JP3033175B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a small and lightweight new secondary battery.

2. Description of the Related Art In recent years, portable electronic devices and cordless electronic devices have been rapidly developed. Accordingly, there is an increasing demand for a small, lightweight secondary battery having a high energy density, which serves as a driving power supply. From such a viewpoint, non-aqueous secondary batteries, especially lithium secondary batteries, are particularly expected to be high-voltage, high-energy-density batteries,
Development is urgent.

Conventionally, manganese dioxide, vanadium pentoxide, titanium disulfide, and the like have been used as a positive electrode active material of a lithium secondary battery. A battery was composed of these positive electrodes, a lithium negative electrode and an organic electrolyte, and charging and discharging were repeated. However, in general, in a secondary battery using lithium metal for the negative electrode, problems such as an internal short circuit due to dendritic lithium generated during charging and a side reaction between the active material and the electrolytic solution are major obstacles to the secondary battery. Further, no satisfactory material has been found in the high rate charge / discharge characteristics and the overdischarge characteristics. In recent years, the safety of lithium batteries has been strictly pointed out, and it is very difficult to ensure safety in battery systems using lithium metal or lithium alloy for the negative electrode.

On the other hand, a new type of electrode active material utilizing the intercalation reaction of a layered compound has attracted attention, and graphite intercalation compounds have been used as electrode materials for secondary batteries since ancient times.

In particular, graphite intercalation compounds incorporating anions such as ClO ₄ ⁻ , PF ₆ ⁻ and BF ₄ ⁻ ions are used as the positive electrode, while Li
Graphite intercalation compounds incorporating cations such as ⁺ and Na ⁺ are considered as negative electrodes. However, graphite intercalation compounds incorporating cations are extremely unstable, and when natural graphite or artificial graphite is used as the negative electrode, they generally lack stability as a battery and have a low capacity. Furthermore, since the decomposition of the electrolytic solution is involved, it cannot be used as a substitute for a lithium anode.

Recently, it has been found that a cation-doped body of a pseudo-graphite material obtained by carbonizing various hydrocarbons or polymer materials is effective as a negative electrode, has a relatively high utilization factor, and has excellent stability as a battery. Was. Research is being actively conducted on small and lightweight secondary batteries using the same.

On the other hand, with the use of a carbon material for the negative electrode, the positive electrode active material has a higher voltage, and LiCoO ₂ and LiMn ₂ O ₄ , which are compounds containing Li, or a part of these Co and Mn. It has been proposed to use a composite oxide substituted with another element, for example, Fe, Co, Ni, Mn, or the like.

Problems to be Solved by the Invention When a pseudo-graphite material having a certain turbostratic structure as described above was used for the negative electrode material, when the amount of occlusion and release of lithium was determined, only the capacity of 100 to 150 mAh / g carbon was obtained. It cannot be obtained, and the polarization of the carbon electrode accompanying charging and discharging increases.

Therefore, it is difficult to obtain a satisfactory capacity and voltage when combined with a positive electrode such as LiCoO ₂ . On the other hand, it has been reported that when a highly crystalline graphite material is used as a negative electrode material, gas is generated due to decomposition of an electrolytic solution on the surface of the graphite electrode during charging, and the intercalation reaction of lithium hardly proceeds. However, the coke high-temperature fired body has a relatively high capacity (200 to 250 mA) although gas is generated.
h / g). However, due to the large expansion and contraction of graphite in the C-axis direction due to charge and discharge, the molded body swells and cannot maintain its original shape.
Therefore, there is a problem in cycle characteristics. In addition, the graphite electrode has poor wettability with the electrolytic solution, so that the reaction is not uniform at the time of initial charging, and all of lithium is not intercalated, and deposition of metallic lithium is partially observed on the electrode surface. There was a problem that was.

SUMMARY OF THE INVENTION An object of the present invention is to provide a non-aqueous electrolyte secondary battery having a high voltage, a high capacity, and excellent cycle characteristics by solving the above-mentioned conventional problems.

Means for Solving the Problems In order to solve these problems, the present invention provides a graphitic material obtained by heat-treating coke on a negative electrode and a vapor-grown carbon fiber (hereinafter referred to as a carbon fiber).
Abbreviated as VGCF. The use of the composite carbon material of (1) prevents swelling and destruction of the molded article due to charge and discharge, and improves wettability of the electrode.

