JP3064662B2 - 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 its 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. Further, in JP-A-63-285872, for the purpose of improving the high capacity of the cycle characteristics, the d (002) is 3.35～3.8A, wherein L _c is 10
250 Å, the size L _a in the a-axis direction of crystallites 15-25
0 °, and the specific surface area by the BET method is 50 m ² /
g of carbon black is disclosed. Japanese Patent Application Laid-Open No. 63-112248 discloses a BET for improving the self-discharge characteristic and the cycle characteristic.
Specific surface area by the method is 0.1 to 100 m ² / g, true density ρ is 1.70 to 2.18 g / cm ³ , and L _c is 10 <L
_c (Å) <120ρ-189, and 0.1 to 50
A particulate carbonaceous material having a particle size distribution of 90% or more by volume in the range of μm is disclosed. Further, JP-A-2-
Japanese Patent Publication No. 284354 discloses a carbonaceous material in which the particle size distribution of the carbonaceous material is 5 μm or less, the volume ratio is 5% or less, and the average particle size is 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 such a problem, and provides a high-capacity graphite material having a large amount of intercalating lithium and preventing the generation of lithium dendrites during charging. A non-aqueous electrolyte secondary battery with improved safety is provided.

[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) determined by the X-ray diffraction method is 3.37 ° or less, the crystallite size L _{c in the} c-axis direction is 500 ° or more, and a
Size L _a in the axial direction of the crystallites is not less 500Å or more and an average particle diameter of the spherical graphite is in a range of 3 to 50 [mu] m.

[0011]

The carbonaceous material obtained by liquid-phase carbonization of coal tar pitch or the like is generally graphitizable soft carbon, and a highly crystalline structure can be obtained by graphitization by high-temperature heat treatment. In this method, the mesophase microspheres generated in the liquid phase carbonization process of these pitches are separated and extracted to graphitize them.

Therefore, since the carbon precursor before graphitization has a structure in which thin layers of polycyclic aromatic molecules are already stacked, a graphitic material which is easy to graphitize and has high crystallinity can be obtained. Obtainable.

[0013] Further, since dense spherical graphite is formed by graphitization, if this is used for the negative electrode, a high-capacity negative electrode with a large amount of intercalation of lithium can be provided.

[0014]

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 of the present invention
The secondary battery is made of stainless steel case 1 and stainless steel
A sealing plate 2, a negative electrode 3 using a graphite material,
Method, material separator 5, positive electrode 6, organic electrolyte and absolute
The edge gasket 7 is constituted. Here, the negative electrode 3
The graphitic material used was coal tar pitch converted to liquid phase carbon.
Graphitized the mesophase microspheres formed during the process
Spheroidal graphite, and interlayer distance d (00
2) is 3.36 °, and the crystallite size L in the c-axis direction is _cIs 6
30 °, crystallite size L in a-axis direction_aIs 3100Å
Things. The particle size distribution is 20 μm in average particle size,
2% by volume of small particles with a diameter of 6μm or less
Is mixed with a fluororesin binder at a weight ratio of 90:10,
Formed on a negative electrode current collector 4 made of stainless steel to produce the negative electrode 3
did. Next, the current collector 4 is spot-welded to the sealing plate 2.
And 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. Black and a fluororesin as a binder were mixed at a weight ratio of 80:10:10 and molded into a pellet having a diameter of 14.5 mm and 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 liquid-phase carbonization of coal tar pitch was used. Was prepared. The obtained powdery carbonaceous material has an interlayer distance d (002) of 3.36 ° by X-ray diffraction and a crystallite size L _{c in the} c-axis direction of 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.
On the other hand, 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, 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 from coal tar pitch, but the spheroidal graphite of the present invention is produced during the liquid phase carbonization process of coal tar pitch. It is considered that because mesophase microspheres having extremely high orientation were selected and graphitized, the characteristics of the amorphous portion or the surface state of the particles, which are not revealed by X-ray diffraction, were improved.

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 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, the relationship between the discharge specific capacity and size L _c of the c-axis direction of the crystallite in Figure 4, Figure 5 and the size L _a in the a-axis direction of crystallites discharge The relationship with the specific capacity is shown.

[0023] As shown in FIGS. 3, 4, 5, when the interlayer distance of the spheroidal graphite d (002) and the case is less than 3.37 Å L _c and L _a is greater than or equal 500Å respectively, discharge specific capacity It was as large as 250 mAh / g.

