode for secondary battery and secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for inexpensively producing a silicic acid compound which, when used as e.g., a positive electrode active substance of a secondary battery, can enhance its average discharge voltage and its discharge capacity per unit mass.SOLUTION: The silicic acid compound having a composition represented by AMSGeXO(wherein f is a number which depends on a, b, c, d, e, the valence of M, and the valance of X and satisfies electrical neutrality) is produced by heating a pulverization product of a solid obtained by cooling a melt having a composition represented by AMSiGeXO(wherein A is Li, Na, or K; M is Fe, Mn, Co, or Ni; X is P, Ta, Nb, or V; 0.8<a≤2.7; 0.6≤b≤1.4; 0.8≤c≤1.1; 0.01≤d≤0.8; 0≤e≤0.1; and f1 is a number which depends on a, b, c, d, e, the valence of M, and the valance of X and satisfies electrical neutrality).

Description

  The present invention relates to a silicate compound, a positive electrode for a secondary battery, and a method for producing a secondary battery.

  In recent years, plug-in hybrid vehicles and electric vehicles have been developed and spread from the viewpoint of global carbon dioxide emission regulations and energy saving. In order to realize these, it is an issue to increase the capacity, increase the energy, and increase the size while maintaining the safety of the secondary battery used. As a positive electrode material for the next-generation lithium ion secondary battery, an orthosilicate compound has attracted attention because of its advantages in terms of resources, safety, cost, and stability.

In Patent Document 1, as a candidate material for a positive electrode for a secondary battery, k Li is included in a unit formula, and [SiO ₄ ], [SO ₄ ], [PO ₄ ], [GeO ₄ ], [VO] ₄ ], [AlO ₄ ], [BO ₄ ] and the like have been proposed compounds having an orthosilicate structure having a wide general composition.
Patent Document _{2, Li a M b Si c} O 4 (M is Mn, Fe, at least one transition metal selected from the group consisting of Co and Ni, a is 1 <a ≦ 3, b is 0.5 ≦ a lithium transition metal silicate represented by b ≦ 1.5, c is 0.5 ≦ c ≦ 1.5), a metal compound containing M, a lithium compound, and a silicon-based polymer compound such as polysilane or polysilazane. There has been proposed a method for producing a mixture containing the mixture by firing.

JP 2001-266882 A International Publication No. 2008/123311

Among the electrode materials disclosed in Patent Document 1, it is possible to confirm that the polyanion contains [SiO ₄ ] and a part thereof is substituted with another element as Li _1.7 Mn _0.7 Fe _0.3 Si _0.7 P _0.3 O. Only ₄ . The compound is produced by a solid phase reaction in which Li ₂ MnSiO ₄ and LiFePO ₄ produced individually are mixed and pulverized, sealed in a tube, and heated. The manufacturing process is complicated, the manufacturing cost is high, mass production is difficult, and control of composition and particle size is not easy.
Patent Document 2 shows that the discharge voltage is increased by changing the atom M to Co, but Li ₂ CoSiO ₄ is not obtained as a single phase, and the discharge capacity is low.

  The present invention is a silicate compound capable of increasing the average discharge voltage and the capacity per unit mass (discharge capacity) when used as a positive electrode material for a secondary battery, and the composition and particle size of the silicate compound. It is an object of the present invention to provide a production method capable of controlling the temperature well and inexpensively. An object of this invention is to provide the positive electrode for secondary batteries which is excellent in a battery characteristic and reliability, and the manufacturing method of a secondary battery.

The present invention is the following [1] to [14].
[1] A melting step for obtaining a melt having a composition represented by the following formula (1):
A cooling step of cooling the melt to obtain a solidified product,
Crushing the solidified product to obtain a pulverized product, and heating the pulverized product to obtain a silicate compound having a composition represented by the following formula (2) in this order: A method for producing a silicic acid compound.
_{A a M b Si c- (d} + e) Ge d X e O f1 (1)
(In the formula (1), A is at least one atom selected from the group consisting of Li, Na and K, M is at least one atom selected from the group consisting of Fe, Mn, Co and Ni, and X is P , Ta, Nb and V, at least one atom selected from the group consisting of a, 0.8 <a ≦ 2.7, b is 0.6 ≦ b ≦ 1.4, and c is 0.8 ≦ c ≦ 1.1, d is 0.01 ≦ d ≦ 0.8, e is 0 ≦ e ≦ 0.1, f1 is the numerical value of a, b, c, d and e, and the valence of M And a number that depends on the valence of X and satisfies the electrical neutrality.)
_{A a M b Si c- (d} + e) Ge d X e O f (2)
(In the formula (2), A, M and X represent the same kind of atoms as described above, a, b, c, d and e represent the same numerical values as described above, and f represents a, b, c, d and e. And a number that depends on the valence of M and the valence of X and satisfies the electrical neutrality.)
[2] The method for producing a silicate compound according to [1], wherein the melting step is a step of obtaining a melt by heating the raw material formulation.
[3] In the raw material formulation,
Atom A is A carbonate, A bicarbonate, A hydroxide, A silicate, A phosphate, A borate, A fluoride, A chloride, A At least one selected from the group consisting of nitrates of A, sulfates of A, and organic acid salts of A (however, some or all of the at least one may each form a hydrate salt). Included as
Atom M is M oxide, M oxyhydroxide, M silicate, M phosphate, M borate, metal M, M fluoride, M chloride, M nitrate And at least one selected from the group consisting of M sulfate, M alkoxide, and M organic acid salt,
Si is included as at least one selected from the group consisting of silicon oxide, A or M silicate, and Si alkoxide,
Ge is included as at least one selected from the group consisting of germanium oxide, A or M germanate, and Ge alkoxide;
Atom X is X oxide, X hydroxide, X silicate, phosphoric acid, phosphate, A or M vanadate, A or M niobate, A or M tantalate The method for producing a silicic acid compound according to [2], which is contained as at least one selected from the group consisting of a salt, an alkoxide of X, and an organic acid salt of X.
[4] The method for producing a silicate compound according to [1] to [3], wherein a cooling rate in the cooling step is −10 ³ to −10 ¹⁰ ° C./second.
[5] The method for producing a silicate compound according to [1] to [4], wherein an average particle diameter of the pulverized product obtained in the pulverization step is 10 nm to 10 μm in terms of a median diameter in terms of volume.
[6] In the pulverization step, the solidified product includes at least one carbon source selected from the group consisting of an organic compound and a carbon-based conductive active material, and a carbon equivalent amount (mass) in the carbon source is The method for producing a silicic acid compound of [1] to [5], which is 0.1 to 20% by mass relative to the total mass of the mass of the solidified product and the carbon equivalent (mass) in the carbon source. .
[7] The method for producing a silicate compound according to [1] to [6], wherein the heating step is performed in an inert gas or a reducing gas at 500 to 1,000 ° C.
[8] The method for producing a silicate compound according to [1] to [7], wherein a crystallite diameter of the silicate compound obtained in the heating step is 10 to 50 nm.
[9] The method for producing a silicate compound according to any one of [1] to [8], wherein the silicate compound obtained in the heating step is an olivine type crystal particle.
[10] The method for producing a silicate compound of [1] to [9], wherein the silicate compound obtained in the heating step has a composition represented by the following formula (3).
_{Li 2 Fe 1-g Mn g} Si c- (d + e) Ge d X e O f (3)
(In formula (3), X represents the same kind of atom as described above, c, d, e and f represent the same numerical values as described above, and g is 0 ≦ e ≦ 1.)

[11] A silicate compound is obtained by the production method of [1] to [10], and then a positive electrode for a secondary battery is produced using the silicate compound as a positive electrode material for a secondary battery. A method for producing a positive electrode for a secondary battery.
[12] A method for producing a secondary battery, comprising: obtaining a positive electrode for a secondary battery by the production method according to [11], and then producing a secondary battery using the positive electrode for a secondary battery.

[13] A silicic acid compound having a composition represented by the following formula (2).
_{A a M b Si c- (d} + e) Ge d X e O f (2)
(In Formula (2), A is at least one atom selected from the group consisting of Li, Na and K, M is at least one atom selected from the group consisting of Fe, Mn, Co and Ni, and X is P , Ta, Nb and V, at least one atom selected from the group consisting of a, 0.8 <a ≦ 2.7, b is 0.6 ≦ b ≦ 1.4, and c is 0.8 ≦ c ≦ 1.1, d is 0.01 ≦ d ≦ 0.8, e is 0 ≦ e ≦ 0.1, f is a numerical value of a, b, c, d and e, and valence of M And a number that depends on the valence of X and satisfies the electrical neutrality.)
[14] The silicate compound of [13] is a particle containing a conductive material derived from a carbon source, and the conductive material is 0.1 to 0.1% of the total mass of the silicate compound and the conductive material. Particles containing 20% by mass.

