JPH0719621B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery using a specific iron compound as a positive electrode active material.

In this specification, the terms "%" and "parts" mean "wt%" and "parts by weight", respectively.

[0003]

2. Description of the Related Art In recent years, lithium secondary batteries having high energy density have been attracting attention, and various compounds have been proposed as positive electrode active materials and their characteristics have been investigated. A lithium secondary battery using a layered compound of a transition metal chalcogenide such as titanium disulfide or molybdenum disulfide as a positive electrode active material has been commercialized until now, but it has not been widely used. . This is because the potential of the battery using these substances at the time of discharging is low, so that it is difficult to improve the energy density, and the resource reserves of these substances are not sufficient.

Further, when metallic lithium is used for the negative electrode, there is a problem that the cycle life of the battery is short due to the generation of dendrites. To solve this problem, alloys, carbon materials and the like have come to be used as negative electrode materials. Along with this, improvement of the energy density of the entire battery cannot be achieved unless a material having a high discharge potential is used even on the positive electrode side, and therefore, the development of a positive electrode active material made of a metal oxide material is being promoted. .

[0005]

A rechargeable battery using lithium is a power source for portable electronic devices and a high-performance battery for distributed small battery power storage systems such as electric vehicles, which is a means for mitigating environmental problems. It is expected as a performance battery. In order to be widely used as a practical battery for such applications, it is essential that (a) it has excellent safety, (b) the material used has abundant resource reserves, and the cost be low. In addition, (c) a long cycle life is also required.

When a lithium cobalt composite oxide having a high discharge potential, which is currently being developed, is used as the positive electrode active material, in the region where the potential exceeds 4 V, the solvent content of the non-aqueous electrolyte solution is increased. Since oxidative decomposition may occur, there is a risk of battery rupture due to gas generation during overcharge. Further, it is difficult to say that metals such as cobalt and lithium are expensive and have abundant resources.
Also in terms of life, it is presumed that the composite oxide such as lithium cobalt is a crystalline substance, and the battery life is shortened due to the collapse of the crystals accompanying the entry and exit of lithium ions.

The inventor of the present invention has previously found that iron oxychloride (FeOC)
The aniline-containing amorphous FeOOH obtained by reacting l) with aniline is used as a positive electrode active material.
We have developed a non-aqueous electrolyte secondary battery using propylene carbonate as an electrolyte, in which lithium fluoroborate (hereinafter referred to as LiBF ₄ ) is dissolved at a concentration of 1.0 mol / dm ³ .
However, the capacity of this battery was found to deteriorate from a relatively early cycle. Further, the stability of the electrolytic solution is a factor that affects the cycle life, and is also related to the safety of the battery. However, the electrolyte solution is LiB, which is a solute.
It was found that the anion in F ₄ has a defect that it easily decomposes with respect to water as an impurity.

Therefore, the main object of the present invention is to improve the safety and improve the cycle life of a non-aqueous electrolyte secondary battery using aniline-containing amorphous FeOOH as a positive electrode active material.

[0009]

Means for Solving the Problems The present inventor has conducted further research in view of the current state of the art of non-aqueous electrolyte secondary batteries as described above, and as a result, the above items (a) to (c) Regarding the terms, we have found the following.

Regarding item (a): iron oxychloride (FeO)
Cl) and aniline are used as the positive electrode active material obtained by reacting aniline-containing amorphous FeOOH, the potential of the battery is slightly lower than that of the composite oxide, and due to the action of the remaining organic substance, overcharge occurs. It was found that it is excellent in safety because it exerts a buffering effect at the time.

Regarding item (b): Iron is an element that is present in large amounts on the earth, and is easily available at low cost. Therefore, even when a large number of electric vehicles are expected to spread in the future, a sufficient amount of raw materials for producing aniline-containing amorphous FeOOH can be obtained.

Regarding the item (c); the mixing ratio (volume ratio) of lithium hexafluoroarsenate or lithium hexafluorophosphate, which is more stable than LiBF ₄ , is higher than that of ethylene carbonate and the ether solvent. A solution prepared by dissolving 1.0 to 1.5 mol / dm ³ of a solvent in a solvent of 2: 1 to 1: 2 is used as an electrolytic solution, and aniline-containing amorphous Fe.
It has been found that when OOH is used as the positive electrode active material, it can be expected to improve the cycle life and safety of the non-aqueous electrolyte secondary battery.

The present invention has been completed based on these findings.

