JP2010527132A - Anode material for secondary battery and secondary battery using the same - Google Patents

Abstract

The present invention relates to a negative electrode material for a secondary battery and a secondary battery using the same. The negative electrode material for a secondary battery of the present invention is obtained by firing a core material carbon material coated with a low crystalline carbon material, and having a compression density ratio of 0.9 or more. And
According to the present invention, the protection function against the reaction with the electrolytic solution on the negative electrode material surface can be improved, and the efficiency and cycle capacity of the secondary battery can be improved.

Description

  The present invention relates to a negative electrode material for a secondary battery and a secondary battery using the same, and more specifically, by adjusting the ratio of the compression density of the negative electrode material to 0.9 or more, The present invention relates to a secondary battery negative electrode material and a secondary battery using the same, which can improve the protection function against the reaction with the electrolyte and improve the efficiency and cycle capacity of the secondary battery.

  Various types of portable electronic devices such as video cameras, wireless telephones, cellular phones, and notebook PCs are rapidly spreading in daily life, and the demand for secondary batteries used as a power supply source is greatly increasing. Among them, lithium secondary batteries are currently used in the widest range of secondary batteries because of their excellent battery characteristics with large capacity and high energy density.

A lithium secondary battery basically includes a positive electrode, a negative electrode, and an electrolyte. Therefore, research and development on a lithium secondary battery is largely divided into research on a positive electrode, a negative electrode material, and an electrolyte.
Among these, natural graphite used as a negative electrode material for lithium secondary batteries has excellent initial capacity, but is inefficient in efficiency and cycle capacity. This is known to be due to the decomposition reaction of the electrolytic solution generated at the edge portion of highly crystalline natural graphite.

  In order to overcome such characteristics, surface treatment (coating) of a low crystalline carbon material on natural graphite and heat treatment at 1,000 ° C. or more to coat the surface of natural graphite with low crystalline carbide There is. According to this method, although the initial capacity of the battery is reduced by a small amount, a negative electrode active material with improved efficiency and cycle capacity characteristics can be obtained. However, since the negative electrode active material is used as an electrode, there is a problem that the carbide coated in the step of applying pressure to the electrode current collector such as a copper foil and then pressing is cracked. Moreover, the edge part of highly crystalline natural graphite reacts with electrolyte solution through the part which the said carbide | carbonized_material cracked, and there existed a problem that the coating effect of actual carbide fell.

Japanese Patent Laid-Open No. 2002-084836 (Patent Document 1) discloses the characteristics of graphite in which part or all of the edge portion of the crystal of the carbon material of the core is coated with a carbon material for coating formation.
The above-mentioned Japanese patent describes the amount of coating-forming carbon material to be coated on natural graphite, the heat treatment temperature, and the contents of natural graphite coated with the coating-forming carbon material, such as X-ray diffraction and Raman analysis. However, there was no description of the effect of cracking of the carbide coated during the crimping process when actually applied to an electrode.

  In addition, when graphite is used as an active material for a lithium secondary battery, a phenomenon occurs in which the active material breaks due to a volume change in the process of repeated charging and discharging. However, Patent Document 1 also mentions the effect of such a phenomenon. Not done.

  Therefore, efforts to solve the above-mentioned problems of the prior art have been sustained in related industries, and the present invention has been devised under such a technical background.

JP 2002-084836 A (Patent Document 1)

  The technical problem to be solved by the present invention is that, when actually applied to an electrode, the problem is that the coated carbide cracks during the crimping process, and when used as an active material of a lithium secondary battery, charging / discharging is performed. The present invention is to provide a negative electrode material for a secondary battery and a secondary battery using the same that can achieve such a technical problem. There is a purpose.

  A negative electrode material for a secondary battery for achieving the technical problem to be solved by the present invention is a core material made of a carbon material coated with a low crystalline carbon material and fired, and its compression density The ratio is 0.9 or more.