  According to the production method of the present invention, when used as a positive electrode material for a secondary battery or the like, a silicate compound capable of increasing the average discharge voltage and the discharge capacity can be produced. Furthermore, the composition and particle size of the silicate compound can be controlled well. Therefore, a silicic acid compound useful as a positive electrode material can be provided inexpensively and efficiently. Moreover, the positive electrode for secondary batteries and secondary battery which are excellent in a battery characteristic and reliability can be manufactured by using the silicic acid compound of this invention. Furthermore, according to the present invention, a novel silicic acid compound is provided.

3 is a diagram showing an X-ray diffraction pattern of a positive electrode material for a secondary battery manufactured in Example 1. FIG. 4 is a diagram showing an X-ray diffraction pattern of a positive electrode material for a secondary battery manufactured in Example 2. FIG. 4 is a diagram showing an X-ray diffraction pattern of a positive electrode material for a secondary battery manufactured in Example 3. FIG. 6 is a diagram showing an X-ray diffraction pattern of a positive electrode material for a secondary battery manufactured in Example 4. FIG. 6 is a diagram showing an X-ray diffraction pattern of a secondary battery positive electrode material manufactured in Example 21. FIG. 6 is a diagram showing an X-ray diffraction pattern of a positive electrode material for a secondary battery produced in Example 22. FIG. It is a figure which shows the relationship between the Ge substitution amount of Si in a silicate compound, and the crystal lattice constant of a axis | shaft.

In the following description, A represents at least one atom selected from the group consisting of Li, Na and K. A represents an atom of the above three alkali metal elements. A may consist of a combination of two or more atoms. M represents at least one atom selected from the group consisting of Fe, Mn, Co and Ni. M represents an atom of the above four transition metal elements. M may consist of a combination of two or more atoms. In addition, chemical formulas, such as Formula (1), Formula (2), Formula (3), represent an average composition.
Further, an olivine type crystal is hereinafter referred to as an olivine type crystal, and a particle containing the olivine type crystal is hereinafter also referred to as an olivine type crystal particle. The olivine-type crystal particle may partially include a crystal structure other than the olivine-type crystal structure, or may partially include an amorphous structure. It is preferable that substantially all of the olivine type crystal particles are made of olivine type crystals.

[Method for producing silicic acid compound]
In the method for producing a silicate compound of the present invention, the following melting step (I) to heating step (IV) are performed in this order. Other steps may be performed before, between and after the melting step (I) to the heating step (IV) as long as each step is not affected.

Melting step (I): Formula _{(1): A a M b} Si c- (d + e) Ge d X e O f1 ( symbols in the formula, A, M and X have the same kind of atoms and the , A, b, c, d, e and f1 have the same numerical values as described above), and a step of obtaining a melt (hereinafter also referred to as melt (1)).
Cooling step (II): a step of cooling the melt to obtain a solidified product,
Pulverization step (III): a step of pulverizing the solidified product to obtain a pulverized product;
Heating step (IV): by heating the ground product, the formula _{(2): A a M b} Si c- (d + e) Ge d X e O f ( symbols in the formula, A, M and X are A silicate compound having the composition represented by the same kind of atom as described above, wherein a, b, c, d, e, and f have the same numerical values as described above (hereinafter also referred to as silicate compound (2)). Step to obtain.

At least part of the surface of the silicate compound (2) may be covered with a conductive material. The conductive material is preferably conductive carbon. Conductive carbon is formed from a carbon source. The carbon source is at least one selected from the group consisting of an organic compound and a carbon-based conductive active material. The organic compound is thermally decomposed by heating, and at least a part of the organic compound becomes a carbide, and the carbide becomes conductive carbon. The thermal decomposition of the organic compound is preferably performed at 400 ° C. or lower, and the carbonization is preferably performed at 700 ° C. or lower. When heating is performed at 700 ° C. or lower, in addition to carbonization of the organic compound, volume change associated with the thermal decomposition reaction can be reduced, so that the conductive carbon made of the carbide is the particle surface of the silicate compound (2) or the compound particles. It can be bonded uniformly and firmly to the interface between them and functions as a conductive material. The carbon-based conductive active material functions as conductive carbon.
The carbon source is preferably included in at least one of the pulverization step (III) and the heating step (IV). When a carbon source is included in the pulverization step (III), it is preferable to obtain a pulverized product including a solidified product and a carbon source. When the carbon source is included in the heating step (IV), the pulverized product is heated to obtain a compound, then the pulverized product containing the compound and the carbon source is obtained, and then the pulverized product is inactivated. Heating in a gas or a reducing gas is preferred.
The carbon source may be included in the silicic acid compound (2) obtained in the heating step (IV) and heated. The heating temperature is preferably 500 to 1,000 ° C.
Hereinafter, each step will be specifically described.

(Melting step (I))
The melting step (I) is a step of obtaining a melt having a composition represented by the above formula (1). In the melting step (I), a raw material formulation is prepared in which raw materials containing each atomic source (atom A, atom M, Si, Ge, and atom X) are adjusted to have the composition represented by formula (1). It is preferable to do this.

  In the formula (1), a is 0.8 <a ≦ 2.7, b is 0.6 ≦ b ≦ 1.4, c is 0.8 ≦ c ≦ 1.1, and d is 0.01 ≦ d ≦. When 0.8 and e are 0 ≦ e ≦ 0.1, the raw material preparation can be melted well, and a uniform melt can be obtained. Further, a silicate compound can be obtained in the subsequent heating step (IV), and further, a silicate compound containing an olivine type crystal structure, particularly a silicate compound consisting only of an olivine type crystal structure, is preferred.

  The atom A is at least one atom selected from the group consisting of Li, Na and K. In particular, when the silicic acid compound produced by the production method of the present invention is used as a positive electrode material for a secondary battery, it is preferable to make Li essential, and it is particularly preferable to use only Li. The silicate compound containing Li can increase the capacity per unit mass of the secondary battery.

  The atom M is at least one atom selected from the group consisting of Fe, Mn, Co, and Ni. The atom M is preferably composed of only one kind or two kinds. In particular, when the silicate compound produced by the production method of the present invention is used for a positive electrode material for a secondary battery, the atom M is composed only of Fe, Mn, or Fe and Mn. This is preferable. The valence of the atom M is a numerical value that can be changed in each step of the production method of the present invention, and is preferably in the range of +2 to +4. The valence of atom M is +2, +8/3, or +3 when atom M is Fe, +2, +3, or +4 when M is M, +2, +8/3, or +3 when Co is Ni, Is preferably +2 or +4. The valence of the atom M in the formula (1) (hereinafter, the valence of the atom M in the formula (1) is represented by N1 ′) is preferably 2 to 2.5, and preferably 2 to 2.2. Is particularly preferred. On the other hand, the valence of the atom M in the formula (2) (hereinafter, the valence of the atom M in the formula (2) is represented by N1) is preferably 2 to 2.2 and particularly preferably 2. .

  a, b and c are more preferably 1.7 ≦ a ≦ 2.3, 0.8 ≦ b ≦ 1.2 and 0.9 ≦ c ≦ 1.1, 1.8 ≦ a ≦ 2.2, 0.9 ≦ b ≦ 1.1 and 0.95 ≦ c ≦ 1.05 are particularly preferred. When a, b, and c are within the above ranges, a silicate compound exhibiting a multi-electron type reaction (a reaction that extracts more than 1 mol of atoms A per unit number of moles) is obtained, and this silicate compound is used for a secondary battery. When used as a positive electrode material, the theoretical electric capacity of the secondary battery can be increased.

  Ge is contained by substituting a part of Si. Since Ge has a larger ionic radius than Si, it has the effect of expanding the crystal lattice constant of silicate compound crystals, particularly olivine crystals. Therefore, when a silicate compound is used as a positive electrode material for a secondary battery, ions of atoms A such as Li ions are easily diffused. The average discharge voltage of the secondary battery using the silicate compound as the positive electrode material for the secondary battery can be increased.

  When d is 0.01 ≦ c ≦ 0.8, the crystal lattice constant can be expanded by the above-described Ge while stably maintaining the crystal structure of the silicate compound. d is more preferably 0.01 ≦ d ≦ 0.5, and particularly preferably 0.05 ≦ d ≦ 0.2. Within the above range, the stabilization of the crystal structure and the expansion of the crystal lattice constant can be achieved more satisfactorily. Note that when all of Si is replaced with Ge, a desired crystal structure such as an olivine type crystal cannot be obtained. Therefore, Ge replaces a part of Si.