In general, the amount of lithium that can be chemically intercalated between graphite layers is reported to be the upper limit of the first stage graphite intercalation compound C ₆ Li in which one atom of lithium is inserted into six atoms of carbon. In this case, the active material has a capacity of 372 mAh / g. When the above-described pseudo-graphite material is used, the amount of lithium that can be intercalated is small because the layered structure of graphite is undeveloped, and the charge / discharge reaction is around 1.0 V, which is noble to metallic lithium. , And was not suitable as a negative electrode material. When a high temperature fired body of coke is used for the negative electrode, the initial 200 to 250 mAh /
It is known that the molded product has a capacity of g, but the molded product swells and the capacity deterioration accompanying the cycle becomes large. Also, the wettability with the electrolyte is poor. On the other hand, when VGCF is used for the negative electrode, almost no swelling of the molded body is observed and the wettability is good. However, since the bulk density is very small compared to other carbon materials such as coke, an electrode plate in which carbon paste is applied to a long core material as shown in the embodiment is considered. ,
The filling amount (filling density) becomes extremely small. Therefore, the capacity as a battery decreases. Thus, the present inventors have solved the above-mentioned problems by making use of the features of both, and using a composite carbon material.

In that case, the mixing ratio of the high temperature fired body of coke and VGCF is important, and the amount of VGCF added is preferably from 5% by weight to 20% by weight, more preferably from 5% by weight to 10% by weight. If it is less than 5% by weight, the effect of VGCF cannot be utilized, and the cycle characteristics will be poor. If it exceeds 20% by weight, the packing density of the carbon material in the electrode plate is reduced, and the capacity as a battery is reduced. The degree of graphitization is an important factor in both the graphite material and the VGCF used in the present invention, and it is required that the plane spacing (d002) between the 002 planes is 3.40 ° or less and 3.45 ° or less, respectively. When a carbon material having the above-mentioned plane spacing is used, the capacity is small and the polarization of the carbon electrode is large as in the case of other pseudo-graphite materials.

On the other hand, the positive electrode is Li, a compound containing lithium ions.
CoO ₂ , LiMn ₂ O ₄ , and a composite oxide obtained by substituting a part of Co or Mn with another element, for example, Co, Mn, Fe, Ni, or the like can be used. The composite oxide can be easily obtained by mixing and calcining, for example, a lithium or cobalt carbonate or oxide as a raw material according to a desired composition. Of course, the synthesis can be performed in the same manner when other raw materials are used.

Usually, the firing temperature is set between 650 ° C and 1200 ° C.

The electrolytic solution and separator are not particularly limited, and any conventionally known ones can be used.

The composite carbon material of the high-temperature coke fired body and VGCF according to the present invention makes use of the features of both.

VGCF is a carbon fiber formed by thermally decomposing a hydrocarbon in the gas phase, like carbon black and the like, and therefore has good wettability with an electrolytic solution. In addition, carbon / carbon fiber composite materials are generally used as materials having high strength and high elastic modulus when considering carbon materials as structural materials. In the present invention, this concept is applied to battery electrode materials. It was done. As a result, swelling and destruction of the electrode plate due to charge and discharge were prevented by using the composite carbon material, and the wettability of the electrode plate was further improved. Therefore, by combining with a positive electrode made of a lithium-containing composite oxide, a secondary battery having high voltage, high capacity, and excellent cycle characteristics can be obtained.

Examples Hereinafter, the present invention will be described in detail with reference to Examples.

FIG. 1 shows a longitudinal sectional view of the cylindrical battery used in this example. In the drawing, reference numeral 1 denotes a battery case formed by processing an organic electrolyte resistant stainless steel sheet, 2 denotes a sealing plate provided with a safety valve, and 3 denotes an insulating packing. Reference numeral 4 denotes an electrode plate group, in which a positive electrode and a negative electrode are wound and stored a plurality of times via a separator. A positive electrode lead 5 is drawn from the positive electrode and connected to the sealing plate 2, and a negative electrode lead 6 is drawn from the negative electrode and connected to the bottom of the battery case 1. Reference numeral 7 denotes an insulating ring provided on the upper and lower portions of the electrode group. Hereinafter, the positive and negative electrode plates, the electrolytic solution and the like will be described in detail.

For the positive electrode, Li ₂ CO ₃ and CoCO ₃ are mixed and baked at 900 ° C. for 10 hours. 100 parts by weight of LiCoO ₂ powder, 3 parts by weight of acetylene black, 4 parts by weight of graphite, a fluororesin binder 7 parts by weight were mixed and suspended in an aqueous solution of carboxymethyl cellulose to make a paste. This paste was coated on both sides of an aluminum foil having a thickness of 0.03 mm, dried and rolled to obtain an electrode plate having a thickness of 0.19 mm, a width of 40 mm and a length of 250 mm.

The negative electrode was a coke subjected to a heat treatment at 2800 ° C. (d0002 = 3.3
8Å) and VGCF heat-treated at 2200 ℃ (d002 = 3.42Å)
Was mixed at a mixing ratio as shown in Table 1, 100 parts by weight of a carbon material was mixed with 10 parts by weight of a fluororesin binder, and suspended in an aqueous solution of carboxymethyl cellulose to form a paste. . The paste is coated on both sides of a copper foil having a thickness of 0.02 mm, dried, and then rolled to obtain a thickness of 0.20 mm, a width of 40 mm, and a length of 270 mm.
Electrode plate.

