JPH07235294A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To provide a nonaqueous electrolyte secondary battery with less gas evolution in high temperature storage, high safety by using high capacity graphite, and high capacity. CONSTITUTION:Graphite in which R value (I1360/I1580) indicating by the ratio of peak intensity (I1360) of spectrum in a 1360+ or -100cm<-1> wave length region to peak intensity (I1580) of spectrum in a 1580+ or -100cm<-1> wave region in spectrum analysis using argon ion laser beam whose wave length is 5145Angstrom is 0.15 or more, preferably 0.20 or more, and half-power band width DELTAnu1580 of spectrum in a 1580+ or -100cm<-1> wave region is less than 25c<-1>, preferably less than 23cm<-1> is used as a negative active material in a nonaqueous electrolyte secondary battery.

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 using a carbon material as a negative electrode active material.

[0002]

2. Description of the Related Art A lithium secondary battery using an organic electrolytic solution and an alkali metal such as lithium as a negative electrode active material has attracted attention because it has a higher energy density than an aqueous secondary battery. However, when metallic lithium is used as the negative electrode, active lithium generated by charging reacts with the organic solvent of the electrolytic solution, and the deposited lithium grows in the form of dendrite, and the insulating layer is formed by the reaction between the deposited lithium and the solvent. Lithium, which has no electronic conductivity, is generated (R. Selim and Bro, J. Electroche
m. Soc, 121, 1457 (1974)), the charge / discharge efficiency of the lithium electrode is poor. In addition, there is a problem that an internal short circuit of the battery occurs due to lithium grown in a dendrite shape, and a practically sufficient lithium secondary battery has not been obtained. In addition, a non-aqueous electrolyte secondary battery using a carbon material as a negative electrode has been proposed as a battery system which can avoid such a problem and which can be expected to have a high energy density. This battery system has already been partially commercialized, and LiCoO ₂ is used for the positive electrode.

The role of the carbon material of the negative electrode in the lithium secondary battery is to take in Li by charging and release it by discharging, which is an active material that acts as a base material carrying Li.

Generally, there are many kinds of carbon materials such as graphite type, carbon black type and amorphous type, and their forms and physical properties are greatly different. Since attention has been paid to a carbon material as a negative electrode material for a lithium secondary battery, many attempts have been made to select an optimum carbon material for the negative electrode, and much discussion has been made on the physical properties of the carbon material and the negative electrode characteristics.
For example, the charge / discharge capacity means the amount of Li that can be reversibly supported in the carbon material, and in the case of graphite, ideal graphite can theoretically support Li up to C ₆ Li between the graphite layers. It is said that. In addition, many applications have been filed regarding carbon materials as negative electrode active materials for lithium secondary batteries, and many specify the physical properties of the carbon materials. For example, those relating to particle size, purity and surface area, those relating to d002 interplanar spacing and crystallite size L _c value which are crystallinity parameters obtained from X-ray diffraction analysis, and further Raman intensity ratio (1580 cm) of Raman spectrum. ^-1 spectrum intensity and 1
The ratio of 360 to the spectrum intensity of cm ^-1 ; R value (JP-A-63-124380, JP-A-2-
267873).

[0005]

