JPH10241690A - Negative electrode for lithium secondary battery - Google Patents

Abstract

(57) [Problem] To provide a negative electrode for a lithium secondary battery capable of obtaining a lithium secondary battery having high discharge capacity, low irreversible capacity and excellent cycle characteristics. SOLUTION: This is a negative electrode for a lithium secondary battery in which lithium 2 is occluded in a negative electrode active material, wherein the negative electrode active material contains 0.01 to 10% of phosphorus (% by weight, the same applies hereinafter) and 0.01% of oxygen. It is composed of an amorphous carbon material 1 containing unavoidable impurities of up to 15% by weight and graphite of 5 to 50%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a negative electrode for a lithium secondary battery having excellent charge / discharge capacity.

[0002]

2. Description of the Related Art In recent years, miniaturization and cordlessness of electronic devices such as mobile phones have been rapidly progressing. In addition, development and diffusion of electric vehicles are desired due to environmental problems and energy problems. Accordingly, a secondary battery having a high energy density is required.

Conventionally, nickel cadmium batteries, nickel-metal hydride batteries, and lead-acid batteries have been known as secondary batteries. However, these batteries are heavy and have low energy density. On the other hand, lithium secondary batteries are expected to be lightweight, high in energy density, high performance batteries for mobile phones, batteries for electric vehicles, and the like.

However, when lithium metal is used for the negative electrode in a lithium secondary battery, the lithium metal precipitates in an active state such as dendrite or powder on the surface of the negative electrode during charge / discharge, so that the lithium metal penetrates through the separator. Short circuit with positive electrode or react with electrolyte. As a result, the charge / discharge efficiency of the lithium secondary battery decreases.

[0005] Therefore, graphite, which can intercalate lithium ions between layers when charged and suppress dendrite deposition and reaction with an electrolytic solution, can be used as a negative electrode active material in the negative electrode for a lithium secondary battery.
In addition, graphite has a small electric resistance and a flat charge / discharge voltage, and is considered promising as a negative electrode active material. However, graphite has insufficient capacity as a negative electrode of a lithium secondary battery that requires a high capacity density.

Therefore, as an attempt to realize a high capacity,
For example, as described in JP-A-3-137010, 5-74457, a carbonaceous material containing phosphorus has been proposed. That is, when an organic material is carbonized into a carbonaceous material, a phosphorus compound is added to obtain a carbonaceous material having a large lithium doping amount.

[0007]

However, even with such a carbonaceous material containing phosphorus, although the initial capacity is high, the irreversible capacity, which is the difference between the charge amount and the discharge amount, is extremely high (see the comparative example in FIG. 2). reference). Further, the constant current charging or the constant current-constant voltage charging, which is generally performed, has a disadvantage that the discharge capacity is greatly reduced after only a few cycles, and the cycle characteristics are poor. It is known that non-graphite carbonaceous materials are inferior in electrical conductivity to graphite.

The present invention has been made in view of the above problems, and has as its object to provide a negative electrode for a lithium secondary battery capable of obtaining a lithium secondary battery having a high discharge capacity, a low irreversible capacity, and excellent cycle characteristics. Things.

[0009]

According to a first aspect of the present invention, there is provided a negative electrode for a lithium secondary battery in which lithium is occluded in the negative electrode active material, wherein the negative electrode active material is 5 to 50% (% by weight, hereinafter the same). And the balance is amorphous carbon containing phosphorus, oxygen and unavoidable impurities, and the content of phosphorus to the whole amorphous carbon is 0.01 to 10%, and the content of oxygen is 0.1 to 10%. 0.1 to 15% of the negative electrode for a lithium secondary battery.

