JP3489600B2 - Carbon material for negative electrode of non-aqueous solvent 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 carbon material which is a negative electrode for a non-aqueous solvent lithium secondary battery having a large capacity and excellent charge / discharge characteristics, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, miniaturization and weight reduction have been remarkable in the field of information and electronic equipment, and accordingly, there is an increasing need for a secondary battery that is small, lightweight, has a large capacity, and can be recharged and reused. Among them, lithium secondary batteries are expected to replace conventional lead-acid batteries and nickel-cadmium batteries. However, when metallic lithium was used as the negative electrode material, it was not possible to avoid the risk of ignition due to dendrites caused by repeated charging and discharging. Then, a method of intercalating lithium using graphite as a negative electrode material was found. However, with graphite, the electrolytic solvent deteriorated quickly and there was a risk of ignition.

Further, in intercalation into graphite, it is not easy to manufacture a battery because the expansion and contraction of the negative electrode material is large, and the practical capacity is considerably lower than this. Therefore, carbon materials that do not reach the level of graphite, that is, carbon materials with a low degree of graphitization have been proposed as safe and high-capacity negative electrode materials. And
Regarding carbon suitable as a negative electrode material, numerical values such as hydrogen / carbon ratio, true density, X-ray diffraction, Raman spectroscopy, and specific surface area have been specified as a scale showing the carbonization degree and physical properties.

However, although the values showing the carbonization degree and the physical properties are rough requirements for obtaining a preferable carbon, the amount of lithium occluding or the capacity even if the carbon having the similar carbonization degree and the physical property values is used. In many cases, it was not possible to say that it was a necessary and sufficient condition. The type of raw material used and the proportion of each element are important. Even if the firing carbonization temperature is the same, the heating rate, atmosphere,
This is because the capacity of the negative electrode carbon material was significantly affected by conditions such as the filling rate. For example, JP-A-58-931
76, JP-A-62-122066 or JP-A-6-8.
9721 and the like disclose firing in the presence of an inert gas, in a reduced pressure or in a vacuum atmosphere in the firing and carbonizing step. However, even if a good carbon material is obtained when firing a small amount of samples in a laboratory, the carbonization degree and physical property values are desired when firing a large amount industrially only by the firing conditions described. Despite the value, it became unsuitable as a negative electrode material for lithium secondary batteries due to its low battery capacity.

[0005]

DISCLOSURE OF THE INVENTION The inventors of the present invention have found that in the above calcination carbonization step, an anode carbon material having industrially stable performance cannot be obtained only by the preferable calcination conditions shown in the prior art. I knew and aimed to solve this problem.

[0006]

In order to achieve the above object, the inventors of the present application have made detailed studies on the conditions of calcined carbonization and the performance of the obtained carbon material. As a result, they have found that the carbon material obtained under the following conditions has extremely high performance as a battery negative electrode, and have completed the present invention.

That is, the atomic ratio of H / C is 0.5 to 0.
No. 05, the carbon content of 85 wt% or more and the average particle size of 100 microns or less, in the firing of the raw carbon powder, the vapor partial pressure in the firing furnace due to the volatile components from the carbon powder at the firing temperature of 800 to 1500 ° C. It was a carbon powder obtained by firing while maintaining it at 30 mmHg or less, and when this was used as a negative electrode, a lithium secondary battery with desired performance could be obtained.

As described above, even with carbon having the same degree of carbonization and physical properties, the amount of occlusion of lithium (hereinafter referred to as "capacity") that can be used as a battery is greatly affected by different starting materials and firing conditions. It is presumed that the difference in the fine structure and the chemical state of the surface which are not shown in X-ray diffraction are different. As a result of a detailed study of the relationship between the firing conditions and performance, it is possible to obtain a carbon material with the desired performance in a stable manner even if the firing amount increases, if the carbonization raw material selection and firing atmosphere are strictly controlled. Found.

