JP5729647B2 - Negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Description

The present invention relates to lithium titanate (hereinafter sometimes abbreviated as Li ₄ Ti ₅ O ₁₂ ) and titanium oxide having a sodium bronze type crystal structure (hereinafter sometimes abbreviated as TiO ₂ (B)) as a negative electrode. The present invention relates to a negative electrode for a lithium ion secondary battery contained as an active material and a lithium ion secondary battery using the negative electrode.

  Lithium-ion secondary batteries, which are non-aqueous electrolyte secondary batteries, are widely used in portable electronic devices such as video cameras, portable audio players, mobile phones, and notebook computers that are becoming smaller, lighter, and higher in performance. Has been. In addition, large lithium ion secondary batteries are becoming increasingly important in the fields of electric vehicles, hybrid vehicles, and power load leveling systems.

Patent Document 1 proposes Li ₄ Ti ₅ O ₁₂ having a specific particle size as a negative electrode active material for a lithium ion secondary battery having good rate characteristics.
Patent Document 2 proposes TiO ₂ (B) that defines an integral intensity ratio of a specific peak of X-ray diffraction as a negative electrode active material for a lithium ion secondary battery having a high discharge capacity and excellent rate characteristics and cycle characteristics. Has been.

Japanese Patent No. 4249727 JP 2010-55855 A

When Li ₄ Ti ₅ O ₁₂ of Patent Document 1 is used as a negative electrode active material of a lithium ion secondary battery, the rate characteristics are good, but the discharge capacity is 167 mAh / g at the highest in the examples. Since the theoretical discharge capacity of Li ₄ Ti ₅ O ₁₂ is about 175 mAh / g, even if it is a value close to the theoretical discharge capacity, it cannot be said that the discharge capacity is sufficient. Moreover, since expensive Li is used, there also exists a problem that cost is high.
When TiO ₂ (B) of Patent Document 2 is used as the negative electrode active material of the lithium ion secondary battery, the discharge capacity is high at 232 mAh / g in the examples. However, the rate characteristics of TiO ₂ (B) are not sufficient compared to Li ₄ Ti ₅ O ₁₂ .

An object of the present invention is to improve rate characteristics, which is a drawback, while maintaining a large discharge capacity, which is an advantage when using TiO ₂ (B) powder as a negative electrode active material of a lithium ion secondary battery. To do.

In order to improve the rate characteristics of the TiO ₂ (B) powder, the present inventors examined the use of a mixture with Li ₄ Ti ₅ O ₁₂ powder, and maintained a large discharge capacity especially during charge / discharge at a high rate. The present invention has been completed.

According to the present invention, a negative electrode for a lithium ion secondary battery containing at least a TiO ₂ (B) powder and a Li ₄ Ti ₅ O ₁₂ powder and a lithium ion secondary battery using the negative electrode are provided.

  The negative electrode for a lithium ion secondary battery and the lithium ion secondary battery of the present invention have a large discharge capacity and excellent rate characteristics.₂(B) Powder and Li₄Ti₅O ₁₂Despite the use of mixed powder, the electric potential during discharge is constant at about 1.6V. Therefore, it can be preferably used for small electronic devices, particularly electric vehicles, hybrid vehicles, power load leveling systems, and the like that require safety. TiO not using Li₂(B) Since powder can be used, the cost can be reduced.

4 is a copy of an SEM observation image of TiO ₂ (B) powder produced by an ion exchange method used in Reference Example 1 .

  Hereinafter, the present invention will be described in more detail.
  The negative electrode for a lithium ion secondary battery of the present invention has at least TiO₂(B) Powder and Li ₄Ti₅O₁₂Contains powder.
  TiO₂(B) is one of the metastable phases of titanium oxide and is known to have a monoclinic crystal structure belonging to the space group C2 / m. In the present invention, TiO₂When (B) is written, TiO as the main phase₂(B) means a titanium oxide having a crystal structure, and includes those having a crystal structure such as anatase type as a subphase. TiO as the main phase₂The titanium oxide having the crystal structure (B) is TiO 2 by X-ray diffraction measurement.₂The maximum peak intensity derived from (B) is greater than the maximum peak intensity derived from another crystal structure. TiO used in the present invention₂(B) Powder is TiO₂A single phase (B) is preferred. TiO used in the present invention₂(B) The powder may contain elements other than Ti and oxygen. Examples of elements other than Ti and oxygen include alkali metals, alkaline earth metals, transition metals, B, and halogen.

