JPH03252053A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To increase the discharge capacity and to improve the cycle life property by using a carbonaceous material made by carbonizing a petrolic pitch and regulating the morphologic parameter in a specific scope. CONSTITUTION:As a negative electrode, a carbonaceous material obtained by carbonizing a petrolic pitch, in which the surface interval of the (002) surface is 3.70 A or more, the true density is less than 1.70g/cm<3>, and there is no heating peak more than 70 deg.C in the differential thermal analysis in the air current is used, while a substance including Li which is doped and dope-released to the negative electrode is used as a positive electrode, and a nonaqueous electrolyte is used. When the carbonaceous material used as the negative electrode has the surface interval of the (002) surface less than 3.70Angstrom , the discharge capacity is reduced and the cycle life is deteriorated. In the same manner, when its true density exceeds 1.70g/cm<3>, the discharge capacity and the cycle life are deteriorated. And when the heating peak is at 700 deg.C or higher in the result of the differential thermal analysis, the battery property is deteriorated. Consequently, the cycle life property can be improved, and the discharge capacity can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a non-aqueous electrolyte secondary battery, and particularly to improvement of its negative electrode.

[Summary of the invention]

The present invention aims to provide a non-aqueous electrolyte secondary battery with high capacity and excellent cycle life characteristics by specifying the characteristics of the carbonaceous material of the negative electrode and using a positive electrode containing sufficient Li. It is something.

[Conventional technology]

BACKGROUND ART With the miniaturization of electronic devices, there is a demand for higher energy density batteries, and in order to meet this demand, various non-aqueous electrolyte batteries such as so-called lithium batteries have been proposed.

However, for example, batteries using lithium metal in the negative electrode have the following drawbacks, especially when used as secondary batteries. That is, (1) Charging usually requires 5 to 10 hours and is poor in rapid charging performance, (2) Cycle life is short, and so on.

All of these are caused by the lithium metal itself, including changes in the lithium form that occur with repeated charging and discharging, and the formation of dendrite-like lithium.

Irreversible changes in lithium are said to be the cause.

Therefore, as a method to solve these problems, it has been proposed to use a carbonaceous material for the negative electrode.

This takes advantage of the fact that carbon intercalation compounds of lithium can be easily formed electrochemically. For example, when carbon is used as a negative electrode and charged in a non-aqueous electrolyte, lithium in the positive electrode electrochemically forms. The negative electrode carbon is doped between the eyebrows.

Then, the carbon doped with lithium acts as a lithium electrode, and with discharge, lithium is dedoped from between the carbon layers and returned to the positive electrode.

[The problem that the invention attempts to solve! ! ] By the way, at this time, the current capacity per unit weight of carbon (
mAll/g) is determined by the amount of lithium doped, so in such a negative electrode, it is desirable to increase the amount of lithium doped as much as possible.

(Theoretically, the upper limit is the ratio of 1 L+ atom to 6 carbon atoms.) Conventionally, carbonaceous materials for negative electrodes include, for example, JP-A-62-
Publication No. 122066 or JP-A-62-90863
As disclosed in the publication, the spacing between (002) planes is about 3.40 to 3.60, and the density is about 1.70 to 2.20 g.
/aJ is used.

However, in reality, the amount of lithium doped in such carbonaceous materials is insufficient and is only about half of the theoretical value.

Therefore, the present invention was proposed in view of the above-mentioned conventional situation, and aims to develop a carbonaceous material with a large amount of lithium doped, which not only has excellent cycle life characteristics but also has a high discharge capacity. The purpose is to provide a large non-aqueous electrolyte secondary battery.

[Means to solve the problem]

In order to achieve the above-mentioned object, the present invention is obtained by carbonizing petroleum pitch, and has a (002) plane spacing of 3.70 Å.
As described above, a negative electrode made of a carbonaceous material whose true density is less than 1.70 g/cd and which does not have an exothermic peak at 700° C. or higher in differential thermal analysis in an air stream, and Li doped and dedoped into the negative electrode. It is characterized by having a positive electrode containing the above, and a non-aqueous electrolyte.

If the spacing between the (002) planes of the carbonaceous material used for the negative electrode is less than 3.70, the discharge capacity will decrease and the cycle life will deteriorate to the same level as conventional ones.

Similarly, even if the true density exceeds 1.70 g/cm3, deterioration in discharge capacity and cycle life are observed.

Further, after conducting various experiments, it was found that the results of differential thermal analysis greatly affect the battery characteristics, and that it is necessary that the battery does not have an exothermic peak at 700° C. or higher.

As carbonaceous materials having such characteristics, specific H/C
Introducing a functional group containing oxygen into a petroleum pinch that has an atomic ratio (
(so-called oxygen crosslinking) and then carbonized by a method such as sintering.