That is, it is essential that the carbonization raw material has a sufficiently high degree of carbonization and is a powder. Those that melt during firing are unsuitable, such as pitch, corrugated,
Carbon precursors such as organic polymers must be pulverized after increasing the degree of carbonization by carbonization reaction or removal of volatile components. Specifically, the raw material carbon powder has a carbon content of at least 85 wt% and an H / C of 0.
It is 5 to 0.05, preferably 0.3 to 0.1. The average particle size of the powder is 100 microns or less, preferably 50 to 5 microns. The use of such a raw material carbon powder is for avoiding poisoning of itself by volatile components generated from the carbon powder during firing.

Therefore, the volume of the carbon material produced when firing is performed in the firing furnace such that the vapor partial pressure from the volatile components of the carbon powder is high, and the initial efficiency is low, while the vapor partial pressure is low. When both were fired so that they were always low, both showed high values. For example, a small amount of carbon powder is fired in an atmosphere of inert gas at a slow heating rate. That is, a preferable result was obtained by decreasing the generation rate of volatile components. However, in order to industrially carry out the firing treatment, it is necessary to perform a large amount of firing in a short time, and it was not economical to suppress the generation of volatile matter as much as possible.

As a method of lowering the vapor partial pressure, firing is performed. A method of diluting volatile vapors by flowing an inert gas ,. A method of reducing the pressure in the firing furnace with a vacuum device was effective. But that was not enough. This is because the amount of volatile matter vapor generated is not constant following the temperature rising rate of the firing furnace. The total amount of carbon dioxide generated varies depending on the nature of the carbonization raw material, and the condensation reaction may proceed rapidly at a certain temperature and a large amount of volatile matter may be generated. Therefore, it is necessary to select the optimum heating rate, heating pattern, inert gas flow rate, or vacuum pump evacuation rate depending on the raw material and the processing amount. Also, if the type and size of the firing furnace are different,
Naturally, the firing conditions must be changed.

As a result of various trials and errors, the present inventors have made it possible to reproducibly obtain a desired carbon material by keeping the vapor partial pressure due to the volatile matter in the firing furnace at 30 mmHg or less at a temperature of 800 to 1500 ° C. I found that I could get it well. The volatile components mentioned here are non-condensable gases such as hydrogen and methane generated in the reaction process of carbonization and condensable gases having higher boiling points than the light boiling components in the raw material, Is the total volatile content, which is the weight obtained by subtracting the weight of the carbonized material after firing from the weight of the charged raw material.

It is described in the prior art that carbonization is carried out in an inert gas atmosphere, reduced pressure or vacuum. However, when firing an industrial amount, it was impossible to avoid contamination of the product carbon powder due to an increase in the vapor partial pressure due to the volatile gas that is randomly generated in large amounts during the heating process. That is, as in the present invention, the vapor partial pressure with a low volatile content could not always be kept below 30 mmHg.

In the above method, in order to keep the vapor partial pressure in the firing furnace due to volatile matter below 30 mmHg,
In this method, the temperature range in which the amount of volatile matter generated sharply increases is measured beforehand by differential thermal balance analysis. Then, the rate of temperature rise in that temperature region is reduced or held, and the amount of inert gas is increased. The exhaust gas from the firing furnace is cooled and the volatile components are collected. For the gas that has not been collected by cooling, measure the exhaust gas flow rate with a dry gas meter and analyze the gas components other than the inert gas by gas chromatography. From this and the amount and composition of the volatile matter collected at regular intervals, the vapor partial pressure of the volatile matter in the firing furnace can be calculated and it can be checked whether the firing has been performed under the predetermined conditions. Since this checking method is complicated, a firing experiment is performed in advance with a small amount of sample, and a firing pattern such as an inert gas flow rate and a heating rate is set so that the vapor pressure due to volatile matter is 30 mmHg or less. Then, the collection cooler is switched at regular time intervals and the weight of the collection tar is measured to confirm that the firing is performed in a predetermined manner.

In the method of (1), the temperature rising conditions are grasped in the same manner as in (1), and the exhaust speed is increased when the gas generation increases. For example, it is effective to use a mechanical booster together with a vacuum pump. In the vacuum firing, since it is possible to control while watching the vacuum gauge, it is relatively easy to operate at a predetermined vapor pressure.