The TiO ₂ (B) powder used in the present invention has a D50 of preferably 0.1 to 20 μm, more preferably 0.1 to 10 μm, when measured by a laser diffraction method. The shape of the TiO ₂ (B) powder is not particularly limited, but is usually granular or whisker-like. In addition, as the TiO ₂ (B) powder, a tube-like or fibrous shape having a large anisotropy can be used. These are mainly synthesized by the hydrothermal method described later, and TiO ₂ (B) powder synthesized by the hydrothermal method is known to have high crystallinity and large charge / discharge capacity. Therefore, a negative electrode or a battery having a large charge / discharge capacity can be obtained by using tube-like or fibrous TiO ₂ (B) powder.
By using the TiO ₂ (B) powder in the above preferred range, the Li moving distance is shortened, so that a negative electrode and a battery having a large charge / discharge capacity and excellent rate characteristics can be obtained.

The TiO ₂ (B) powder can be produced by a known method. For example, the layered titanate K ₂ Ti ₄ O ₉ can be obtained by a so-called ion exchange method in which H ₂ Ti ₄ O ₉ · H ₂ O obtained by acid treatment with hydrochloric acid or the like is heated and dehydrated. FIG. 1 shows an SEM observation image of the TiO ₂ (B) powder produced by the ion exchange method used in Example 1 described later. Other methods include hydrothermal treatment of titanium oxide in an alkaline environment and hydrothermal treatment of titanium glycolate complex in an acidic environment.

Li ₄ Ti ₅ O ₁₂ is known to have a spinel crystal structure belonging to the space group Fd3m. In the present invention, when Li ₄ Ti ₅ O ₁₂ is described, it means a titanium oxide having a crystal structure of Li ₄ Ti ₅ O ₁₂ as a main phase, and an anatase type, LiTi ₂ O ₄ or the like as a subphase. Includes what you have. Titanium oxide having a crystal structure of Li ₄ Ti ₅ O ₁₂ as the main phase means that the intensity of the maximum peak derived from Li ₄ Ti ₅ O ₁₂ is determined by X-ray diffraction measurement and the intensity of the maximum peak derived from another crystal structure The bigger one. The Li ₄ Ti ₅ O ₁₂ powder used in the present invention is preferably a single phase of Li ₄ Ti ₅ O ₁₂ . The Li ₄ Ti ₅ O ₁₂ powder used in the present invention may contain elements other than Ti and oxygen. Examples of elements other than Ti and oxygen include alkali metals, alkaline earth metals, transition metals, B, and halogen. It is known that rate characteristics are improved by containing an alkali metal, Mg, and Al.

The Li ₄ Ti ₅ O ₁₂ powder used in the present invention preferably has a D50 of 0.1 to 30 μm, more preferably 0.1 to 15 μm, and most preferably 0.1 to 10 μm when measured by a laser diffraction method. And smaller than the TiO ₂ (B) powder used for mixing.
By using Li ₄ Ti ₅ O ₁₂ powder in the above preferred range, a negative electrode or a battery excellent in rate characteristics can be obtained. In the case of the above-mentioned most preferable range, a negative electrode and a battery having particularly excellent rate characteristics can be obtained from the rate characteristics expected from the mixing ratio of the TiO ₂ (B) powder and the Li ₄ Ti ₅ O ₁₂ powder.

Li ₄ Ti ₅ O ₁₂ powder can be produced by a known method. For example, it can be obtained by a so-called solid phase reaction method in which anatase-type titanium oxide and Li salt such as lithium carbonate and lithium hydroxide are mixed and baked at about 700 to 1000 ° C.

The negative electrode for a lithium ion secondary battery of the present invention can contain other types of active materials of TiO ₂ (B) powder and Li ₄ Ti ₅ O ₁₂ powder. An active material whose charge / discharge potential is close to TiO ₂ (B) and Li ₄ Ti ₅ O ₁₂ is preferable, and for example, H ₂ Ti ₁₂ O ₂₅ powder can be used.