The petroleum pitch is coal tar, ethylene bottom oil,
It is obtained from tars, asphalt, etc. obtained by high-temperature pyrolysis of crude oil, etc., by distillation (vacuum distillation, atmospheric distillation, steam distillation), thermal polycondensation, extraction, chemical polycondensation, etc.

At this time, the H/C atomic ratio of petroleum pitch is important, and in order to make it a non-graphitizable carbon, this H/C atomic ratio should be 0.6-0.
It needs to be 8.

The specific means for introducing oxygen-containing functional groups into these petroleum pitches is not limited, but for example, nitric acid, mixed acids, sulfuric acid,
A wet method using an aqueous solution such as hypochlorous acid, a dry method using an oxidizing gas (air, oxygen), and a reaction using a solid reagent such as sulfur, ammonia nitrate, ammonia persulfate, or ferric chloride are used.

Petroleum pitch into which oxygen-containing functional groups have been introduced using the method described above is carbonized and used as a negative electrode material, but the carbonization conditions must be set so as to yield a carbonaceous material that satisfies the characteristics described above. For example, in a nitrogen stream, 300-70
After carbonization at 0'C, the heating rate was 1 to 20 in a nitrogen stream.
℃, the final temperature is 900 to 1300°C, and the holding time at the final temperature is about θ to 5 hours. Of course, the carbonization operation may be omitted depending on the case.

In addition, the obtained carbonaceous material is crushed and classified to be used as a negative electrode material, but this crushing is performed before and after carbonization.

It may be carried out either after firing.

Such carbonaceous materials are also described in, for example, Japanese Patent Publication No. 53-31116, but here, by optimizing the oxygen content, the interplanar spacing d0°2 of the (002) plane is set to 3.7.
A carbonaceous material having a temperature of 0 Å or more and having no exothermic peak at 700° C. or more in differential thermal analysis (DTA) in an air stream is used as the negative electrode material.

That is, the amount of oxygen introduced into the oil pitch is (00
2) Interplanar spacing d. For example, by setting the oxygen content of a simply crosslinked petroleum pitch precursor to 10% by weight or more, d. 2 can be set to 3.70 Å or more. Therefore, the oxygen content of the precursor is preferably 10% by weight or more, and practically 10% by weight or more.
-20% by weight. In particular, d. 2 is 3.72
Since it is preferable from the point of view of charging/discharging efficiency that the oxygen content be .ANG. or more, it is desirable to set the oxygen content in consideration of this point.

In addition, by adding a phosphorus compound or a boron compound during the firing of the petroleum pitch mentioned above, the capacity can be increased from 470 to 5.
It has been confirmed that it is possible to set it to about 00 mAh/g.

When the above-mentioned carbonaceous material is used as the negative electrode of a non-aqueous electrolyte secondary battery, the positive electrode preferably contains a sufficient amount of Li, for example, the general formula LiMOg (where M is at least one of Co and Ni). Composite metal oxides represented by ) and intercalation compounds containing Li are suitable, especially LiCo0t
It exhibits good characteristics when used.

Since the non-aqueous electrolyte secondary battery of the present invention aims to achieve high capacity, the positive electrode has a steady state of 1! (For example, after repeating charging and discharging about 5 times) L equivalent to a charge/discharge capacity of 250 mAh or more per pin of negative electrode carbonaceous material
i, preferably contains Li equivalent to a charge/discharge capacity of 300 mAh or more, more preferably contains Li equivalent to a charge/discharge capacity of 350 mAh or more,
It should be noted that Li does not necessarily need to be supplied entirely from the positive electrode material; in short, 25 Li per 1 pin of the negative electrode carbonaceous material in the battery system.
It is sufficient that Li equivalent to a charge/discharge capacity of 0 mAh or more is present, and the amount of Li is determined by measuring the discharge capacity of the battery.

The nonaqueous electrolyte is prepared by appropriately combining an organic solvent and an electrolyte, and any organic solvent or electrolyte that is used in this type of battery can be used.

To illustrate, examples of organic solvents include propylene carbonate, ethylene carbonate, 1.2-dimethoxyethane, 1.2-':;ethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1.3-dioxolane, 4- These include methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, anisole, and the like.

As the electrolyte, LiCo0t, , LiAsFi, 1i
PF*, LiBFa, L i B (CaHs)4, L
i Cj! XL i Br, CH, So, L i.

CFsSOsLi etc.

(Function) Carbon doped with lithium has a distance between the eyebrows ((002
) is known to have a surface spacing of d0°2 of 3.70 people. Therefore, like the conventionally used carbonaceous materials, d. 2 is 3.40 to 3.60 people,
It is thought that the interlayer distance increases when lithium is doped. That is, do. In a carbonaceous material with t<3.10, it is considered that doping with lithium becomes difficult as the interlayer space must be widened, and this is considered to reduce the amount of doping.

