JP3052565B2 - 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 nonaqueous electrolyte secondary battery, and more particularly to a graphite material capable of intercalating and deintercalating lithium.

[0002]

2. Description of the Related Art A lithium secondary battery using an organic electrolyte and using lithium metal as a negative electrode active material has attracted attention because of its high energy density and excellent low-temperature characteristics as compared with an aqueous secondary battery. I am collecting.

However, active lithium produced by charging reacts with the organic solvent of the electrolyte, or the reaction between the lithium grown and deposited in the form of dendrites and the solvent forms an insulating layer, which causes electron conductivity. No lithium is produced (R. Selim and Bro, J. Electrochem. Soc,
121, 1457 (1974)), there has been a problem that the negative electrode using lithium metal has poor charge / discharge efficiency. In addition, there is a problem in safety, such as an internal short circuit of the battery caused by lithium grown in a dendrite shape, and a practically sufficient lithium secondary battery has not been developed.

Conventionally, in order to solve the problems of the negative electrode using lithium metal, Japanese Patent Application Laid-Open No. 57-20807 has been proposed.
No. 9, JP-A-59-143280, and the like, it has been proposed to use a carbonaceous material such as graphite as a negative electrode.

For example, Japanese Patent Application Laid-Open No. 62-122066 discloses an organic polymer, a condensed polycyclic hydrocarbon, and a polycyclic heterocyclic compound for the purpose of improving self-discharge characteristics, cycle characteristics, and storage stability of lithium. A hydrogen (H) / carbon (C) atomic ratio of less than 0.15, and an interlayer distance d (00) of the (002) plane determined by an X-ray diffraction method.
2) is 3.37 ° or more and the crystallite size L _{c in the} c-axis direction
Discloses a carbonaceous material having a pseudographite structure of 150 ° or less. Japanese Patent Application Laid-Open No. 63-285872 discloses a method for improving the cycle characteristics and increasing the capacity.
(002) is 3.35~3.8Å, L _c is 10 to 250
Å, size L _a in the a-axis direction of crystallites is 15～250A, and a BET specific surface area of carbon black is disclosed is 50 m ² / g or more. JP-A-63-112248 discloses that the specific surface area by the BET method is 0.1 to 100 m ² / g and the true density ρ is 1.
70 to 2.18 g / cm ³ , L _c is 10 <LC (Å) <12
A powdery carbonaceous material having a particle size distribution of 0 ρ-189 and having a particle size distribution of 90% or more by volume in the range of 0.1 to 50 μm is disclosed. Further, Japanese Patent Application Laid-Open No. 2-284354 discloses that, in order to improve the self-discharge characteristics of a battery and the filling rate of a carbonaceous material, a carbonaceous material having a particle size distribution of 5 μm or less has a volume ratio of 5% or less. The following discloses a carbonaceous material having an average particle size in the range of 25 to 100 μm.

As described above, as the carbonaceous material for the negative electrode, a material obtained by carbonizing and grinding various hydrocarbons or polymer materials, or a pseudo-graphite material having an appropriate turbostratic structure and not too high in crystallinity is used. In addition, lithium intercalation and deintercalation have been proposed.

[0007]

However, when the above-described pseudo-graphite material is used for the negative electrode, the amount of intercalated lithium is low because the layer structure of graphite is undeveloped and has low crystallinity. However, there was a problem that the capacity was reduced.

In addition, when a graphitized graphitic material having advanced graphitization is used as a negative electrode, gas is generated by decomposition of an electrolytic solution on the surface of the negative electrode during charging, and lithium intercalation reaction is reduced. Thus, there is a problem that the capacity is reduced and lithium dendrite is generated on the surface of the negative electrode.

The present invention has been made to solve the above problems, and provides a high-capacity graphitic material having a large amount of lithium intercalation and the generation of lithium dendrite on the surface of a negative electrode during charging. It is intended to provide a non-aqueous electrolyte secondary battery in which the safety is improved by preventing the problem.

[0010]

In order to solve the above-mentioned problems, a graphitic material used for a negative electrode of a nonaqueous electrolyte secondary battery of the present invention comprises a mesophase microparticle formed in a liquid phase carbonization process of pitches. Spheroidal graphite obtained by graphitizing a sphere,
The interlayer distance d (002) by X-ray diffraction method is 3.37 ° or less, the crystallite size Lc in the c-axis direction is 500 ° or more, and the particle size distribution of the spherical graphite is 6 μm or less. Less than 3% and the average particle size is 15 μm
２５25 μm.

[0011]

[Function] A carbonaceous material obtained by liquid-phase carbonization of coal tar, pitch, and the like is generally graphitizable soft carbon, and a highly crystalline structure can be obtained by graphitization by high-temperature heat treatment. In the present invention, the mesophase microspheres generated in the liquid phase carbonization process of these pitches are separated and extracted, and then they are graphitized.

[0012] Therefore, since the carbon precursor before graphitization has a structure in which thin layers of polycyclic aromatic molecules are already stacked, graphitization becomes easy when a graphitization heat treatment is performed. Thus, a graphitic material having high crystallinity can be obtained.