  TiO₂(B) Powder and Li₄Ti₅O₁₂The mixing ratio of the powder is not particularly limited, but the weight ratio of TiO₂(B) Powder: Li₄Ti₅O₁₂Powder = 10 to 90%: 10 to 90% is preferable. More preferably TiO₂(B) Powder: Li₄Ti₅O₁₂Powder = 25-75%: 75-25%, most preferably TiO₂(B) Powder: Li₄Ti₅O₁₂Powder = 60-80%: 20-40%. In the above preferred range, TiO₂(B) High capacity and Li₄Ti₅O₁₂Good rate characteristics can be achieved at the same time. In addition, even though different active materials are used in mixture, the charge and discharge potentials of both are close, so the potential at the time of discharge is constant at about 1.6 V, so it can be used widely in various applications. . In the most preferred range, TiO₂(B) Powder or Li₄Ti ₅O₁₂Larger discharge capacity can be maintained from a relatively low rate to a higher rate than when the powder is used alone, and Li-free TiO₂(B) Since a large amount of powder can be used, the cost can be reduced.

The TiO ₂ (B) powder and the Li ₄ Ti ₅ O ₁₂ powder are preferably in a uniform mixed state. Mixing can be performed by a known method. For example, a rotary type mixer such as a double cone or V type, a stirring type mixer such as a blade type or a screw type, or a pulverizer such as a ball mill or an attritor mill is used, and TiO ₂ (B) powder and Li ₄ Ti are used. It is also possible to mix while pulverizing a part of the ₅ O ₁₂ powder. The mixing of the TiO ₂ (B) powder and the Li ₄ Ti ₅ O ₁₂ powder may be performed only with these active materials, or may be performed together with a conductive additive, a binder, a solvent, and the like in the production process of the negative electrode. And you can do both.

In the negative electrode of the present invention, TiO ₂ (B) powder and Li ₄ Ti ₅ O ₁₂ powder, which are negative electrode active materials, a conductive agent, a binder, and the like are kneaded and slurried with an organic solvent, applied to an electrode plate, dried, and then roller And is obtained by a known method of rolling and cutting to a predetermined size. Generally, the negative electrode has a thickness of 20 to 200 μm.

  Known materials such as a conductive agent, a binder, an organic solvent, and an electrode plate can be used. Examples of the conductive agent include carbonaceous materials such as natural graphite, artificial graphite, ketjen black, and acetylene black. The binder is fluorine resin such as polytetrafluoroethylene and polyvinylidene fluoride, polyvinyl acetate, polymethyl methacrylate, ethylene-propylene-diene copolymer, styrene-butadiene copolymer, acrylonitrile butadiene copolymer, carboxymethyl cellulose. Etc. Examples of the organic solvent include N-methylpyrrolidone, tetrahydrofuran, ethylene oxide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, dimethylformamide, dimethylacetamide and the like.

  Examples of the electrode plate include metal foils such as Al and Cu. A metal foil having a thickness of 10 to 30 μm is preferred.

  The lithium ion secondary battery of the present invention uses the above-described negative electrode for a lithium ion secondary battery of the present invention. By using the negative electrode for a lithium ion secondary battery of the present invention, a lithium ion secondary battery having a large discharge capacity, excellent rate characteristics, and low cost can be obtained.

  The battery of the present invention is mainly composed of a positive electrode, a negative electrode, an organic solvent, an electrolyte, and a separator. A solid electrolyte may be used in place of the organic solvent and the electrolyte. Known materials can be used for the positive electrode, the organic electrolyte, the electrolyte, and the separator. For example, the positive electrode is made of cobalt, nickel, lithium oxide of manganese, lithium phosphate, or the like as the positive electrode active material. Similar binders, electrode plates and the like are used.

  For example, as an organic solvent, carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, ethers such as 1,2,1,3-dimethoxypropane, tetrahydrofuran and 2-methyltetrahydrofuran, methyl acetate, Examples thereof include esters such as Γ-butyrolactone, nitriles such as acetonitrile and butyronitrile, amides such as N, N-dimethylformamide and N, N-dimethylacetamide. For example, as an electrolyte, LiClO₄, LiPF ₆, LiBF₄Etc.