The true density ρ has a close relationship with the interlayer distance, and ρ>1
．． When it becomes 70 g/d, it becomes difficult to secure the above-mentioned distance between the eyebrows, and the amount of dope also decreases.

Furthermore, the discharge capacity and cycle life characteristics are affected by the structure of the carbonaceous material, and although differential thermal analysis showed that materials with no peaks above 700° C. showed good results, the details of the structure are unknown.

The carbonaceous material obtained by oxygen crosslinking petroleum pitch having a predetermined H/C atomic ratio and then firing satisfies all of these characteristics. By using it as the negative electrode of a liquid secondary battery, efficient charging and discharging can be performed.

Further, a battery using carbon as a negative electrode requires a shorter charging time than a battery using lithium metal as a negative electrode, and the battery of the present invention maintains this characteristic.

〔Example〕

The present invention will be explained below based on specific experimental results.

First, petroleum pitch (H/C atomic ratio 0.6-0.8) was oxidized, and the oxygen content was 2.2% by weight and 6.7% by weight10
Five types of carbon precursors, 68% by weight, 15.4% by weight, and 16.3% by weight, were prepared and carbonized at 500° C. for 5 hours in a nitrogen stream.

Next, the expanded piece was pulverized in a mill, placed in a crucible, and heated in a nitrogen stream at a temperature increase rate of 5°C/min, a maximum temperature of 1100°C, and a holding time of 1 hour at the maximum temperature. It was fired in

After cooling, it was ground in a mortar and classified into particles of 38 μm or less using a mesh.

The spacing d between the (002) planes of the obtained carbonaceous material. R,
L co. 8 and main peak temperature T in differential thermal analysis
p is shown in Table 1, and changes in true density due to oxygen content are shown in FIG. 1, and the interplanar spacing d. 2 is calculated from the 2θ value determined by the tangential method from the X-ray diffraction measurement results, and Lc+. , is the half of the (002) peak 411II
Based on ii, it was calculated using the Scheler equation. Differential thermal analysis was performed in an air stream of 200 m/min at a heating rate of 10° C./min, and the true density was measured by a vicinometer method.

Table 1 Looking at Table 1 and Figure 1, the interplanar spacing d. ! ,
LC@. 8. Both the main beak temperature Tp and true density in differential thermal analysis are greatly affected by the oxygen content, especially the interplanar spacing d. Regarding No. 2, it can be seen that the oxygen content becomes 3.70 Å or more only when the oxygen content is 10% by weight or more.

Therefore, these carbonaceous materials were evaluated using a test cell.

When preparing the test cell, first, immediately before preparing the negative electrode mix, the carbonaceous material was subjected to preheat treatment in an Ar atmosphere at a temperature increase rate of about 0°C/min and a final temperature of 600°C and a holding time of 1 hour. After the application, polyvinylidene fluoride was added as a binder in an amount equivalent to 10% by weight of the carbonaceous material, mixed using dimethylformamide as a solvent, and dried to prepare a negative electrode mix. Thereafter, the 37 square centimeters were formed into a pellet with a diameter of 15.5 square centimeters together with a Ni mesh serving as a current collector to produce a carbon electrode. Furthermore, the configuration of the test cell is as follows.

Cell configuration Coin-shaped cell (diameter 20cm, thickness 2.5cm) Opposite electrode
: Li metal separator: Porous membrane (polypropylene) electrolyte
: 1 mol/f! of LiCIO4 in a mixed solvent of propylene carbonate and dimethoxyethane (1:1 by volume). dissolved at the rate of

Current collector: copper foil The test cell with the above configuration was repeatedly charged and discharged five times at a current flow condition of 1 mA (current density 0.53 mA/cm3).
When a steady state was reached, the discharge capacity per gram of negative electrode carbonaceous material was measured. The results are shown in Table 2.

Table 2 shows the change in discharge capacity and the spacing d of the (002) plane when the temperature is varied in the range of 1000 to 1300°C. ! , L
C@Is, the main peak temperature TP in differential thermal analysis was measured. The results are shown in Table 3.

Table 3 As is clear from Table 2, the discharge capacity increases as the oxygen content in the raw petroleum pitch increases.

Next, we investigated the influence of firing temperature when firing petroleum pitch to produce a carbonaceous material.

In other words, if we fix the oxygen content of petroleum pitch at 15.4% by weight and look only at the maximum temperature reached during the previous firing, we can see that as the firing temperature increases, Lc...1 increases and a
S. 2 has become smaller, indicating that the growth and stacking of the carbon layer is progressing. Therefore, the capacity decreases as the firing temperature increases.

Based on the results of the above preliminary experiments, we actually assembled a non-aqueous electrolyte secondary battery and investigated its characteristics.