As described above, even in the case of the graphitic material obtained by subjecting the same mesophase microspheres to graphitization, if the heat treatment conditions are different and the crystallinity is different, their charge / discharge characteristics, that is, the intercalation of lithium between the graphite layers, is not possible. It can be seen that the states of calate and deintercalate are different.

In general, a graphite intercalation compound forms a crystal structure called a stage structure, and lithium ions and the like do not randomly enter the interlayer but enter with a certain regularity. , Is called in-plane ordering. Accordingly, graphite having a crystal structure in which not only the crystallinity in the c-axis direction but also the crystallinity in the a-axis direction is developed has a higher stacking order and in-plane order, and thus has a larger amount of penetration sites. That is, it is considered that the charge / discharge capacity was increased and the above result was obtained.

Further, the average particle size obtained by classifying the graphite material in which the particle size of the mesophase microspheres, that is, the particle size of the spherical graphite was changed by changing the heat treatment conditions for liquid phase carbonization, and the artificial graphite of the comparative example were obtained. About artificial graphite with different diameters,
The same evaluation as described above was performed, and the results are shown in FIG. From FIG. 6, in the artificial graphite of the comparative example, the larger the particle size is, the larger the specific discharge capacity is and the influence of the particle size is small. On the other hand, in the spherical graphite of the present invention, the average particle size is 3 to 50 μm.
, The discharge specific capacity is large, about 200 m
Ah / g or more. This is thought to be because the spherical graphite of the present invention greatly affects the essential properties of graphite by changing the average particle size according to the heat treatment conditions for liquid-phase carbonization.

That is, when the temperature of the heat treatment conditions for the liquid phase carbonization is higher and the time is longer, the obtained spherical graphite is formed by fusing small particles into large particles, and the microparticles in the a-axis and c-axis directions are formed. It is considered that, when the general orientation is deteriorated and the particle diameter is increased, the bulk density is reduced and the filling property of the graphite material is deteriorated, so that the capacity is reduced. On the other hand, when the temperature of the heat treatment conditions for liquid phase carbonization is lower and the time is shorter, the obtained spherical graphite has a small particle size and a large surface area, so that the spherical graphite is preferable, and excellent surface characteristics of the particles are obtained. It is considered that the effect of the surface structure is reduced, the reaction with the solvent as seen in the artificial graphite of the comparative example occurs, and the intercalation of lithium between the graphite layers is hindered, so that the capacity is reduced. Therefore, it is considered that spherical graphite having an average particle diameter in the range of 3 to 50 μm has the largest discharge capacity.

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

[0029]

As described above, the graphite material used for the negative electrode of the non-aqueous electrolyte secondary battery of the present invention is spherical graphite obtained by graphitizing mesophase microspheres generated in the liquid phase carbonization process of pitches. And the interlayer distance d (002) is 3.3
7Å or less, c-axis direction of the size L _c of the crystallites 500Å or more, the size L _a crystal axis in the a-axis direction is not less 500Å or more and an average particle diameter of the spherical graphite in the range of 3~50μm Therefore, a high-capacity negative electrode having a large amount of intercalated lithium can be obtained and a high-capacity non-aqueous 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 / discharge characteristics of nonaqueous electrolyte secondary 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

Diagram showing the relationship between the size L _a and the discharge specific capacity of the a-axis direction of crystallites of spherical graphite of the present invention; FIG

FIG. 6 is a graph showing the relationship between the average particle size and the specific discharge capacity of graphite of the present invention and a comparative example.

[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 (56) References JP-A-4-188559 (JP, A) JP-A-4-115457 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/38-4/58 H01M 10/40

Claims (1)

(57) [Claims] 1. A negative electrode comprising a graphite material as a main material, a positive electrode comprising a lithium-containing metal oxide as an active material, an organic electrolytic solution, and a separator. Spheroidal graphite obtained by graphitizing the mesophase microspheres formed in the carbonization process, wherein the interlayer distance d (002) by X-ray diffraction is 3.37 ° or less, and the crystallite size L _{c in the} c-axis direction is 50
0Å above, the size L _a in the a-axis direction of crystallites is not less 500Å or more and an average particle diameter of the spherical graphite 3~50μm
Non-aqueous electrolyte secondary battery in the range.