  The atom X is at least one atom selected from the group consisting of P, Ta, Nb and V. A part of Si may be substituted with an atom X. Substituting a part of Si with the atom X improves the electronic conductivity of the resulting silicate compound and stabilizes the crystal structure. The valence of atom X is a numerical value that can be changed in each step of the production method of the present invention, and is preferably in the range of +2 to +5. The valence of the atom X in the formula (1) (hereinafter, the valence of the atom X in the formula (1) is represented by N2 ′) is basically +5 in the case of P, +2 or +5 in the case of Ta, Nb Is +2 or +5 for V, and +3 or +5 for V. The valence of the atom X in the formula (2) (hereinafter, the valence of the atom X in the formula (2) is represented by N2) is basically +5 for P, +2 or +5 for Ta, In the case of Nb, it is +2 or +5, and in the case of V, it is +3 or +5.

  When the valence N2 of the atom X is pentavalent, the carrier density of the silicate compound is increased. The conductivity is improved, and insertion / extraction reaction of ions of atoms A such as Li ions can be more smoothly advanced, which is particularly preferable. Since the resistance component decreases by smoothly proceeding with the insertion / extraction reaction of the ions of the atom A, the discharge capacity of the secondary battery using the silicate compound as the positive electrode material for the secondary battery can be increased.

  When e is 0 ≦ e ≦ 0.1, it is possible to obtain the above-described effect of improving the electronic conductivity and further the effect of improving the carrier density of the silicate compound while stabilizing the crystal structure of the silicate compound. . e is more preferably 0 ≦ e ≦ 0.05, particularly preferably 0.0001 ≦ e ≦ 0.01. Within the above range, the stabilization of the crystal structure and the effect of improving the electron conductivity can be more satisfactorily achieved. Furthermore, precipitation of impurities and the like can be suppressed.

  The value of f1 is a number that depends on the numerical values of a, b, c, d, e, the valence N1 'of the atom M, and the valence N2' of the atom X, and satisfies the electrical neutrality. It is a value that can change in the heating step (IV) and is a value that becomes f after the heating step (IV). For example, when the value of f1 increases or decreases due to oxidation / reduction or volatilization of the components in the heating step (IV), it is preferable to set the value in consideration of the increase / decrease. The value of f in the composition of the silicic acid compound obtained in the heating step (IV) depends on the numerical values of a, b, c, d and e, and the valence N1 of atom M and the valence N2 of atom X. It is a number that satisfies the electrical neutrality. If a = 2, b = 1, c = 1, d = 0.2, e = 0.01, N1 = + 2, N2 = + 5, f = 4.005. When the valence N2 of the atom X is pentavalent and acts as a carrier donor in the crystal, the value of f does not depend on the value of e and the valence N2 of the atom X.

  The melt may contain atoms other than atom A, atom M, silicon (Si), germanium (Ge), atom X, and oxygen (O) (hereinafter also referred to as atom T). By containing the atom T, the melt can be easily melted. As for the content of atoms T (the total amount in the case of a plurality of atoms), the oxide equivalent amount (unit: mol%) of each atom when it becomes a melt is preferably 0.1 to 3%.

  In the melting step (I), it is preferable to first select and mix each atomic source so as to obtain a melt, thereby obtaining a raw material formulation. The raw material preparation is composed of a compound containing atom A, a compound or metal containing atom M, a compound containing Si, a compound containing Ge, a compound or metal containing atom X, etc., and if necessary, a compound containing atom T It is preferable to contain.

Examples of the compound containing the atom A include A carbonate (A ₂ CO ₃ ), A hydrogen carbonate (AHCO ₃ ), A hydroxide (AOH), A silicate (A ₂ O · 2SiO _2). A ₂ O · SiO ₂ , 2A ₂ O · SiO _2, etc.), A phosphate (A ₃ PO ₄ ), A borate (A ₃ BO ₃ ), A fluoride (AF), A Chlorides (ACl), A nitrates (ANO ₃ ), A sulfates (A ₂ SO ₄ ), and organic acids such as A acetates (CH ₃ COOA) and oxalates ((COOA) ₂ ) At least one selected from the group consisting of salts (however, a part or all of the at least one may each form a hydrated salt) is preferable. Of these, A ₂ CO ₃ and AHCO ₃ are particularly preferable because they are inexpensive and easy to handle.

Examples of the compound containing atom M include oxides of M (FeO, Fe ₃ O ₄ , Fe ₂ O ₃ , MnO, Mn ₂ O ₃ , MnO ₂ , CoO, Co ₃ O ₄ , Co ₂ O ₃ , NiO, etc.) M oxyhydroxide (MO (OH), etc.), M silicate (MO · SiO ₂ , 2MO · SiO ₂ etc.), M phosphate (M ₃ (PO ₄ ) ₂ etc.), M Borate (M ₃ (BO ₃ ) ₂ etc.), M fluoride (MF ₂ , MF ₃ etc.), M chloride (MCl ₂ , MCl ₃ etc.), M nitrate (M (NO ₃ )) ₂ , M (NO ₃ ) ₃ etc.), M sulfate (MSO ₄ , M ₂ (SO ₄ ) ₃ etc.), M alkoxide (M (OCH ₃ ) ₂ , M (OC ₂ H ₅ ) ₂ etc.) And at least one selected from the group consisting of organic acid salts such as M acetate (M (CH ₃ COO) ₂ ) and oxalate (M (COOH) ₂ ). In view of availability and cost, at least one compound selected from the group consisting of Fe ₃ O ₄ , Fe ₂ O ₃ , MnO, Mn ₂ O ₃ , MnO ₂ , Co ₃ O ₄ and NiO is more preferable. In particular, when the atom M is Fe, the compound is preferably Fe ₃ O ₄ and / or Fe ₂ O _{3, and} when the atom M is Mn, the compound is preferably MnO ₂ and / or MnO. The compound containing atom M may be one type or two or more types.

Examples of the compound containing Si include silicon oxide (SiO _{2 and the} like), A silicate, M silicate, and silicon alkoxide (Si (OCH ₃ ) ₄ , Si (OC ₂ H ₅ ) _{4 and the} like). At least one selected from the group consisting of As the compound containing Si, silicon oxide is particularly preferable from the viewpoint of inexpensiveness. The compound containing Si may be crystalline or amorphous.

Examples of the compound containing Ge include germanium oxide (GeO _{2 and the} like), germanium salt of A (A ₂ O · 2GeO ₂ , A ₂ O · GeO ₂ , 2A ₂ O · GeO _{2 and the} like)) and M germanate. (MO · GeO ₂ , 2MO · GeO ₂ etc.) and germanium alkoxide (Ge (OCH ₃ ) ₄ , Ge (OC ₂ H ₅ ) ₄ etc.) are preferred. As the compound containing Ge, germanium oxide is particularly preferable from the viewpoint of low cost. The compound containing Ge may be crystalline or amorphous.

Examples of the compound containing atom X include oxides of X (P ₂ O ₅ , Ta ₂ O ₅ , Ta ₂ O ₃ , Nb ₂ O ₅ , Nb ₂ O ₃ , V ₂ O ₅ , V ₂ O _3, etc.), X hydroxide (X (OH) ₅ etc.), X silicate (X ₂ O ₅ .SiO ₂ etc.), phosphoric acid (H ₃ PO ₄ etc.), phosphate ((NH ₄ ) ₃ PO ₄ , (NH ₄ ) ₂ HPO ₄ , NH ₄ H ₂ PO ₄ , A or M phosphate, etc.) A or M tantalate, A or M niobate, A or M vanadate, X alkoxide (X (OCH ₃ ) ₅ , X (OC ₂ H ₅ ) _5, etc.), and organic compounds such as X acetate (X (CH ₃ COO) ₅ ) and oxalate (X (COO) ₅ ) At least one selected from the group consisting of acid salts is preferred. In view of availability and cost, at least one compound selected from the group consisting of NH ₄ H ₂ PO ₄ , Ta ₂ O ₅ , Nb ₂ O ₅ and V ₂ O ₅ is particularly preferred.

A suitable combination of the respective raw materials is that the compound containing atom A is carbonate or hydrogen carbonate of A; the compound containing atom M is an oxide of M; the compound containing Si is silicon oxide; the compound containing Ge is oxidized Germanium; a combination where the compound containing atom X is an oxide or phosphate of X;
In principle, the composition of the raw material formulation corresponds theoretically to the composition of the melt obtained from the raw material formulation. However, since the raw material preparation contains a component (for example, Li) that is easily lost due to volatilization or the like during melting, the composition of the resulting melt has an oxide-based mole calculated from the charged amount of each raw material. % May be slightly different. In such a case, it is preferable to set the charging amount of each raw material in consideration of the amount lost due to volatilization or the like. In the production method of the present invention, it is preferable that the charged amount of each raw material is set to a value 0.9 to 1.2 times the composition of the melt.

The purity of each raw material in the raw material formulation is not particularly limited. Considering reactivity and physical properties of the positive electrode positive electrode material, the purity excluding hydration water is preferably 99% by mass or more.
As each raw material, it is preferable to use a pulverized raw material. Each raw material may be pulverized and mixed, or may be pulverized after mixing. The pulverization is preferably performed by a dry method or a wet method using a mixer, a ball mill, a jet mill, a planetary mill or the like, and a dry method is preferable because a solvent removal step is unnecessary. The particle size of each raw material in the raw material preparation is not limited as long as it does not adversely affect the mixing operation, the filling operation of the mixture into the melting container, the meltability of the mixture, and the like.