Nitrogen, argon and helium are preferable as the inert gas in the above method. The firing furnace may be either a batch type or a continuous type, and a muffle furnace, a pusher furnace, a rotary kiln furnace, etc. are used. It is preferable to introduce a small amount of inert gas even in the vacuum firing according to the above method. This is because volatile matter adheres to the exhaust line,
This is to prevent trouble such as blockage.

Volatile components other than gas are recovered by a high temperature collector controlled at 60 to 100 ° C. and a low temperature collector regulated at 30 ° C. or lower. Final firing temperature is 1000-1500
It is ℃, but the process to reach this temperature is very important. As described above, the heating pattern is set so as to avoid sudden gas generation with reference to the differential thermal analysis. Since the raw material carbon powder is pre-carbonized, it is generally 500
The temperature rises relatively quickly up to around 0 ° C, and at 500 ° C or higher, the temperature rise rate is 1 to 5 ° C / min in a temperature range where gas generation is small, and 1 ° C / min or lower in a region where gas generation is high. preferable. The vapor partial pressure due to volatile matter in the firing furnace is better as it is lower than 800 ° C, 200 mmH
It is desirable to keep it below g, and when it is exposed to volatile matter in excess of 200 mmHg, the capacity of the product carbon powder tends to decrease. Since carbon powder is easily adversely affected at 800 ° C or higher, it is essential to set it to 30 mmHg or less, preferably 10 mmHg or less, more preferably 5 mmHg or less.
It is less than or equal to mmHg.

The carbon was calcined by the above-mentioned method as a negative electrode, and a battery was constructed and evaluated. The amount of lithium absorbed and released electrochemically was converted into the amount of electricity, and the theoretical capacity of ordinary graphite was calculated. More than 372 mAh / g was obtained. Moreover, no precipitation of lithium metal was observed by NMR analysis or visual observation. Even when the vapor partial pressure of volatile matter is operated low, there is almost no difference in the apparent carbon density, specific surface area, X-ray diffraction, etc., obtained when the vapor partial pressure is operated high. The difference is surprising, but the reason is unknown.

The effects of the present invention will be described in detail below with reference to Examples and Comparative Examples. The following examples are for the purpose of specifically explaining the present invention. It is not intended to limit the scope or scope of the invention. Further, various analysis methods and analysis conditions for the negative electrode material in this example are described below.

The particle size distribution measuring apparatus used was LA-500, a laser diffraction type particle size distribution measuring apparatus manufactured by Hikiba Seisakusho. The measurement was carried out by adding 3 drops of surfactant to 100 ml of pure water, adding a sample to this to give a predetermined concentration, and performing ultrasonic dispersion for 10 minutes, and then measuring the resulting median diameter. And

(Elemental analysis) 2400CH manufactured by Perkin Elmer Co. as an analyzer for simultaneous analysis of carbon, hydrogen and nitrogen
An N-type elemental analyzer was used. The measurement was carried out by precisely weighing 1.5 ± 0.2 mg of the negative electrode material of the sample in a tin container, burning it at a temperature of 975 ° C. for 5 minutes, and detecting by TCD with a He gas carrier. In addition, before measuring the sample,
Corrected with standard acetanilide (2.0 ± 0.1 mg).

[0022]

【Example】

Example 1 The carbon content of the pre-carbonized calcination raw material carbon powder was 88.0.
wt%, hydrogen content 2.3 wt%, nitrogen content 4.1w
It was t%. Therefore, the H / C atomic ratio was 0.136. After this carbon powder was pulverized to an average particle size of 15 microns, four 30 L graphite containers were placed in a vacuum firing furnace having an internal volume of 230 L, and each container was charged with 10 kg and 40 kg in total. Replace with vacuum and nitrogen gas, oxygen concentration 100p
After adjusting to pm or less, the degree of vacuum is adjusted to 200 mmHg or less while introducing nitrogen gas, and the rate of temperature rise is 5 ° C./min.
Raise the temperature to 00 ° C. From 500 ° C, vacuum degree is 150m
The temperature is raised to 800 ° C. at a heating rate of 1 ° C./min, with mHg or less. From 800 ° C., the supply of nitrogen is stopped, the exhaust amount is increased by a mechanical booster, and the temperature is raised to 1200 ° C. at a heating rate of 1 ° C./min at 4 mmHg. 1m at 1200 ℃
A vacuum degree of mHg or less was maintained for 4 hours. After that, the fan was forcibly cooled while gradually lowering the degree of vacuum. The generated volatile matter was collected as tar by a cooling trap provided in the exhaust system. After cooling, the weight of the obtained carbon powder is about 33.
It was Kg. The carbon content of this carbon powder is 92.1 wt.
%, Hydrogen content 0.13 wt%, nitrogen content 0.74%
Met. Therefore, the atomic ratio of H / C was 0.017.

