JP2006202552A - Lithium cell and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a secondary battery which is superior in charge and discharge characteristics by preventing a voltage rise with the elapse of time in the low voltage secondary battery. <P>SOLUTION: A positive electrode active material in a lithium battery is made as lithium titanate in which a BET specific surface area is 3 m<SP>2</SP>/g or less, and in which a composition formula is expressed by LixTiyOz (0≤x≤4, 1≤y≤5, and 2≤z≤12). The positive electrode active material in which the BET specific surface area of lithium titanate is 1-3 m<SP>2</SP>/g is more preferable. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a lithium battery and a method for manufacturing the same.

  Coin-type (button-type) lithium batteries are increasingly used as a power source for device backup due to their high energy density and light weight. Conventional lithium secondary batteries for memory and clock backups are mainly used with a nominal voltage of 3V or more.

  However, in order to save power, the operating voltage of small electronic devices such as mobile phones has been lowered, and secondary batteries for memory and clock backup are also required to have a high capacity in a low voltage region.

In a low voltage region (region less than 3 V), a lithium secondary battery using lithium titanate that exhibits excellent charge / discharge characteristics as a positive electrode active material and silicon oxide as a negative electrode active material is known (see, for example, Patent Document 1). ing.
Japanese Patent Laid-Open No. 10-162828 (Items 2 to 3 and FIG. 2)

  A lithium battery using lithium titanate as a positive electrode active material has a large capacity when the discharge voltage is around 1.5 V, and has a large capacity and is excellent. However, the battery voltage, which was 1.8 V at the time of manufacture, gradually increases to 3 V if left unattended. Thus, the conventional lithium battery has a problem that the voltage increases with time.

  The titanium compound doped with lithium has a large charge / discharge capacity in a low voltage region, and is suitable as a positive electrode active material for a low voltage secondary battery. However, in a secondary battery using a titanium compound as a positive electrode active material, when the initial voltage of 1.8 V ± 0.1 V at the time of manufacture is left at room temperature, the voltage gradually increases from the initial voltage, and 120 There was a problem that the voltage increased by 1V or more when days passed. The discharge capacity in the region of 1.8 V or more is about 4% of the total discharge capacity, and is slight, but since there is a variation in the rising voltage, it is necessary to carry out a discharge process at the time of product shipment to make the initial voltage constant. there were.

  The capacity of 1.8V or more is as small as 4% or less of the total capacity, and the electric potential is lowered to 1.8V just by discharging with a resistance of 10Ω for 5 seconds. Thus, it is considered that an electric double layer is formed. The voltage increase mechanism is considered to be because an electric double layer is formed over time between the titanium compound of the positive electrode active material and the electrolytic solution.

Considering the electrolyte, since the solute is LiClO ₄ and the solvents propylene carbonate (PC) and ethylene carbonate (EC), the dielectric constant of the solvent is quite high, and the electrolyte is ionized into lithium ions and perchlorate ions. Yes. The ion radius of lithium ions is small, and it is thought that three PCs are coordinated. Here, when considering graphite as the conductive agent and the active material lithium titanate, it can be estimated that lithium intercalation into graphite easily occurs. The driving force is considered to be due to the subsequent doping of the surface active active material lithium titanate having a large surface area. Considering the chemical potential of lithium ions, it is presumed that the lithium in the solid is lower in potential between the coordinated state in the solvent and the state doped with spinel lithium titanate. From the knowledge that there is almost no change in the structure during the insertion and release of lithium ions into the spinel structure of lithium titanate, the insertion (doping) of lithium ions is considered to occur easily.

Since the inserted lithium ions are positively charged, the surface of the lithium titanate is charged with a positive charge. ClO ₄ ⁻ collects on the surface of the lithium titanate as counter ions to cancel the charge. Thus, it is considered that a Helmholtz electric double layer is generated on the surface of the positive electrode active material. It is considered that the anion is adsorbed on the positively charged portion of the positive electrode active material and the amount of adsorption increases with time, so that the voltage also increases with time. Assuming that the dielectric constant of the electric double layer is almost constant, lithium ion insertion occurs until the solid active site is filled and equilibrium is reached, and as the lithium ion increases, anions are adsorbed on the surface of lithium titanate. It is considered that an electric double layer is formed, causing a voltage increase with time. For this reason, it is considered that the battery voltage gradually increases by the potential of the electric double layer.

  As described above, the increase in charge / discharge capacity due to voltage increase is negligible and does not contribute to the increase in charge / discharge capacity of the secondary battery. On the other hand, the voltage of the secondary battery varies, and it is necessary to perform the discharge process to make the initial voltage constant, which is complicated.

  An object of the present invention is to provide a secondary battery that solves the above problems and has excellent charge / discharge characteristics.

