JP5482303B2 - Non-aqueous secondary battery active material and non-aqueous secondary battery - Google Patents

Description

  The present invention relates to an active material for a non-aqueous secondary battery and a non-aqueous secondary battery.

Conventionally, silicic acid-based polyanion compounds represented by Li ₂ MnSiO ₄ , Li ₂ FeSiO _{4 and the} like are known as active materials for non-aqueous secondary batteries (see, for example, Patent Document 1). Such a silicic acid-based polyanion compound contains two lithium atoms per one transition metal and has a larger theoretical capacity than a phosphoric acid-based polyanion compound represented by LiFePO ₄ or the like. It is expected to contribute to capacity. However, for example, in a battery using a compound represented by Li ₂ MnSiO ₄ as a battery active material, the amount of lithium used at the time of initial charge and discharge is about 1.2 Li per Mn, and a sufficient capacity can be obtained. It has been reported (see, for example, Non-Patent Document 1).

JP 2001-266882 A

Journal of Power Sources, 2007, 174, 528-532.

  Thus, although the conventional silicic acid-based polyanion compound has a high theoretical capacity, it may not be able to exhibit a sufficient capacity in practice. For this reason, what has a capacity closer to the theoretical capacity, that is, a capacity capable of further increasing the capacity has been desired.

  The present invention has been made to solve such a problem, and a main object of the present invention is to provide a non-aqueous secondary battery active material and a non-aqueous secondary battery that can further increase the capacity.

In order to achieve the above-mentioned object, the present inventors changed a part of Mn of the compound represented by the general formula Li ₂ MnSiO ₄ to a divalent ion (for example, Zn) having an ionic radius smaller than that of Mn. As a result, a lithium secondary battery was fabricated using a cathode active material as a positive electrode active material. As a result, it was found that the capacity could be increased and the present invention was completed.

That is, the non-aqueous secondary battery active material of the present invention is represented by the general formula Li ₂ Mn _1-x D _x SiO ₄ (D is a divalent ion having an ionic radius smaller than Mn, 0 <x <1). Is.

  The non-aqueous secondary battery of the present invention includes a positive electrode having the above-described active material for a non-aqueous secondary battery, a negative electrode having a negative electrode active material, an ion that is interposed between the positive electrode and the negative electrode and conducts lithium. A conductive medium.

In this non-aqueous secondary battery active material and non-aqueous secondary battery, the capacity can be further increased. The reason why such an effect can be obtained is not clear, but Li ₂ MnSiO ₄ having a structure in which a part of Mn is a divalent ion having an ionic radius smaller than that of Mn has a lithium conduction path in the active material crystal. This is probably because the size is appropriate and the lithium diffusion resistance in the active material is low, so that the polarization during charging and discharging is reduced.

2 is a cross-sectional view illustrating a schematic configuration of a coin-type battery 20. FIG. It is an initial discharge curve of Example 1 and Comparative Example 1. It is a graph showing the relationship between the value and the discharge capacity of x in the general formula _{Li 2 Mn 1-x D x} SiO 4.

  The nonaqueous secondary battery of the present invention includes a positive electrode having an active material for a nonaqueous secondary battery, a negative electrode having a negative electrode active material, an ion conductive medium that is interposed between the positive electrode and the negative electrode and conducts lithium, It is equipped with.