As a result of earnest studies by the present inventors, it has been found that among carbon materials, graphite (a carbon material having advanced crystallinity, such as artificial graphite or natural graphite) has a relatively high capacity. I understood. Therefore, we have been focusing our attention on graphite in particular. Generally, there are natural graphite that is naturally produced and artificial graphite that is artificially synthesized. Natural graphite is a fossil fuel graphitized under high temperature and high pressure, and generally has low purity. Therefore, it is often used after being subjected to a high-purification treatment. Artificial graphite is made of easily graphitizable (prone to become graphite) materials such as petroleum pitch, coal pitch, and coke as starting materials.
It is obtained by heat treatment (also referred to as graphitization treatment) at a high temperature of 500 ° C. or higher. Both are carbon materials with developed crystallinity.
However, even with graphite, the crystallinity parameter d002 (according to the (002) X-ray diffraction method) is different due to the difference in the starting materials, the graphitization processing conditions including the processing temperature and the processing time.
The surface spacing) and the L _C value (the size of the crystallite in the C-axis direction) are not uniform, and there are differences in parameters such as particle size, surface area, and purity. I know it depends. For example, X
Although the crystallinity parameters obtained by the line diffraction method are almost the same, the results show that the capacities differ greatly. Since graphite alone has such a wide range, it is necessary to dig deeper into the relationship between the negative electrode characteristics and the physical properties of graphite. However, some directions have been empirically derived from the studies on graphite that the present inventors have conducted so far. One is that at least d002 is 3.37Å or less and L _c value is 500Å or more in order to obtain a high capacity, and in order to obtain stable negative electrode characteristics, it is at least 99.5% or more in purity. And that starting materials are preferably petroleum-based materials. However, it is not sufficient to specify graphite to some extent in this way, and even if the graphite contained in these categories exhibits a relatively high capacity, there is still a clear difference in capacity as compared with each other. Furthermore, in addition to the capacity, in a battery using graphite as the negative electrode active material, when the battery is stored at a high temperature in a charged state (state in which lithium is supported in graphite) (for example, stored at 60 ° C. for 20 days), the graphite surface From the results, it was found that there was a case where remarkable gas generation occurred and the internal pressure of the battery increased. In particular, when the internal pressure rises remarkably, there is a risk of the battery bursting, which is a very important problem. Safety is also a priority issue, and currently, in order to avoid this problem, mechanical devises such as explosion-proof valve actuating when the internal pressure reaches a certain point and releasing gas are made. It is not an essential solution.

Conventionally, some proposals have been made regarding carbon materials used as negative electrode active materials, but in particular, regarding the above-mentioned problem that gas is generated when high temperature storage is performed in a charged state, not only a solution but also a sufficient solution is sufficient. I can't find anything that explains it.

The present invention solves this problem, and an object of the present invention is to provide a carbon material, especially graphite, which has a high capacity and produces a small amount of gas during high temperature storage in a charged state.

[0008]

In order to solve the above-mentioned problems, the non-aqueous electrolyte secondary battery of the present invention has a Raman spectrum of 1580 ± 100 cm ^-1 in Raman spectrum analysis using an argon ion laser beam having a wavelength of 5145Å. 1360 ± with respect to the peak intensity (I ₁₅₈₀ ) of the spectrum in the wavelength range
The R value (I ₁₃₆₀ / I ₁₅₈₀ ) indicated by the ratio of the peak intensity (I ₁₃₆₀ ) of the spectrum in the wavelength range of 100 cm ⁻¹ is 0.15 or more, and half of the spectrum in the wavelength range of 1580 ± 100 cm ^−1. Price range Δν ₁₅₈₀ is 25 cm ^-1
Graphite, which is less than, was used as the negative electrode active material.

The R value (I ₁₃₆₀ / I ₁₅₈₀ ) is 0.
20 or more and the full width at half maximum Δν ₁₅₈₀ is 23 cm ⁻¹
It is preferably less than.

As a result, not only is the capacity high, but
It is possible to provide a lithium secondary battery having a high energy density and an excellent safety, which can realize a negative electrode with less gas generation during high temperature storage.

[0011]