That is, the present invention provides the following non-aqueous electrolyte secondary battery: An iron compound synthesized by the method shown in the following item (A) and having the characteristics shown in the following item (B). And a negative electrode active material composed of a lithium host compound are used as battery active materials, and the mixing ratio (volume ratio) of ethylene carbonate (hereinafter referred to as EC) and the ether solvent is 2: 1 to 1: 2. Lithium hexafluoroarsenate (hereinafter referred to as LiAsF ₆ ) or lithium hexafluorophosphate (hereinafter referred to as LiPF ₆ ) in a mixed solvent is 1.0 to
Non-aqueous electrolyte secondary battery characterized by using a solution dissolved at a concentration of 1.5 mol / dm ³ as an organic electrolyte: (A) Iron oxychloride (FeOCl) and aniline 4:
Mix at a molar ratio of 1 and stir at 25-35 ° C. in water; (B) the density as powder is about 2.3 g / cm ³ and Cu
In X-ray diffraction using Kα rays, 2θ = 14 °, 27
A substance with low crystallinity in which organic substances remained, having peaks similar to γ-FeOOH at ° and 44 °.

The iron compound used as the positive electrode active material in the present invention is iron oxychloride (FeOCl) and aniline 4: 4.
It is manufactured by mixing in a molar ratio of 1 and stirring in water at 25 to 35 ° C. This iron compound has a density of about 2.3 g / cm ³ as a powder, and is 2θ = 14 °, 27 ° and 44 ° in γ-F by X-ray diffraction using CuKα rays.
It is a substance with low crystallinity in which an organic substance remains, having a peak similar to that of eOOH. Since the iron compound is originally amorphous, it is hardly affected by the structural change during use. Such an iron compound (aniline-containing amorphous FeOOH, hereinafter referred to as a-FeOOH (An)) in an appropriate amount of a conductive agent (for example, a carbon material such as acetylene black or carbon black), a binder (for example, polytetrafluoroethylene (PTFE)) Powder, polyethylene powder, etc.) and molded into a positive electrode material.

The lithium host compound used as the negative electrode active material in the present invention is a carbon material which is carbonized or graphitized by heat-treating pitch-based carbon or petroleum coke-based material at an appropriate temperature of about 800 to 2800 ° C. Examples include metals and alloys that alloy with lithium. The method for producing the negative electrode active material is not particularly limited, but the following method is exemplified. (A) When using a carbon material obtained in powder form,
After mixing an appropriate amount of a binder and the like and molding, an appropriate current density (for example, 0.3 to
Electrode lithium is electrochemically inserted until the composition calculated from the amount of electricity is close to C ₆ Li and the potential for lithium metal is within 20 mV, with metallic lithium as the counter electrode at about 5 mA · cm ⁻² ). Manufacture active material. (B) A material obtained in a form other than powder, such as a metal or an alloy, is punched into an appropriate shape, and when the material itself contains lithium, it is directly used as the negative electrode active material. (C) In the above (b), when the material does not contain lithium, the punched material is treated in the same manner as in the above (a) to manufacture the negative electrode active material.

As the electrolytic solution in the present invention, especially Li
AsF ₆ or LiPF ₆ in a mixed solvent of EC and an ether-based solvent (volume ratio of about 2: 1 to 1: 2) is 1.0 to.
A solution dissolved at a concentration of 1.5 mol / dm ³ is used. Examples of ether solvents include 2-methyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoethane. When the mixing ratio of EC and the ether-based solvent is out of the above range, the conductivity is lowered. Also,
Even when the concentration of LiAsF ₆ or LiPF ₆ is out of the above range, the conductivity decreases.

[0018]

According to the present invention in which a specific positive electrode active material and an electrolytic solution are used in combination, the following effects are achieved. (1) Even if overcharge of 3 times the discharge capacity or more is performed, the flat portion, which is considered to be the oxidation of the residual organic substance in the positive electrode active material, is around 4.2 to 4.5 V, so the electrolytic solution There is no danger of the battery exploding due to the decomposition of the electrolytic solution, because the decomposition potential of 1 is not reached. (2) Since a large discharge capacity per mol of iron can be obtained,
Even though the discharge potential is lower than that of lithium cobalt composite oxide, if calculated from the polarization curve at low rate discharge,
A secondary battery having a high weight energy density of about ¹ Wh · g ⁻¹ only for the positive electrode active material can be obtained. (3) Since the polarization during charging and discharging is small and stable, the energy density of the entire battery is improved. (4) The cycle life is improved. (5) Therefore, according to the present invention, a non-aqueous electrolyte secondary battery having excellent practicability can be obtained.

[0019]

EXAMPLES Examples and comparative examples will be shown below to further clarify the features of the present invention. Example 1 EC and 2-methyltetrahydrofuran (hereinafter 2-MeT
LiAsF in a mixed solvent with a volume ratio of 1: 1
₆ was dissolved at a concentration of 1.0 mol / dm ³ to obtain an electrolytic solution.

