JP2002110155A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control capacity deterioration in a nonaqueous electrolyte secondary battery composed of lithium manganese composite oxide for a positive pole and carbon material for a negative pole, which is caused when it is full- charged and left under high temperature circumstance. SOLUTION: This nonaqueous electrolyte secondary battery comprises the positive pole composed of lithium manganese composite oxide with spinel structure, a nonaqueous electrolyte and the negative pole composed of carbon material which is enabled to electrochemically dope and dedope lithium. In this case, carbon material is low crystal carbon in which lattice spacing (d002) of 002 plane is >=0.360 nm and <=0.390 nm, and capacity density of the negative pole in full-charged is >=150 Ah/kg and <=250 Ah/kg. It enables obtaining the nonaqueous electrolyte secondary battery which is superior in high temperature stability.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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 lithium ion secondary battery having manganese oxide as a positive electrode.

[0002]

2. Description of the Related Art In recent years, lithium-ion secondary batteries have been put into practical use as power sources for driving portable electronic devices such as mobile phones, notebook computers, and video camcorders as secondary batteries having a high operating voltage and a high energy density. The production volume has continued to increase as a battery system leading small secondary batteries. However, most of the positive electrode materials of these lithium ion secondary batteries use a composite oxide of lithium and cobalt (LiCoO ₂ ). LiCoO ₂ cathode is a high-performance cathode material with high voltage, high energy density, and excellent high-temperature stability and cycle life characteristics. However, cobalt is scarce in resources and expensive because of limited production areas. I am worried about stability.

Recently, large-sized lithium ion secondary batteries for power storage and electric vehicles have been developed, and a lithium manganese composite oxide having a spinel structure which is cheaper and has abundant resources as a cathode material. (LiMn
₂ O ₄ ) is promising. LiMn ₂ O ₄ has been known for a long time as a material exhibiting 4V-class discharge (Japanese Patent Publication No. 58-58).
Japanese Patent Application Laid-Open No. 34414/1989), which has been partially put into practical use, but has a cycle life characteristic and high-temperature stability in which LiCoO _{2 is used.}
It is inferior to the positive electrode, and various efforts have been made to improve their performance. For example, attempts have been made to stabilize the crystal structure during charge and discharge by replacing some of the manganese atoms with other transition metal elements such as cobalt, chromium, and nickel. Although improved, high temperature stability has not yet been sufficiently improved.

Although the mechanism of capacity deterioration of a battery at such a high temperature has not been completely elucidated, it is said that one of the causes is that the dissolution of manganese from a positive electrode active material at a high temperature causes capacity deterioration. . Therefore, it is disclosed in Japanese Patent Application Laid-Open No. HEI 11-112400 that the control of the production conditions and physical properties of LiMn ₂ O ₄ and the optimal combination of the composition of the electrolytic solution suppress the amount of manganese dissolved.
-297322, and at high temperature, Li
Japanese Patent Application Laid-Open No. 2000-58134 discloses a method of using a battery that suppresses capacity deterioration by performing charging and discharging while maintaining the voltage of the positive electrode of Mn ₂ O ₄ at a high voltage portion.

[0005]

On the other hand, as a negative electrode material, a carbon material is the most common, and its type and physical properties have been studied in detail. It is roughly classified into a case where a graphite material utilizing a reaction is used and a case where a low-crystalline carbon material having almost no graphite layer structure is used. Are different in the degree of side reaction between the electrolyte and lithium.

In a carbon material having a developed layer structure such as graphite, lithium is intercalated between graphite layers by an intercalation reaction, and lithium is stored in an ion state in an extremely anisotropic state called a stage structure. ing. In such a system, in the lithium manganese composite oxide positive electrode system, the side reaction between the electrolyte and lithium selectively proceeds at the edge portion of the graphite crystal where lithium enters and exits under a high temperature environment, and the capacity deterioration of the battery increases. .

On the other hand, when low-crystalline carbon is used for the negative electrode, the proportion of lithium stored in the void portion of the carbon crystal structure is much larger than that stored in the interlayer due to the intercalation reaction. Is stored uniformly in the ion state isotropically, so that side reactions do not proceed selectively even in a high temperature environment.

However, the concentration of lithium in the negative electrode, that is, the negative electrode capacity density is important for this side reaction, and the higher the concentration of lithium, the stronger the interaction between lithium and lithium in carbon. In other words, a portion of the charge that is shallow with respect to the metallic lithium counter electrode shows a very noble potential, but a portion of the charge that is deeply charged exhibits a very low potential, which is extremely close to the potential of metallic lithium. , The decomposition of the electrolyte solution is promoted, and a side reaction is likely to occur.

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above-described problems, and minimizes capacity deterioration even when a battery is fully charged or discharged in a high-temperature environment or left unattended. It is an object of the present invention to provide a non-aqueous electrolyte secondary battery excellent in stability and high-temperature stability.

