JPH0773868A - Nonaqueous electrolyte secondary battery and manufacture of negative electrode thereof - Google Patents

Abstract

PURPOSE:To provide a nonaqueous electrolyte secondary battery with high energy density, low deterioration in performance during storage, and high reliability by using natural graphite with specific graphitizable carbon material on the surface as a negative electrode. CONSTITUTION:A nonaqueous electrolyte secondary battery consists of a positive electrode capable of reversibly charging/discharging, a nonaqueous electrolyte containing a lithium salt, and a negative electrode made of a carbon material. Natural graphite having on the surface a carbonaceous material layer with a spacing of (002) planes as determined by X-ray wide angle diffraction of 3.40Angstrom or less and a crystallite size in the direction of c axis (Lc) of 100-1000Angstrom is used as the negative electrode. The negative electrode is manufactured in such a way that a mixture of natural graphite with at least one organic material selected from the group comprising pitch, coal tar, and coke is heated to carbonize or graphitize the organic material. The temperature of heating is preferably 2000-3000 deg.C.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery,
Particularly, it relates to improvement of the negative electrode.

[0002]

2. Description of the Related Art Non-aqueous electrolyte secondary batteries using lithium as a negative electrode have high electromotive force and are expected to have higher energy density than conventional nickel-cadmium storage batteries and lead storage batteries, and many studies have been conducted. However, when metallic lithium is used for the negative electrode, a dendrite is generated during charging, a short circuit is likely to occur, and the battery has low reliability. In order to solve this problem, the use of an alloy negative electrode of Li, aluminum and lead has been studied. When these alloy negative electrodes are used, Li is occluded in the negative electrode alloy by charging,
A highly reliable battery that does not generate dendrites. However, the discharge potential of the alloy negative electrode is about 0.5 V compared to metallic Li.
Since it is noble, the voltage of the battery also drops by 0.5 V, which also reduces the energy density of the battery.

[0003] On the other hand, researches using an intercalation compound of a carbon material such as graphite and Li as a negative electrode active material have been actively conducted. In this compound negative electrode as well, Li enters the carbon layer and does not generate dendrite upon charging. Discharge potential is metallic Li
Since it is about 0.1 V higher than that of the above, the decrease in battery voltage is small. Therefore, it can be said that it is a more preferable negative electrode. Usually, carbonaceous materials contain organic substances in an inert atmosphere at about 4
It is obtained by decomposing by heating at 00 to 3000 ° C., carbonizing, and further graphitizing. The starting material of the carbonaceous material is almost always an organic substance, and by heating up to around 1500 ° C. which is a carbonization step, almost only carbon atoms remain and heating up to a high temperature near 3000 ° C. develops a graphite structure.

As the organic raw material, pitch, coal tar, or a mixture of coke and pitch is used in the liquid phase, and in the solid phase, wood raw material, furan resin, cellulosic, polyacrylonitrile, lacquer. -Yong can be mentioned. In the gas phase, hydrocarbon gas such as methane and propane is used. So-called graphitizable carbon materials having a graphite structure that has been developed by firing petroleum pitch or the like as a starting material and generally at a high temperature of 2000 ° C. or higher, and thermosetting resins such as furan resin have been developed. Attempts have been made to use, as a starting material, a so-called non-graphitizable carbon material, which has a turbostratic structure and is fired at a relatively low temperature of 2000 ° C. or less, as a negative electrode material for a non-aqueous electrolyte secondary battery that occludes and releases lithium. There is.

Further, many studies have been conducted on the use of natural graphite as a negative electrode active material. Natural graphite generally has a graphite structure having a perfect crystal structure, and the interlayer distance of (002) plane is d ₀₀₂ = 3.35 angstrom, and the crystallite size is Lc> 1000 angstrom. When such natural graphite absorbs and releases lithium, it is possible to reversibly charge and discharge a larger electric capacity than other carbon materials. At present, theoretically, the highest storage state is C ₆ Li, that is, the electric capacity is 372 Ah /
It is known to reach values very close to kg, for example 350 Ah / kg.