For example, examples of the solid electrolyte include polymer electrolytes such as polyethylene oxide, and sulfide-based electrolytes such as Li ₂ S—SiS ₂ , Li ₂ S—P ₂ S ₅ , and Li ₂ S—B ₂ S ₃ . Moreover, what is called a gel type which hold | maintained the nonaqueous electrolyte solution in the polymer | macromolecule can also be used.

  For example, examples of the separator include porous polymer films such as polyethylene and polypropylene, and ceramic-coated porous sheets.

  The lithium ion secondary battery of the present invention can have various shapes such as a cylindrical shape, a stacked shape, and a coin shape. Regardless of the shape, the above-described components are housed in a battery case, and the space between the positive electrode and the negative electrode to the positive electrode terminal and the negative electrode terminal is connected using a current collecting lead or the like, and the battery case is sealed.

EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention in detail, this invention is not limited to these.
Reference example 1
TiO ₂ (B) powder (D50 1.5 μm) was prepared by an ion exchange method, and Li ₄ Ti ₅ O ₁₂ powder (D50 15.1 μm) was prepared by a solid phase method. When X-ray diffraction measurement was performed on the TiO ₂ (B) powder and the Li ₄ Ti ₅ O ₁₂ powder, it was confirmed that each of the TiO ₂ (B) and Li ₄ Ti ₅ O ₁₂ had a crystal structure having a main phase. . TiO ₂ (B) powder and Li ₄ Ti ₅ O ₁₂ powder (1: 1 by weight) 0.800 g, Ketjen black 0.100 g, polyvinylidene fluoride 0.100 g, and weighed an appropriate amount of N-methylpyrrolidone. In addition, the mixture was well kneaded in a mortar to obtain an electrode paste. The obtained electrode paste was applied to a copper foil having a thickness of 25 μm by a doctor blade method, dried, press-formed with a press, and then cut into a predetermined size to obtain a negative electrode. A lithium metal foil is used as the counter electrode, a 25 μm-thick polypropylene non-woven fabric is used as the separator, and an electrolyte is mixed with ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1 so that lithium hexafluorophosphate is 1M. A coin-type lithium ion secondary battery was manufactured using the dissolved electrolyte solution. About the manufactured lithium ion secondary battery, TiO ₂ (at a cut-off potential of 3.0 V-1.0 V under a temperature condition of 20 ° C. Based on the theoretical capacity of 335 mAh / g of B), charging / discharging was performed 5 times each for C / 6, 0.5C, 1C, 2C, 5C, and 10C. The average discharge capacity at each rate was 202.3 mAh / g, 194.3 mAh / g, 185.5 mAh / g, 174.5 mAh / g, 153.0 mAh / g, 128.4 mAh / g. .

Example 2
A negative electrode and a lithium ion secondary battery were produced in the same manner as in Reference Example 1 except that the weight ratio of the TiO ₂ (B) powder and the Li ₄ Ti ₅ O ₁₂ powder was changed to 3: 1.
The obtained lithium ion secondary battery was subjected to a charge / discharge test in the same manner as in Reference Example 1 . The average values of the discharge capacities at the respective rates of C / 6, 0.5C, 1C, 2C, 5C, and 10C are 217.0 mAh / g, 205.2 mAh / g, 194.4 mAh / g, 179.9 mAh / g, 157.2 mAh / g, 130.8 mAh / g.

Reference example 2
A negative electrode and a lithium ion secondary battery were produced in the same manner as in Reference Example 1 except that the weight ratio of the TiO ₂ (B) powder and the Li ₄ Ti ₅ O ₁₂ powder was changed to 1: 3.
The obtained lithium ion secondary battery was subjected to a charge / discharge test in the same manner as in Reference Example 1 . The average values of the discharge capacities at the respective rates of C / 6, 0.5C, 1C, 2C, 5C, and 10C are 186.9 mAh / g, 182.3 mAh / g, 176.8 mAh / g, and 167.0 mAh / g, 149.2 mAh / g, 125.2 mAh / g.