As shown in the above-mentioned prior art, the present inventors have found that the poor cycle characteristics of a carbonaceous material to which a phosphorus compound has been added is due to the fact that the structure changes due to doping and undoping of lithium ions, and the electrical characteristics of the carbonaceous material change. It was thought that the conductivity was significantly reduced. Therefore, by adding graphite, which has good conductivity and can itself dope and dedope lithium ions, to a non-graphite carbonaceous material to which a phosphorus compound has been added, a negative electrode having a high discharge capacity and good cycle characteristics can be obtained. The inventors have found that the present invention has been achieved, and have achieved the present invention.

In the present invention, the amorphous carbon refers to a non-graphite carbon material. The phosphorus (P) needs to be contained in the amorphous carbon in an amount of 0.01 to 10%. 0.01
%, The effect of increasing the discharge capacity is small.
%, The irreversible capacity monotonically increases despite the saturation of the discharge capacity.

Preferably, oxygen is contained in an amount of 0.01 to 15%. If it is less than 0.01%, there is a problem that the discharge capacity is reduced. On the other hand, if it exceeds 15%, there is a possibility that the problem that the irreversible capacity is significantly increased despite the saturation of the discharge capacity.

Further, the amorphous carbon may contain unavoidable impurities. Inevitable impurities include hydrogen, nitrogen, silicon, sulfur and the like. Amorphous carbon produced from the following petroleum raw coke generally contains hydrogen as an unavoidable impurity, and may further contain nitrogen.

Preferably, the amorphous carbon is contained in the negative electrode active material in an amount of 50 to 95%. If it is less than 50%, there is a problem that the discharge capacity becomes low.
%, The conductivity may be reduced, and the discharge capacity may be reduced.

In the negative electrode active material, the amount of graphite is 5 to 5.
It is necessary to contain 50%. If it is less than 5%, the effect of lowering the conductivity is small and there is a problem that the discharge capacity cannot be increased. On the other hand, if it exceeds 50%, there is a problem that the discharge capacity decreases.

Next, the operation of the present invention will be described below. First, the insertion of lithium into amorphous carbon is performed by inserting lithium ions into the cavities between the layers of carbon crystallites and between the terminals. That is, the surface of amorphous carbon has abundant adsorption sites (cavities),
When a current in the reduction direction is applied to amorphous carbon in an electrolyte in which lithium ions are present, lithium ions are adsorbed on the surface (in the cavity), and the capacity is accumulated like a capacitor. On the other hand, when a current flows in the oxidation direction, lithium ions are released.

As described above, the reason why the amount of stored lithium ions is improved is presumed to be as follows. That is, adding phosphorus is a kind of doping to amorphous carbon having high resistance. Therefore, lithium ions are easily adsorbed on the surface of the amorphous carbon, and the charge / discharge capacity increases. Further, the addition of phosphorus has an effect of increasing the specific surface area of amorphous carbon. That is, the number of cavities increases, the amount of lithium ion adsorbed increases, and the capacity increases.

On the other hand, when examining the effect of adding graphite, in this case, since graphite is a good conductor, the electrical resistance of the entire negative electrode active material is reduced, and the ratio of the electrochemically effective carbon material is reduced. To increase.

In particular, even if the structure of amorphous carbon is changed by repeating the charge / discharge cycle and the electric conductivity is lowered, the conductivity is maintained by the graphite, so that the cycle characteristics are improved. . Further, graphite has an irreversible capacity lower than that of amorphous carbon, and thus has an effect of reducing the irreversible capacity. Thus, by defining the above-mentioned phosphorus, graphite, amorphous carbon and oxygen in the above-mentioned specific ranges, a high discharge capacity can be obtained due to a synergistic effect of the above-mentioned components, while irreversible capacity is reduced and cycle characteristics are also reduced. improves.

Next, the amorphous carbon can be obtained, for example, by subjecting raw coke of petroleum to heat treatment at 500 to 1500 ° C. The above-mentioned petroleum raw coke is obtained, for example, by subjecting petroleum heavy oil to dry distillation at a temperature of about 500 ° C. for a certain period of time (heated without air) to cause a pyrolytic polymerization reaction to proceed together with gas and liquid fractions. .