[0006]

[Problems to be Solved by the Invention] The above-mentioned natural graphite and Li
There was also a big problem when the intercalation compound (1) was used as the negative electrode active material. That is, when natural graphite is used as the negative electrode active material, the storage characteristics of the battery are inferior to when other artificial carbons are used. In particular, when the battery is charged and stored in a high temperature environment such as 60 ° C., the capacity is greatly decreased, and the recovery capacity thereafter is also decreased.

Natural graphite is generally produced in a state of containing iron, silicon, calcium and the like as impurity elements.
Its content varies depending on the place of origin and the like, but is several% at maximum, and several% at least for 100 minutes. We have
As a result of a detailed study on the storage characteristics and impurities of such a battery, the reason why the storage characteristics of the battery using natural graphite as the negative electrode active material is inferior is mainly due to the impurity elements contained at the time of production as described above. I found out that.

Therefore, the present invention makes use of the characteristics of natural graphite by substantially eliminating the adverse effect on the storage characteristics due to the contained impurity elements, has a high energy density, and is highly reliable with little deterioration in performance due to storage. It is an object to provide a negative electrode that provides a non-aqueous electrolyte secondary battery.

[0009]

The present invention provides a non-aqueous electrolyte secondary battery comprising a positive electrode having reversibility for charge and discharge, a non-aqueous electrolyte containing a lithium salt, and a negative electrode made of a carbon material, The negative electrode has a (002) plane spacing of 3.40 angstroms or less and a crystallite size (Lc) in the c-axis direction of 100 to 10 according to a wide-angle X-ray diffraction method.
It is characterized by including natural graphite having a carbonaceous layer of 00 angstrom as a surface layer.

Further, the method for producing the negative electrode of the present invention comprises a pitch,
It is characterized in that a mixture of at least one organic substance selected from the group consisting of coal tar and coke and natural graphite is heated to carbonize or graphitize the organic substance. The heating temperature is 2000 to 30.
It is preferably in the range of 00 ° C.

[0011]

The negative electrode using the carbon material according to the present invention, like the negative electrode using the conventional carbon material, occludes lithium by charging and releases the occluded lithium as ions in the electrolyte upon discharging. Therefore, lithium does not deposit in a metallic state due to charging, and an internal short circuit of the battery due to the dendrite does not occur. Since the discharge potential is about 0.1 V higher than that of metallic Li, the decrease in battery voltage is small.

Moreover, since the higher-purity graphitizable carbon material is formed on the surface of the natural graphite, the impurity element contained in the natural graphite is not formed on the surface without lowering the high capacity of the natural graphite. Without being exposed, the electrolyte does not come into contact with impurities. That is, it can be considered that impurities in the surface layer portion of natural graphite, which adversely affects the storage characteristics, are improved to a state substantially similar to that by removing the highly pure graphitizable carbon material. Therefore, it is possible to substantially eliminate the adverse effect on the storage characteristics due to such impurities contained in the natural graphite.

Further, the crystallinity of the artificial graphite formed on the surface layer is slightly lower than that of natural graphite, so that lithium is easily inserted during charging, resulting in a discharge capacity of 372 Ah / A theoretical capacity of natural graphite of kg can be obtained. According to the present invention, the electric capacity of charge and discharge is increased as compared with the conventional carbon materials and graphite, and the storage characteristics of the battery are improved.

[0014]

EXAMPLES Examples of the present invention will be described below. Example 1 First, the crystal structure of the negative electrode material used will be described. Natural graphite having a purity of 98.0% was used, and the surface layer had a (002) plane spacing (d ₀₀₂ ) of 3.36 angstroms and a crystallite size (Lc) in the c-axis direction. Has a 600 Å carbonaceous layer as the test material. This material was added to 100 parts by weight of natural graphite having an average particle size of 10 μm and 20
After adding 10 parts by weight of pitch containing tar by weight and mixing them well, 2600 in an argon gas atmosphere
It was obtained by heating to 0 ° C., and the heating time is 100 hours.

As a comparative example, a carbon material having a substantially single crystal structure is used. That is, Comparative Example 1 is a natural graphite having the above-mentioned purity of 98.0%, Comparative Example 2
Has an interplanar spacing (d ₀₀₂ ) of the (002) plane of 3.36 angstroms and a crystallite size (Lc) in the c-axis direction of 60.
It is a carbonaceous material of 0 angstrom.