100 parts by weight of this powdery negative electrode material was mixed with 5 parts by weight of polytetrafluoroethylene powder [binder] and mixed to prepare a flexible molded body which was compression molded into a disk shape. Using this test piece for evaluation, lithium perchlorate and propylene carbonate and 1,2-
A half cell was prepared using a solution [concentration 1.0 mol / l] dissolved in an equal volume mixture with dimethoxyethane as an electrolytic solution and a polypropylene microporous membrane having a thickness of 50 μm as a separator. A lithium metal having a diameter of 16 mm and a thickness of 0.5 mm was used as the counter electrode. In addition, a small piece of lithium metal was used as the reference electrode in the same manner as the counter electrode.

The initial circuit potential of the half cell thus obtained was 3.18 V (volt). Then current density 1.0
The electrode potential of the test piece for evaluation with respect to the reference electrode is 1 at mA / cm ^2.
Constant-current charging was performed up to mV, and constant-potential charging was further performed for 20 hours at an electrode potential of 1 mV. Charging capacity is 608mAh /
It was g. Subsequently, when electricity was applied at a current density of 1.0 mA / cm ² , it was 328 mAh / cm ² at an electrode potential of the evaluation test piece of 0.2 V with respect to the reference electrode, and 50 at an electrode potential of 1.5 V.
When discharging to 5 mAh / cm ² and further to 3.0 V, a discharge capacity of 520 mAh / cm ² was confirmed. The capacity loss during charging / discharging was 88 mAh / cm ² .

Comparative Example 1 A carbon powder obtained by firing in the same manner as in Example 1 was evaluated in exactly the same manner as in Example 1 except that only the degree of vacuum during firing from 800 ° C. to 1200 ° C. was kept constant at 35 mmHg. . The initial circuit potential of the obtained half cell is 3.15.
It was V (volt). Next, current density 1.0 mA / cm ²
Then, constant current charging was performed until the electrode potential of the evaluation test piece with respect to the reference electrode was 1 mV, and further constant potential charging was performed for a total of 20 hours at an electrode potential of 1 mV. The charge capacity was 600 mAh / g. Subsequently, when discharging was performed at a current density of 1.0 mA / cm2, the electrode potential of the evaluation test piece with respect to the reference electrode was 0.
309 mAh / cm ² up to 2V, 470 mAh up to 1.5V
/ cm ² , further discharged to 3.0V, 4
A discharge capacity of 90 mAh / cm ² was confirmed. The capacity loss during charge and discharge increased to 130 mAh / cm ² .

Example 2 The carbon content of the carbon powder for calcination prepared by the preliminary carbonization reaction was 85.0% by weight, the hydrogen content was 3.4% by weight, and the nitrogen content was 4.9% by weight. Therefore, the H / C atomic ratio is 0.
It was 480. This carbon powder was crushed to an average particle size of 30 microns, placed in a 6 L graphite container in an inert gas firing furnace having an internal volume of 10 L, and charged with 1 kg. After decompressing and replacing with nitrogen gas to make the oxygen concentration 100 ppm or less, the nitrogen gas flow rate was 20 L / min, and the heating rate was 5
C./min. To 500.degree. From 500 ° C, the flow rate of nitrogen gas is 30 L / min, and the temperature rise rate is 2 ° C / min.
Raise the temperature to ℃. Nitrogen gas flow rate is 40L from 800 ℃
The temperature is raised to 1300 ° C. at a heating rate of 0.5 ° C./min. The same gas flow rate was maintained for 5 hours at 1300 ° C.
After that, natural cooling was started, the flow rate of nitrogen gas was gradually reduced, and forced cooling was performed with a fan.

