JP2002231308A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery, such as lithium ion secondary battery using graphite as a negative active material and dimethoxy ethane as the organic solvent of the electrolyte, having high charging/ discharging characteristics, high charging/discharging efficiency, and high reliability, by using specified graphite as the negative active material. SOLUTION: This nonaqueous electrolyte secondary battery has a negative electrode using graphite as the active material, the organic electrolyte containing dimethoxy ethane, and a positive electrode. Graphite has an R value [I'/I], which is the ratio of the peak intensity [I'] at 1.350 cm-1, measured by Raman scattering method, to the peak intensity [I] at 1.580 cm-1 of 0.150 or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly, to a non-aqueous electrolyte secondary battery having improved charge / discharge characteristics.

[0002]

_2. Description of the Related Art At present, commercially available lithium ion secondary batteries generally use a lithium-containing metal oxide such as LiCoO ₂ as a positive electrode active material and graphite as a negative electrode active material. In many cases, an electrolyte is dissolved in an organic solvent obtained by mixing a cyclic carbonate such as, for example, with a non-cyclic carbonate such as diethyl carbonate. Such an electrolytic solution depends on the electrolytic solution selectivity of graphite, which is a negative electrode active material, and the acyclic carbonate is required for lowering the viscosity of the electrolytic solution, as opposed to the cyclic carbonate required for highly efficient charging and discharging. ing.

On the other hand, the use of dimethoxyethane as an organic solvent for an electrolytic solution other than those described above has been studied. This solvent cannot be used in a 4 V class battery using LiCoO ₂ for the positive electrode because of its low oxidation resistance, but has advantages such as a high dielectric constant and a low viscosity as compared with the acyclic carbonate described above. By using a lot of this solvent,
Attempts have been made to obtain secondary batteries having excellent load characteristics and storage characteristics.

[0004]

However, it is known that dimethoxyethane intercalates graphite in the same manner as Li. This is described in J.A. Electroc
hem. Soc. vol. 145, No. 7, P239
As reported in FIG. 6, 0.6 V (vs. Li / Li
⁺ ) It is also evident from the appearance of a shoulder with an inflection point around it.
There is a problem that not only the expansion rate of the electrode becomes larger than that of only the intercalate, but also the charge / discharge characteristics deteriorate.

On the other hand, if low-crystalline carbon, which is the same carbonaceous material, is used instead of graphite, co-intercalation of dimethoxyethane is not observed, but the volume energy density is lower than that of graphite, and The potential also decreases.
In order to overcome this, recently, graphite having a so-called core-shell structure in which graphite surface is coated with low-crystalline carbon has been developed. However, this kind of graphite has a difference in the expansion coefficient of the core shell due to the intercalation of Li, and has a problem in the reliability of the shell.

In view of such circumstances, the present invention provides a lithium ion secondary battery and the like using graphite as a negative electrode active material and dimethoxyethane as an organic solvent for an electrolytic solution, which contributes to improvement of load characteristics and storage characteristics. In the non-aqueous electrolyte secondary battery of the above, by using a specific graphite as the above, excellent charge and discharge characteristics, high charge and discharge efficiency is obtained, to provide a highly reliable secondary battery With the goal.

[0007]

[Means for Solving the Problems] Graphite is largely classified into natural graphite and artificial graphite. Artificial graphite includes coke fired products and mesocarbon microbeads obtained by graphitizing mesophase components depending on the raw material. There is also a core-shell type. The same artificial graphite and natural graphite have recently been used for lithium-ion secondary batteries.
In order to improve the shape of graphite particles, various graphites having different grinding methods have been developed.

The inventors of the present invention have examined the charge and discharge characteristics of a secondary battery using these various graphites as a negative electrode active material and dimethoxyethane as an organic solvent for an electrolytic solution. According to the graphite of 0.150 or less, it was found that the shoulder near 0.6 V due to the co-intercalate of dimethoxyethane at the time of the first charge disappeared, and excellent charge / discharge efficiency was obtained.

In Raman scattering of graphite, E2g has Raman activity in a normal hexagonal space group, and a peak appears at 1,580 cm ^-1 . In addition to this, there is a Raman-inactive peak at 1,360 cm ⁻¹ depending on the low crystallinity.
Defines a peak intensity ratio of the 1,360Cm ^-1 as R value to the peak intensity of 580 cm ^-1 (e.g.,
J. Electrochem. Soc. vol. 14
2, No. 1, p. 20) It is known that the smaller the R value, the higher the crystallinity.

The crystallinity of graphite is generally evaluated using an X-ray diffraction method (hereinafter referred to as XRD). However, since XRD has a high energy value, it easily penetrates graphite particles and reflects the crystallinity of the entire graphite. are doing. On the other hand, the above-mentioned Raman scattering reflects the crystallinity near the particle surface, that is, the edge structure of graphite having a layer structure, and this point is different from XRD. For example, graphite having an R value of 0.3 by Raman scattering and graphite having an R value of 0.1 also show exactly the same interlayer distance [d ₀₀₂ ] and Lc ( ₀₀₂ ) when compared by XRD. There may be no difference in gender.