In order to study the characteristics of these carbonaceous materials as electrodes, the test cell shown in FIG. 1 is prepared as follows. 1 g of polyethylene powder as a binder was mixed with 10 g of carbon material, and 0.1 g of this mixture was added to a diameter of 17.5 mm.
A carbon electrode is manufactured by pressure molding on the disk. This electrode 1
Is placed in the center of the case 2, a microporous polypropylene separator 3 is placed thereon, and then 1 mol / l of lithium perchlorate (LiClO ₄ ) dissolved in ethylene carbonate and dimethoxyethane in a volume ratio of 1: 1. A non-aqueous electrolytic solution consisting of the mixed solution of is poured onto the separator. Next, a disk-shaped metallic lithium 4 having a diameter of 17.5 mm is attached to the inside, and a sealing plate 6 having a polypropylene gasket 5 attached to the outer periphery is combined and sealed to form a test cell.

Regarding the cells using the carbonaceous materials obtained as described above, at a constant current of 2 mA, the cathode polarization was performed until the carbon electrode became 0 V with respect to the Li counter electrode (when the carbon electrode is viewed as a negative electrode, Charging) and then anodic polarization (corresponding to discharging) until the carbon electrode reaches 1.0V.
After repeating this cycle of cathodic polarization and anodic polarization for 10 cycles at 20 ° C., after completion of the 11th cycle of charging, it is stored at 60 ° C. for 4 weeks, stored and returned to 20 ° C., and discharged and charged under the same conditions.

Table 1 shows the discharge capacity (initial capacity) of the first cycle measured as described above, and the capacity retention rate and capacity recovery rate defined below. Capacity maintenance rate = 100 x (11th cycle discharge electricity quantity)
/ (10th cycle discharge electricity quantity) Capacity recovery rate = 100 x (12th cycle discharge electricity quantity)
/ (Discharge electricity at 10th cycle)

[0019]

[Table 1]

The discharge capacity at the first cycle is large in the cells of Example and Comparative Example 1 and small in the cells of Comparative Example 2. In addition, the cell of Comparative Example 1 shows a very large decrease in capacity with storage at 60 ° C. for 4 weeks. On the other hand, the cells of Example and Comparative Example 2 have a capacity retention rate of 90% or more and a capacity recovery rate of 95% or more, which is a high value. As described above, the use of the surface layer of natural graphite having the carbonaceous layer satisfying the specific condition has an effect of suppressing the capacity decrease due to the high temperature storage.

Example 2 Average particle size 6 μm, Purity 98.
By adding 10 parts by weight of pitch containing 10% by weight of tar to 100 parts by weight of 0% natural graphite and mixing them well, by heating in an argon gas atmosphere at a temperature of up to 3000 ° C. for 100 hours, As shown in Table 2,
The surface spacing (d) of the (002) plane by the X-ray wide-angle diffraction method on the surface layer (d
₀₀₂ ) has a carbonaceous layer of 3.355 to 3.45 angstroms and a crystallite size (Lc) in the c-axis direction of 100 to 1000 angstroms. As a comparative example, natural graphite itself which does not form graphitizable carbon on the surface layer is used.

[0022]

[Table 2]

The characteristics of these carbonaceous materials as electrodes are evaluated in exactly the same manner as in Example 1. When the test cell was disassembled after completion of the 11th cycle of cathodic polarization in each test cell, no deposition of metallic Li was observed.

As for the initial discharge capacity, the surface carbonaceous material has a d ₀₀₂ of 3.355 to 3.40 angstroms and a crystallite size (Lc) in the c-axis direction of 100 to 1000 angstroms. It showed almost the same value as that using. However, the initial discharge capacity was small when the d ₀₀₂ was 3.45 Å and the crystallite size (Lc) in the c-axis direction was 100 Å. It is considered that since the graphitizable carbon formed in the surface layer has insufficient crystallinity, the charge / discharge capacity at this portion becomes small, and as a result, the capacity per unit weight of the entire carbon is lowered. Moreover, when Lc was 1500 angstrom, the initial capacity was lowered. This is because the crystallinity is considerably high, and it is considered that the invasion of lithium into natural graphite was hindered. Capacity retention rate of each cell after 4 weeks,
Table 3 shows the capacity recovery rate.