As the raw coke, coal is used
Raw coal coke (semi-coke) carbonized at a temperature of about 00 ° C. or pitch raw coke obtained by carbonizing coal tar pitch can be used in the same manner. It is preferable that the phosphorus is subjected to the simultaneous heating treatment by adding a phosphorus-containing compound such as P ₂ O ₅ to the raw coke of petroleum or coal as described above. Thereby, phosphorus can be contained in the amorphous carbon.

When the heat treatment temperature is lower than 500 ° C., the conductivity of the amorphous carbon becomes small, and a phenomenon occurs that the closed circuit voltage (terminal voltage) is lower than the open circuit voltage due to the IR drop of the electrode. As a result, charging and discharging of the lithium secondary battery may be insufficient. The IR drop refers to a voltage drop that occurs when a current flows in the carbon electrode.

When the heat treatment temperature exceeds 1500 ° C., the atomic ratio H / C of hydrogen atoms and carbon atoms in the amorphous carbon becomes smaller than 0.1, and each carbon atom is charged during charging. There is a possibility that the amount of lithium cluster generated in the cavity between the ends of the crystallite (see FIG. 1) may be reduced.

Further, the lower limit of the preferable heat treatment temperature is 60.
The upper limit of the temperature is 0 ° C., more preferably 1200 ° C. Further, the preferable time of the heat treatment is not particularly limited. Among them, a range of 30 minutes to 3 hours is particularly preferable because a very large charge / discharge capacity can be obtained.

It is preferable that the heating be performed in an inert atmosphere. Thereby, the oxidation of the raw coke can be prevented, and as shown in FIG. 1 described later, amorphous carbon composed of carbon crystallites can be obtained. The inert atmosphere includes, for example, a vacuum atmosphere, a rare gas, N ₂
And the like.

The amorphous carbon is composed of carbon crystallites as shown in FIG. Although the above carbon crystallites are larger than the carbon crystallites in raw coke used as a raw material, the terminal of the carbon crystallites is increased as compared with normal coke (manufactured at 1200 to 1400 ° C.). Many cavities are formed between the ends.

The carbon crystallites are mainly composed of hydrocarbons and have a six-membered ring network planar structure, and a part of them has a layer structure similar to that of crystalline graphite. The terminal of the carbon crystallite is in a state in which hydrogen is bonded to carbon.

Then, in the carbon crystallite, lithium ions are occluded in the state of lithium ions between the layers having the same layer structure as the crystalline graphite. Further, lithium ions are absorbed into the cavities formed between the ends of the carbon crystallites while forming lithium clusters. Therefore, more lithium ions and lithium clusters can be stored.

Further, as described above, the phosphorus can be added to the raw coke when producing amorphous carbon. Preferably, phosphorus is added to the raw coke in the form of a phosphorus-containing compound such as P ₂ O ₅ (phosphor pentoxide), phosphoric acid, and phosphate. In this case, as described above, the phosphorus-containing compound is decomposed during the heat treatment for converting the raw coke to amorphous carbon, and phosphorus remains in the amorphous carbon. In addition, part of the oxygen generated by this decomposition remains in the amorphous carbon. Further, the phosphorus-containing compound is contained in amorphous carbon as phosphorus in an amount of 0.01 to 10%.
%.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 An anode for a lithium secondary battery according to an embodiment of the present invention will be described with reference to FIG. The negative electrode for a lithium secondary battery of this example is a negative electrode for a lithium secondary battery in which lithium is occluded in the negative electrode active material, and the negative electrode active material is phosphorus 0.01 to 10% (% by weight, hereinafter the same). ) And oxygen 0.01
~ 15% and amorphous carbon containing unavoidable impurities 50 ~
It consists of 95% and 5-50% graphite. And the said amorphous carbon consists of amorphous carbon obtained by heating a petroleum raw coke at 500-1500 degreeC.