  A secondary battery for achieving the technical problem to be solved by the present invention is characterized by including a negative electrode made of the negative electrode material described above.

  Hereinafter, preferred embodiments of the present invention will be described in detail. Prior to this, the terms and words used in the specification and claims should not be construed in a normal or lexicographic sense, and the inventor will explain his invention in the best possible way. Therefore, in accordance with the principle that the concept of a term can be appropriately defined, it should be interpreted as a meaning and a concept consistent with the technical idea of the present invention. Therefore, the configuration of the embodiment described in this specification is only the most preferred embodiment of the present invention, and does not represent all of the technical idea of the present invention, and can be substituted at the time of the present application. It should be understood that various equivalents and variations are possible.

The present invention is characterized by improving the protection function against the reaction with the electrolyte on the surface of the negative electrode material in order to prevent the decomposition reaction between the natural graphite and the electrolyte.
The present invention relates to a ratio of compression density (P.D. [coated natural graphite] / determined by measuring the compressed density (PD) of natural graphite coated under a specific pressure. Depending on the PD (natural graphite)), the degree to which the coverage of the electrode active material actually applied to the battery is maintained up to the electrode state varies, and the charge / discharge characteristics of the battery differ accordingly. It was done.

  The present invention relates to a negative electrode material for a secondary battery, wherein the negative electrode material is fired after a core carbon material is coated with a low crystalline carbon material, and a compression density ratio thereof is 0.9. It is the above.

  The ratio of the compression density of the negative electrode material is determined by measuring the compression density of natural graphite coated with a low crystalline carbon material in a specific pressure state. At this time, the pressurizing condition for measuring the compression density is desirably in the range of 20 to 130 MPa. In the case where the numerical range of the pressurizing condition is satisfied, it is desirable that the adhesive strength of the electrode plate and the battery capacity are sufficient, the side reaction with the electrolytic solution can be efficiently prevented, and the impregnation property is sufficient.

  The compression density is obtained according to the following equation 1, and the ratio of the compression densities is obtained according to the following equation 2.

In the above Equation 1, m is the weight (g) of natural graphite coated or uncoated in a specific pressure state, and V is coated or uncoated in a specific pressure state. The volume of natural graphite (cm ² ).

In Equation 2, PDc is the compression density of the coated natural graphite, and PDn is the compression density of the natural graphite.
The ratio of the compression density of the negative electrode material determined as described above is desirably 0.9 or more. Regarding the numerical range of the compression density ratio, when the compression density ratio is 0.9 or more, the initial efficiency is 93.5% or more, and the discharge capacity (holding capacity) at the 30th cycle is 95%. This is desirable. However, when the compression density ratio is less than 0.9, the initial efficiency is less than 93.5%, and the discharge capacity at the 30th cycle is less than 95, which is not desirable.

In addition, the negative electrode material for a secondary battery of the present invention can be produced by coating a carbon material as a core material with a low crystalline carbon material and firing it according to a conventional method practiced in the art.
As the carbon material of the core material, natural graphite, artificial graphite or a mixture thereof can be used, and it is particularly preferable to use natural graphite.

As the low crystalline carbon material, pitch, tar, phenol resin, furan resin, or the like can be used.
That is, the present invention provides a low-crystalline material in which the ratio of the compression density of the negative electrode material produced by coating part or all of the edge portion of the core carbon material with a low-crystalline carbon material is 0.9 or more. A negative electrode material excellent in efficiency and cycle characteristics can be produced by selecting and covering the type and process conditions of the carbonaceous material.

A conductive material and a binder can be selectively added in a small amount, if necessary, to the slurry for producing an electrode plate containing the negative electrode material produced as described above.
The use amount of the conductive material and the binder can be appropriately adjusted and used as much as is normally used in the art, and the range does not affect the present invention.

  As the conductive material, any electronic conductive material that does not cause a chemical change in the battery constructed can be used. For example, examples of the conductive material include carbon black such as acetylene black, ketjen black, furnace black, and thermal black; natural graphite; artificial graphite; conductive carbon fiber; and particularly carbon black, graphite powder, or carbon. It is desirable to use fibers.