In general, graphite is an aggregate of unit crystallites composed of layered crystals in which layers composed of hexagonal mesh planes of carbon are stacked as shown in FIG. The surface perpendicular to the layer surface is the edge surface (A in FIG. 1), and the parallel surface is the basal surface (B in FIG. 1).
I am calling. The R value obtained from the Raman spectrum is a parameter relating to the degree of graphitization and crystal orientation, and is also a parameter indicating the degree of exposure of the edge surface and basal surface of graphite. In this case, if the R value is large, many edge surfaces are exposed, and if the R value is small, many basal surfaces are exposed. The above-mentioned gas generation is caused by the reaction on the surface of graphite. As a result, the larger the R value, the smaller the gas generation amount. The fact is that the gas generation amount tends to decrease as the ratio of the edge surface of the graphite surface increases. Shows. That is, it is considered that gas generation occurs selectively with the involvement of the basal plane. Although the detailed mechanism of gas generation caused by the involvement of the basal plane is not clear, there is a high electron density state in which the π electrons are widely spread on the basal plane, and these electrons are involved in the gas generation reaction. It seems that Therefore, it is considered that graphite whose basal plane is not exposed to the surface as much as possible is preferable. In addition, since the charge / discharge reaction when graphite is used for the negative electrode is an insertion / release reaction of Li between the graphite layers, it can be said that it is more preferable that the inlet of the layer, that is, the edge surface is exposed to the outside. A large R value is advantageous not only in terms of generation but also in terms of electrode reaction. On the other hand, the capacity is determined by the amount of Li that can enter the graphite layers. The higher the degree of perfection of the layered crystal, the theoretical capacity of 372 mAh / g.
It is considered to approach (corresponding to C ₆ Li). That is,
It is expected that the higher the crystallinity of the unit crystallite of graphite, the larger the capacity. Therefore, the graphite material in which the crystallinity of each unit crystallite is high and the edge surface is exposed a lot can be an excellent negative electrode material having a high capacity and less gas generation during storage at high temperature. The optimum values of R value and Δν ₁₅₈₀ will be described in detail in the section of Examples.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 shows a diameter 20 used in the embodiment of the present invention.
FIG. 3 is a cross-sectional view of a coin-shaped battery having a size of mm and a height of 1.6 mm.
1 is a stainless steel case, 2 is a stainless steel sealing plate, 3
Is pressed against the inner surface of the sealing plate 2 with metallic lithium. Four
Is a polypropylene separator. In No. 5, graphite of the negative electrode active material and fluororesin of the binder were mixed at a weight ratio of 95: 5, a thickener was added to form a paste, and the paste was applied to the copper foil 6 with a thickness of 0.2 mm. It is a negative electrode plate that is minutely dried, rolled with a rolling roller to a thickness of 0.1 mm, and punched to have a diameter of 12.5 mm. 7 is a polypropylene gasket. The electrolytic solution is a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) mixed at a volume ratio of 50:50, and lithium hexafluorophosphate (L) as an electrolyte.
iPF ₆ ) is dissolved in a concentration of 1 mol / l.
Charging / discharging using this coin-shaped battery is performed by charging with a constant current of 0.1 mA (direction in which the voltage decreases due to the reaction of Li being inserted into graphite), 0 V, and discharging (the reaction of releasing Li from the graphite). (The direction in which the voltage increases) The final voltage was set to 1V. In addition, in the case of this type of battery, a part of the charged electricity quantity may work irreversibly in the initial stage, and the charge / discharge capacity becomes stable.
The discharge capacity at the cycle was used as the capacity of graphite for comparison in this example.

FIG. 3 is a cross-sectional view of an apparatus for collecting gas generated during high temperature storage of a negative electrode in a charged state used in an embodiment of the present invention. Reference numeral 11 is a Teflon container, 12 is a graduated cylinder for collecting gas, 13 is a graphite negative electrode in a charged state taken out from the coin battery, and 14 is an electrolytic solution (same as in the coin battery). The amount of gas generated was measured by configuring a gas collecting device, sealing the Teflon container 11 and storing it in a constant temperature bath at 85 ° C. for 3 days, during which the gas generated from the graphite negative electrode 13 was calibrated for gas collection. It was collected with the louver 12 and went.