The present inventors have found that the co-intercalate of dimethoxyethane depends on the crystallinity near the particle surface of graphite, and the R value by Raman scattering reflecting this crystallinity is not more than 0.1.
When graphite having a high crystallinity of 150 or less is used, the above-mentioned cointercalate is eliminated, and a high-performance, highly-reliable secondary battery having excellent charge-discharge characteristics and high charge-discharge efficiency is obtained. It is the first time that it has been found.

That is, the present invention relates to a nonaqueous electrolyte secondary battery having a negative electrode using graphite as an active material, an organic electrolyte containing dimethoxyethane, and a positive electrode, wherein the graphite is measured by a Raman scattering method. peak intensity of 1,350Cm ^-1 peak intensity [I '] and 1,580Cm ^-1 [I]
The non-aqueous electrolyte secondary battery has an R value [I '/ I] of 0.150 or less. Further, according to the present invention, in the above graphite, the interlayer distance [d ₀₀₂ ] after the charge / discharge is performed at least once is _{0.1 mm} .
A non-aqueous electrolyte secondary battery having the above-described configuration of 45 nm or less;
In the above organic electrolyte, the non-aqueous electrolyte secondary battery having the above structure in which the volume ratio of dimethoxyethane in the organic solvent is 5 to 80%, and the positive electrode potential during charging and discharging (vs. Li / Li ⁺ )
Is 4.0 V or less.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, examples of graphite having an R value of 0.150 or less measured by Raman scattering include natural graphite and mesophase graphitized products having a lamellar structure with a well-developed edge structure. . Even natural graphite coated with low-crystalline carbon thinly has a large R value and is not suitable for the present invention. When the R value is 0.15 or less, high charge / discharge efficiency can be obtained. However, the lower the R value, the better, and particularly preferably, the R value is 0.11 or less. Note that R
Even with graphite having a value exceeding 0.15, high charge / discharge efficiency can be expected for an electrolytic solution containing dimethoxyethane by coating the particle surface of the graphite with graphite having an R value of 0.15 or less. be able to.

When such a graphite is used as a negative electrode active material, the interlayer distance [d ₀₀₂ ] is not less than 0.1 even after charging and discharging the electrolyte containing dimethoxyethane at least once.
It becomes 45 nm or less. That is, in the case of ordinary graphite, the interlayer distance [d ₀₀₂ ] is increased by about 10% from about 0.36 nm by intercalation of Li and several times by co-intercalation of dimethoxyethane. In the graphite of the present invention, there is no co-intercalate of dimethoxyethane, and the intercalate of Li alone is used for 0.1%.
It indicates an interlayer distance of 45 nm or less. As a result, the irreversible capacity, which is the difference between the initial charge / discharge capacities, is about 100 mAh / g or more in ordinary graphite, but is suppressed to 50 mAh / g or less in the graphite of the present invention.

In the present invention, dimethoxyethane is indispensable as an organic solvent for the electrolytic solution. Usually, dimethoxyethane is used in combination with a cyclic carbonate such as ethylene carbonate or propylene carbonate which contributes to high-efficiency charge / discharge.
However, it is not limited to these. The volume ratio of dimethoxyethane in the organic solvent is desirably 5 to 80%. 5
%, The effect of using dimethoxyethane to improve the load characteristics and storage characteristics is hardly obtained, and if it exceeds 80%, the charge / discharge efficiency tends to decrease even when the specific graphite is used. When ethylene carbonate is used in combination as the cyclic carbonate, the volume ratio of dimethoxyethane in the organic solvent is preferably 50% or less from the viewpoint of the relative dielectric constant and the viscosity characteristics. For such an organic solvent, L such as Li ₂ CF ₃ SO ₃
An organic electrolyte is prepared by dissolving a predetermined amount of a known electrolyte such as i-salt.

In the present invention, in the current lithium ion battery having a positive electrode potential of 4.0 V or more, the decomposition of dimethoxyethane occurs, so that the positive electrode potential (vs. Li / Li ⁺ ) at the time of charging and discharging is 4.0 V. Or less, for example, Li ₄ Ti ₅ so that the charge / discharge voltage is 4.0 V or less.
O ₁₂ , LiNbO, NaV ₃ O _{8 and the} like are used,
Of course, LiCoO ₂ , LiNiO ₂ , LiMn
There is no particular problem if O ₂ , LiMn ₂ O ₄ or the like is used and driven at 4.0 V or less.

The nonaqueous electrolyte secondary battery of the present invention is different from a conventional battery except that it has a negative electrode using the above-mentioned specific graphite as an active material, an organic electrolyte containing dimethoxyethane, and a positive electrode. However, for example, the preparation of the negative electrode and the positive electrode may be performed according to a conventional method, and the battery can is inserted into a battery can through an appropriate separator between the two electrodes and sealed according to a conventional method. Thus, a battery can be manufactured.