Reference example 3
As Li ₄ Ti ₅ O ₁₂ powder, except using D50 1.2 [mu] m of _Li 4 _Ti 5 _{O 12} powder prepared by the solid phase method was prepared in the same manner as the negative electrode and a lithium ion secondary battery as in Reference Example 1.
When X-ray diffraction measurement was performed on the Li ₄ Ti ₅ O ₁₂ powder, it was confirmed that it had a crystal structure having Li ₄ Ti ₅ O ₁₂ as a main phase.
The obtained lithium ion secondary battery was subjected to a charge / discharge test in the same manner as in Reference Example 1 . The average values of the discharge capacities at the respective rates of C / 6, 0.5C, 1C, 2C, 5C, and 10C are 202.0 mAh / g, 197.9 mAh / g, 189.7 mAh / g, 177.9 mAh / g, 156.4 mAh / g, 131.6 mAh / g.

Example 5
The negative electrode and lithium were the same as in Reference Example 1 except that the Li ₄ Ti ₅ O ₁₂ powder used in Reference Example 3 was used and the weight ratio of TiO ₂ (B) powder to Li ₄ Ti ₅ O ₁₂ powder was 4: 1. An ion secondary battery was produced.
The obtained lithium ion secondary battery was subjected to a charge / discharge test in the same manner as in Reference Example 1 . The average values of the discharge capacities at the respective rates of C / 6, 0.5C, 1C, 2C, 5C, and 10C are 219.3 mAh / g, 211.5 mAh / g, 198.6 mAh / g, and 183.3 mAh /. g, 157.8 mAh / g, 130.8 mAh / g.

Reference example 4
The negative electrode and lithium were the same as in Reference Example 1 except that the Li ₄ Ti ₅ O ₁₂ powder used in Reference Example 3 was used and the weight ratio of TiO ₂ (B) powder to Li ₄ Ti ₅ O ₁₂ powder was 1: 4. An ion secondary battery was produced.
The obtained lithium ion secondary battery was subjected to a charge / discharge test in the same manner as in Reference Example 1 . The average values of the discharge capacities at the respective rates of C / 6, 0.5C, 1C, 2C, 5C, and 10C are 182.2 mAh / g, 178.5 mAh / g, 175.5 mAh / g, 168.4 mAh / g, 153.1 mAh / g, 129.6 mAh / g.

Comparative Example 1
A negative electrode and a lithium ion secondary battery were produced in the same manner as in Reference Example 1 except that only TiO ₂ (B) powder was used.
The obtained lithium ion secondary battery was subjected to a charge / discharge test in the same manner as in Reference Example 1 . The average values of the discharge capacities at the respective rates of C / 6, 0.5C, 1C, 2C, 5C, and 10C are 230.3 mAh / g, 207.4 mAh / g, 192.1 mAh / g, 174.9 mAh / g, 146.8 mAh / g, 120.1 mAh / g.

Comparative Example 2
A negative electrode and a lithium ion secondary battery were produced in the same manner as in Reference Example 1 except that only Li ₄ Ti ₅ O ₁₂ powder was used.
The obtained lithium ion secondary battery was subjected to a charge / discharge test in the same manner as in Reference Example 1 . The average values of the discharge capacities at the respective rates of C / 6, 0.5C, 1C, 2C, 5C, and 10C are 170.2 mAh / g, 168.9 mAh / g, 165.8 mAh / g, 159.0 mAh / g, 146.3 mAh / g, 124.8 mAh / g.

  From the above examples and comparative examples, the negative electrode for lithium ion secondary batteries and the lithium ion battery of the present invention are useful because they maintain a large discharge capacity, particularly during charge / discharge at a high rate.

Claims (3)

It contains at least TiO ₂ (B) powder and Li ₄ Ti ₅ O ₁₂ powder, and the weight ratio of TiO ₂ (B) powder to Li ₄ Ti ₅ O ₁₂ powder is TiO ₂ (B) powder: Li ₄ Ti ₅ O. ₁₂ powder = 60-80%: 20-40% negative electrode for lithium ion secondary battery. Li ₄ Ti ₅ O ₁₂ powder having an average particle size (D50) of the _TiO 2 (B) according to claim 1 negative electrode for a lithium ion secondary battery, wherein a smaller than the average particle size of the powder. Lithium ion secondary battery using the anode according to claim 1 or 2.