JP2001216962A - Megative electrode for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode capable of constituting a lithium secondary battery having high energy density and superior durability, by solving the problems on a lithium secondary battery using a carbon material as a negative electrode active material, which are retention, cycling deterioration and capacity decreases during high-temperature storage. SOLUTION: The carbon material as a main active material and a lithium- titanium combined oxide represented by a composition formula LixTiyO4 (0.5 <=x<=3, 1<=y<=2.5), as a subsidiary active material are used as active materials to form the negative electrode for the lithium secondary battery. Namely, the lithium-titanium oxide is subsidiarily added to the carbon material to form the negative electrode active material which is used to form the negative electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for storing and storing lithium.
The present invention relates to an electrode that can be used for a lithium secondary battery utilizing a desorption phenomenon.

[0002]

2. Description of the Related Art In the field of communication equipment and information-related equipment, lithium secondary batteries have already been put to practical use and spread widely because of the high energy density accompanying the miniaturization of mobile phones and notebook personal computers. Has reached. On the other hand, in the field of automobiles, early commercialization of electric vehicles is desired due to environmental problems such as air pollution and an increase in carbon dioxide, and the use of lithium secondary batteries as power sources for electric vehicles has been studied. I have.

At present, lithium ion secondary batteries using a carbon material as a negative electrode active material are mainly used for lithium secondary batteries for safety reasons such as no dendrite deposition on the negative electrode surface. However, in a lithium secondary battery using a carbon material as a negative electrode active material, a problem of retention (irreversible capacity) occurs in which the carbon material is trapped in the initial discharge and does not contribute to the subsequent battery reaction, and the discharge capacity decreases. Is one of the causes. Further, the oxidation / reduction potential of the carbon material is as low as 0.1 V with respect to the potential of Li / Li ⁺ , so that the nonaqueous electrolyte solution decomposes on the negative electrode surface (active material surface), reacts with lithium, reacts with lithium, and reacts with lithium. For example, a lithium compound is generated during the charging and discharging operation, and lithium that contributes to charging and discharging is deactivated as charging and discharging are repeated, thereby causing a problem of cycle deterioration such that the discharge capacity is reduced. In a high-temperature environment, a battery reaction is activated. Therefore, when a lithium secondary battery is stored at a high temperature, a large amount of lithium is deactivated and the capacity is reduced. As disclosed in Japanese Patent Application Laid-Open Publication No. H10-163, the use of a lithium-titanium composite oxide as a negative electrode active material that is structurally stable and has good cycle characteristics is also being studied. Further, as disclosed in JP-A-10-069922, a lithium-titanium composite oxide is used as a main active material, and an active material having a lower oxidation / reduction potential is added in an auxiliary manner, to prevent overcharge and overcharge. Negative electrodes with improved discharge characteristics are also being studied.

[0004] In the lithium secondary battery constructed using the lithium-titanium composite oxide as the main negative electrode active material, the oxidation / reduction potential of the negative electrode is relatively high at 1.5 V with respect to the potential of Li / Li ^+. In addition, the decomposition of the electrolyte is unlikely to occur, and the stability of the crystal structure results in a lithium secondary battery having good cycle characteristics. However, the lithium-titanium composite oxide has a problem that its capacity per unit weight is as small as about の that of a carbon material, and only a lithium secondary battery having a low energy density can be constituted.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the problems of retention, cycle deterioration, and capacity reduction in high-temperature storage of a lithium secondary battery using a carbon material as a negative electrode active material. In order to utilize the advantages of the above-mentioned lithium-titanium composite oxide, a lithium secondary battery having a high energy density and excellent durability is obtained by adding a lithium-titanium composite oxide to a carbon material as an active material. It is an object of the present invention to provide a negative electrode which can constitute a negative electrode.

[0006]

Means for Solving the Problems The negative electrode for a lithium secondary battery of the present invention comprises a carbon material as a main active material and a composition formula Li _x Ti _y O ₄ (0.5 ≦) as an auxiliary active material. x
≦ 3, 1 ≦ y ≦ 2.5) as an active material. That is, the negative electrode of the present invention is one in which lithium titanium oxide is supplementarily added to a carbon material to form a negative electrode active material, and a negative electrode is formed using the negative electrode active material.