When the lithium titanate having a BET specific surface area of 3 m ² / g or less in a lithium battery using lithium titanate as a positive electrode active material is used, the present inventors Was found to be less than 3V.

Albeit at estimating electrolytic solution of the nonaqueous electrolyte, lithium _perchlorate, solvent propylene carbonate (PC), a highly ethylene carbonate (EC) tends to form an electric double layer, such as dielectric constant solvent used for, for example, a solute . When the positive electrode, the negative electrode, and the electrolytic solution are placed in the battery container, the solute in the electrolytic solution is adsorbed on the surface of the positive electrode active material due to the potential difference between the positive electrode and the negative electrode to form an electric double layer. It is thought that the amount of adsorption increases with time, and the voltage also increases with time. The amount of solute adsorbed in the electrolyte depends on the BET specific surface area of the positive electrode active material. It was estimated that the greater the BET specific surface area, the greater the amount of adsorption, that is, the greater the voltage rise over time. The capacity of the electric double layer due to the voltage rise is very small and hardly contributes to the increase in capacity of the lithium battery. However, since it exceeds the withstand voltage of a memory or the like, it is necessary to perform a discharge process after assembling the battery to lower the voltage, which is complicated.

A lithium titanate having a BET specific surface area of 3 m ² / g or less is prepared as follows. Lithium hydroxide or lithium carbonate and titanium dioxide are mixed in the starting material and calcined at 700 to 1000 ° C. When the firing temperature is 700 ° C. or less, there is unreacted titanium dioxide, which is not preferable because the battery capacity is small. Moreover, since the optimal crystal structure cannot be obtained at 1000 ° C. or higher, the battery capacity is reduced. Further, the higher the firing temperature, the smaller the BET specific surface area due to melting of the starting material. Therefore, the BET specific surface area of the starting titanium dioxide is preferably 10 m ² / g or less, depending on the firing temperature.

By using lithium titanate having a BET specific surface area of 3 m ² / g or less according to the present invention, it is possible to prevent a voltage increase with time and to provide a lithium battery excellent in charge / discharge characteristics.

The present invention is characterized in that lithium titanate having a BET specific surface area of 3 m ² / g or less is used as the positive electrode active material. The composition of lithium titanate is represented by LixTiyOz (0 ≦ x ≦ 4, 1 ≦ y ≦ 5, 2 ≦ z ≦ 12), Li ₂ TiO ₃ , Li ₄ Ti ₅ O ₁₂ , anatase TiO ₂ , rutile TiO such as _2, it may be one or more of the mixtures. The negative electrode active material is not particularly limited, and conventionally known graphite, silicon, silicon oxide, lithium-aluminum alloy, and the like can be used.

The electrolyte is an organic solvent such as 縺 | butyrolactone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate, 1,2-dimethoxyethane, tetrahydrofuran, dioxolane, dimethylformamide, etc. A non-aqueous (organic) electrolytic solution in which a lithium ion dissociable salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N or the like is dissolved in a single or mixed solvent as an electrolyte Any lithium ion conductive non-aqueous electrolyte may be used, such as a polymer solid electrolyte in which a lithium salt or an organic solvent is dissolved in a polymer such as ethylene oxide or a crosslinked polyphosphazene. In particular, a mixed solvent of a cyclic alkyl carbonate such as propylene carbonate, ethylene carbonate or butylene carbonate and a chain alkyl carbonate such as dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate, such as LiPF ₆ , LiClO ₄ , LiBF ₄ or LiCF ₃ SO ₃ The use of an organic electrolytic solution in which a salt is dissolved is particularly preferable because a battery having excellent charge / discharge characteristics and a long cycle life can be obtained.

  A cross-sectional view of a lithium secondary battery according to the present invention is shown in FIG. A positive electrode 101 made of a positive electrode active material, a positive electrode case 102, a negative electrode 103 made of a negative electrode active material, a negative electrode case 104, metallic lithium 105, a separator 106 separating the positive electrode and the negative electrode, a gasket 107, and an electrolytic solution 108. First, the positive electrode 101 is attached to the positive electrode case 102 and the negative electrode 103 is attached to the negative electrode case 104 with a conductive adhesive. Next, the separator 106 is mounted on the positive electrode, and the electrolytic solution 108 is injected. Metal lithium 105 is attached to the negative electrode 103, the negative electrode case 104 and the positive electrode case 102 are assembled via the gasket 107, and the positive electrode case is caulked and sealed. The battery has an outer diameter of 6.8 mm and a thickness of 2.1 mm.