  In the melting step (I), the raw material mixture obtained by the above method is then heated to melt. The heating is preferably performed by putting the raw material preparation into a container or the like and heating and melting it using a heating furnace. Examples of the container include alumina, carbon, silicon carbide, zirconium boride, titanium boride, boron nitride, carbon, platinum, platinum alloy containing rhodium, refractory bricks, and reduction. A container made of a material such as a material (for example, graphite) can be used. The container is preferably fitted with a lid in order to prevent volatilization and evaporation in the heating furnace. The heating furnace is preferably a resistance heating furnace, a high frequency induction furnace, or a plasma arc furnace. The resistance heating furnace is preferably an electric furnace provided with a heating element made of an alloy such as a nichrome alloy, silicon carbide, or molybdenum silicide.

  The heating temperature is preferably 1,300 to 1,600 ° C, particularly preferably 1,400 to 1,550 ° C. Here, melting means that each raw material is melted and is in a transparent state visually. When the heating temperature is not less than the lower limit of the above range, melting becomes easy, and when it is not more than the upper limit, the raw material is hardly volatilized. The heating time is preferably 0.2 to 2 hours, particularly preferably 0.5 to 2 hours. When the heating time is not less than the lower limit of the above range, the uniformity of the melt is sufficient, and when it is not more than the upper limit, the raw material is hardly volatilized. In the melting step (I), stirring may be performed to increase the uniformity of the melt. Further, the melt may be clarified at a temperature lower than the melting temperature until the next cooling step (II) is performed.

Heating is preferably carried out in air, in an inert gas or in a reducing gas. Melting conditions can be changed as appropriate depending on conditions such as the type of container or heating furnace and the heating method such as a heat source. The pressure may be any of normal pressure, pressurization (1.1 × 10 ⁵ Pa or more), and reduced pressure (0.9 × 10 ⁵ Pa or less). The melting condition is preferably in a reducing gas. It may be in oxidizing gas. In the case of melting in an oxidizing gas, it is preferable to perform reduction (for example, change from M ³⁺ to M ²⁺ ) in the subsequent heating step (IV).

Here, the inert gas is 99% by volume or more of at least one inert gas selected from the group consisting of nitrogen gas (N ₂ ) and rare gases such as helium gas (He) and argon gas (Ar). Contains gas. The reducing gas refers to a gas that is substantially free of oxygen by adding a reducing gas to the above-described inert gas. Examples of the reducing gas include hydrogen gas (H ₂ ), carbon monoxide gas (CO), and ammonia gas (NH ₃ ). The amount of the reducing gas in the inert gas is preferably 0.1% by volume or more, and more preferably 1 to 10% by volume of the reducing gas contained in the total gas volume. The oxygen content is preferably 1% by volume or less, and particularly preferably 0.1% by volume or less in the gas volume.

(Cooling step (II))
The cooling step (II) is a step of cooling the melt obtained in the melting step (I) to near room temperature (20 to 25 ° C.) to obtain a solidified product. The solidified product is preferably an amorphous material, but a part of the solidified product may be a crystallized product. When the solidified product contains an amorphous material, the next pulverization step (III) can be easily performed, and the composition and particle size of the silicate compound can be easily controlled. In the case where the solidified product contains a crystallized product, the crystallized product becomes a crystal nucleus in the heating step (IV), and crystallization is facilitated. The amount of the crystallized product in the solidified product is preferably 0 to 30% by mass with respect to the total mass of the solidified product. When a large amount of crystallized material is contained, it becomes difficult to obtain a granular or flaky solidified material. Moreover, the wear of the cooling device is accelerated, and the burden of the subsequent pulverization step (III) is increased.

  It is preferable to cool the melt in the air, in an inert gas, or in a reducing gas because the equipment is simple.

The cooling rate of the melt is preferably not less than -1 × 10 ³ ℃ / sec, -1 × 10 ⁴ ℃ / sec or more is particularly preferable. In the present specification, a temperature change per unit time (ie, cooling rate) in the case of cooling is indicated by a negative value, and a temperature change per unit time in case of heating (ie, the heating rate) is indicated by a positive value. When the cooling rate is higher than this value, an amorphous material is easily obtained. The upper limit of the cooling rate is preferably about −1 × 10 ¹⁰ ° C./second from the viewpoint of production equipment and mass productivity, and 1 × 10 ⁸ ° C./second is particularly preferable from the viewpoint of practicality.
The melt is cooled from 1,300 to 1,600 ° C., which is the heating temperature in the melting step (I) of the previous step, to around room temperature (20 to 25 ° C.) at a cooling rate of −1 × 10 ³ to −1. It is preferable to cool at × 10 ¹⁰ ° C / second.

  As a method for cooling the melt, a method in which the melt is dropped between twin rollers rotating at a high speed, a method in which the melt is dropped on a rotating single roller to cool, a carbon plate that has cooled the melt, A method of cooling by pressing on a metal plate is preferred. Among these, a cooling method using twin rollers is particularly preferable because the cooling rate is high and a large amount of processing can be performed. As the double roller, it is preferable to use one made of metal, carbon or ceramic.

  The solidified product obtained in the cooling step (II) is preferably flaky or fibrous. In the case of flakes, the average thickness is preferably 200 μm or less, particularly preferably 100 μm or less. The average diameter of the plane perpendicular to the flaky average thickness is not particularly limited. In the case of a fibrous form, the average diameter is preferably 50 μm or less, particularly preferably 30 μm or less. When the average thickness or the average diameter is not more than the upper limit value, the burden of the pulverization step (III) can be reduced, and the crystallization efficiency in the heating step (IV) can be increased. The average thickness and average diameter can be measured with a caliper or a micrometer. The average diameter can also be measured by microscopic observation.

(Crushing step (III))
The pulverization step (III) is a step of pulverizing the solidified product obtained in the cooling step (II) to obtain a pulverized product. Since the solidified product usually contains a large amount of amorphous material or consists of an amorphous material, there is an advantage that it can be easily pulverized. Further, there is an advantage that pulverization can be performed without imposing a burden on an apparatus used for pulverization and the particle size can be easily controlled. On the other hand, in the conventional solid phase reaction, pulverization is performed after the heating step. However, the present inventor has noticed that there is a problem that residual stress is generated by the pulverization and battery characteristics and the like are deteriorated. In the production method of the present invention, since the pulverization step (III) is performed before the heating step (IV), the residual stress generated in the pulverization step (III) can be reduced or removed in the heating step (IV).

  The pulverization is preferably performed using a cutter mill, jaw crusher, hammer mill, ball mill, jet mill, planetary mill or the like. Moreover, pulverization can be efficiently advanced by using each method stepwise depending on the particle diameter. For example, it is preferable to preliminarily make it fine with a hand massage or a hammer because the burden on the grinding process is reduced. Further, after preliminary pulverization with a cutter mill, pulverization with a planetary mill or ball mill is preferable because the time required for pulverization can be shortened. From the viewpoint of productivity, it is particularly preferable to use a ball mill.

As the grinding media, it is preferable to use zirconia balls, alumina balls, glass balls or the like. In particular, zirconia balls are preferable because they have a low wear rate and can suppress the mixing of impurities. The diameter of the grinding media is preferably 0.1 to 30 mm. After pulverization is performed in multiple stages and pulverization is performed with a large pulverization medium, the pulverization medium and the pulverized product may be separated and pulverized using a smaller pulverization medium. With this method, the remaining of unground particles can be suppressed.
The pulverization container is not particularly limited, but the pulverization efficiency is good when the pulverization media and the solidified material are placed in the container up to 30 to 80% of the container height. When using a ball mill, the grinding time is preferably 6 to 360 hours, more preferably 6 to 120 hours, and particularly preferably 12 to 96 hours. If the pulverization time is not less than the lower limit of the above range, the pulverization can be sufficiently advanced, and if it is not more than the upper limit, excessive pulverization can be suppressed.

  The pulverization may be performed either dry or wet, but is preferably performed wet from the viewpoint of pulverization particle size. As a solvent used for pulverization (hereinafter also referred to as pulverization solvent), water or an organic solvent such as ethanol, isopropyl alcohol, acetone, hexane, or toluene can be used. Water is preferable from the viewpoint of cost and safety. On the other hand, when a problem such as elution of the solidified product occurs in the polar solvent, the organic solvent is preferable. When the grinding solvent contains grinding media and is filled to 30 to 80% of the container height, grinding efficiency is improved. When the pulverization is performed by a wet process, it is preferable to carry out the heating step (IV) after removing the pulverization solvent by sedimentation, filtration, drying under reduced pressure, drying by heating, or the like. However, when the pulverizing solvent is small, particularly when the ratio of the mass of the solid content to the mass of the pulverized product is 30% or more, the pulverized product containing the pulverizing solvent may be used for the heating step (IV). .