[0013] In addition, since dense graphite is formed by graphitization, it can be used as a negative electrode to provide a high-capacity negative electrode with a large amount of intercalation of lithium.

Further, the particle size of the spheroidal graphite is 15-25 μm.
And by mixing particles having a particle size of 6 μm or less smaller than this in a volume ratio of 3% or less, the filling properties of the spherical graphite can be further improved and the dendrite growth of lithium can be prevented. Thus, a lithium secondary battery having a large charge / discharge capacity can be obtained.

[0015]

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

FIG. 1 shows a 20 mm diameter and a 1.6 mm height of the present invention.
1 is a cross-sectional view of a mm coin-type nonaqueous electrolyte secondary battery.

As shown in FIG. 1, the non-aqueous electrolyte secondary battery of the present invention comprises a stainless steel case 1, a stainless steel sealing plate 2, a negative electrode 3 using a graphite material, It is composed of a separator 5, a positive electrode 6, an organic electrolyte and an insulating gasket 7 of dimensions and materials. Here, the graphitic material used for the negative electrode 3 is a spherical graphite obtained by separating and extracting mesophase microspheres generated in the process of liquid-phase carbonization of coal tar pitch and graphitizing the same. interlayer distance d (002) is 3.36 Å, the size L _c of the c-axis direction of crystallites 630 Å, the size L _a in the a-axis direction of crystallites is of 3100A. The negative electrode 3 is obtained by mixing spherical graphite having a particle size distribution of 20% in average particle diameter and 2% by volume of small particles having a particle diameter of 6 μm or less with a binder of a fluororesin in a weight ratio of 90:10, It was fabricated by molding on a negative electrode current collector 4 made of stainless steel. Then, the current collector 4 was spot-welded to the sealing plate 2 to be electrically connected.

The positive electrode 6 is prepared by mixing lithium carbonate and cobalt tetroxide in equimolar amounts and calcining the mixture at 900 ° C. for 10 hours to produce a positive electrode active material LiCoO ₂ . Carbon black and a fluororesin as a binder are mixed at a weight ratio of 80:10:10, and the diameter is 14.5 m.
m, and molded into a pellet having a height of 0.8 mm.

The electrolyte is ethylene carbonate (E
C) and diethyl carbonate (DEC) in a volume ratio of 50:
A solution obtained by dissolving lithium hexafluorophosphate so as to have a concentration of 1 mol / l as an electrolyte in the mixed solvent mixed at 50 was used.

As a comparative example, a battery similar to that of the present invention was used except that powder obtained by pulverizing artificial graphite obtained by graphitizing coke obtained by carbonizing coal tar pitch into liquid phase was used as a negative electrode. Was prepared. Here, the obtained carbonaceous material of the granular material has an interlayer distance d (002) determined by X-ray diffraction.
Is 3.36 °, and the crystallite size L _{c in the} c-axis direction is 600.
Å, the size L _a in the a-axis direction of crystallites is 600 Å,
The average particle size was 35 μm.

Next, the batteries of the present invention and the comparative example were set to 0.5 mA.
At a constant current of 4.1 V and a discharge end voltage of 3.0 V
A charge / discharge cycle test was performed in the range of V. FIG. 2 shows the charge / discharge characteristics of the battery of the present invention and the comparative example at the fifth cycle.
As is clear from FIG. 2, the battery of the present invention has a large discharge capacity. In addition, the battery of the comparative example using artificial graphite has a relatively large charge capacity but a small discharge capacity, and a decomposition reaction of the solvent of the electrolytic solution occurs during charging as conventionally known. It is thought that there is.
However, when the spheroidal graphite of the present invention is used, the capacity difference between charge and discharge is small, decomposition reaction of the solvent is prevented, and intercalation of lithium ions between graphite layers,
It can be seen that deintercalation is performed smoothly.

Although the crystallinity of carbonaceous materials is almost the same, such a difference is observed.
It can be considered as follows. That is, the artificial graphite of the comparative example is produced by further performing a graphitization heat treatment on the coke obtained by liquid-phase carbonization of coal tar pitch, but the spheroidal graphite of the present invention is produced during the liquid-phase carbonization process of coal tar pitch. This is because the generated highly oriented mesophase microspheres were selected and graphitized, and this was thought to be due to improvements in the properties of the amorphous part or the surface state of the particles, which were not revealed by X-ray diffraction. Can be

Next, by changing the graphitizing heat treatment conditions for the mesophase microspheres, a battery similar to that described above was formed using a graphitic material having a different crystallinity, and the discharge capacity was evaluated under the above-described charge / discharge conditions. Was. Here, the discharge capacity is represented by a discharge specific capacity obtained by dividing the capacity of the battery at the time of discharge by the weight of the graphite material of the negative electrode. As a result, FIG. 3 shows the interlayer distance d (00
The relationship 2) and the discharge specific capacity, respectively the relationship between the discharge specific capacity and size L _c of the c-axis direction of the crystallite in FIG. As shown in FIGS. 3 and 4, the interlayer distance d (002) of the spheroidal graphite
Is less than 3.37 ° or when the crystallite size L _{c in the} c-axis direction is 500 ° or more, the discharge specific capacity is 25%.
It was as large as 0 mAh / g or more. As described above, even if the same mesophase microspheres are graphitized, the heat treatment conditions are different, and the crystallinity is different. It can be seen that the intercalation state is different. If the graphite intercalation compound is formed by intercalation of lithium between the graphite layers, the charge / discharge capacity will naturally increase if the graphite crystal structure is developed, since the amount of intercalated lithium will increase. Is considered to have increased.

