JP5493330B2 - Water-based lithium secondary battery - Google Patents

Description

  The present invention relates to an aqueous lithium secondary battery.

Conventionally, an aqueous lithium secondary battery using an aqueous solution as an electrolytic solution is known (see, for example, Patent Document 1). This aqueous lithium secondary battery has the following advantages with respect to the problems of non-aqueous lithium secondary batteries. That is, the water-based lithium secondary battery does not basically burn because an organic solvent is not used for the electrolyte. In addition, since a dry environment is not required in the manufacturing process, manufacturing costs can be significantly reduced. Furthermore, since an aqueous electrolyte generally has higher conductivity than a non-aqueous electrolyte, an aqueous lithium secondary battery has a lower internal resistance than a non-aqueous lithium secondary battery. On the other hand, the water-based lithium secondary battery is required to be used in a potential range where no water electrolysis reaction occurs, and therefore, the electromotive force is smaller than that of the non-aqueous lithium secondary battery. When calculated from the electrolysis voltage of water, the limit of electromotive force is about 1.2V. However, in reality, overvoltage is necessary to generate gas by electrolysis, so about 2V is expected to be the limit. Is done. Thus, in a water based lithium secondary battery, high safety, low cost, and low internal resistance are ensured at the expense of high voltage and high energy density. For this reason, the water-based lithium secondary battery is suitable for uses such as an electric vehicle, a hybrid electric vehicle, and a home-use distributed power source that require a large-sized battery with a relatively high priority on cost. In such an aqueous lithium secondary battery, Patent Document 1 proposes a battery having a larger battery capacity by using LiFePO ₄ as a positive electrode active material.
JP 2002-260722 A

  However, the water-based lithium secondary battery described in Patent Document 1 has a problem in that the initial discharge capacity is small, and the capacity is reduced when repeated charging and discharging are performed.

  This invention is made | formed in view of such a subject, and it aims at providing the water-system lithium secondary battery which can improve an initial stage discharge capacity and charging / discharging cycling characteristics more.

  As a result of diligent research to achieve the above-described object, the present inventors have found that a lithium-transition metal-silicon composite oxide (provided that the transition metal is at least one of Fe, Co, Ni, and Mn) is active in the positive electrode. As a material, it was found that the initial discharge capacity and charge / discharge cycle characteristics can be further improved, and the present invention has been completed.

That is, the aqueous lithium secondary battery of the present invention is
A positive electrode having a positive electrode active material containing a lithium-transition metal-silicon composite oxide (wherein the transition metal is at least one of Fe, Co, Ni, and Mn);
A negative electrode having a negative electrode active material capable of inserting and extracting lithium;
An aqueous electrolyte containing a lithium salt as a main electrolyte;
It is equipped with.

In this water based lithium secondary battery, the initial discharge capacity and charge / discharge cycle characteristics can be further improved. The reason why such an effect is obtained is not clear, but is presumed as follows. For example, a lithium-transition metal-silicon composite oxide that is a positive electrode active material is Li ₂ MeSiO ₄ (where Me is at least one of Fe, Co, Ni, and Mn). includes two Li ⁺ for, showing a large charge-discharge capacity than such a LiFePO ₄ comprising one of Li ⁺ for one transition metal. In addition, since the lithium-transition metal-silicon composite oxide has a high crystal structure stability, the elution of the transition metal is suppressed, and it is assumed that the charge / discharge cycle is repeated more stably.

  The aqueous lithium secondary battery of the present invention includes a positive electrode having a positive electrode active material containing a lithium-transition metal-silicon composite oxide (wherein the transition metal is at least one of Fe, Co, Ni, and Mn), lithium A negative electrode having a negative electrode active material that can be occluded / released, and an aqueous electrolyte containing a lithium salt as a main electrolyte.