The negative electrode for a lithium secondary battery is manufactured, for example, as follows. That is, as will be described in detail later, 100% by weight of petroleum raw coke is contained in the phosphorus-containing compound 2.
The mixture containing 100% to 100% is heated at 500 to 1500 [deg.] C. to form amorphous carbon. Graphite is added thereto and mixed to form a negative electrode active material component. Further, this paste is mixed with a binder to obtain a paste-like negative electrode mixture. Next, this negative electrode mixture is applied to a current collector to form a negative electrode for a lithium secondary battery.

The operation and effect according to this embodiment will be described below. The negative electrode active material according to this example is made of amorphous carbon containing phosphorus, oxygen, and unavoidable impurities, and graphite, which are in the above range. As shown in FIG. 1, the amorphous carbon 1 is composed of larger carbon crystallites 10 in which carbon crystallites constituting raw coke are partially decomposed and generated by heat. In the negative electrode for a lithium secondary battery, lithium ions are absorbed into the amorphous carbon 1 as described below.

That is, the carbon crystallite 10 has a layer structure similar to that of graphite in a part thereof, and the lithium ions 2 are directly intercalated between the layers 13 in the layer structure. The cavity 1 is located between the terminal 12 of the carbon crystallite 10 and the terminal 12 of another carbon crystallite 10.
1 is formed, and the lithium cluster 20 generated from the lithium ion 2 is occluded in the cavity 11.

[0034] As described above, since the above-mentioned phosphorus tends to be negatively charged, a large number of donor sites are formed in the coke by the addition of phosphorus. Then, it is considered that this donor site becomes an adsorption site for lithium ions, and the amount of occluded lithium ions increases. Further, it is considered that the amount of the cavity is increased because the specific surface area of the amorphous carbon is increased by the addition of phosphorus.

Thus, more lithium ions can be stored. Therefore, a negative electrode for a lithium secondary battery having high discharge capacity, low irreversible capacity, and excellent cycle characteristics can be obtained.

Embodiment 2 Next, the performance of the negative electrode for a lithium secondary battery according to the present invention will be described with reference to FIGS. First, a method for producing a negative electrode active material as a sample will be described. 2 for 1 part (100% by weight) of petroleum raw coke
2.9% of phosphorus pentoxide (P ₂ O ₅ ) was added, and the mixture was heated and baked at 900 ° C. under an argon stream to obtain an amorphous carbon material (amorphous carbon). For this amorphous carbon, the content of phosphorus (P) was measured by inductively coupled plasma (ICP) emission spectroscopy, and the content of oxygen (O) was measured by carrying out melting and thermal conductivity in an inert gas. As a result, the phosphorus content was 6.65% and the oxygen content was 6.13%.

To 75% of this carbon material, 25% of graphite (“SP8W” made by Nippon Graphite) was added and mixed in a mortar. Further, with respect to 96% of the negative electrode active material composed of these mixtures,
4% of polytetrafluoroethylene was added as a binder. Sample No. of the negative electrode mixture in the form of a film was further kneaded.
It was set to 1. Next, using a hydraulic press, the film-shaped negative electrode mixture was pressure-bonded to a current collector SUS304 mesh to obtain a negative electrode for evaluation.

On the other hand, as a comparative example, a sample N using a negative electrode active material to which graphite was not added in the above negative electrode mixture was used.
o. Sample No. C1 using the negative electrode active material prepared without heating the same raw coke as above without adding the phosphorus-containing compound and also heating the amorphous coke to form amorphous carbon. A negative electrode using C2 was produced. An aprotic conductor in which 1 mol% of LiPF ₆ is dissolved in a solution in which ethylene carbonate and diethyl carbonate are mixed at a weight ratio of 1: 1 is used as an electrolyte, and lithium is used as a counter electrode. The battery for evaluation was constructed using a polyethylene porous film as a separator.