[0025]

[Table 3]

The comparative example shows a very large decrease in capacity with storage at 60 ° C. for 4 weeks. On the other hand, the cells of the examples show high values such as a capacity retention rate of 90% or more and a capacity recovery rate of 95% or more. By using the natural graphite having the carbonaceous layer satisfying a specific condition on the surface layer in this manner, there is an effect of suppressing the capacity decrease due to high temperature storage. From the viewpoint of both the initial capacity and the storage stability, the graphitizable carbon of this surface layer is a crystal in the c-axis direction when the interplanar spacing (d ₀₀₂ ) of the (002) plane by the X-ray wide angle diffraction method is 3.40 angstroms or less. It can be seen that a child size (Lc) of 100 to 1000 angstrom is desirable.

[Example 3] In this example, the results of evaluation of natural graphite having a graphitizable carbon layer obtained by heating various organic substances on the surface layer will be described. Average particle size 12 μm,
100 parts by weight of natural graphite having a purity of 98.0% and various organic substances 1
After adding 0 part by weight and mixing with the filling part, the graphitizable layer is formed on the surface layer of natural graphite by heating in an argon gas atmosphere at a temperature up to 2800 ° C. for 200 hours. Organic substances added to Table 4 and d of the obtained carbonaceous layer
₀₀₂ and Lc are shown.

[0028]

[Table 4]

Table 5 shows the results of evaluating the characteristics of the carbonaceous material obtained as described above as an electrode in exactly the same manner as in Example 1.

[0030]

[Table 5]

When the organic material forming the surface carbonaceous layer was pitch, talter or coke, the initial capacity was the same as that of natural graphite. On the other hand, the capacity retention rate and capacity recovery rate after high temperature storage are 65% and 70%, respectively, in the case of the comparative example.
On the other hand, the carbon materials according to the examples had a capacity retention rate of 90% or more and a capacity recovery rate of 95% or more. Thus, the organic matter forming the surface layer of natural graphite is pitch,
It can be seen that the one selected from the coulter and the coke is desirable.

[Embodiment 4] In this embodiment, the result of studying the heating temperature for obtaining the graphitizable carbon layer forming the surface layer of natural graphite will be described. 10 parts by weight of pitch was added to 100 parts by weight of natural graphite having an average particle diameter of 12 μm and a purity of 98.0%, mixed well, and then heated in an argon gas atmosphere at various temperatures for 200 hours to form a surface layer of natural graphite. To do. Heating temperature and d of the obtained carbonaceous layer
₀₀₂ and Lc are shown in Table 6.

[0033]

[Table 6]

Table 6 shows the results of evaluating the characteristics of the obtained carbonaceous material as an electrode in exactly the same manner as in Example 1.

[0035]

[Table 7]

When the heating temperature is 2000 ° C. or higher, the initial capacity is almost the same as that of natural graphite. In addition, the capacity retention rate and the recovery rate after storage at high temperature show higher values than those of natural graphite in the entire temperature range examined. Furthermore, 3000 ° C
The heating under higher temperature conditions was not studied because it is difficult to carry out industrially from the viewpoint of the constituent material of the heating furnace and the economical efficiency of the heating process. From the above results, it is understood that the heating temperature is preferably 2000 ° C to 3000 ° C.

[Embodiment 5] The results of examining the characteristics of a cylindrical battery having the carbon material of the present invention as a negative electrode will be described. Purity 9
After adding 10 parts by weight of pitch containing 20% by weight of tar to 100 parts by weight of natural graphite having an average particle diameter of 8.0 μm and an average particle size of 12 μm, and thoroughly mixing, 100 at a temperature of up to 2600 ° C. in an argon gas atmosphere. By heating for a long time, the surface of natural graphite was subjected to the X-ray wide-angle diffraction method (002).
The surface spacing (d ₀₀₂ ) is 3.36 angstroms and c
A carbonaceous layer having an axial crystallite size (Lc) of 600 Å is formed. Further, as a comparative example, the above-mentioned natural graphite having a purity of 98.0% is used.

In order to study the characteristics of these carbonaceous materials as electrodes, a cylindrical battery having the structure shown in FIG. 2 is manufactured as follows. First, the above-mentioned carbonaceous material and a polyfluorinated ethylene resin as a binder were mixed in a weight ratio of 100: 5, and water was added to this to form a paste, which was applied to a copper core material. It is dried at 100 ° C. to obtain a negative electrode plate.