  As the binder, a thermoplastic resin, a thermosetting resin, or a mixture thereof can be used. The binder is preferably polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), and more preferably polyvinylidene fluoride.

  As described above, a slurry for producing an electrode plate containing a negative electrode active material and optionally at least one of a conductive material and a binder is applied to the electrode current collector and then dried to remove the solvent, the dispersion medium, and the like. Thus, the active material is bound to the current collector and the active material is bound.

  The electrode current collector is not particularly limited as long as it is made of a conductive material, but it is particularly desirable to use a foil manufactured from copper, gold, nickel, a copper alloy, or a combination thereof.

  In addition, the present invention is a secondary battery including a positive electrode, a negative electrode, a separation membrane interposed between the two electrodes, and an electrolyte, comprising the negative electrode manufactured using the negative electrode material manufactured according to the manufacturing method described above. .

The secondary battery of the present invention can be manufactured by inserting a porous separation membrane between a positive electrode and a negative electrode and injecting an electrolyte according to a conventional method known in the art.
The electrolyte is a non-aqueous electrolyte containing a lithium salt and an electrolyte solution compound. As the lithium salt, LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ and LiN (CF ₃ SO ₂ ). One or more compounds selected from the group consisting of ₂ can be used. Examples of the electrolyte compound include ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and methyl propyl carbonate ( One or more compounds selected from the group consisting of MPC) can be used.

The battery separation membrane of the present invention is preferably a porous separation membrane, and examples thereof include polypropylene-based, polyethylene-based, and polyolefin-based porous separation membranes.
The secondary battery of the present invention can be manufactured in various forms without limitation on the outer shape, and examples thereof include a cylindrical shape using a can, a square shape, a pouch shape, and a coin shape.

The secondary battery of the present invention as described above has a charge / discharge efficiency of 93.5% or more and a discharge capacity of 95% or more in the 30th cycle.
[Example]
Hereinafter, in order to help understanding of the present invention, preferred examples and comparative examples for comparison will be described in more detail.

Example 1
Spherical natural graphite carbon material and pitch were prepared.
First, pitch dissolved in tetrahydrofuran was mixed with spherical natural graphite at 5% by weight. The above mixture was wet-stirred for 2 hours or more at normal pressure, mixed and then dried to produce a mixture. The mixture was subjected to primary and secondary firing for 1 hour at 1,100 ° C. and 1,500 ° C. for 1 hour, respectively, and fine powders were removed to produce a negative electrode material having a compression density ratio of 0.9.

Example 2
In Example 1, the negative electrode material having a compression density ratio of 0.98 by adjusting the pitch content to 3% by weight, the heat treatment temperature to 1,000 ° C., and the heating rate to 0.14 ° C./min. Manufactured.

Example 3
In Example 1, the negative electrode material having a compression density ratio of 1.12 by adjusting the pitch content to 1 wt%, the heat treatment temperature to 1,000 ° C., and the heating rate to 0.14 ° C./min. Manufactured.

Comparative Example 1
In Example 1 above, a negative electrode material having a compression density ratio of 0.71 was manufactured by adjusting the heat treatment temperature to 1,000 ° C. and the heating rate to 10 ° C./min.

Comparative Example 2
In Example 1 above, a negative electrode material having a compression density ratio of 0.82 was manufactured by adjusting the heat treatment temperature to 1,000 ° C. and the temperature increase rate to 10 ° C./min.

The battery characteristics were evaluated by the following methods for the negative electrode materials manufactured in Examples 1 to 3 and Comparative Examples 1 and 2.
First, 2 g of natural graphite was put into a φ1.4 cm hole, and a force of 0.5 t was applied to the φ1.4 cm area for 2 seconds using a press machine (that is, pressurized for 2 seconds at a pressure of 31,852 KPa). The compression height was determined by measuring the height of the hole with a micro gauge in the pressurized state.