[0018]

Next, an embodiment of the present invention will be described in more detail. In the following, “parts” means “parts by weight”.

Example 1 A natural graphite having an R value of 0.111 was used as a negative electrode active material, and a pellet electrode was prepared using this graphite as a binder with polyvinylidene fluoride at a weight ratio of 5 to 10% by weight. This electrode is fixed to the copper foil that will be the lead wire with a conductive adhesive,
A three-electrode cell in which the counter electrode and the reference electrode were Li foils was manufactured. Ethylene carbonate (hereinafter referred to as EC)
): Propylene carbonate (hereinafter, referred to as PC): dimethoxyethane (hereinafter, referred to as DME) = 2:
Li _{2 as an} electrolyte is added to a 2: 1 (volume ratio) mixed organic solvent.
An organic electrolyte in which CF ₃ SO ₃ was dissolved at 1.2 mol / L was used. The charge and discharge conditions were as follows: after the above-mentioned cell was produced, the cell was charged at a current value of 100 μA / g, a constant current and a constant voltage of 0.01 V, and discharged at a constant current of the same current value to 1.5 V.

Example 2 As a negative electrode active material, mesophase pitch was graphitized to obtain R
Example 1 except that graphite having a value of 0.085 was used.
In the same manner as in Example 1, a three-electrode cell was prepared, and constant-current and constant-voltage charging and constant-current discharging were performed in the same manner as in Example 1.

Example 3 A coke graphitized product having an R value of 0.145 was used as a negative electrode active material, and EC: DME =
Li ₂ CF ₃ SO ₃ was added to a 3: 1 (volume ratio) mixed organic solvent.
Was prepared in the same manner as in Example 1 except that an organic electrolytic solution in which was dissolved 1.2 mol / L was used.
In the same manner as in Example 1, constant-current constant-voltage charging and constant-current discharging were performed.

Comparative Example 1 A three-electrode cell was prepared in the same manner as in Example 1 except that a coke graphitized product having an R value of 0.289 was used as the negative electrode active material. Then, constant current and constant voltage charging and constant current discharging were performed.

Comparative Example 2 A three-electrode cell was prepared in the same manner as in Example 1 except that mesocarbon microbeads having an R value of 0.352 were used as the negative electrode active material. Then, constant current and constant voltage charging and constant current discharging were performed.

Comparative Example 3 A three-electrode cell was prepared in the same manner as in Example 1, except that natural graphite having an R value of 0.192 and coated with low-crystalline carbon was used as the negative electrode active material. In the same manner as in Example 1, constant-current constant-voltage charging and constant-current discharging were performed.

Comparative Example 4 The same procedure as in Example 1 was carried out except that a coke graphitized product having an R value of 0.313 was used as the negative electrode active material and the same organic electrolytic solution as in Example 3 was used as the electrolytic solution. Similarly, 3
A pole cell was prepared and charged and discharged at a constant current and a constant voltage in the same manner as in Example 1.

The charging and discharging characteristics of the triode cells of Examples 1 to 3 and Comparative Examples 1 to 4 are shown in Table 1.
Was as shown in FIG. In Table 1, the R value of the negative electrode active material used and the configuration of the organic electrolyte are also shown for reference.

Table 1

As is clear from the results in Table 1, the DM
By using graphite having an R value of 0.150 or less as a negative electrode active material with respect to the organic electrolyte containing E as in Examples 1 to 3, a high charge / discharge efficiency of 90% or more can be obtained. It can be seen that it is possible to greatly contribute to increasing the capacity of the nonaqueous electrolyte secondary battery.

[0029]

As described above, in the non-aqueous electrolyte secondary battery using graphite as a negative electrode active material and dimethoxyethane as an organic solvent of an electrolyte, which contributes to improvement of load characteristics and storage characteristics, By using graphite having an R value of 0.150 or less as measured by a Raman scattering method, the secondary battery having excellent charge-discharge characteristics and high charge-discharge efficiency can be obtained. Can be provided.

Continued on the front page F-term (reference)

Claims (4)

[Claims] 1. A negative electrode for a graphite as an active material, 1,350Cm organic electrolyte containing dimethoxyethane and the non-aqueous electrolyte secondary battery having a positive electrode, the above graphite, measured by Raman scattering ^- nonaqueous secondary to 'R value which is the ratio of the peak intensity of 1,580Cm ^-1 [I] [I ¹ of the peak intensity (I)' / I] is characterized in that it is 0.150 or less Next battery. 2. The graphite was charged and discharged at least once.
Later interlayer distance [d ₀₀₂Is less than 0.45 nm.
The non-aqueous electrolyte secondary battery according to claim 1. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein a volume ratio of dimethoxyethane in the organic solvent in the organic electrolyte is 5 to 80%. 4. Positive electrode potential during charging / discharging (vs. Li / Li ⁺ )
The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, which has a voltage of 4.0 V or less.