Petroleum pitch appropriately selected from a range of 0.6 to 0.8 with a direct IJ0-H/C atomic ratio was ground and oxidized in an air stream to obtain a carbon precursor. Quinoline solubility of this carbon precursor (JIS
Centrifugal method: K2425-1983) was 80%, and the oxygen content (according to organic elemental analysis) was 15.4% by weight.

This carbon precursor was carbonized by being held at 500° C. for 5 hours in a nitrogen stream, and then heated to 1100° C. for 1 hour.

The following battery was constructed using the carbonaceous material thus obtained.

The carbonaceous material is crushed in a mortar and classified with a sieve, and is
The following materials were used.

100 μm of polyvinylidene fluoride was added as a binder to 1 g of this carbonaceous material, made into a paste using dimethylformamide, applied to a stainless steel mesh, and then pressed under a dry pressure of 5 tons/d. This was punched out to make a negative electrode, but the net carbonaceous material at this time was Li, while the positive electrode was made of Li as an active material.
Using N1a, *Coo, *(h, the LiN1o
, zcoo, sow 6 g of graphite powder and 3 g of polytetrafluoroethylene were added to 91 g, and after mixing thoroughly, the Ig was taken and put into a mold, and 2 tons/
The mixture was molded at a pressure of cm3 to obtain a disc-shaped electrode.

Using the above positive and negative electrodes, LiCj! A coin-shaped battery was prototyped using polypropylene nonwoven fabric as a separator, in which O. was dissolved at a ratio of 1 mol/l. In addition, this battery uses the amount of active material used as the electrochemical equivalent of the positive electrode.
〉It was configured to be a negative electrode, and the negative electrode was regulated.

A prototype coin-shaped battery was produced in the same manner as in Example 1 except that the oxygen content of the 1m carbon precursor was 10.8% by weight.

It was 32.4 ■.

Example 1u11 A coin-shaped battery was prototyped in the same manner as in Example 1 except that the oxygen content of the carbon precursor was 16.3% by weight.

The same petroleum pitch as in Example 1 was oxidized to obtain a carbon precursor. The quinoline solubility of this carbon precursor was 8%, and the oxygen content was 6.7% by weight.

Using this carbon precursor, a coin-shaped battery was experimentally produced in the same manner as in Example 1 except for the above.

A prototype coin-shaped battery was produced in the same manner as in Example 1 except that the oxygen content of the carbon precursor was 2.2% by weight.

First, discharge curves were drawn for Example 1 and Comparative Example 2. The results are shown in Figure 2.

It can also be seen from FIG. 2 that the higher the oxygen content, the greater the capacity.

Next, for each example and comparative example, the cycle characteristics when charged at 320 mAh per gram of negative electrode carbonaceous material were investigated. During the charge/discharge test, the current density was 0.5 for both charging and discharging.
The discharge was performed at a constant current of 3 mA/d, and the discharge cutoff voltage was 1.
．． It was set to 5■. The results are shown in Figure 3.

Each example in which the oxygen content of the carbon precursor is 10% by weight or more exhibits good cycle characteristics, but as the oxygen content decreases, the discharge capacity decreases as the number of cycles increases. Comparative Example 1 where is 6.7%
Also, when the number of cycles exceeds 20, the discharge capacity gradually decreases, and when the number of cycles exceeds 40 to 50, it rapidly decreases.

〔Effect of the invention〕

As is clear from the above description, in the present invention,
Since the negative electrode is made of carbonized petroleum pitch and a carbonaceous material with morphological parameters defined within a predetermined range is used, it is possible to provide a non-aqueous electrolyte secondary battery with a large discharge capacity and a long cycle life. It is.

Furthermore, since the non-aqueous electrolyte secondary battery of the present invention uses a carbonaceous material as the negative electrode, the advantage of short charging time is maintained, making it possible to provide a highly practical non-aqueous electrolyte secondary battery. It is possible.

[Brief explanation of drawings]

FIG. 1 is a characteristic diagram showing the relationship between the oxygen content and true density of a carbon precursor. FIG. 2 is a characteristic diagram showing a discharge curve of an example to which the present invention is applied in comparison with that of a comparative example. FIG. 3 is a characteristic diagram showing the cycle characteristics of batteries in which the oxygen content of the carbon precursor is changed. Oxygen content (Deaf!”/,) Figure 1

Claims (1)

[Claims] A product obtained by carbonizing petroleum pitch, having a spacing of (002) planes of 3.70 Å or more, a true density of less than 1.70 g/cm^3, and differential thermal analysis in an air stream. at 700℃
A nonaqueous electrolyte secondary battery comprising: a negative electrode made of a carbonaceous material that does not have an exothermic peak; a positive electrode containing Li that is doped and dedoped into the negative electrode; and a nonaqueous electrolyte.