Graphite material for negative electrodes of lithium ion secondary battery, manufacaturing method for said material, and lithium ion secondary battery using same

Abstract

A graphite material for a negative electrode is provided which can suppress capacity degradation due to repeated charging and discharging cycles, storage in a charged state, and floating charging.A method of manufacturing a graphite material for a negative electrode of a lithium ion secondary battery is provided in which an atomic ratio H / C of hydrogen atoms H and carbon atoms C in the raw coke composition is in a range of 0.30 to 0.50 and a microstrength of the raw coke composition is in a range of 7 wt % to 17 wt %.

Description

TECHNICAL FIELD[0001]The present invention relates to a graphite material used for a negative electrode of a lithium ion secondary battery and a manufacturing method thereof. More particularly, the present invention relates to a method of manufacturing a graphite material used for a negative electrode of a lithium ion secondary battery with high durability and with suppressed capacity degradation, and a lithium ion secondary battery including a negative electrode formed of the graphite material.BACKGROUND ART[0002]Since a lithium ion secondary battery has a low weight and excellent input and output characteristics in comparison with a nickel-cadmium battery, a nickel-hydrogen battery, and a lead battery, which are secondary batteries in the related art, the lithium ion secondary battery is recently anticipated as a power source for electric cars or hybrid cars. In general, such a type of battery has a structure in which a positive electrode (cathode) including lithium which can be r...

AI Technical Summary

Benefits of technology

The invention relates to a method for manufacturing a graphite material that can improve the performance of lithium-ion batteries. The method involves partially introducing areas of low crystallinity into a highly crystalline structure of the graphite material, which can help to prevent the decomposition of the electrolyte and improve the storage characteristics of the battery. The resulting graphite material can also have a reduced number of edge portions, which can further enhance the stability and reliability of the battery. Overall, this method can provide a high-quality graphite material that can be used as a negative electrode material in lithium-ion batteries.

Problems solved by technology

This is because it is not possible in principle to set the charging and discharging efficiencies of the positive electrode reaction (Formula 1) and the negative electrode reaction (Formula 2) to be completely equal to each other and the charging and discharging efficiency of the negative electrode is lower.

As described above, when such a type of battery repeats the charging and discharging cycles, there is a problem in that the capacity obtained within a predetermined voltage range (within the range of the discharging cutoff voltage and the charging cutoff voltage) degrades due to the change in operational range of the capacity of the positive and negative electrodes.

The positive electrode potential and the negative electrode potential of which the operational ranges once changed are irreversible and cannot be returned to the original state, and the fact that there is no means for recovering the capacity makes this problem severe.

First, when a battery is stored in a charged state, it is known that the capacity consumed in side reactions and competitive reactions occurring in the charged state, that is, the self-discharged capacity, is larger in the negative electrode than in the positive electrode and thus the operational ranges of capacity of the positive and negative electrodes are changed before and after storage, thereby causing the battery capacity after storage to degrade (Non-Patent document 3).

In this way, when the floating charging is performed, there is a problem in that the operational ranges of capacity of the positive electrode and the negative electrode are irreversibly changed and the battery capacity degrades.

Since this disturbance of the crystalline structure remains as unorganized carbon even after the graphitization as the final process, the initial charging and discharging efficiency of the negative electrode can be improved (see Paragraph of Patent document 2), but there is a problem in that the reliability of the battery cannot be improved.

In a lithium ion secondary battery using the graphite material as the negative electrode material, since the electrolyte in the edge portions exposed from the particle surfaces of the negative electrode is decomposed to increase the leak current in the negative electrode and to increase the difference from the leak current in the positive electrode, there is a problem in that the capacity retention rate (storage characteristics) is greatly lowered when the battery is held at high temperatures or normal temperatures for a long time.

The charging and discharging efficiency of the negative electrode is lowered by the side reactions and the competitive reactions due to the decomposition of the electrolyte between the graphite layers, thereby causing capacity degradation.

On the surface of the carbon material of the negative electrode, that is, in the interface between the electrolyte and the carbon material, in the charging process, it is thought that side reactions and competitive reactions due to the reductive decomposition of the electrolyte based on the catalytic action of the localized electrons occur in addition to the original charging reaction in which lithium is inserted into the graphite crystals, thereby lowering the charging and discharging efficiency of the negative electrode.

That is, even when an area having a low crystallinity is introduced into the particle surfaces to suppress the decomposition of the electrolyte due to the solvent co-intercalation, the crystallites in the introduced areas having a low crystallinity are in the isotropic state and thus the edge portions are exposed from the surface to increase the reductive decomposition of the electrolyte, thereby causing the capacity degradation.

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