The positive electrode of the lithium secondary battery of the present invention may be prepared by mixing a positive electrode active material, a conductive material, and a binder, for example, and may be pressure-bonded to the surface of the current collector. A paste obtained by adding a solvent may be applied and dried on the surface of the current collector, and may be compressed to increase the electrode density as necessary. In the water based lithium secondary battery of the present invention, the positive electrode has a positive electrode active material containing a lithium-transition metal-silicon composite oxide. This positive electrode active material is a composite oxide containing lithium, a transition metal, and silicon. For example, the basic composition may be Li ₂ MeSiO ₄ . Here, the transition metal (Me) is at least one of Fe, Co, Ni, and Mn. Of these lithium-transition metal-silicon composite oxides, Li ₂ MnSiO ₄ is more preferably used as the positive electrode active material. In this way, the initial discharge capacity can be further increased in a preferable potential region, and more stable cycle characteristics can be obtained. This positive electrode active material may be used by mixing two or more kinds of complex oxides such as Li ₂ MnSiO ₄ and Li ₂ FeSiO ₄ . _{Further, Li 2 Mn 1-x Fe} x SiO 4 (X is a positive number) may be obtained by substitution of one transition metal in the other transition metals as.

  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), styrene butadiene rubber (SBR), polyacrylonitrile (PAN), etc. alone or as a mixture of two or more Can 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. 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. As the current collector, aluminum, titanium, stainless steel, nickel, baked carbon, a conductive polymer, conductive glass, or the like can be used. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group. The thickness of the current collector is, for example, 1 to 500 μm.

The negative electrode of the lithium secondary battery of the present invention may be, for example, a negative electrode material obtained by mixing a negative electrode active material, a conductive material, and a binder, and may be pressure-bonded to the surface of the current collector. A paste obtained by adding a solvent may be applied and dried on the surface of the current collector, and may be compressed to increase the electrode density as necessary. The negative electrode active material is not particularly limited as long as it is a material that occludes and releases lithium ions, but it can reversibly occlude and release as much lithium ions as possible in a potential range where hydrogen is not generated by electrolysis of water. Those are preferred. For example, the negative electrode active material includes an inorganic compound such as a composite oxide containing a transition metal, and among these, an oxide or water containing a transition metal such as vanadium, iron, titanium, manganese, molybdenum, or tungsten. It is preferable to use oxides or composite oxides of these metals and lithium. Examples of such negative electrode active materials include oxides containing vanadium (LiV ₂ O ₄ , LiV ₃ O ₈ , VO _2, etc.) and phosphate compounds containing titanium (LiTi ₂ (PO ₄ ) ₃ , TiP ₂ O _7, etc.). Among them, it is more preferable that the negative electrode active material is one or more selected from LiTi ₂ (PO ₄ ) ₃ , TiP ₂ O ₇ and LiV ₂ O ₄ . In addition, as the conductive material, binder, solvent, and the like used for the negative electrode, those exemplified for the positive electrode can be used. For the current collector of the negative electrode, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, or the like can be used. The shape of the current collector can be the same as that of the positive electrode.

In the aqueous lithium secondary battery of the present invention, the aqueous electrolyte solution is not particularly limited as long as it has a lithium salt as a main electrolyte. Examples of the lithium salt include LiNO ₃ , Li ₂ SO ₄ , LiOH, LiCl, and CH ₃ COOLi. Among these, LiNO ₃ is preferable from the viewpoint of solubility. These lithium salts can be used alone or in combination of two or more. The pH of the aqueous electrolyte is preferably 4-10. When the pH of the aqueous electrolyte is 4 or more, generally, chemical oxidation of the positive electrode active material and the negative electrode active material does not easily occur, and inhibition of Li insertion / release by protons hardly occurs, and the battery capacity and charge / discharge cycle characteristics are improved. On the other hand, when the pH is 10 or less, the charge / discharge reaction of the positive electrode active material proceeds at an electrolysis potential of water, that is, a potential at which almost no oxygen is generated, so that generation of oxygen at the positive electrode can be further suppressed.

  In the aqueous lithium secondary battery of the present invention, a separator may be provided between the positive electrode and the negative electrode. The separator is not particularly limited as long as it is a material that can withstand the use range of the lithium secondary battery, such as a microporous film of a polymer compound. For example, polyethylene, polypropylene, polyvinylidene fluoride, polyvinylidene chloride, polyacrylonitrile, polyacrylamide, polytetrafluoroethylene, polysulfone, polyethersulfone, polycarbonate, polyamide, polyimide, polyethylene oxide, polypropylene oxide and other polyethers, carboxymethylcellulose And celluloses such as hydroxypropyl cellulose, polymer compounds mainly composed of poly (meth) acrylic acid and various esters thereof, derivatives thereof, and films made of copolymers or mixtures thereof. Moreover, these may be used independently and may be used as a multilayer film which piled up the some film. Further, for example, an additive for enhancing ion conductivity and various additives for enhancing strength and corrosion resistance may be added to these films. Of these microporous films, polyethylene, polypropylene, polyvinylidene fluoride, polysulfone and the like are preferably used. This separator is preferably subjected to a hydrophilic treatment or microporosity so that the aqueous electrolyte solution can permeate and ions can easily pass therethrough.