  The average particle size of the pulverized product is preferably 10 nm to 5 μm, particularly preferably 10 nm to 500 nm, in terms of volumetric median diameter. It is preferable for the average particle size to be equal to or greater than the lower limit of the above range because the pulverized products do not sinter and become too large in the heating step (IV). It is preferable for it to be not more than the upper limit of the above range because the heating temperature and time in the heating step (IV) can be reduced, and the particle size of the resulting silicate compound can be reduced. The average particle diameter can be measured by, for example, a sedimentation method, a laser diffraction / scattering particle diameter measuring apparatus, or a flow image analyzer, and is preferably measured by a laser diffraction / scattering particle diameter measuring apparatus from the viewpoint of workability.

Since the silicate compound in the present invention is an insulating material, when used as a positive electrode material for a secondary battery, it is preferable to coat at least a part of the surface of the silicate compound with a conductive material. In the pulverization step (III), the solidified product preferably contains a carbon source serving as a conductive material. The carbon source is preferably at least one selected from the group consisting of organic compounds and carbon-based conductive active materials. As the carbon source, only an organic compound, only a carbon-based conductive active material, or a combination of an organic compound and a carbon-based conductive active material may be used, but it is particularly preferable to use an organic compound and a carbon-based conductive active material in combination. By using in combination, the distribution of the carbon-based conductive active material in the silicate compound becomes uniform, and the contact area with the organic compound and its thermal decomposition product (carbide) increases. By these, the electroconductivity of the positive electrode material for secondary batteries using a silicate compound can be improved.
The pulverization step (III) is preferably a step of obtaining a pulverized product of a solidified product and a carbon source. The pulverization step (III) includes a step of mixing the solidified product and the carbon source and then pulverizing, a step of mixing the solidified product and the carbon source after pulverizing, and a step of adding the carbon source after pulverizing the solidified product. It is more preferable that In addition, when a carbon source is only an organic compound, it can mix with a solidified material, without grind | pulverizing.

  The ratio of the mass of the carbon source is such that the carbon conversion amount (mass) in the carbon source is 0.1 to 20 with respect to the total mass of the mass of the solidified product and the carbon conversion amount (mass) in the carbon source. An amount of mass% is preferred, an amount of 0.1-10 mass% is more preferred, and an amount of 2-10 mass% is particularly preferred. By setting the amount of the carbon source to be equal to or more than the lower limit of the above range, the electrical conductivity can be sufficiently increased when the silicate compound is used as the positive electrode material for the secondary battery. When the thickness of the conductive material covering the silicate compound does not become too thick by setting it to the upper limit of the above range or less, the positive electrode for the secondary battery is manufactured using the silicate compound. The electrolyte can be sufficiently distributed to the material.

<Organic compounds>
The organic compound as the carbon source is preferably a compound that undergoes a thermal decomposition reaction when heated in an inert gas or a reducing gas, and carbonizes by releasing oxygen and hydrogen. The organic compound is selected from the group consisting of saccharides, alkanes, amino acids, peptides, aldehydes, ketones, carboxylic acids, terpenes, heterocyclic amines, fatty acids and aliphatic acyclic polymers having functional groups. At least one is preferred. 1 type may be used for this organic compound, or 2 or more types may be used for it.
Examples of the saccharide include monosaccharides such as glucose, fructose and galactose, oligosaccharides such as sucrose, maltose, cellobiose and trehalose, invert sugar, polysaccharides such as dextrin, amylose, amylopectin and cellulose, ascorbic acid and the like.
As the alkane, paraffin having 10 to 40 carbon atoms is preferable.
Examples of amino acids include amino acids such as alanine and glycine.
Peptides include low molecular weight peptides having a molecular weight of 1,000 or less.

As the aldehydes, aromatic aldehydes having 7 to 20 carbon atoms are preferable. Preferable examples include benzaldehyde, cinnamaldehyde, perillaldehyde and the like.
Examples of carboxylic acids include acetic acid, propionic acid, butyric acid, oxalic acid, benzoic acid, phthalic acid, maleic acid and the like.
As the ketones, aromatic ketones having 6 to 20 carbon atoms are preferable. Preferred examples include acetophenone and cyclohexanone.
As the terpenes, bicyclic monoterpenes or derivatives thereof are preferable. A preferred example is camphor.
As the heterocyclic amines, compounds having 1 to 3 amino groups in the molecule are preferable. A preferred example is melamine.
As the fatty acid, a saturated fatty acid having 10 to 30 carbon atoms is preferable, and a saturated fatty acid having 10 to 20 carbon atoms is particularly preferable. Preferable examples include stearic acid, oleic acid, linoleic acid and the like.

The aliphatic acyclic polymer having a functional group preferably has at least one selected from the group consisting of a hydroxyl group, a carboxy group, a sulfo group, derivatives thereof, and an etheric oxygen atom. The aliphatic acyclic polymer is a polymer obtained by polymerizing an aliphatic acyclic monomer having at least one selected from the group consisting of a hydroxyl group, a carboxy group, a sulfo group, a derivative thereof, and an etheric oxygen atom. Is preferred. Examples of the monomer include vinyl alcohol, acrylic acid, vinyl sulfonic acid, ethylene glycol, cyclic ether (ethylene oxide, propylene oxide, butylene oxide, etc.) and the like. From the viewpoint of adhesion to a silicic acid compound, at least one selected from the group consisting of vinyl alcohol, acrylic acid and ethylene glycol is preferable.
Preferable examples of the aliphatic acyclic polymer include polyethylene glycol, polyvinyl alcohol, polyacrylic acid and the like.

  Although the number average molecular weight of an organic compound is not specifically limited, 100-50,000 are preferable and 100-20,000 are especially preferable. When the number average molecular weight is not less than the lower limit of the above range, the organic compound does not volatilize in the heating step (IV), and conductive carbon tends to remain. When the amount is not more than the upper limit of the above range, the organic compound is likely to adhere to the surface of the pulverized product in the pulverization step (III).

  Organic compounds include glucose, sucrose, glucose-fructose invert sugar, caramel, paraffin, starch, pregelatinized starch, carboxymethylcellulose, ethylcellulose, camphor, melamine, stearic acid, oleic acid, linoleic acid, ascorbic acid, polyethylene glycol, More preferable is at least one selected from the group consisting of polyvinyl alcohol, polyacrylic acid, and phenolic resin, from glucose, sucrose, ethyl cellulose, camphor, melamine, stearic acid, ascorbic acid, polyethylene glycol, polyvinyl alcohol, and polyacrylic acid. Particularly preferred is at least one selected from the group consisting of

<Carbon-based conductive active material>
The carbon-based conductive active material as the carbon source is preferably at least one selected from the group consisting of carbon black, graphite, acetylene black, carbon fiber, and amorphous carbon. As the amorphous carbon, those in which a C—O bond peak or a C—H bond peak causing a decrease in conductivity of the positive electrode material is not substantially detected in the FTIR analysis are preferable.

  The pulverization step (III) in the case where an organic compound is included as a carbon source in the solidified product is preferably performed by a wet method in order to uniformly disperse the pulverized product on the surface. The pulverization step (III) in the case where only the carbon-based conductive active material is included as a carbon source in the solidified product is preferably dry.

(Heating step (IV))
The heating step (IV) is a step of heating the pulverized product obtained in the pulverizing step (III) to obtain the silicate compound (2). In the heating step (IV), it is preferable to obtain the silicic acid compound (2) as a particulate substance. The particulate silicic acid compound (2) is more preferably a crystal particle, and particularly preferably an olivine type crystal structure. It is preferable that the silicic acid compound (2) does not contain an amorphous substance. That the silicic acid compound (2) does not contain an amorphous substance can be confirmed by the fact that the halo pattern is not detected by X-ray diffraction.

  In the heating step (IV), since the pulverized product is heated, the relaxation of the residual stress generated by the pulverization is promoted. In addition, since the generation of crystal nuclei and grain growth are performed by heating, the composition, particle diameter, and distribution of the silicate compound can be easily controlled. Further, the heating step (IV) when the carbon source is included in the pulverization step (III) can be a step of bonding a conductive material to the surface of the product, preferably the crystal grains of the product. The organic compound in the carbon source is thermally decomposed in the heating step (IV), becomes a carbide, and functions as a conductive material. When the pulverization step (III) is performed in a wet manner, the pulverization solvent may be removed simultaneously with the heating step.