The positive electrode of the non-aqueous secondary battery of the present invention is obtained by mixing a positive electrode active material, a conductive material and a binder, and adding a suitable solvent to form a paste-like positive electrode material on the surface of the current collector. It may be formed by coating and drying, and compressing to increase the electrode density as necessary. In the non-aqueous secondary battery of the present invention, the positive electrode active material is an active material for a non-aqueous secondary battery represented by a general formula Li ₂ Mn _1-x D _x SiO ₄ . Here, D is a divalent ion having an ionic radius smaller than that of Mn, and x is a number satisfying 0 <x <1. Thus, with Li ₂ MnSiO ₄ having a structure in which a part of Mn is a divalent ion having an ionic radius smaller than that of Mn, the lithium conduction path in the active material crystal is considered to be small. And, the size of this conduction path is appropriate, and the lithium diffusion resistance in the active material is low, so that the polarization during charging and discharging is reduced and the insertion and release of lithium is performed smoothly. It can be increased. Here, the ion radius is R.I. D. Refers to the ionic radius (effective ionic radius) by Shannon (see Acta Cryst. (1976). A32, 751). Note that, since Mn and D in Li ₂ Mn _1-x D _x SiO ₄ are surrounded by four O ions, a four-coordinate value is used. According to this, the ionic radius of Mn is 0.66 Å. And as D component which is a divalent ion whose ion radius is smaller than Mn, for example, Zn (0.60Ｚｎ), Fe (0.63Å), Co (0.58Å), Ni (0.55Å), Cu (0.57 cm). Of these, Zn having an ionic radius of 0.60Å is preferable. In this way, the capacity can be further increased. x may satisfy 0 <x <1, but preferably satisfies 0.02 ≦ x ≦ 0.20, and preferably satisfies 0.05 ≦ x ≦ 0.18. It is more preferable that 0.07 ≦ x ≦ 0.15 is satisfied. If x is 0.02 or more, the lithium conduction path (conduction path) can be reduced, and if it is 0.20 or less, the lithium conduction path does not become too small, and the battery capacity directly increases. This is because it is considered that Mn involved in the increase / decrease in the amount is not too small. Note that “represented by the general formula Li ₂ Mn _1-x D _x SiO ₄ ” means that the raw material composition is synthesized so as to have such a composition. For example, Li, Mn, A part of Si may be substituted with another element, or a non-stoichiometric composition in which part of the element is deficient or excessive.

The method for producing the active material for a non-aqueous secondary battery represented by the general formula Li ₂ Mn _1-x D _x SiO ₄ is not particularly limited, and a solid phase method, a liquid phase method, a gas phase method, etc. are used. Can be. For example, it may be obtained as follows. First, an organic acid salt (acetate, oxalate, etc.) of each component metal is added to a water / alcohol mixed solvent bubbled with an inert gas so as to have a predetermined composition ratio. Reflux for several hours under an inert atmosphere and collect the precipitate. The precipitate is dried under an inert atmosphere, and further fired in an inert atmosphere or a reducing atmosphere for several hours, whereby the intended silicon oxide powder can be obtained. Here, argon gas, helium gas, nitrogen gas, or the like can be used as the inert gas, and the inert atmosphere can be an argon atmosphere, a helium atmosphere, a nitrogen atmosphere, or a vacuum atmosphere. it can. The reducing atmosphere may be pure hydrogen gas or the like in addition to a mixed gas of hydrogen gas and the above-described inert gas.

  The conductive material contained in the positive electrode is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the positive electrode. , Carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) or a mixture of two or more thereof can be used. Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coatability. The binder serves to bind the active material particles and the conductive material particles. For example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluorine-containing resin such as fluorine rubber, or polypropylene, Thermoplastic resins such as polyethylene, ethylene-propylene-dienemer (EPDM), sulfonated EPDM, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. In addition, an aqueous dispersion of cellulose or styrene butadiene rubber (SBR), which is an aqueous binder, can also be used. Examples of the solvent for dispersing the positive electrode active material, the conductive material, and the binder include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, and N, N-dimethylaminopropyl. Organic solvents such as amine, ethylene oxide, and tetrahydrofuran can be used. Moreover, a dispersing agent, a thickener, etc. may be added to water, and an active material may be slurried with latex, such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more. Examples of the application method include roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain an arbitrary thickness and shape. Current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, and aluminum, copper, etc. for the purpose of improving adhesion, conductivity, and oxidation resistance. A surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group.

  In the nonaqueous secondary battery of the present invention, the negative electrode has a negative electrode active material. This negative electrode active material is preferably capable of occluding and releasing lithium. Here, examples of the material capable of occluding and releasing lithium include metal lithium and lithium alloys, metal oxides, metal sulfides, and carbonaceous substances that occlude and release lithium ions. Examples of the lithium alloy include alloys of lithium with aluminum, silicon, tin, magnesium, indium, calcium, and the like. Examples of the metal oxide include tin oxide, silicon oxide, lithium titanium oxide, niobium oxide, and tungsten oxide. Examples of the metal sulfide include tin sulfide and titanium sulfide. As the carbonaceous material that occludes and releases lithium ions, for example, hard carbon or soft carbon can be used, or natural or artificial graphite, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber, gas phase. A mixture of carbon fiber, a fired organic compound such as phenol resin, coke, or the like can be used.