The shape of the aqueous lithium 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. A plurality of such water-based lithium secondary batteries may be connected in series to serve as a power source for an electric vehicle. Examples of the electric vehicle include an electric vehicle that is driven only by a battery, a hybrid electric vehicle that combines an internal combustion engine and a motor drive, and a fuel cell vehicle that generates power using a fuel cell. An example of this water-based lithium 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 having a positive electrode active material provided at a lower portion of the battery case 21, and a negative electrode active material having a positive electrode 22 via a separator 24. The battery case 21 is disposed in the opening of the battery case 21 via the gasket 25, the negative electrode 23 provided in the opposite position, the aqueous electrolyte solution 28 containing the electrolyte, the gasket 25 formed of an insulating material, and the gasket 25. A sealing plate 26 for sealing. Here, the positive electrode 22 has Li ₂ MeSiO ₄ (wherein Me is at least one of Fe, Co, Ni, and Mn) as a positive electrode active material.

The water-based lithium secondary battery of the present embodiment described in detail above has Li ₂ MeSiO ₄ (wherein Me is at least one of Fe, Co, Ni, and Mn) as the positive electrode active material, and the initial discharge capacity. In addition, the charge / discharge cycle characteristics can be further improved. This is considered to be due to the fact that the unit cell contains two Li ⁺ and exhibits a large charge / discharge capacity, or the contained Si acts to suppress elution of transition metals. .

  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.

  The following is a specific example of a water-based lithium secondary battery including a positive electrode using a lithium-transition metal-silicon composite oxide (wherein the transition metal is at least one of Fe, Co, Ni, and Mn) as a positive electrode active material. Examples prepared in the following will be described as examples. First, a method for manufacturing an active material will be described.

<Preparation of Li ₂ MnSiO ₄ >
To 255 mL of the water / ethanol mixed solvent subjected to Ar bubbling, 0.0375 mol of CH ₃ COOLi, 0.0188 mol of (CH ₃ COO) ₂ Mn, 0.01875 mol of Si (OC ₂ H ₅ ) ₄ were added, and Ar atmosphere was added. Was refluxed at 80 ° C. for 6 hours to obtain a precipitate. The precipitate was collected by a rotary evaporator, vacuum-dried at 105 ° C. for 12 hours, and then baked at 600 ° C. for 8 hours in an Ar atmosphere containing 4 vol% H ₂ to prepare Li ₂ MnSiO ₄ .

<Production of LiV ₂ O ₄ >
Lithium carbonate (Li ₂ CO ₃ ) and vanadium pentoxide (V ₂ O ₅ ) were weighed according to the stoichiometric ratio of the above composition formula and mixed in an automatic mortar for 20 minutes. Thereafter, 2 parts by weight of carbon black (TB-5500 manufactured by Tokai Carbon Co., Ltd.) was added to 100 parts by weight of the mixture, and further mixed for 20 minutes in an automatic mortar. The mixture was baked in an argon stream at 750 ° C. for 24 hours, and then rapidly cooled to prepare LiV ₂ O ₄ .

<Preparation of LiTi ₂ (PO ₄ ) ₃ >
Titanium oxide (TiO ₂ (anatase)), lithium carbonate (Li ₂ CO ₃ ), and ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) were weighed so as to have a reciprocal composition and mixed thoroughly in a mortar. The mixed powder was formed into pellets and calcined at 300 ° C. for 6 hours in the atmosphere. The pellet was taken out and sufficiently pulverized in a mortar, then formed into a pellet again and calcined at 600 ° C. for 24 hours in the atmosphere. The pellets were taken out and sufficiently pulverized in a mortar, and then formed into pellets again, and then subjected to main firing at 900 ° C. for 24 hours in the air to produce LiTi ₂ (PO ₄ ) ₃ .