  The heating temperature is preferably 500 to 1,000 ° C. When the heating temperature is 500 ° C. or higher, a reaction is likely to occur, and a crystal of a silicate compound is easily generated. When the heating temperature is 1,000 ° C. or less, the pulverized product is difficult to melt, and the crystal system and particle diameter can be easily controlled. The heating temperature is particularly preferably 600 to 900 ° C. When the heating temperature is used, crystal particles having appropriate crystallinity, particle diameter, particle size distribution, etc. are easily obtained, and further, silicate compound crystal particles having an olivine type crystal structure are easily obtained.

  The heating is not limited to being held at a constant temperature, and may be performed by setting the holding temperature in multiple stages. When the heating temperature is increased, the particle diameter of the generated particles tends to increase. Therefore, it is preferable to set the heating temperature according to the desired particle diameter. The heating time (holding time depending on the heating temperature) is preferably 1 to 72 hours in consideration of a desired particle size. Heating is preferably carried out in a box furnace, tunnel kiln furnace, roller hearth kiln, rotary kiln furnace, microwave heating furnace or the like.

Heating is preferably performed in air, an inert gas, or a reducing gas, and particularly preferably performed in an inert gas or a reducing gas. The conditions of the inert gas and the reducing gas are the same as those in the melting step (I). The pressure may be normal pressure, increased pressure (1.1 × 10 ⁵ Pa or more), and reduced pressure (0.9 × 10 ⁵ Pa or less). Further, when a container containing a reducing agent (for example, graphite) and pulverized material is loaded in a heating furnace, reduction of M in the pulverized material (for example, change from M ³⁺ to M ²⁺ ) is performed. Can be promoted. Thereby, the silicic acid compound (2) can be obtained with good reproducibility.

After the heating step (IV), it is usually cooled to room temperature. The cooling rate in the cooling is preferably -30 to -300 ° C / hour. By setting the cooling rate within this range, distortion due to heating can be removed, and when the product contains a crystalline substance, the target product can be obtained while maintaining the crystal structure. Further, the cooling may be left to cool to room temperature. The cooling is preferably allowed to cool to room temperature. Cooling is preferably performed in an inert gas or a reducing gas.
The cooling is preferably performed from the heating step (IV) of 500 to 1,000 ° C. to room temperature at a cooling rate of −30 to −300 ° C./hour.

As a preferable aspect of the present invention, an aspect in which a carbon source to be a conductive material is included in the pulverization step (III) can be mentioned. Furthermore, in this embodiment, after the pulverized product obtained in the pulverization step (III) is heated in an inert gas or a reducing gas to obtain the silicate compound (2), the silicate compound (2) and An embodiment in which a pulverized product with a carbon source is obtained and then the pulverized product is heated in an inert gas or a reducing gas is preferable.
The types and functions of the carbon source are as described above.
The method for obtaining a pulverized product of the silicate compound (2) and the carbon source is a method of mixing the silicate compound (2) and the carbon source and then pulverizing the silicate compound (2) and the carbon source. The method of mixing after grind | pulverizing, or the method of including a carbon source after grind | pulverizing a silicic acid compound (2), without grind | pulverizing a carbon source is mentioned.

The ratio of the mass of the carbon source is such that the carbon equivalent amount (mass) in the carbon source is 0 with respect to the total mass of the mass of the silicate compound (2) and the carbon equivalent amount (mass) in the carbon source. The amount of 1 to 20% by mass is preferable, the amount of 0.1 to 10% by mass is more preferable, and the amount of 2 to 10% by mass is particularly preferable. The reason for defining the amount of the carbon source is as described in the pulverization step (III).
The amount of carbon source used is selected to satisfy the above. 1-50 mass% is preferable with respect to the total amount of the mass of a silicic acid compound (2), and the mass of a carbon source, and 3-45 mass% is especially preferable.

  Preferable pulverization conditions for obtaining a pulverized product of the silicic acid compound (2) and the carbon source are the same as the pulverization conditions in the pulverization step (III).

  The pulverization may be performed either dry or wet, but is preferably performed in a wet manner from the viewpoint that the silicate compound (2) and the carbon source can be uniformly mixed. When the pulverization of the silicic acid compound (2) and the carbon source is carried out in a wet manner, the type and amount of the solvent (grinding solvent) are preferably the same as the conditions for carrying out the pulverization step (III) in a wet manner. When the pulverization of the silicic acid compound (2) and the carbon source is performed in a wet manner, it is preferable to carry out the following heating after removing the pulverizing solvent by sedimentation, filtration, drying under reduced pressure, drying by heating, and the like. However, when the ratio of the solid content with respect to the grinding solvent is 30% or more, the pulverized product containing the grinding solvent may be used for heating.

  The pulverized product of the silicate compound (2) and the carbon source is preferably heated in an inert gas or a reducing gas. When the carbon source is an organic compound, the organic compound is carbonized during heating to cover the surface of the silicic acid compound (2), so that the adhesion of the conductive carbon to the surface of the silicic acid compound (2) is increased. be able to. Even when a carbon-based conductive active material is used as the carbon source, the pulverized product of the silicate compound (2) and the carbon source is heated in an inert gas or a reducing gas to thereby produce a silicate compound of conductive carbon. The adhesion to the surface of (2) can be enhanced. The composition of the silicate compound (2) whose surface is coated with conductive carbon may change due to oxidation or volatilization of components during heating.

The heating temperature of the pulverized product of the silicate compound (2) and the carbon source is preferably 300 to 800 ° C, particularly preferably 500 to 700 ° C. When the heating temperature is at least the lower limit of the above range, the carbonization reaction of the organic compound is likely to proceed, and the adhesion of the carbon-based conductive active material to the surface of the silicate compound (2) is improved. When the amount is not more than the upper limit of the above range, it is possible to suppress deterioration in characteristics due to reduction of the silicate compound (2), and it is possible to suppress increase in particle size due to sintering of the silicate compounds (2).
Preferred conditions for the heating time, the heating atmosphere, the pressure during heating, and the heat source used for heating are the same as those in the heating step (IV).
After heating, it is usually cooled to room temperature. The preferable conditions for the cooling are the same as the cooling after heating in the heating step (IV).

[Silica compound]
The silicic acid compound obtained by the production method of the present invention is a useful compound as a positive electrode material for a secondary battery. Since the silicate compound having the composition represented by the formula (2) has high conductivity and exhibits a multi-electron type reaction as described above, the capacity per unit mass when used as a positive electrode material for a secondary battery. Becomes larger. Furthermore, since a part of Si is replaced by Ge, the crystal lattice constant of the silicate compound crystal is expanded, and ions of atoms A such as Li ions are easily diffused. This increases the average discharge voltage when used as a positive electrode material for a secondary battery.

  The silicic acid compound (2) preferably contains olivine-type crystal particles, and particularly preferably comprises only olivine-type crystal particles. The crystal particles include both primary particles and secondary particles. When secondary particles are present in the resulting silicic acid compound, they may be crushed and pulverized as long as the primary particles are not destroyed.

As the silicate compound (2), a silicate compound having a composition represented by the following formula (3) is more preferable.
_{Li 2 Fe 1-g Mn g} Si c- (d + e) Ge d X e O f (3)
(In formula (3), X represents the same kind of atom as described above, c, d, e and f represent the same numerical values as described above, and g is 0 ≦ e ≦ 1.)
The silicic acid compound having the composition represented by the formula (3) is preferable because it exhibits good characteristics when used as a positive electrode material for a secondary battery and the manufacturing cost is low.

  In the production method of the present invention, when at least one carbon source selected from the group consisting of an organic compound and a carbon-based conductive active material is included, the conductive material derived from the carbon source on the surface of the silicate compound Can be bonded uniformly and firmly. The silicic acid compound to which the conductive material is bonded can be used as it is as a positive electrode material for a secondary battery. When an organic compound is used as the carbon source, the organic compound is carbonized in the heating step (IV), and the carbide covers at least a part of the surface of the silicate compound as a conductive material. When a carbon-based conductive active material is used as the carbon source, the material covers at least a part of the surface of the silicate compound as the conductive material.

  The particles of the silicate compound contain a conductive material, and 0.1 to 20% by mass of the conductive material on the surface of the particle or the interparticle interface with respect to the total mass of the silicate compound and the conductive material. It is preferable to contain 0.1 to 10% by mass, more preferably 2 to 10% by mass.

The crystallite diameter of the silicate compound of the present invention is preferably 10 to 50 nm, particularly preferably 10 to 35 nm. By making the crystallite diameter within this range, the conductivity of the silicate compound becomes higher. The crystallite diameter is obtained from the half-width of the strongest peak using Scherrer's equation. The average particle diameter of the silicic acid compound is preferably 10 nm to 10 μm, and particularly preferably 10 nm to 2 μm, in terms of volume median diameter. The method for measuring the average particle diameter is as described above. The specific surface area of the silicic acid compound is preferably 0.2~200m ² / g, more preferably 1~200m ² / g, 1~50m more preferably _² / g, ^{1~10m 2} / g is particularly preferred. By making a specific surface area into this range, the electroconductivity of a silicate compound becomes high. The specific surface area can be measured by, for example, a specific surface area measuring apparatus using a nitrogen adsorption method. The average particle size and specific surface area of the silicate compound are the average particle size and specific surface area when the silicate compound does not contain a conductive material.
The average particle diameter of the silicate compound coated with the conductive material is preferably 10 nm to 10 μm, particularly preferably 10 nm to 2 μm, in terms of volume median diameter. By making the average particle diameter within this range, the conductivity of the silicate compound becomes higher. The specific surface area of the silicic acid compound is preferably 0.2~200m ² / g, more preferably ^{1~200m 2 / g, 1~50m 2 /} g is particularly preferred. By making a specific surface area into this range, the electroconductivity of a silicate compound becomes high.