For the positive electrode plate, LiMn ₂ O _{4 as} an active material, acetylene black as a conductive agent, and polyfluorinated ethylene resin as a binder were mixed in a weight ratio of 100: 5: 5, and water was added to the paste. The material is applied to a titanium core material and dried to form a structure. The weight of LiMn ₂ O _{4 in} the positive electrode is 5 g. Microporous polypropylene is used for the separator. The electrode body is formed by inserting a strip-shaped separator 13 wider than both electrode plates between a positive electrode plate 11 having a positive electrode lead 14 and a negative electrode plate 12 having a negative electrode lead 15 made of the same material as the core material attached by spot welding. It is constructed by winding in a spiral.

Further, insulating plates 16 and 17 made of polypropylene are arranged on the upper and lower sides of the electrode body, respectively, and a metal battery case 18 is provided.
To form a step on the upper part of the battery case 18, and then as a non-aqueous electrolyte, 1 mol / l of lithium perchlorate (LiC
lO ₄₎ at a volume ratio of dissolved ethylene carbonate and dimethoxyethane in a 1: 1 mixed solution was injected, the sealing plate 1
The battery is sealed with 9 to form a battery. 20 is a positive electrode terminal.

With respect to the battery having the above structure, after 10 cycles of charging and discharging at 20 ° C., after completion of the 11th cycle of charging, it was stored at 60 ° C. for 4 weeks, stored, returned to 20 ° C., discharged, and further charged and discharged. did. The charging / discharging conditions were a voltage range of 4.3 V to 3.0 V and a current density of 0.5 mA / cm ^2.
And

FIG. 3 shows the discharge curve at the first cycle (before storage), the discharge curve after storage (maintenance after storage), and the discharge curve after storage and storage after storage (storage recovery) as described above. Shown in. Further, a similar discharge curve of the battery of the comparative example is shown in FIG.

The battery of the comparative example shows a very large decrease in capacity with storage at 60 ° C. for 4 weeks. On the other hand, the battery of this example had a high capacity retention rate and a high capacity recovery rate, and the capacity retention rate calculated by the same calculation method as in Example 1 was 90% or higher, and the capacity recovery rate was 95% or higher. Thus, it can be seen that the battery using the negative electrode according to the present invention has a high discharge voltage, a high capacity, and excellent storage characteristics. In the examples, LiMn ₂ O ₄ was used as the positive electrode active material, but the negative electrode according to the present invention is not limited to LiCoO ₂ ,
It goes without saying that the same effect can be obtained when combined with a positive electrode having reversibility for charge and discharge, such as LiNiO ₂ , LiFeO ₂ , γ-type LiV ₂ O _{5 and the} like.

[0044]

As described above, according to the present invention, it is possible to obtain a non-aqueous electrolyte secondary battery having high energy density, short-circuiting due to dendrite, and excellent storage stability.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a test cell for evaluating electrode characteristics of the present invention.

FIG. 2 is a vertical sectional view of a cylindrical battery according to an embodiment of the present invention.

FIG. 3 is a discharge curve before and after high temperature storage of a cylindrical battery according to an example of the present invention.

FIG. 4 is a discharge curve before and after high temperature storage of a cylindrical battery of a comparative example.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuji Ito 1006, Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. In the company

Claims (3)

[Claims] 1. A positive electrode having reversibility for charge and discharge, a non-aqueous electrolyte containing a lithium salt, and a negative electrode made of a carbon material, wherein the negative electrode has a (002) plane obtained by an X-ray wide angle diffraction method. The interplanar spacing is 3.40 angstroms or less, and the crystallite size (Lc) in the c-axis direction is 100 to 10
A non-aqueous electrolyte secondary battery comprising natural graphite having a carbonaceous layer of 00 Å on its surface. 2. A step of heating the mixture of at least one organic material selected from the group consisting of pitch, cotter roll and coke and natural graphite to carbonize or graphitize the organic material. A method for producing a negative electrode for a non-aqueous electrolyte secondary battery, which is characterized. 3. The method for producing a negative electrode for a non-aqueous electrolyte secondary battery according to claim 2, wherein the heating temperature is 2000 to 3000 ° C.