[0039] Then, the charging current density (lithium doping) during 0.5 mA · cm ^-2, discharge a charge-discharge test was conducted as (lithium dedoping) sometimes 1 mA · cm ^-2.
The result is shown as Sample No. according to the embodiment of the present invention. 1
And Sample No. of Comparative Example. Figure 2 for C1 (without graphite)
The sample No. C1 and Comparative Sample No. FIG. 3 shows the number of charge / discharge cycles on the horizontal axis and the capacity on the vertical axis for C2 (only amorphous carbon).

As can be seen from FIG. 2, the present invention (sample N
o. According to 1), the discharge capacity per 1 g of the electrode is as high as 400 mAh in one cycle, and is 296 mAh in 10 cycles, indicating that the cycle characteristics are excellent. The irreversible capacity of the difference between the charge capacity and the discharge capacity in one cycle is 153 mAh. Considering that about 50 mAh is caused by polytetrafluoroethylene, the negative electrode active material It can be seen that the irreversible capacity based on this is as low as about 100 mAh.

On the other hand, in the comparative example, the sample No. to which phosphorus pentoxide was added but graphite was not added. C1 has a high discharge capacity at one cycle of 318 mAh, but drops sharply as the cycle is repeated.
It dropped to 60 mAh. The irreversible capacity is as large as 240 mAh. Further, sample No. In C2, as shown in FIG. 3, the discharge capacities at one cycle and ten cycles were 176, 1 and 1, respectively.
It is extremely low at 34 mAh.

Embodiment 3 This embodiment shows the discharge capacity and irreversible capacity when the addition amount of P ₂ O ₅ as a phosphorus-containing compound and the firing temperature of petroleum raw coke are changed. To one part (100% by weight) of petroleum raw coke, 5.7 to 45.8% by weight of phosphorus pentoxide (2.5 to 20% in terms of P amount) is added, and the mixture is added under an argon stream. And heated and fired at 900 to 1200 ° C. to obtain an amorphous carbon material (amorphous carbon).

25% of graphite ("SP8W" made by Nippon Graphite) was added to 75% of the carbon material and mixed in a mortar to obtain a negative electrode active material. In addition, Carboxymeth as a binder
ylCellose Sodium salt (CMC
An aqueous solution of Na) was added so as to have a CMCNa amount of 6%, and further kneaded to obtain a paste-like negative electrode mixture. The electrode material from which the paste-like negative electrode mixture was dried to remove water was composed of a negative electrode active material (a total of 94%) composed of amorphous carbon containing phosphorus and oxygen and graphite, and 6% of the binder.

Next, the negative electrode active material was applied to a thickness of about 100 μm on a copper foil having a thickness of 20 μm, and dried by heating at 70 ° C. for 1 hour. 0.5 ton · cm ^-2 at 170 ° C
Hot pressing was performed, and vacuum drying was performed at 200 ° C. for 2 hours to obtain a negative electrode for evaluation. Next, a battery for evaluation was constructed under the same conditions as those of Embodiment 2 except for the counter electrode, the electrolytic solution, and the separator.

[0045] Each of the current density at the time of charging and discharging 0.5 mA · cm ^-2, the constant current of 1 mA · cm ^-2, a charge-discharge test was conducted for 10 cycles end voltage as 0V and 1.5V. Table 1 shows the results.

Further, following the 10 cycles of the charge / discharge, constant current / constant voltage charging with a current density of 1 mA · cm ⁻² and an end voltage of 0 V, while a current density of 1 mA · cm ⁻² ,
A charge / discharge test was performed by constant current discharge at a cutoff voltage of 1.5V. Table 2 shows the results. In both tables, the results of measuring the amounts of P ₂ O ₅ added to the raw coke and the amounts of phosphorus and oxygen contained in the amorphous carbon by the same method as in Embodiment 2 are also shown.