[0007] The negative electrode of the present invention has an advantage that the energy density can be kept high because a carbon material is used as a main active material. As described above, the lithium-titanium composite oxide has a relatively stable crystal structure and a relatively high oxidation / reduction potential of the negative electrode of 1.5 V with respect to the potential of Li / Li ⁺ , By adding this, the oxidation / reduction potential of the entire negative electrode can be increased to some extent, and cycle deterioration due to deactivation of lithium due to decomposition of the nonaqueous electrolyte due to charge / discharge can be suppressed.

Further, lithium is trapped in the carbon material to generate an irreversible capacity, and the lithium titanium composite oxide has a smaller irreversible capacity than the carbon material. Therefore, the addition of the lithium-titanium composite oxide acts to reduce the irreversible capacity of the entire negative electrode,
It is possible to efficiently alleviate the retention due to the initial charge / discharge and the capacity reduction accompanying the subsequent cycle progress. further,
At high temperatures, the battery reaction is activated, so that the capacity is significantly reduced due to the deactivation of lithium. Therefore, the high-temperature cycle characteristics and the high-temperature storage characteristics are sufficiently improved.

Due to such an effect, the negative electrode for a lithium secondary battery according to the present invention, in which a lithium-titanium composite oxide is added to a carbon material as an active material, has a large discharge capacity,
That is, the negative electrode has a high energy density, a good cycle characteristic, a good high-temperature storage characteristic, and a long life lithium secondary battery having excellent durability.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of a negative electrode for a lithium secondary battery according to the present invention will be described with reference to a carbon material serving as a main active material, a lithium-titanium composite oxide serving as an auxiliary active material, and a structure of a negative electrode. And the production and the lithium secondary battery using the negative electrode of the present invention will be described in detail in this order.

<Carbon Material> The carbon material serving as the main active material in the negative electrode for a lithium secondary battery of the present invention is not particularly limited as long as it can absorb and desorb lithium. Examples of carbon materials that can be used include natural graphite, spherical or fibrous artificial graphite, non-graphitizable carbon, and organic compound fired substances such as phenolic resins, and powdered substances of easily graphitizable carbon such as coke. Can be mentioned. These carbon materials have their respective advantages, and may be selected according to the characteristics of the lithium secondary battery to be manufactured. One type can be used alone, or two or more types can be used in combination.

Of these, natural and artificial graphites have a high true density and are excellent in conductivity, so that they constitute a lithium secondary battery having a large capacity (high energy density), good power characteristics and good rate characteristics. There is an advantage that you can. When producing a lithium secondary battery taking advantage of this advantage, it is desirable that the graphite used has high crystallinity,
The plane distance d ₀₀₂ of the (002) plane is 3.4 ° or less, and c
It is preferable to use one having an axial crystallite thickness Lc of 1000 ° or more. Note that artificial graphite can be produced by, for example, heat-treating graphitizable carbon at a high temperature of 2800 ° C. or higher. In this case, the easily graphitizable carbon as a raw material includes coke and optically anisotropic small spheres (mesocarbon microbeads: MCMB) obtained in the process of heating pitches at about 400 ° C. .

Particularly, the graphitized MCMB has a spherical shape and a small specific surface area, so that the decomposition of the non-aqueous electrolyte can be minimized and the packing density in the negative electrode can be improved. It has the advantage that. Furthermore, the crystallites are oriented in a lamellar shape in the spherical particles, and the crystallite end faces are exposed on the entire surface of the particles, so that lithium can be absorbed and desorbed smoothly and is suitable for large current discharge applications. Thus, it becomes an active material that can constitute a lithium secondary battery having more excellent power characteristics and rate characteristics. In consideration of these points, it is more desirable to use the graphitized MCMB as the main active material.