Volatile components other than the generated non-condensable gas are collected as tar by a switching type cooling trap provided in the exhaust system, and the collected tar is weighed at regular intervals, and the vapor partial pressure is 30 mmHg or less. I confirmed that. After cooling, the weight of carbon powder obtained was about 1.5 Kg. This carbon powder has a carbon content of 92.1 wt% and a hydrogen content of 0.13.
The wt% and the nitrogen content were 0.74 wt%. Therefore, the atomic ratio of H / C was 0.017.

Example 1 based on this powdery negative electrode material
A half cell was prepared in the same manner as in. The initial circuit potential of the half cell was 3.14 V (volt). Then, constant current charging was performed at a current density of 1.0 mA / cm ² until the electrode potential of the evaluation test piece with respect to the reference electrode was 1 mV, and further constant voltage charging was performed at an electrode potential of 1 mV for a total of 20 hours. Charge capacity is 5
It was 20 mAh / g. Next, current density 1.0 mA / cm
^When discharge was performed at ^{2, the} evaluation test piece electrode potential for the reference electrode was 279 mAh / cm ² , 1.5, up to 0.2 V.
When the battery was discharged to 434 mAh / cm ² up to V, and further to 3.0 V, a discharge capacity of 449 mA / cm ² was confirmed. The capacity loss during charging / discharging was 71 mAh / cm ² .

Comparative Example 2 Firing was performed in the same manner as in Example 2 except that the flow rate of nitrogen gas in each firing temperature region was reduced to 1/2. Cooling trap
Le collection amount is 800 ~ 900 ℃, vapor partial pressure 30m
Distillation at mHg was above the calculated amount. The half circuit obtained in the same manner as in Example 2 had an initial circuit potential of 3.10 V (volt). Next, current density 1.0
The electrode potential of the test piece for evaluation with respect to the reference electrode is 1 at mA / cm ^2.
Constant-current charging was performed up to mV, and constant-potential charging was further performed at an electrode potential of 1 mV for a total of 20 hours. Charge capacity is 516mAh /
It was g. Then, when discharging was carried out at a current density of 1.0 mA / cm ² , the electrode potential of the evaluation test piece with respect to the reference electrode was 258 mAh / cm ² at a voltage of 0.2 V and 3 at a voltage of 1.5 V.
When the battery was discharged to 98 mAh / cm ² and further 3.0 V, the discharge capacity decreased to 415 mAh / cm ² , while the capacity loss during charging / discharging increased to 101 mAh / cm ² .

[0030]

According to the present invention, a high performance carbon material for a negative electrode of a non-aqueous solvent secondary battery can be obtained by an industrially stable production method.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-6-89721 (JP, A) JP-A-8-64207 (JP, A) JP-A-8-115723 (JP, A) JP-A-8- 222273 (JP, A) JP-A-6-145669 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 4/58 H01M 4/02

Claims (4)

(57) [Claims] 1. A calcination temperature of 800 to 1500 in calcination of a raw carbon powder having an H / C atomic ratio of 0.5 to 0.05, a carbon content of 85 wt% or more, and an average particle size of 100 microns or less. A carbon material for a negative electrode of a non-aqueous solvent secondary battery obtained by firing while maintaining a vapor partial pressure in a firing furnace at 30 ° C. due to volatile matter from the carbon powder at 30 mmHg or less. 2. The negative electrode for a non-aqueous solvent secondary battery according to claim 1, which is obtained by firing while flowing an inert gas so that the vapor partial pressure in the firing furnace due to the volatile components from the raw material carbon powder is 30 mmHg or less. Carbon material. 3. The non-claim according to claim 1, obtained by firing while maintaining the degree of vacuum in the firing furnace at 30 mmHg or less so that the vapor partial pressure in the firing furnace due to the volatile components from the raw material carbon powder becomes 30 mmHg or less. Carbonaceous material for negative electrode of water-solvent secondary battery. 4. The carbon material for a non-aqueous solvent secondary battery negative electrode according to claim 1, wherein the H / C atomic ratio of the carbon material obtained by firing is 0.02 or less.