<Preparation of LiFePO ₄ >
As a starting material, iron oxalate having a valence of iron, lithium carbonate, and ammonium dihydrogen phosphate are mixed so that a molar ratio of Li: Fe: P is 1.2: 1: 1, and pelletized. LiFePO ₄ was produced by molding into 650 ° C. and firing in an Ar atmosphere for 24 hours.

[Example 1]
A 2016 type coin battery was prepared as a water based lithium secondary battery. 70% by weight of Li ₂ MnSiO ₄ as a positive electrode active material, 25% by weight of carbon 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. LiV ₂ O ₄ produced as described above was used as the negative electrode active material. LiV ₂ O ₄ was 70% by weight, carbon as a conductive agent was 25% by weight, and PTFE as a binder was 5% by weight and mixed well. This mixed powder 21.5 mg (active material amount 15 mg) was pressure-bonded at about 0.6 ton / cm ² onto a SUS mesh previously welded to the inside of the coin cell to obtain an electrode. Using a polyethylene separator (E25MMS manufactured by Tonen Chemical Co., Ltd.) with a hydrophilic treatment, the positive electrode and the negative electrode were separated, and the electrolyte contained 6 mol / L LiNO ₃ , 0.1 mol / L CH ₃ COOH,. An aqueous solution containing 9 mol / L CH ₃ COOK was used. The electrolytic solution was introduced into a coin battery and sealed to obtain a test coin battery.

[Example 2]
A coin battery was produced in the same manner as in Example 1 except that the above LiTi ₂ (PO ₄ ) ₃ was used as the negative electrode active material.

[Comparative Examples 1 and 2]
A coin battery was produced in the same manner as in Example 1 except that LiFePO ₄ was used as the positive electrode active material. A coin battery was produced in the same manner as in Example 2 except that LiFePO ₄ was used as the positive electrode active material.

(Battery performance evaluation)
Under an environment of 20 ° C., the Li insertion / extraction test was repeated 20 times with a constant current method and a current density of 0.162 mA / cm ² . The cut-off voltage was 0.8 V to 1.4 V in Example 1, 0.7 V to 1.3 V in Example 2 and Comparative Example 1, and 0.6 V to 1.2 V in Comparative Example 2. When the discharge capacity at the first cycle of the charge / discharge cycle test is the initial discharge capacity CAP ₁ (mAh / g) and the discharge capacity at the _nth cycle is CAP _n (mAh / g), the capacity maintenance ratio CAPma (% ) Was calculated using the formula CAP _n / CAP ₁ × 100.

(Experimental result)
Table 1 shows the electrode configurations, cut-off voltages, initial discharge capacities, and 20th cycle capacity retention rates of Examples 1 and 2 and Comparative Examples 1 and 2. FIG. 2 shows the relationship between the number of cycles and the capacity retention rate of the battery. As shown in Table 1, compared to Comparative Examples 1 and 2 using LiFePO ₄ as the positive electrode and less than 115 mAh / g, an aqueous lithium secondary battery using Li ₂ MnSiO ₄ (Example) 1, 2) showed a high initial discharge capacity of 130 mAh / g or more. In addition, as shown in FIG. 2, regardless of the type of the negative electrode, the batteries (Examples 1 and 2) using Li ₂ MnSiO ₄ for the positive electrode have better cycle characteristics than Comparative Examples 1 and 2 using LiFePO _4. showed that. From the above results, it was found that an aqueous lithium secondary battery using Li ₂ MnSiO ₄ as a positive electrode active material can realize an aqueous lithium secondary battery having an unprecedented large discharge capacity and high cycle stability. .

  The present invention is applicable to the battery industry.

2 is a cross-sectional view illustrating a schematic configuration of a coin-type battery 20. FIG. It is a figure showing the relationship of the capacity maintenance rate of the battery output with respect to the cycle number.

Explanation of symbols

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

Claims (2)

A positive electrode having a positive electrode active material containing a lithium-transition metal-silicon composite oxide (wherein the transition metal is at least one of Fe, Co, Ni, and Mn);
A negative electrode having one or more negative electrode active materials selected from LiTi ₂ (PO ₄ ) ₃ and TiP ₂ O ₇ capable of inserting and extracting lithium;
An aqueous electrolyte containing a lithium salt as a main electrolyte;
A water based lithium secondary battery. The water based lithium secondary battery according to claim 1, wherein the positive electrode uses Li ₂ MnSiO ₄ as a lithium-transition metal-silicon composite oxide as a positive electrode active material.

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