In the present embodiment, at least d002 of 3. is selected from artificial graphite and natural graphite obtained from a plurality of carbon material manufacturers as the negative electrode active material, according to the above-mentioned empirical rule.
Twenty kinds of graphite having an L _c value of 37 Å or less, an L _c value of 500 Å or more and a purity of at least 99.5% or more were selected and used. In the case of artificial graphite, the starting material selected was a petroleum-based material. However, the starting material for natural graphite is naturally unknown. The graphite selected here was completely randomly selected except for the conditions defined based on the above empirical rule. Of the above 20 kinds of graphite (sample No. A to T, written in alphabetical order), No.
15 kinds of A to O are artificial graphite, and 5 kinds of No. P to T are natural graphite.

Table 1 shows the above 20 kinds of graphite.
Results obtained by Ar laser Raman spectrum (R
Value, Δν ₁₅₈₀ ), the discharge capacity per weight of the active material obtained by the coin-shaped battery, and the amount of gas measured by using the gas collecting device.

[0017]

[Table 1]

FIG. 4 is a plot of the relationship between the R values of all samples and Δν ₁₅₈₀ . Looking at FIG. 4, R value and Δ
There is a slight tendency for Δν ₁₅₈₀ to increase as the R value increases between ν ₁₅₈₀ , but it is not a clear correlation, but rather can be regarded as factors independent of each other. However, compared with artificial graphite, natural graphite (No.
In the case of P to T), both the R value and Δν ₁₅₈₀ tend to be relatively small. Also, the R value of the graphite selected here is 0.08 to
It is in the range of 0.37, and Δν ₁₅₈₀ is 19.7 to 29.1.
Was in the range. Since graphite is selected almost at random, it is considered that carbon materials classified as graphite fall within this range.

FIG. 5 is a plot of the relationship between the R value and the discharge capacity, and there is no clear correlation between the R value and the discharge capacity.

FIG. 6 is a plot of the relationship between the R value and the gas generation amount, and the correlation is clearly seen. As is clear from FIG. 6, the gas generation amount tends to decrease as the R value increases. In particular, it was found that the gas generation amount can be roughly divided into three regions that correlate with the R value. One is a region where the gas generation amount is the largest, and corresponds to a region where the R value is less than 0.15. The next is a region where the gas generation amount is relatively small, and corresponds to a region where the R value is 0.15 or more and less than 0.20. Further, it is a region where the R value is 0.20 or more, which is a region where the gas generation amount is extremely small. Therefore, from the viewpoint of the amount of generated gas, the R value is 0.15 or more, preferably 0.20 or more.

FIG. 7 shows Δν₁₅₈₀Plot the relationship between
However, the correlation is clearly seen. Clear from Figure 7
As you can see, Δν₁₅₈₀Becomes larger, the capacity becomes smaller
Tend to Especially for the capacity Δν₁₅₈₀Correlates to
It was found that it can be roughly divided into three areas. One
Is the region with the largest capacity, and Δν₁₅₈₀Is 23 cm^-1Not yet
Corresponds to the full area. Next is a relatively large area
And Δν₁₅₈₀Is 23 cm ^-1Above, 25cm^-1Less than the area
Equivalent to. And Δ, which is a region with a relatively small capacity
ν₁₅₈₀Is 25 cm^-1The above is the area. Therefore,
From a viewpoint, Δν₁₅₈₀Is 25 cm^-1Less than, preferably
Is 23 cm^-1It is desirable to be less than.

FIG. 8 is a plot of the relationship between Δν ₁₅₈₀ and the gas generation amount, and no clear correlation can be seen.

FIG. 9 summarizes the above results, the horizontal axis
To Δν₁₅₈₀, Take the R value on the vertical axis and
It shows a preferable region from both viewpoints of production.
It That is, the R value is 0.15 or more, and Δν₁₅₈₀But
25 cm^-1Graphite in the area below (shaded area in Figure 9)
For example, in the samples used in this example, No. B, C, K,
N, O, and R correspond to this), the capacity is large.
It becomes a negative electrode active material that is sharp and generates a small amount of gas. Furthermore
Preferably, the R value is 0.20 or more, and Δν ₁₅₈₀But
23 cm^-1Graphite in the area below (mesh line part in Fig. 9) (example
For example, in the samples used in this example, No. B and K are
It is equivalent to this).