[0047]

[Table 1]

[0048]

[Table 2]

First, as shown in Table 1, the discharge capacity increases as the amount of added phosphorus pentoxide increases. As an example, looking at the case where the raw coke is heat-treated at 1000 ° C., when the addition amount of phosphorus pentoxide increases from 5.7% to 45.8% (the phosphorus content in amorphous carbon is 2%). .47
% To 6.96%), and the discharge capacity per gram of the electrode increases from 273 mAh to 340 mAh in one cycle.
251 mAh to 325 mA even at 10 cycles
h. The irreversible capacity has increased from 63 mAh to 100 mAh.

When the firing temperature of the raw coke is increased, the discharge capacity is slightly reduced, but the irreversible capacity can be reduced.
For example, as seen when 45.8% of phosphorus pentoxide is added, when the firing temperature is increased to 900 to 1200 ° C.,
The irreversible capacity drops significantly from 175 mAh to 68 mAh. On the other hand, although the discharge capacity has also decreased,
Discharge capacity at 1 and 10 cycles even after firing at
It is as large as 07,271 mAh.

Next, as a comparative example, 94% of a negative electrode active material obtained by calcining raw coke without adding phosphorus pentoxide at 1000 ° C. and 75% of graphite and 25% of graphite, and CMC of a binder were used.
When the negative electrode mixture composed of 6% of Na was evaluated, 1,10
The discharge capacity during cycling was as low as 290,260 mAh. 5.7% phosphorus pentoxide in 100% raw coke
In the negative electrode mixture composed of 94% of the negative electrode active material composed of the raw coke calcined material alone and calcined at 900 ° C. and 6% of the binder CMCNa, the discharge capacity at 1,10 cycles was 3%.
It was as low as 00,264 mAh, and the irreversible capacity was as high as 84 mAh.

Table 2 shows the results of evaluation by constant current / constant voltage charging and constant current discharging.
It can be seen that the battery has a high discharge capacity of at least mAh.
Further, there is almost no change in the discharge capacity between 1 cycle and 10 cycles, and the cycle characteristics are good. The tendency of the change in the discharge capacity and the irreversible capacity depending on the amount of phosphorus added and the firing temperature is the same as in the case of Table 1.

Embodiment 4 In this embodiment, as shown in FIG. 4, the relationship between the added amount of graphite and the charge / discharge capacity is shown. 5.7% phosphorus pentoxide is added to 1 part (100%) of petroleum raw coke, and the mixture is baked at 1000 ° C under an argon stream to obtain 100% of an amorphous carbon material. Was. The amount of phosphorus contained in the amorphous carbon was 2.47%, and the amount of oxygen was 0.39%. Graphite ("LF20A" manufactured by Chuetsu Graphite Manufacturing Co., Ltd.) was added to the carbon material and mixed in a mortar to obtain a negative electrode active material. The amount of graphite in the negative electrode active material was changed from 0 to 100%.

Carboxymethyl as a binder
Cellulose Sodium salt (CM
An aqueous solution of CNa) was added to 94% of the above mixture so that the amount of CMCNa became 6%, and the mixture was further kneaded to obtain a paste-like negative electrode mixture. Next, the negative electrode mixture was applied to a thickness of about 100 μm on a copper foil having a thickness of 20 μm, and dried by heating at 70 ° C. for 1 hour. 0.5 to additional pressure at 170 ° C
Hot pressing was performed at n · cm ^{−2 and} vacuum drying was performed at 200 ° C. for 2 hours to obtain a negative electrode for evaluation.

Next, a battery for evaluation was constructed under the same conditions as those of Embodiment 2 except for the counter electrode, the electrolyte and the separator. Constant current with current density of 1 mA · cm ^-2 and end voltage of 0 V
Constant voltage charging, current density 1 mA · cm ^-2 , final voltage 0
A charge / discharge test was performed by constant current discharge at V. FIG. 4 shows the results. As can be seen from the figure, the addition of graphite increased the discharge capacity, reaching a maximum at 35%. However, the discharge capacity decreased when the amount of graphite was further increased. On the other hand, it can be seen that the irreversible capacity decreases monotonously with the addition of graphite.