Attach leads to each of the positive and negative plates,
It was wound through a polypropylene separator having a thickness of 0.025 mm, a width of 46 mm and a thickness of 700 mm, and housed in a battery case having a diameter of 12.8 mm and a height of 50 mm. For the electrolyte, use a mixed solvent of propylene carbonate and ethylene carbonate in equal volume Used at a rate of 1 mol / mol.

A charge / discharge operation was performed before sealing the battery, the generated gas was sufficiently degassed under vacuum, and the battery was sealed to obtain a test battery.

Then, these test batteries were subjected to a constant current charge / discharge test under the conditions of a charge / discharge current of 100 mAh, a charge end voltage of 4.1 V, and a discharge end voltage of 3.0 V. FIG. 2 shows a comparison of the cycle characteristics. In the battery 1 not using VGCF, the initial capacity is as large as 500 mAh or more, but the capacity is significantly deteriorated due to the cycle. on the other hand,
It can be seen that in the batteries 2 and 3 in which VGCF was mixed at 5% by weight or 10% by weight, the capacity deterioration due to the cycle was extremely small while maintaining the high capacity. 25% by weight of VGCF and
In the batteries 4 and 5 of 100% by weight, the cycle characteristics are good, but the capacity is extremely small. This is due to the fact that VGCF became dominant and the mixture loading was reduced. The average discharge voltage was about 3.7 V in each case.

After sealing test batteries 1 to 5 configured under the same conditions, 1
The test was stopped after the completion of charging in the cycle, the battery was disassembled, and the negative electrode plate was observed. As a result, in the battery 1, the electrode plate and the electrolyte solution were insufficiently wet, the central portion was not wet at all, and there was an unreacted portion, and slight precipitation of lithium metal was observed around the periphery. In the batteries 2 to 5, the electrode plates were sufficiently wet and reacted uniformly, and no noticeable change such as deposition of lithium was observed.

Comparative Example 1 The same procedure was performed as in the Example except that a composite carbon material mixed with 5% by weight of a commercially available acetylene black (d002 = 3.48%) was used in place of the VGCF as the negative electrode material. The battery of Example 1 was used.

Comparative Example 2 In Example, the heat treatment temperature of VGCF was 1200 ° C. (d002 =
3.55%), and a battery of Comparative Example 2 was made under the same conditions as in Example except that 5% by weight was mixed.

The batteries of Comparative Examples 1 and 2 were subjected to a charge / discharge test under the same conditions as the examples. In each case, the wettability of the electrode plate was good, but the capacity was as small as 400 mAh or less, and the average discharge voltage was as low as 3.5 V. This is acetylene black and
This is because the degree of graphitization of VGCF (1200 ° C treated product) is insufficient.

Effect of the Invention As is apparent from the above description, the nonaqueous electrolyte secondary battery according to the present invention using the composite carbon material composed of the coke high-temperature fired body and the vapor-grown carbon fiber for the negative electrode has a high performance. There is an effect that a nonaqueous electrolyte secondary battery having a voltage, a high capacity, and excellent cycle characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a cylindrical battery according to an embodiment of the present invention, and FIG. 2 is a diagram showing a comparison of cycle characteristics. 1 ... battery case, 2 ... sealing plate, 3 ... insulating packing, 4 ... electrode plate group, 5 ... positive electrode lead, 6 ... negative electrode lead
C, 7 ... An insulating ring.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-129664 (JP, A) JP-A-1-311565 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/58 H01M 10/40

Claims (3)

(57) [Claims] 1. A positive electrode comprising a lithium-containing composite oxide,
A non-aqueous electrolyte and a non-aqueous electrolyte secondary battery including a rechargeable negative electrode; wherein the negative electrode is a composite carbon material including a graphite material obtained by heat-treating coke and a vapor-grown carbon fiber; In addition, the graphite material has a surface spacing (d00
2) is 3.40Å or less, and the vapor-grown carbon fiber is
A nonaqueous electrolyte secondary battery characterized in that (d002) is 3.45 mm or less. 2. The non-aqueous electrolyte solution according to claim 1, wherein a mixing ratio of the vapor growth type carbon fiber in said composite carbon material is not more than 20% by weight with respect to said graphite material. Next battery. 3. The positive electrode is at least one selected from the group consisting of LiCoO ₂ , LiMn ₂ O ₄ , and a composite oxide obtained by substituting a part of Co and Mn with another element. 2. The non-aqueous electrolyte secondary battery according to claim 1.