[Positive electrode for secondary battery and method for producing secondary battery]
By using the silicate compound obtained by the production method of the present invention as a positive electrode material for a secondary battery, a positive electrode for a secondary battery and a secondary battery can be produced. Examples of the secondary battery include a metal lithium secondary battery, a lithium ion secondary battery, and a lithium polymer secondary battery, and a lithium ion secondary battery is preferable. The battery shape is not limited, and various shapes and sizes such as a cylindrical shape, a square shape, and a coin shape can be appropriately employed.

  The positive electrode for a secondary battery of the present invention can be manufactured according to a known electrode manufacturing method except that the silicate compound obtained by the manufacturing method of the present invention is used. For example, a known binder (polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, ethylene propylene diene polymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluororubber, Obtained by mixing with polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, etc.) and, if necessary, known conductive materials (acetylene black, carbon, graphite, natural graphite, artificial graphite, needle coke, etc.). The mixed powder may be pressure-formed on a support made of stainless steel or filled in a metal container. Further, for example, the mixed powder is mixed with an organic solvent (N-methylpyrrolidone, toluene, cyclohexane, dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran. Etc.) can also be employed such as applying the slurry obtained by mixing with a metal substrate such as aluminum, nickel or stainless steel.

  As the structure of the secondary battery, a structure in a known secondary battery can be adopted except that the positive electrode for a secondary battery obtained by the production method of the present invention is used as an electrode. The same applies to separators, battery cases, and the like. As the negative electrode, a known negative electrode active material can be used as the active material, and at least one selected from the group consisting of carbon materials, alkali metal materials, and alkaline earth metal materials is preferably used. As the electrolytic solution, a non-aqueous electrolytic solution is preferable. That is, as the secondary battery obtained by the production method of the present invention, a nonaqueous electrolyte lithium ion secondary battery is preferable.

  The present invention will be specifically described with reference to examples, but the present invention is not limited to the following description. Examples 1 to 9 are Examples, and Examples 21 to 22 are Reference Examples.

[Examples 1-9, 21-22]
(Melting step (I))
The composition of the melt is Li ₂ O, FeO, MnO, SiO ₂ , GeO ₂ , P ₂ O ₅ , Ta ₂ O ₅ and Nb ₂ O ₅ equivalent amounts (unit: mol%), respectively, and the ratios shown in Table 1 Lithium carbonate (Li ₂ CO ₃ ), triiron tetroxide (Fe ₃ O ₄ ), manganese dioxide (MnO ₂ ), silicon dioxide (SiO ₂ ), germanium oxide (GeO ₂ ), ammonium hydrogen phosphate ( NH ₄ H ₂ PO ₄ ), tantalum oxide (Ta ₂ O ₅ ), and niobium oxide (Nb ₂ O ₅ ) were weighed, mixed and pulverized in a dry process to obtain a raw material formulation.

The obtained raw material mixture was filled in a platinum alloy crucible containing 20% by mass of rhodium. Next, the crucible was placed in an electric furnace (manufactured by Motoyama, apparatus name: NH-3035) having a heating element made of molybdenum silicide. The electric furnace was heated at a rate of + 300 ° C./hour and heated at 1,450 to 1,500 ° C. for 0.5 hours while flowing N ₂ gas at a flow rate of 1 L / min. After confirming that it became transparent visually, a melt was obtained.

(Cooling step (II))
The melt in the crucible obtained in the melting step (I) is passed through a stainless steel double roller having a diameter of about 15 cm rotating at 400 revolutions per minute to cool the melt to room temperature at −1 × 10 ⁵ ° C./second, A flaky solidified product was obtained. The obtained solidified product was a glassy substance.

(Crushing step (III))
10 g of the flaky solidified product obtained in the cooling step (II), 2.5 g of camphor (manufactured by Tokyo Chemical Industry Co., Ltd., reagent) as an organic compound, 400 g of zirconia balls having a diameter of 5 mm as a grinding medium, and a grinding solvent 75 mL of acetone was put into a 250 mL polypropylene bottle and pulverized at 280 rpm for 72 hours using a desktop pot mill mount (manufactured by ASONE, apparatus name: PM-001). Next, the obtained pulverized product was separated from zirconia balls having a diameter of 5 mm. Next, the pulverized product and 400 g of a zirconia ball having a diameter of 1 mm were placed in a 250 mL polypropylene bottle, and were similarly pulverized at 280 rpm for 72 hours using a desktop pot mill frame. The mass of the camphor is 20% by mass with respect to the total amount of the mass of the solidified product and the mass of the camphor.

(Heating step (IV))
The pulverized product obtained in the pulverization step (III) was dried and then placed in an alumina pot. Next, the alumina pot was placed in a reducing atmosphere firing furnace (manufactured by Motoyama, apparatus name: SKM-3035F-SP). While circulating N ₂ gas at 1.5 L / min in the firing furnace, heating was performed at 700 ° C. for 8 hours to obtain a positive electrode material for a secondary battery. The positive electrode material for a secondary battery is obtained by coating at least a part of the surface of a crystal particle of a silicate compound with a conductive material derived from camphor.

(Particle size distribution)
The average particle size of the pulverized material obtained in Examples 1-9 and 21-22 was measured with a laser diffraction / scattering particle size distribution measuring device (manufactured by Horiba, Ltd., device name: LA-920). The results are shown in Table 1.

(X-ray diffraction)
The mineral phase of the obtained positive electrode material for a secondary battery was examined using an X-ray diffraction apparatus (manufactured by Rigaku Corporation, apparatus name: RINT TTRIII). The particles of Examples 1-9 and 21-22 were all orthorhombic, and it was confirmed that the crystal particles covered with the conductive material were olivine-type crystal particles. The X-ray diffraction patterns of the crystals obtained in Examples 1, 2, 3, 4, 21, and 22 are shown in FIGS. 1, 2, 3, 4, 5, and 6, respectively. In addition, the crystallite diameter was calculated from the diffraction peak having the highest intensity using analysis software (manufactured by Rigaku Corporation, JADE7). Furthermore, the a-axis crystal lattice constant was calculated using six diffraction peaks from the low angle side. The results are shown in Table 2. FIG. 7 shows the relationship between the Ge substitution amount of Si in the silicate compound and the a-axis crystal lattice constant.

(Composition analysis)
The chemical composition of the obtained positive electrode material for secondary batteries was measured. First, the material was heated and sealed and decomposed with a 2.5 mol / L KOH solution at 120 ° C., and the decomposition solution was dried under hydrochloric acid acidity. Next, after filtration as a hydrochloric acid acidic solution, a filtrate and a residue were obtained. Fe, Mn, Si, Ge, Ta, Nb and P in the filtrate were quantified using an inductively coupled emission spectroscopic analyzer (manufactured by Seiko Instruments Inc., apparatus name: SPS3100). Li in the filtrate was quantified using an atomic absorption photometer (manufactured by Hitachi High-Technologies Corporation, apparatus name: Z-2310). Fe, Mn, Si, Ge, Ta, from quantitative values of Nb and P, were calculated FeO, MnO, the amount of _{SiO 2, GeO 2, P 2} O 5, Ta 2 O 5 and Nb ₂ O _5. Further, the residue was ashed and then decomposed with hydrofluoric acid-sulfuric acid, and the weight loss due to this treatment was defined as the amount of SiO ₂ . The total amount of SiO ₂ was the sum of the amount calculated from the weight loss value and the amount of SiO ₂ in the filtrate. Table 2 shows quantitative values of the chemical compositions of the positive electrode materials for secondary batteries obtained in Examples 1 to 9 and Examples 21 to 22.

(Remaining carbon content)
The amount of residual carbon in the obtained positive electrode material for secondary batteries was measured using a carbon analyzer (manufactured by Horiba, Ltd., device name: EMIA-321V). The results are shown in Table 2. The residual carbon amount is a value obtained by subtracting the mass of the silicate compound (2) from the total mass of the positive electrode material for secondary batteries, and the ratio of the residual carbon amount in the total mass of the positive electrode material for secondary batteries ( Mass%) was calculated.