Further, the graphitic material in which the particle diameter of the mesophase microspheres of the present invention, that is, the particle diameter of the spherical graphite is changed by changing the heat treatment conditions for liquid phase carbonization,
The same evaluation as described above was performed, and the results are shown in FIG. As shown in FIG. 5, the graphite material of the present invention has a discharge specific capacity of 250 when the average particle size is in the range of 15 to 25 μm.
mAh / g or more. This is because, as the particle diameter increases, the bulk density decreases and the fillability of the graphitic material is poor, so that the capacity decreases.At the same time, when the particle diameter decreases, the surface area increases, and the reaction with the solvent occurs, and the lithium ion graphite is formed. It is considered that the capacity was reduced due to interfering between the layers. Therefore, it is considered that the graphitic material of the present invention most preferably has an average particle size in the range of 15 to 25 μm.

Further, regarding the particle size distribution of the spheroidal graphite which is the graphitic material of the present invention, the filling property of the spheroidal graphite was examined by mixing spheroidal graphite having an average particle diameter of 20 μm with spheroidal graphite having a particle diameter of 6 μm or less. FIG. 6 shows the result. As shown in FIG. 6, it can be seen that when the particles having a particle size of 6 μm or less have a volume ratio of 3% or less, the filling property is improved. This is because, in the case of the spherical graphite of the present invention, since it is a dense sphere, it can be cubic close-packed or hexagonal close-packed, and further 4 particles or particles formed in close-packed It is considered that fine spherical graphite can enter the gap surrounded by the six particles, so that the packing density can be improved and the discharge capacity has increased. Also, from the model structure, a sphere of about 6 μm can be inserted into a gap surrounded by 6 particles of a 20 μm sphere, and the insertion amount is about 3 μm.
%, Which was consistent with the above results. However, when the amount of the fine spherical graphite is increased more than necessary, it is considered that the close-packed structure cannot be obtained, the filling property of the spherical graphite deteriorates, and the discharge capacity decreases.

Although a coin-type battery was used in the present embodiment, the shape of the battery may be cylindrical, square, or the like. Was obtained.

[0027]

As described above, in the non-aqueous electrolyte secondary battery of the present invention, the graphitic material used for the negative electrode is a sphere obtained by graphitizing mesophase microspheres generated in the liquid phase carbonization process of pitches. Graphite, the interlayer distance d (002) is 3.37 ° or less, and the crystallite size L _{c in the} c-axis direction is 50
0 ° or more, and the particle size distribution of the spherical graphite is 6
Since those having a volume fraction of 3 μm or less have a volume fraction of 3% or less and an average particle size in a range of 15 μm to 25 μm, a large amount of lithium can be intercalated and a high capacity negative electrode can be provided. A high-capacity nonaqueous electrolyte secondary battery capable of preventing generation of lithium dendrites can be provided.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery of the present invention.

FIG. 2 is a diagram showing charge and discharge characteristics of batteries of the present invention and a comparative example.

FIG. 3 is a diagram showing the relationship between the interlayer distance d (002) and the specific discharge capacity of the spheroidal graphite of the present invention.

Shows the relationship between the discharge specific capacity and size L _c of the c-axis direction of crystallites of spherical graphite of the present invention; FIG

FIG. 5 is a graph showing the relationship between the average particle size and the specific discharge capacity of the spherical graphite of the present invention.

FIG. 6 is a view showing the relationship between the volume ratio and the specific discharge capacity of the spherical graphite of the present invention having a particle size of 6 μm or less.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Case 2 Sealing plate 3 Negative electrode 4 Negative electrode collector 5 Separator 6 Positive electrode 7 Insulating gasket

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Sho Ota 1006 Oaza Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-4-188559 (JP, A) JP-A-2- 284354 (JP, A) JP-A-4-115457 (JP, A) JP-A-5-89879 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/02-4 / 04 H01M 4/58 H01M 10/40

Claims (1)

(57) [Claims] 1. A negative electrode comprising a graphite material intercalated with lithium as a main material, a positive electrode comprising a lithium-containing metal oxide as an active material, an organic electrolyte and a separator, wherein the graphite material comprises: Spheroidal graphite obtained by graphitizing mesophase microspheres generated in the liquid phase carbonization process of pitches, wherein the interlayer distance d (002) by X-ray diffraction method is 3.37 ° or less and crystallites in the c-axis direction Size L _c is 50
0 ° or more, and the particle size distribution of the spherical graphite is 6
A non-aqueous electrolyte secondary battery characterized in that the volume of the battery is 3 μm or less and the volume ratio is 3% or less and the average particle size is in the range of 15 μm to 25 μm.