  The compression density for the coated natural graphite produced in Examples 1 to 3 and Comparative Examples 1 and 2 was determined by the same method as above. Subsequently, the ratio of the compression density of each of the negative electrode materials manufactured in Examples 1 to 3 and Comparative Examples 1 and 2 was determined according to Formula 2 above. The ratio of the compression density to the compression density obtained is shown in Table 1 below.

100 g of the negative electrode materials produced in Examples 1 to 3 and Comparative Examples 1 and 2 were placed in a 500 ml reactor, and a small amount of N-methylpyrrolidone (NMP) and polyvinylidene fluoride (PVDF) as a binder were added. Next, the above mixture was kneaded using a mixer, and pressed and dried on a copper foil to be used as an electrode. At this time, the density after electrode crimping was uniformed to 1.65 g / cm ² . The charge / discharge efficiency of this electrode was evaluated using a coin cell.

Charging and discharging test was charged to a 0.01V at a charging current 0.5 mA / cm ² while restricting the potential range of 0 to 1.5 V, the charging current while maintaining the voltage of 0.01V 0 It continued to charge until the .02mA / cm ^2. A discharge current of 0.5 mA / cm ² was discharged up to 1.5 V. The test results are shown in Table 2 below. In Table 2 below, the charge / discharge efficiency indicates the ratio of the discharged electric capacity to the charged electric capacity.

As it can be seen from Table 2, to Examples 1 was prepared by adjusting the 0.9 or the ratio of the compression density of the negative electrode material according to the present invention 3, (efficiency of 1 ^st cycle) Initial efficiency is 93.5 or more The retention capacity (discharge capacity at 30 ^th cycle) was 95% or more. On the other hand, in Comparative Examples 1 and 2, the initial efficiencies were 90.8% and 92.1%, respectively, and the retention capacities were 83.3% and 90.9%, respectively, which were confirmed to be low.

From Table 1 and Table 2 above, there was no correlation between the compression density ratio and the initial efficiency, but it was confirmed that the smaller the compression density ratio, the more the battery efficiency and cycle performance deteriorated.
From these results, the smaller the compression density ratio, the more the surface of the natural graphite coated with the pitch breaks into the electrolyte solution by cracking the carbon layer applied and heat treated during the crimping process to match the electrode density. It is thought that it was exposed and caused a decomposition reaction.

  As described above, the present invention has been described with reference to the limited embodiments. However, the present invention is not limited thereto, and the technical idea of the present invention can be determined by those who have ordinary knowledge in the technical field to which the present invention belongs. It goes without saying that various modifications and variations are possible within the equivalent scope of the claims.

  According to the present invention, when actually applied to an electrode, the problem is that the coated carbide breaks during the crimping process, and when used as an active material for a lithium secondary battery, due to volume change in the process of repeated charge and discharge. It can prevent the decomposition reaction between natural graphite and electrolyte due to the phenomenon of cracking of the active material, improve the protection function against the reaction with the electrolyte on the negative electrode material surface, and improve the efficiency and cycle capacity of the secondary battery .

Claims (5)

In the negative electrode material for secondary batteries,
The negative electrode material is obtained by firing a low carbon material coated on a core carbon material, and the compression density ratio is 0.9 or more. Negative electrode material.   2. The negative electrode material for a secondary battery according to claim 1, wherein the carbon material of the core material is a single material selected from the group consisting of natural graphite and artificial graphite or a mixture thereof.   2. The secondary battery according to claim 1, wherein the low crystalline carbon material is a single material or a mixture of two or more selected from the group consisting of pitch, tar, phenol resin, and furan resin. Negative electrode material.   A secondary battery comprising a negative electrode made of the negative electrode material according to claim 1.   5. The secondary battery according to claim 4, wherein the secondary battery has an initial efficiency of 93.5% or more and a discharge capacity in the 30th cycle of 95% or more.