In the non-aqueous secondary battery of the present invention, the ion conductive medium may be a non-aqueous electrolyte solution, an ionic liquid, a gel electrolyte, a solid electrolyte, or the like in which a supporting salt is dissolved in an organic solvent. Of these, a non-aqueous electrolyte is preferable. Examples of the supporting salt include known LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , Li (CF ₃ SO ₂ ) ₂ N, Li (CF ₃ SO ₃ ), LiN (C ₂ F ₅ SO ₂ ), and the like. Supporting salts can be used. The concentration of the supporting salt is preferably 0.1 to 2.0M, and more preferably 0.8 to 1.2M. As an organic solvent, for example, ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (γ-BL), diethyl carbonate (DEC), dimethyl carbonate (DMC) and the like are used for conventional secondary batteries and capacitors. An organic solvent is mentioned. These may be used alone or in combination. Further, the ionic liquid is not particularly limited, but 1-methyl-3-propylimidazolium bis (trifluorosulfonyl) imide, 1-ethyl-3-butylimidazolium tetrafluoroborate, or the like is used. Can do. The gel electrolyte is not particularly limited. For example, a polymer such as polyvinylidene fluoride, polyethylene glycol, or polyacrylonitrile, or a saccharide such as an amino acid derivative or sorbitol derivative is added with an electrolyte containing a supporting salt. And a gel electrolyte. Examples of the solid electrolyte include inorganic solid electrolytes and organic solid electrolytes. As the inorganic solid electrolyte, for example, Li nitride, halide, oxyacid salt, and the like are well known. Among _{them, Li 4 SiO 4, Li 4} SiO 4 -LiI-LiOH, zLi 3 PO 4 - (1-z) Li 4 SiO 4, Li 2 SiS 3, Li 3 PO 4 -Li 2 S-SiS 2, sulfide Examples thereof include phosphorus compounds. These may be used alone or in combination. Examples of the organic solid electrolyte include polyethylene oxide, polypropylene oxide, polyvinyl alcohol, polyvinylidene fluoride, polyphosphazene, polyethylene sulfide, polyhexafluoropropylene, and derivatives thereof. These may be used alone or in combination.

  The nonaqueous secondary battery of the present invention may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the usage range of the secondary battery. Is mentioned. These may be used alone or in combination.

The shape of the nonaqueous secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing etc. which are used for an electric vehicle etc. An example of this non-aqueous secondary battery is shown in FIG. FIG. 1 is a cross-sectional view schematically showing the configuration of the coin-type battery 20. The coin-type battery 20 includes a cup-shaped battery case 21, a positive electrode 22 provided inside the battery case 21, a negative electrode 23 provided at a position facing the positive electrode 22 via a separator 24, A non-aqueous electrolyte solution 28 containing a supporting salt, a gasket 25 formed of an insulating material, and a sealing plate 26 disposed in the opening of the battery case 21 and sealing the battery case 21 via the gasket 25. Yes. The positive electrode 22 includes the above-described positive electrode active material represented by the general formula Li ₂ Mn _1-x D _x SiO ₄ (D is a divalent ion having an ionic radius smaller than that of Mn, 0 <x <1). It is.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  For example, in the above-described embodiment, the non-aqueous secondary battery has been described. However, the above-described positive electrode active material is not limited to that incorporated in the non-aqueous secondary battery, and is a single non-aqueous secondary battery active material. May be.

  Below, the example which produced the non-aqueous secondary battery concretely is demonstrated.

[Synthesis of positive electrode active material]
Example 1
In 255 mL of a water / ethanol mixed solvent subjected to Ar bubbling, 37.50 mmol of CH ₃ COOLi, 16.88 mmol of (CH ₃ COO) ₂ Mn, 1.875 mmol of (CH ₃ COO) ₂ Zn, Si (OC ₂ H _{5 4} ) 18.75 mmol was added and refluxed at 80 ° C. for 6 hours under an Ar atmosphere to obtain a precipitate. The precipitate was collected by a rotary evaporator, vacuum-dried at 105 ° C. for 12 hours, and then calcined at 600 ° C. for 8 hours in an Ar atmosphere containing 4% by volume of H _{2. 2} Mn _0.90 Zn _0.10 SiO ₄ was synthesized.