A-FeOOH (An), acetylene black as a conductive agent and PTFE as a binder are kneaded in a weight ratio of 85: 10: 5 and formed into pellets having a diameter of 16 mm at a pressure of ² ton / cm ^2. It was pressed to obtain a positive electrode material.

3% by weight of PTFE was kneaded with a pitch-based carbon material heat-treated at an appropriate temperature and press-molded into a pellet having a diameter of 16 mm at a pressure of ² ton / cm ² , and then metallic lithium was used as a counter electrode. Current density in electrolyte of about 1mA
In · cm ^-2, the composition calculated from the amount of electricity becomes closer to the C ₆ Li, until the potential to lithium metal is within 20 mV, electrochemically inserting lithium to obtain a negative electrode active material. At this time, the weight of the negative electrode active material was determined so that the negative electrode capacity would be in excess of the positive electrode discharge capacity.

The positive electrode mixture and the negative electrode active material obtained as described above are arranged via a separator made of a porous polypropylene film, and the battery case having a current collector at the bottom of the case is filled with an electrolytic solution, A coin-type battery (CR2016) having a capacity of 36.5 mAh was produced on the assumption that 1 mol of iron reacts with 1 electron in the positive electrode capacity regulation.

The battery obtained above was charged and discharged at room temperature under a load of 2 to 4.2 V with a current value of 2 mA. The results of the long-term cycle test are shown by solid lines in FIGS.

The utilization factor was determined with 36.5 mAh as 100%.

The Coulombic efficiency was obtained by taking the ratio of the discharge capacity and the charge capacity in the cycle.

The result of the fifth cycle in the above cycle test is shown by the solid line in FIG. Comparative Example 1 A battery was manufactured in the same manner as in Example 1 except that propylene carbonate (hereinafter referred to as PC) in which LiBF ₄ was dissolved at a concentration of 1.0 mol / dm ³ was used as the electrolytic solution.

The obtained battery has a current value of 2 mA at room temperature,
Charging / discharging was performed with a load of 2 to 4.2V. The results of the long-term cycle test are shown by broken lines in FIGS. 1 and 2.

In addition, 5 cycles in the above cycle test
The result of the kur-mesh is shown by a broken line in FIG. Consideration on Examples and Comparative Examples In the case of the battery according to the comparative example, immediately after the start of charge / discharge
Utilization is low compared to the example battery, and 100 cycles
The utilization rate (75%) at the time of the progress of
0%) is inferior. Subsequent cycles
However, the battery according to the example is
That's about 15% more.

Regarding the Coulombic efficiency, the batteries of the examples also maintain 100% up to 200 cycles, and it is clear that the long-term cycle characteristics are improved especially under relatively high load conditions.

These are an electrolytic solution using LiAsF ₆ which has a smaller polarization at discharge as compared with an electrolytic solution using LiBF ₄ , and EC and 2-M which are more stable and have a lower viscosity than PC.
This is the effect achieved by combining a-FeOOH (An) with the electrolytic solution prepared by combining eTHF with a mixed solvent having a specific volume ratio. Example 2 A non-aqueous electrolyte secondary battery was manufactured in the same manner as in Example 1 except that LiPF ₆ was used in place of LiAsF ₆ , and the same results as in Example 1 were obtained.

[Brief description of drawings]

FIG. 1 is a graph showing the results (utilization rate) of a long-term cycle test of the batteries obtained in Example 1 and Comparative Example 1.

FIG. 2 is a graph showing the results (Coulomb efficiency) of a long-term cycle test of the batteries obtained in Example 1 and Comparative Example 1.

FIG. 3 is a graph showing charge / discharge results at the 5th cycle of the batteries obtained in Example 1 and Comparative Example 1.

Claims (1)

[Claims] 1. A positive electrode material containing an iron compound and a negative electrode material composed of a lithium host compound, which are synthesized by the method shown in the following item (A) and have the characteristics shown in the following item (B), are used as a battery active material. , 1.0 to 1.5 mol / dm ^{3 of} lithium hexafluoroarsenate or lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and an ether solvent at a mixing ratio of 2: 1 to 1: 2 (volume ratio). A non-aqueous electrolyte secondary battery characterized by using a solution dissolved at a concentration of as an organic electrolyte: (A) Iron oxychloride (FeOCl) and aniline 4:
Mix at a molar ratio of 1 and stir at 25-35 ° C. in water; (B) the density as powder is about 2.3 g / cm ³ and Cu
In X-ray diffraction using Kα rays, 2θ = 14 °, 27
A substance with low crystallinity in which organic substances remained, having peaks similar to γ-FeOOH at ° and 44 °.