As described above, in Raman spectrum analysis using an argon ion laser beam having a wavelength of 5145Å, the peak intensity (I ₁₅₈₀ ) of the spectrum in the wavelength range of 1580 ± 100 cm ^-1 was 1360 ± 100 _c.
The R value (I ₁₃₆₀ / I ₁₅₈₀ ) represented by the ratio of the peak intensity (I ₁₃₆₀ ) of the spectrum in the wavelength range of m ⁻¹ is 0.15 or more, preferably 0.20 or more, and 1
580 is less than 25 cm ^-1 FWHM .DELTA..nu ₁₅₈₀ of spectrum in the wavelength range of ± 100 cm ^-1, preferably 23cm ^-1
By using graphite, which is less than the above, as the negative electrode active material, it is possible to provide a lithium secondary battery having high capacity and excellent safety.

The graphite according to the present invention can be applied to a non-aqueous electrolyte secondary battery having various shapes such as a cylindrical shape and a prismatic shape. In particular, a prismatic battery has an internal pressure higher than that of a cylindrical battery. Since the graphite of the present invention has a drawback that its shape is likely to be deformed due to rising, and that gas generation can be suppressed, it can be said that the graphite is an effective negative electrode active material particularly for prismatic batteries.

In the examples of the present invention, a mixed solvent prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 50:50 as an electrolytic solution and lithium hexafluorophosphate as an electrolyte are used. (LiPF ₆ ) 1 mol /
Although the one dissolved in a concentration of 1 was used, a mixed solvent containing another solvent, for example, methyl ethyl carbonate (EMC) for a carbonate ester system, methyl acetate (MA), and methyl propionate (MP) for a chain ester system. However, the same tendency is obtained in gas generation characteristics and capacity characteristics.

[0027]

As is clear from the above description, by using the graphite of the present invention selected from the viewpoints of surface orientation and crystallinity of unit crystallites as a negative electrode active material,
In addition, a highly safe lithium secondary battery can be provided.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of an assembly of unit crystallites of graphite.

FIG. 2 is a sectional view of a coin battery.

FIG. 3 is a cross-sectional view of a gas collection device.

FIG. 4 is a plot of R value and Δν ₁₅₈₀ of a graphite sample.

FIG. 5 is a diagram showing the relationship between R value and discharge capacity.

FIG. 6 is a diagram showing a relationship between an R value and a gas generation amount.

FIG. 7 is a diagram showing the relationship between Δν ₁₅₈₀ and capacity.

FIG. 8 is a diagram showing the relationship between Δν ₁₅₈₀ and gas generation amount.

FIG. 9 is a diagram showing a preferable range of R value and Δν ₁₅₈₀ .

[Explanation of symbols]

 1 Case 2 Sealing Plate 3 Metal Lithium 4 Separator 5 Negative Electrode Plate 6 Copper Foil 7 Gasket A Edge Surface B Basal Surface

Claims (2)

[Claims] 1. In a Raman spectrum analysis using an argon ion laser beam having a wavelength of 5145Å, 1580
R value represented by the ratio of the peak intensity of the spectrum (I ₁₃₆₀₎ in the wavelength range of 1360 ± 100 cm ^-1 to the peak intensity of the spectrum (I ₁₅₈₀₎ in the wavelength range of _{^{± 100cm -1 (I 1360 / I}} 1580) is 0 .15 or more,
And 1580 non-aqueous electrolyte secondary battery, which comprises using a graphite spectral half width .DELTA..nu ₁₅₈₀ of less than 25 cm ^-1 in the wavelength range of ± 100 cm ^-1 as a negative electrode active material. 2. The R value (I ₁₃₆₀ / I ₁₅₈₀ ) is 0.20.
The non-aqueous electrolyte secondary battery according to claim 1, wherein the half width Δν ₁₅₈₀ is less than 23 cm ⁻¹ .