(Examples 2 to 5)
The positive electrode active material Li ₂ Mn _0.90 Fe _0.10 SiO ₄ of Example 2 was synthesized by performing the same steps as in Example 1 except that (CH ₃ COO) ₂ Fe was used instead of (CH ₃ COO) ₂ Zn. . Further, the same process as in Example 1 was performed except that (CH ₃ COO) ₂ Co was used instead of (CH ₃ COO) ₂ Zn, and the positive electrode active material Li ₂ Mn _0.90 Co _0.10 SiO ₄ of Example 3 was used. Synthesized. Further, the same process as in Example 1 was performed except that (CH ₃ COO) ₂ Ni was used instead of (CH ₃ COO) ₂ Zn, and the positive electrode active material Li ₂ Mn _0.90 Ni _0.10 SiO ₄ of Example ₄ was used. Synthesized. Further, the same process as in Example 1 was performed except that (CH ₃ COO) ₂ Cu was used instead of (CH ₃ COO) ₂ Zn, and the positive electrode active material Li ₂ Mn _0.90 Cu _0.10 SiO ₄ of Example 5 was used. Synthesized.

(Examples 6 to 8)
(CH ₃ COO) ₂ Mn is replaced with 16.88 mmol and (CH ₃ COO) ₂ Zn is replaced with 1.875 mmol, and (CH ₃ COO) ₂ Mn is used for 17.81 mmol and (CH ₃ COO) ₂ Zn is used for 0.937 mmol. The positive electrode active material Li ₂ Mn _0.95 Zn _0.05 SiO ₄ of Example 6 was synthesized by performing the same steps as in Example 1 except that the above was performed. Further, 16.88 mmol of (CH ₃ COO) ₂ Mn and 1.875 mmol of (CH ₃ COO) ₂ Zn were replaced with 15.94 mmol of (CH ₃ COO) ₂ Mn and 2. of (CH ₃ COO) ₂ Zn. The positive electrode active material Li ₂ Mn _0.85 Zn _0.15 SiO ₄ of Example 7 was synthesized by performing the same process as in Example 1 except that 813 mmol was used. Further, 16.88 mmol of (CH ₃ COO) ₂ Mn and 1.875 mmol of (CH ₃ COO) ₂ Zn were used instead of 13.13 mmol of (CH ₃ COO) ₂ Mn and 5. of (CH ₃ COO) ₂ Zn. The positive electrode active material Li ₂ Mn _0.70 Zn _0.30 SiO ₄ of Example 8 was synthesized by performing the same process as in Example 1 except that 625 mmol was used.

(Comparative Example 1)
The (CH ₃ COO) and ₂ Mn and 16.88mmol (CH ₃ COO) ₂ Zn instead 1.875mmol the (CH ₃ COO) except that the ₂ Mn was used 18.75mmol Example 1 the same process The positive electrode active material Li ₂ MnSiO ₄ of Comparative Example 1 was synthesized.

[Preparation of bipolar evaluation cell]
A bipolar evaluation cell was produced using a 2016 coin cell. First, 70% by weight of the positive electrode active material synthesized as described above, 25% by weight of carbon black as a conductive agent, and 5% by weight of polytetrafluoroethylene (PTFE) as a binder were mixed well. . This mixed powder 14.3 mg (active material amount 10 mg) was pressed onto an SUS mesh previously welded to the inside of the coin cell at about 0.6 ton / cm ² to obtain an electrode. Li metal foil was used for the negative electrode. A separator (Tonen Tapils) was used to separate the positive electrode and the negative electrode. An electrolytic solution in which LiPF ₆ was dissolved to 1M in a solvent in which ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 1 was introduced into a coin battery and sealed to obtain a bipolar evaluation battery. .

[Charge / discharge test]
The discharge capacity was evaluated by a constant current charge / discharge test. The charge upper limit voltage was 4.8 V, the discharge lower limit voltage was 2 V, and the charge / discharge current was 0.167 mA.