[Specific Example 1] As shown in FIGS. 5 and 6, the relationship between the amount of graphite added and the specific resistance of the negative electrode active material was measured. By adding 5.7 or 45.8% phosphorus pentoxide to 1 part (100%) of petroleum raw coke, the former is heated to 1000 ° C and the latter is heated to 1200 ° C under a stream of argon to obtain a non-fired product. A crystalline carbon material was obtained.

The measurement was carried out by the same method as in the second embodiment.
The amounts of phosphorus and oxygen contained in the amorphous carbon were 2.47% phosphorus and 0.39% oxygen when 5.7% phosphorus pentoxide was added. When 45.8% phosphorus pentoxide was added, the content was 6.96% of phosphorus and 2% of oxygen.

Graphite ("SP8W" made by Nippon Graphite) is added to the phosphorus-containing amorphous carbon in an amount of 0 to 100% by weight to form a mixture (100%), and then 96% of the mixture is obtained.
Then, 4% of polytetrafluoroethylene was added as a binder and kneaded to obtain a film-shaped negative electrode mixture. Using a hydraulic press, a disk-shaped sample having a diameter of 10 mm and a thickness of 0.5 mm was used for measurement of electrical resistance.

FIG. 5 shows the relationship between the specific resistance and the amount of graphite for a sample containing 5.7% phosphorus pentoxide (phosphorus content in amorphous carbon excluding binder and graphite: 2.47%). FIG. 6 shows the sample containing 45.8% phosphorus pentoxide (the above-mentioned phosphorus 6.96%).
Shown in In both cases, the specific resistance was sharply reduced by adding graphite, and was almost the same as that of graphite when 30 to 40% by weight was added. As described above, since the addition of graphite lowers the resistance of the electrode, it is considered that polarization hardly occurs, doping and undoping of lithium ions becomes easy, and the discharge capacity increases. From the above, it can be seen that the lithium secondary battery having the negative electrode made of the negative electrode active material according to the present invention has a larger charge / discharge capacity.

[0060]

As described above, according to the present invention, there is provided a negative electrode for a lithium secondary battery capable of obtaining a lithium secondary battery having a high discharge capacity, a low irreversible capacity and excellent cycle characteristics. be able to.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a state in which lithium ions and lithium clusters are occluded in amorphous carbon in a negative electrode active material according to Embodiment 1.

FIG. 2 is a diagram showing the relationship between the number of charge / discharge cycles and the charge / discharge capacity according to a second embodiment.

FIG. 3 is a diagram showing the relationship between the number of charge / discharge cycles and the charge / discharge capacity according to a second embodiment.

FIG. 4 is a diagram showing a relationship between an added amount of graphite and a charge / discharge capacity according to a fourth embodiment.

FIG. 5 is a diagram showing the relationship between the amount of graphite added and the specific resistance according to the fifth embodiment.

FIG. 6 is a diagram showing a relationship between the amount of graphite added and the specific resistance according to the fifth embodiment.

[Explanation of symbols]

 1. . . Amorphous carbon, 10. . . 1. carbon crystallites, . . lithium ion,

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akihiko Koiwai 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Laboratory Co., Ltd. 41, Yokomichi, Toyota Central Research Institute, Inc.

Claims (1)

[Claims] 1. A negative electrode for a lithium secondary battery in which lithium is stored in a negative electrode active material, wherein the negative electrode active material comprises:
5 to 50% (% by weight, the same applies hereinafter) of graphite and the balance being amorphous carbon containing phosphorus, oxygen and unavoidable impurities, and the content of phosphorus relative to the entire amorphous carbon is 0.1%.
A negative electrode for a lithium secondary battery, wherein the content of oxygen is from 0.01 to 10% and the content of oxygen is from 0.01 to 15%.