[0010]

In order to achieve the above object, the present invention provides a positive electrode comprising a lithium manganese composite oxide having a spinel structure, a non-aqueous electrolyte, and a carbon material capable of occluding and releasing lithium. Wherein the carbon material has a lattice spacing (d002) of 002 planes of 0.360 nm or more and 0.390 nm or more.
nm or less and the capacity density of the negative electrode when fully charged is 150 Ah / kg or more and 250 Ah / kg or less.
A non-aqueous electrolyte secondary battery is characterized by the following.

The inventors of the present invention have analyzed the capacity degradation mechanism at high temperature of a nonaqueous electrolyte secondary battery comprising the above-mentioned lithium manganese composite oxide cathode and a carbon material anode. Although manganese dissolution occurs,
The capacity deterioration of the positive electrode active material itself is not so large. Rather, the lithium stored in the negative electrode carbon material under the influence of the manganese-based positive electrode or dissolved manganese causes a side reaction in a high-temperature environment and is an inactive lithium compound. And it is found that capacity deterioration is dominated by being unable to participate in the charge / discharge reaction.

According to the present invention, the battery can be fully charged / discharged in a high-temperature environment, or even when left for a long period of time, the capacity degradation can be minimized, and long-term durability,
A non-aqueous electrolyte secondary battery having excellent high-temperature stability and enduring practical use can be obtained.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 of the present invention comprises a positive electrode comprising a lithium manganese composite oxide having a spinel structure, a non-aqueous electrolyte, and a carbon material capable of occluding and releasing lithium. A non-aqueous electrolyte secondary battery comprising a negative electrode, wherein the carbon material is low-crystalline carbon having a lattice spacing (d002) of 002 planes of 0.360 nm or more and 0.390 nm or less, and is fully charged. A non-aqueous electrolyte secondary battery characterized in that the capacity density of the negative electrode at that time is 150 Ah / kg or more and 250 Ah / kg or less.

The negative electrode carbon material used in the non-aqueous electrolyte secondary battery of the present invention has a low degree of graphitization, and the value of the 002 plane distance (d002) is 0.360 nm or more and 0.39 nm or more.
0 nm or less. The value of (d002) can be easily checked by X-ray diffraction measurement of the carbon material powder.
When α-ray is used as a target, a 002 diffraction line is obtained when 2θ is around 23 ° to 27 °. A more precise value can be obtained by adding high-purity silicon powder as an internal standard sample and correcting the angle. If the value of (d002) is less than 0.360 nm, the graphite layer structure develops somewhat, so that the capacity degradation under a high temperature environment becomes large. Conversely, a carbon material exceeding 0.390 nm has not yet developed carbonization, has a large amount of impurity components remaining, and has a property of causing a decrease in capacity and a decrease in high-temperature stability. When such a carbon material is used, the capacity density of the negative electrode at the time of full charge greatly affects the capacity deterioration, and is 150 Ah / kg or more and 250 Ah / k or more.
g. At less than 150 Ah / kg,
It is clear that the battery capacity is greatly reduced even if high-temperature stability can be ensured, and the features of the lithium ion secondary battery cannot be utilized. On the other hand, if it exceeds 250 Ah / kg, the lithium concentration in the negative electrode increases, the state of lithium approaches the metallic state, and when left in a high-temperature environment, a side reaction with the electrolytic solution easily occurs and the capacity deterioration is large. Become. The calculation of the negative electrode capacity density is a value obtained by dividing the discharge capacity of the battery by the weight of the carbon material in the area of the negative electrode facing the positive electrode.

The true density of the negative electrode carbon material used in the nonaqueous electrolyte secondary battery of the present invention is usually 2.2 if it is a graphite material.
Although the density is as high as about g / cm ³ , it is a low-crystalline carbon in which the crystal structure is hardly developed, so that the true density may be 1.5 g / cm ³ or more and 1.8 g / cm ³ or less. preferable. The value of (d002) is 0.360 nm
The thickness is required to be not less than 0.390 nm and preferably not more than 0.370 nm and not more than 0.385 nm, in particular, the effects of the present invention can be obtained. In addition, as the type of carbon material, its raw material, in the manufacturing method, the physical properties are greatly different, and even in low crystalline carbon, the graphitization easily proceeds by performing heat treatment at high temperature and easily graphitizable carbon. Although it is roughly classified into two types of non-graphitizable carbon, the graphitization of which does not progress so much even when subjected to a high-temperature heat treatment, non-graphitizable carbon is particularly effective in the present invention.

[0016]

The present invention will be described below in detail with reference to examples.