[result]
In FIG. 2, the first time discharge curve of Example 1 and Comparative Example 1 is shown. In Example 1 in which a part of Mn of Li ₂ MnSiO ₄ was Zn, the discharge capacity was improved as compared with Comparative Example 1 in which Li ₂ MnSiO ₄ remained as it was. Table 1 shows the initial discharge capacities of Examples 1 to 5 and Comparative Example 1. In Examples 1 to 5 in which a part of Mn in Li ₂ MnSiO ₄ is a different element smaller than the ionic radius of Mn, the discharge capacity is larger than that in Comparative Example 1, and in particular, Example 1 in which a part of Mn is Zn. In particular, the discharge capacity increased. The reason for this is not clear, but was inferred as follows. That is, there is an optimum value for the size of the conduction path of lithium ions, and the size of this conduction path is the ion radius of the D element (hereinafter also referred to as the substitution element D) in the general formula LiMn _1-x D _x SiO ₄ , It is considered that there is a correlation with the value of x in the general formula LiMn _1-x D _x SiO ₄ (hereinafter also referred to as “substitution amount x”). Here, when the substitution amount x is constant, the size of the conduction path is determined by the size of the substitution element D. When a part of Mn is the substitution element D having an ionic radius smaller than Mn, It is considered that the conduction path of lithium ions in the active material becomes small, and unnecessary movement of lithium is suppressed. And in Examples 1-5, it was guessed that the capacity | capacitance was able to be raised because lithium ion conductivity increased and the polarization at the time of discharge fell because the magnitude | size of this conduction path | route was appropriate. In addition, among those in which part of Mn is Zn, Fe, Co, Ni, Cu, the one in which Zn is the largest has the largest discharge capacity. It is inferred that it has a more appropriate conduction path size.

Table 2 shows the relationship between the Zn substitution amount and the discharge capacity. FIG. 3 is a graph showing the relationship between the value of x and the discharge capacity (mAh / g) in the general formula LiMn _1-x D _x SiO ₄ . From FIG. 3, it was inferred that when the substitution element D was Zn, the discharge capacity could be 170 mAg ⁻¹ or more if the substitution amount x was 0.02 or more and 0.20 or less. In particular, it was found that if the substitution amount x is 0.05 or more and 0.18 or more, the discharge capacity can be further increased, and if it is 0.07 or more and 0.15 or less, the discharge capacity can be further increased. The reason for this is not clear, but if the substitution amount x with Zn is 0.02 or more, an effect of reducing the conduction path of lithium ions is obtained, and if it is 0.20 or less, the conduction path of lithium ions is small. It was guessed that it would not be too much. In addition, Zn is redox-inactive in the potential region of 2.0 to 4.8 V (vs. Li / Li ⁺ ), and when a part of redox-active Mn in Li ₂ MnSiO ₄ is Zn, It is conceivable that the capacity of the active material is reduced by the decrease, but if the substitution amount x with Zn is 0.20 or less, the amount of Mn is not decreased too much. And if the substitution amount x with Zn was 0.20 or less, it was speculated that the increase in capacity due to appropriate conduction paths exceeded the decrease in capacity due to redox inactivation. In particular, in Example 1 in which the substitution amount x with Zn is 0.10, the balance between the size of the lithium ion conduction path and the amount of redox-active element (Mn) is particularly excellent, so that the capacity can be further increased. It was inferred that Similarly, if the substitution element D is Fe, Co, Ni, or Cu having an ionic radius comparable to that of Zn, the discharge capacity can be increased if the substitution amount x is 0.02 or more and 0.20 or less. The discharge capacity can be further increased if the replacement amount x is 0.05 or more and 0.18 or less, and the discharge capacity can be further increased if the replacement amount x is 0.07 or more and 0.15 or less. It was guessed.

  20 coin cell, 21 battery case, 22 positive electrode, 23 negative electrode, 24 separator, 25 gasket, 26 sealing plate, 28 non-aqueous electrolyte.

Claims (4)

Represented by the general formula Li ₂ Mn _1-x D _x SiO ₄ (D is a divalent ion having an ionic radius smaller than that of Mn, 0.02 ≦ x ≦ 0.20 ) ,
The D is a non-aqueous secondary battery active material containing Zn . Wherein D is, F e, Co, Ni, in which further comprises one or more selected from Cu, non-aqueous secondary battery active material according to claim 1. The active material for a non-aqueous secondary battery according to claim 1, wherein the x satisfies 0.05 ≦ x ≦ 0.18 . A positive electrode having the active material for a non-aqueous secondary battery according to any one of claims 1 to 3 ,
A negative electrode having a negative electrode active material;
An ion conducting medium interposed between the positive electrode and the negative electrode and conducting lithium;
A non-aqueous secondary battery comprising:

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