The case where lithium titanate (Li ₄ Ti ₅ O _12, BET specific surface area 3.0 m ² / g) is used as the positive electrode active material and SiO is used as the negative electrode active material is described. Lithium titanate, which is a positive electrode active material, is mixed with graphite as a conductive agent and polyacrylic acid as a binder, and mixed at a ratio of lithium titanate: graphite: polyacrylic acid = 85: 10: 5. did. Next, 37.3 mg of this positive electrode mixture was pressure-molded into a pellet shape having a diameter of 6.3 mm and a thickness of 0.57 mm at 2 ton / cm 2 to obtain a positive electrode. The positive electrode obtained in this manner was bonded and integrated with the positive electrode case using a conductive adhesive. Thereafter, it was dried in a vacuum dryer at 120 ° C. for 8 hours.

The negative electrode was produced as follows. A commercially available SiO powder was used as the negative electrode active material. This was mixed with graphite as a conductive agent and polyacrylic acid as a binder at a weight ratio of 45:40:15 to obtain a negative electrode mixture. A negative electrode pellet was obtained by press-forming 9.3 mg of the negative electrode mixture into a pellet having a diameter of 4 mm and a thickness of 0.33 mm at ² ton / cm ² . The negative electrode pellet obtained in this way was bonded and integrated with the negative electrode case using a conductive resin adhesive. It dried in vacuum 120 degreeC 8 hours with the vacuum dryer. A metal lithium punched out to a diameter of 4.0 mm and a thickness of 0.48 mm was pressed onto the dried negative electrode, and lithium and a negative electrode pellet were laminated to form a negative electrode. A non-woven fabric made of glass fibers having a thickness of 0.25 mm was dried and punched out to a diameter of 4.8 mm to form a separator. A gasket made of PP was used. The electrolyte is a lithium battery, in which 1 mol / L of lithium perchlorate is dissolved in a mixed solvent of propylene carbonate (PC): ethylene carbonate (EC): 1,2-dimethoxyethane (DME) in a battery can. did.

A battery was fabricated in the same manner as in Example 1 using Li ₄ Ti ₅ O ₁₂ having a BET specific surface area of 0.9 m ² / g as the positive electrode active material.

Comparative Example 1

A battery was fabricated in the same manner as in Example 1 using Li ₄ Ti ₅ O ₁₂ having a BET specific surface area of 4.2 m ² / g as the positive electrode active material.

Comparative Example 2

A battery was fabricated in the same manner as in Example 1 using Li ₄ Ti ₅ O ₁₂ having a BET specific surface area of 3.2 m ² / g as the positive electrode active material.

表1に実施例１、２及び比較例１、２の電池容量及び経時的な電圧変化を示す。実施例１では、90日後に電圧が1.91Vあり、電圧が0.16V上昇していた。実施例２では同様に電圧が0.07V上昇していた。一方の比較例１、２では電圧がそれぞれ1.08V、0.45V上昇していた。

Table 1 shows the battery capacities and voltage changes over time of Examples 1 and 2 and Comparative Examples 1 and 2. In Example 1, the voltage was 1.91 V after 90 days, and the voltage increased by 0.16 V. Similarly, in Example 2, the voltage increased by 0.07V. On the other hand, in Comparative Examples 1 and 2, the voltages increased by 1.08 V and 0.45 V, respectively.

As can be seen from this, as the BET specific surface area of the positive electrode active material decreases, the voltage increase with time of the battery decreases. It was found that a positive electrode active material having a BET specific surface area of 3 m ² / g or less is suitable for reducing the influence of voltage increase over time.

However, Example 2 in which the positive electrode active material has a BET specific surface area of 0.9 m ² / g has a discharge capacity of 3.5 mAh, and the positive electrode active material has a BET specific surface area of 3.0 m ² / g. The result was 1.2 mAh lower than 4.7 mAh. Those having a BET specific surface area of 1 m ² / g or more are preferred because of their large discharge capacity.

  Note that lithium titanate having an average particle diameter of 0.5 μm to 3 μm is suitable because the voltage rise with time is small and charge / discharge characteristics are excellent.

It is sectional drawing of the secondary battery of this invention.

Explanation of symbols

101 Positive electrode 102 Positive electrode case 103 Negative electrode 104 Negative electrode case 105 Metallic lithium 106 Separator 107 Gasket 108 Electrolyte

Claims (5)

In the lithium battery containing the lithium titanate represented by the composition formula LixTiyOz (0 ≦ x ≦ 4, 1 ≦ y ≦ 5, 2 ≦ z ≦ 12) in the positive electrode active material, the lithium titanate has a BET specific surface area of 3 m ^2. Lithium battery characterized by being less than / g. The lithium battery according to claim 1, wherein the lithium titanate has a BET specific surface area of 1 to ³ m ² / g.   2. The lithium battery according to claim 1, wherein the lithium titanate has an average particle size of 0.5 to 3 μm.   The lithium battery according to claim 1, wherein lithium titanate is used as the positive electrode active material, and the negative electrode active material contains silicon.   The lithium battery according to claim 1, wherein lithium titanate is used as the positive electrode active material, and the negative electrode active material contains silicon oxide.

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