(Manufacture of positive electrode sheet)
The obtained positive electrode material for a secondary battery, acetylene black (conductive material), and a polyvinylidene fluoride solution (solvent: N-methylpyrrolidone) containing 12.1% by mass of polyvinylidene fluoride (binder) are mixed. Further, N-methylpyrrolidone was added to prepare a slurry. The mass ratio of the positive electrode material for secondary batteries, acetylene black, and polyvinylidene fluoride was 80: 12: 8. Next, the slurry was coated on one side of a 20 μm thick aluminum foil using a doctor blade, then dried at 120 ° C., and further subjected to roll press rolling twice to produce a positive electrode sheet.

(Manufacture of secondary batteries)
The positive electrode sheet punched out to a diameter of 18 mm is used as a positive electrode, a metal lithium foil having a thickness of 500 μm is used as a negative electrode, a stainless steel plate having a thickness of 1 mm as a negative electrode current collector, and a porous polypropylene having a thickness of 25 μm as a separator. Furthermore, lithium hexafluorophosphate (LiPF ₆ ) is added at a concentration of 1 mol / L to a mixed solvent (volume ratio, EC: DEC = 1: 1) of ethylene carbonate (EC) and diethylene carbonate (DEC) as an electrolytic solution. Using the dissolved mixed solution, a stainless steel simple sealed cell half-cell was assembled in an argon glove box.

(Charge / discharge characteristic evaluation of positive electrode for Li ion secondary battery)
The obtained half-cell was placed in a constant temperature bath at 25 ° C. and connected to a constant current charge / discharge tester (manufactured by Hokuto Denko Co., Ltd., device name: HJ201B) to conduct a charge / discharge test. The current density was charged / discharged with the current value per mass of the positive electrode material for secondary batteries (the mass excluding the conductive material and the binder) being 15 mA / g. The charge end potential was 4.5 V based on the Li counter electrode, and after reaching the end voltage, charging was performed until the current value reached 1.5 mA / g, and discharge was started after a 30-minute pause. The end-of-discharge voltage was 2 V on the basis of the Li counter electrode. Table 2 shows the initial discharge capacity and the average discharge voltage. The average discharge voltage is an average of the discharge voltage with respect to the discharge time.

  The batteries of Examples 1 to 9 manufactured using the silicate compound of the present invention have a higher average discharge voltage and the same initial discharge capacity as compared to the battery of Example 21 manufactured using the silicate compound not containing Ge. Met. In particular, the batteries of Examples 6 to 9 using only Mn as the atom M had an average discharge voltage exceeding 3 V, and the average discharge voltage was very high. This is considered to be because the crystal lattice constant was expanded by replacing a part of Si of the silicate compound with Ge having a large ionic radius, and Li was easily diffused. As shown in Table 2 and FIG. 7, the crystal lattice constant is increased by substituting part of Si with Ge. It can be confirmed from the a-axis crystal lattice constant in Table 2 that the crystal lattice constant has been expanded by substituting part of Si of the silicate compound with Ge having a large ionic radius. That is, the a-axis crystal lattice constant of Examples 1 to 9 substituted with Ge was larger than the a-axis crystal lattice constant of Example 21 containing no Ge. In addition, the battery of Example 22 using Ge instead of Si had insufficient initial discharge capacity.

  The production method of the present invention can produce a silicate compound capable of increasing the average discharge voltage and the discharge capacity when used as a positive electrode material for a secondary battery or the like. Furthermore, the composition and particle size of the silicate compound can be controlled well. The obtained silicic acid compound is useful when applied to a positive electrode for a secondary battery and further to a secondary battery.

Claims (14)

A melting step for obtaining a melt having a composition represented by the following formula (1):
A cooling step of cooling the melt to obtain a solidified product,
Crushing the solidified product to obtain a pulverized product, and heating the pulverized product to obtain a silicate compound having a composition represented by the following formula (2) in this order: A method for producing a silicic acid compound.
_{A a M b Si c- (d} + e) Ge d X e O f1 (1)
(In the formula (1), A is at least one atom selected from the group consisting of Li, Na and K, M is at least one atom selected from the group consisting of Fe, Mn, Co and Ni, and X is P , Ta, Nb and V, at least one atom selected from the group consisting of a, 0.8 <a ≦ 2.7, b is 0.6 ≦ b ≦ 1.4, and c is 0.8 ≦ c ≦ 1.1, d is 0.01 ≦ d ≦ 0.8, e is 0 ≦ e ≦ 0.1, f1 is the numerical value of a, b, c, d and e, and the valence of M And a number that depends on the valence of X and satisfies the electrical neutrality.)
_{A a M b Si c- (d} + e) Ge d X e O f (2)
(In the formula (2), A, M and X represent the same kind of atoms as described above, a, b, c, d and e represent the same numerical values as described above, and f represents a, b, c, d and e. And a number that depends on the valence of M and the valence of X and satisfies the electrical neutrality.)   The method for producing a silicate compound according to claim 1, wherein the melting step is a step of obtaining a melt by heating the raw material formulation. In the raw material formulation,
Atom A is A carbonate, A bicarbonate, A hydroxide, A silicate, A phosphate, A borate, A fluoride, A chloride, A At least one selected from the group consisting of nitrates of A, sulfates of A, and organic acid salts of A (however, some or all of the at least one may each form a hydrate salt). Included as
Atom M is M oxide, M oxyhydroxide, M silicate, M phosphate, M borate, metal M, M fluoride, M chloride, M nitrate And at least one selected from the group consisting of M sulfate, M alkoxide, and M organic acid salt,
Si is included as at least one selected from the group consisting of silicon oxide, A or M silicate, and Si alkoxide,
Ge is included as at least one selected from the group consisting of germanium oxide, A or M germanate, and Ge alkoxide;
Atom X is X oxide, X hydroxide, X silicate, phosphoric acid, phosphate, A or M vanadate, A or M niobate, A or M tantalate The method for producing a silicic acid compound according to claim 2, which is contained as at least one selected from the group consisting of a salt, an alkoxide of X, and an organic acid salt of X. The manufacturing method of the silicic acid compound as described in any one of Claims 1-3 whose cooling rate in the said cooling process is -10 < ³ >-10 < ¹⁰ > degreeC / sec.   The method for producing a silicate compound according to any one of claims 1 to 4, wherein an average particle diameter of the pulverized product obtained in the pulverization step is 10 nm to 10 µm in terms of volume median diameter.   In the pulverization step, the solidified product includes at least one carbon source selected from the group consisting of an organic compound and a carbon-based conductive active material, and a carbon equivalent amount (mass) in the carbon source is the solidified It is 0.1-20 mass% with respect to the total mass of the mass of a thing, and the carbon conversion amount (mass) in this carbon source of the silicic acid compound as described in any one of Claims 1-5. Production method.   The manufacturing method of the silicate compound as described in any one of Claims 1-6 which performs the said heating process in 500-1,000 degreeC in inert gas or reducing gas.   The manufacturing method of the silicate compound as described in any one of Claims 1-7 whose crystallite diameter of the said silicate compound obtained at the said heating process is 10-50 nm.   The manufacturing method of the silicic acid compound as described in any one of Claims 1-8 whose said silicic acid compound obtained at the said heating process is an olivine type crystal particle. The method for producing a silicate compound according to any one of claims 1 to 9, wherein the silicate compound obtained in the heating step has a composition represented by the following formula (3).
_{Li 2 Fe 1-g Mn g} Si c- (d + e) Ge d X e O f (3)
(In formula (3), X represents the same kind of atom as described above, c, d, e and f represent the same numerical values as described above, and g is 0 ≦ e ≦ 1.)   A silicate compound is obtained by the production method according to any one of claims 1 to 10, and then a positive electrode for a secondary battery is produced using the silicate compound as a positive electrode material for a secondary battery. A method for producing a positive electrode for a secondary battery.   A method for producing a secondary battery, comprising: obtaining a positive electrode for a secondary battery by the production method according to claim 11, and then producing a secondary battery using the positive electrode for the secondary battery. A silicic acid compound having a composition represented by the following formula (2).
_{A a M b Si c- (d} + e) Ge d X e O f (2)
(In Formula (2), A is at least one atom selected from the group consisting of Li, Na and K, M is at least one atom selected from the group consisting of Fe, Mn, Co and Ni, and X is P , Ta, Nb and V, at least one atom selected from the group consisting of a, 0.8 <a ≦ 2.7, b is 0.6 ≦ b ≦ 1.4, and c is 0.8 ≦ c ≦ 1.1, d is 0.01 ≦ d ≦ 0.8, e is 0 ≦ e ≦ 0.1, f is a numerical value of a, b, c, d and e, and valence of M And a number that depends on the valence of X and satisfies the electrical neutrality.)   The silicate compound according to claim 13 is a particle containing a conductive material derived from a carbon source, and the conductive material is 0.1 to 20 with respect to a total mass of the silicate compound and the conductive material. Particles containing mass%.

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