(Example 1) FIG. 1 shows a cutaway perspective view of a cylindrical battery used in this example. In FIG. 1, reference numeral 1 denotes a negative electrode plate to which a negative electrode lead plate 2 is attached, and 3 denotes a positive electrode plate to which a positive electrode lead plate 4 is attached. The negative electrode plate 1 and the positive electrode plate 3 are housed in a battery case 7 serving also as a negative electrode terminal, with an electrode plate group spirally wound via a separator 5 with insulating plates 6 arranged above and below the electrode plate group. The upper edge of the battery case 7 is hermetically sealed via an insulating packing 8 with a sealing plate 9 also serving as a positive electrode terminal provided with a safety valve. Hereinafter, a method for manufacturing the positive and negative electrode plates will be described in detail.

The positive electrode active material includes electrolytic manganese dioxide (Mn)
O ₂ ) and lithium carbonate (Li ₂ CO ₃ ) were mixed at a Li / Mn molar ratio of 0.54, and then 850 ° C.
Lithium manganese composite oxide was synthesized by heat treatment. The crystal structure of the obtained oxide was confirmed to be a spinel-type structure by powder X-ray diffraction, and was then pulverized and classified to obtain a positive electrode active material powder having an average particle size of about 15 μm. To 100 parts by weight of the active material, 3 parts by weight of acetylene black as a conductive material was added, and to this mixture, 40 parts by weight of N-methylpyrrolidone (NMP) and 4 parts by weight of polyvinylidene fluoride (PVdF) as a binder were added. The dissolved solution was kneaded to form a paste. Next, this paste was applied to both sides of an aluminum foil, dried, and then rolled to a thickness of 0.1 mm.
A positive electrode plate having a size of 20 mm, a width of 37 mm, and a length of 350 mm was used.

For the negative electrode, a carbon material obtained by performing carbonization at various heat treatment temperatures using an anisotropic pitch or an isotropic pitch as a raw material was used. Each carbon material was pulverized and classified to obtain a powder having an average particle size of 10 μm to 15 μm. When anisotropic pitch is used as the raw material, it is easily graphitizable carbon that can be easily graphitized by heat treatment at high temperature. When isotropic pitch is used as the raw material, graphitization proceeds even by high temperature heat treatment. It is difficult to graphitize and is classified as non-graphitizable carbon. Table 1 shows the physical properties of the carbon materials used and batteries using these carbon anodes. The production of the negative electrode plate was substantially the same as the production of the positive electrode plate, and a solution in which 8 parts by weight of PVdF as a binder was dissolved in 40 parts by weight of NMP and 100 parts by weight of each carbon powder was kneaded to form a paste. Next, this paste is coated on both sides of the copper foil, dried, and rolled to a thickness of 0.13 to
The negative electrode plate was 0.16 mm, width 39 mm, and length 420 mm.

Since the negative electrode mixture density, irreversible capacity, and reversible capacity after rolling differ depending on the type of carbon material used, full charge is performed by changing the thickness of the negative electrode plate in consideration of the capacity balance between the positive and negative electrodes. The battery was designed such that the capacity density of the negative electrode at that time was 200 Ah / kg in each of the batteries.

Then, leads were attached to the positive and negative plates, respectively, to have a thickness of 0.025 mm, a width of 45 mm, and a length of about 100 mm.
It is spirally wound through a separator made of a microporous membrane made of 0 mm polyethylene and has a diameter of 17 mm and a height of 50 m.
m in a battery case 7.

The electrolyte is ethylene carbonate (EC)
A solution obtained by dissolving 1 mol / l of LiPF ₆ as an electrolyte in a solvent in which dimethyl carbonate (EMC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 30:30:40 was injected. Then, the opening of the battery case was sealed with a sealing plate to obtain a completed battery. Each of these batteries was prepared for 5 cells and charged and discharged. The charge current and the discharge current were both set to 100 mA, and a constant current charge / discharge at a charge end voltage of 4.3 V and a discharge end voltage of 2.5 V was performed for 10 cycles in an environment of 25 ° C., and the discharge capacity at the 9th cycle was defined as the initial capacity.
Then, in a fully charged state, all cells are exposed to
Left for 0 days. Thereafter, the battery was returned to an environment of 25 ° C. and charged and discharged for 5 cycles. Capacity recovery rate (%) for each battery =
(Recovery capacity) / (initial capacity) × 100 was determined. The values shown in Table 1 indicate the average values of the capacity recovery rates of the five cells.

[0023]

[Table 1]

From Table 1, it can be seen that the capacity recovery rate at high temperature was high because the low crystallinity carbon material having a 002 plane spacing of 0.360 nm or more and a true density of 1.8 g / cm ³ or less was used for the negative electrode. It can be seen that in the batteries G and H, which are batteries and use a material with advanced graphitization, the capacity recovery rate is extremely reduced. Also, among the low-crystalline carbon anodes, Battery A, which has an extremely low degree of carbonization, tends to have poor high-temperature stability, and (d)
The battery C, the battery D, and the battery E whose values of 002) are in the range of 0.370 nm or more and 0.385 nm or less have a capacity recovery rate of 90%.
%, Which is the most preferable negative electrode material.

(Example 2) In Example 1, the batteries I to O in which the negative electrode capacity density at full charge was changed using the negative electrode carbon material used in the battery C having the highest temperature stability were used. Produced. The method for producing the positive electrode plate, the negative electrode plate, and the method for producing the battery were all performed in the same manner as in Example 1. Only in the production of the negative electrode plate, the coating weight when the negative electrode mixture paste was applied to the copper foil, the negative electrode The capacity of the negative electrode at full charge was changed by changing the thickness of the sheet after rolling. Then, the same charge and discharge as in Example 1 were performed, and a high-temperature storage test in a 60 ° C. environment was similarly performed. Table 2 shows the relationship between the negative electrode capacity density and the capacity recovery rate of each battery.

[0026]

[Table 2]

It is apparent from Table 2 that the capacity recovery rate greatly depends on the negative electrode capacity density.
The capacity recovery rate exceeding 90% is maintained up to 0 Ah / kg, and the capacity recovery rate further decreases significantly as the negative electrode capacity density increases. This is due to the fact that the lithium concentration in the negative electrode carbon increases and the interaction between lithium becomes stronger and approaches the metallic state, which tends to cause a side reaction with the electrolyte under a high temperature environment. is there. If the negative electrode capacity density is 250 Ah / kg or less at the time of full charge, such a side reaction is minimized and good capacity recovery characteristics are exhibited.

In Example 2, since no difference was found in the battery capacity because the same positive electrode plate was used for all the batteries, especially when low crystalline carbon was used for the negative electrode, 150 Ah / In charging up to about kg, the negative electrode potential is at a very noble potential, so that the potential behavior of the positive electrode changes, and under the same charge / discharge conditions, the battery capacity is significantly reduced. From these facts, the negative electrode capacity density is 150 A
It is desirable to set it to h / kg or more and 250 Ah / kg or less.

In Examples 1 and 2, a mixed solvent of EC, EMC, and DMC was used as a solvent for the electrolytic solution. However, other known carbonate-based solvents such as propylene carbonate and the like, and a 4V-class oxidation-resistant solvent were used. A solvent having a reduction potential can be used alone or as a mixed solvent. Similarly, conventionally known electrolytes such as LiBF ₄ and LiClO ₄ can be used as the electrolyte.

In the first and second embodiments, a small cylindrical battery has been described. However, as for the battery shape, a rectangular battery in which electrodes are wound in an elliptical shape and accommodated in a rectangular battery case, or a thin electrode is used. A similar effect can be obtained by using a prismatic battery in which a plurality of are stacked and stored in a prismatic battery case. Regarding the battery size, it is needless to say that the same effects can be obtained not only for small batteries assumed for small electronic devices, but also for large batteries assumed for power storage, electric vehicles, and hybrid electric vehicles. The same applies to module batteries and battery packs in which a plurality of are mounted.

[0031]

As described above, according to the present invention, a nonaqueous electrolyte secondary battery excellent in high-temperature stability can be obtained in a system using a lithium manganese composite oxide as a positive electrode and a carbon material as a negative electrode. .

[Brief description of the drawings]

FIG. 1 is a cutaway perspective view of a cylindrical battery used in the present embodiment.

[Explanation of symbols]

 Reference Signs List 1 negative electrode plate 2 negative electrode lead plate 3 positive electrode plate 4 positive electrode lead plate 5 separator 6 insulating plate 7 battery case 8 insulating packing 9 sealing plate

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Keisuke Omori 1006 Kadoma Kadoma, Kadoma City, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. (72) Takashi Fujii 1006 Kadoma Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. Terms (reference) 5H029 AJ04 AK03 AL06 AL07 AM03 AM05 AM07 BJ02 BJ14 DJ17 HJ08 HJ13 HJ19 5H050 AA10 BA17 CA09 CB07 CB08 DA03 FA19 HA08 HA13 HA19

Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode made of a lithium manganese composite oxide having a spinel structure, a non-aqueous electrolyte, and a negative electrode made of a carbon material capable of occluding and releasing lithium. The carbon material has a lattice spacing (d002) of 002 planes of 0.360 nm or more and 0.390.
nm or less and the capacity density of the negative electrode when fully charged is 150 Ah / kg or more and 250 Ah / kg or less.
A nonaqueous electrolyte secondary battery characterized by the following. 2. The true density of the carbon material is 1.5 g / cm.
³ or more and 1.8 g / cm ³ or less and (d002) is 0.3
The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte secondary battery is a non-graphitizable carbon material having a thickness of 70 nm or more and 0.385 nm or less.