[0014] Graphitizable carbon is generally obtained from tar pitch obtained from petroleum or coal, and includes coke,
MCMB, mesophase pitch-based carbon fiber, pyrolytic vapor growth carbon fiber, and the like. An organic compound fired body such as a phenol resin can also be used. Since graphitizable carbon is an inexpensive carbon material, it can be an active material that can form a lithium secondary battery that is excellent in cost. Among them, coke has the advantages of low cost and relatively large capacity, and considering this point,
It is desirable to use coke as the main active material. When coke is used, the plane distance d ₀₀₂ of the (002) plane is 3.4 ° or more, and the crystallite thickness L in the c-axis direction is
It is preferable to use those having c of 30 ° or less.

[0015] The non-graphitizable carbon is a so-called hard carbon, and is a carbon material having a structure close to amorphous such as glassy carbon. Generally, it is a material obtained by carbonizing a thermosetting resin, and does not develop a graphite structure even when the heat treatment temperature is increased. Graphitic non-graphitizable carbon has the advantages of high safety and relatively low cost. In view of this, it is desirable to use non-graphitizable carbon as the main active material. In particular,
For example, a phenol resin fired body, a polyacrylonitrile-based carbon fiber, pseudo isotropic carbon, a furfuryl alcohol resin fired body, or the like can be used. More preferably, (0
02) It is preferable to use one having a plane spacing d ₀₀₂ of 3.6 ° or more and a crystallite thickness Lc in the c-axis direction of 100 ° or less.

<Lithium-titanium composite oxide> In the negative electrode for a lithium secondary battery of the present invention, the lithium-titanium composite oxide (hereinafter referred to as “the lithium-titanium composite oxide”) as an auxiliary active material has a composition Formula Li _x Ti _y O
₄ A lithium titanium composite oxide represented by (0.5 ≦ x ≦ 3, 1 ≦ y ≦ 2.5).

The lithium titanium composite oxide is CuKα
According to powder X-ray diffraction using X-rays, the interplanar spacing in the crystal structure was at least 4.84 °, 2.53 °, 2.09 °,
It is preferable to use a diffraction surface (reflection surface) having a diffraction peak at 1.48 ° (± 0.1 ° between the surfaces). This crystal has a spinel structure or a structure derived from it. The lithium-titanium composite oxide having this crystal structure has a stable crystal structure. There is no change in volume due to desorption, and the addition of this can effectively prevent the electrode from peeling off due to expansion and contraction of the negative electrode. In addition, since the potential is stable at around 1.5 V with respect to Li / Li ⁺ , the potential does not suddenly change. From this point, a lithium secondary battery having better cycle characteristics can be obtained. Suitable for configurable auxiliary active material.

Specifically, the composition formula Li_0.8Ti_2.2O_Four,
Li_2.67Ti_1.33O_Four, LiTi_TwoO_Four, Li_1.33Ti
_1.67O_Four, Li_1.14Ti_1.71O_FourWhat is represented by
One or more of these may be used alone or in combination.
It is desirable to use a mixture of the above. inside that
Also Li_0.8Ti_2.2O_Four, LiTi_TwoO_Four, Li_1.33Ti
_1.67O_FourHas a spinel structure and more stable crystal structure
are doing. Furthermore, the volume change due to charge and discharge is small and
In terms of the most stable crystal structure, the composition
Formula Li_1.33Ti_1.67O_FourIt is possible to use what is represented by
More desirable. By the way, the composition formula Li_0.8Ti_2.2O_Four,
Li_2.67Ti_1.33O_Four, Li_1.33Ti_1.67O_Four, Li_1.14
Ti_1.71O_FourIs the composition formula Li_FourTi₁₁O₂₀, Li
_TwoTiO_Three, Li _FourTi_FiveO₁₂, Li_TwoTi_ThreeO₇To represent
Can also.

Depending on the composition of the lithium-titanium composite oxide, there is an oxide such as LiTi ₂ O ₄ which can proceed in both the direction of occluding Li and the direction of desorbing Li. . In the case of adding such a substance that progresses in both directions, the lithium-titanium composite oxide is desorbed even when irreversible capacity occurs due to, for example, trapping of lithium in the carbon material that is the main active material. Releasable Li can be released into the battery system to compensate for this irreversible capacity. Therefore, in the negative electrode of the present invention, by adding the lithium-titanium composite oxide having a composition that proceeds in both the occlusion and desorption directions, Li
Can be suppressed from being reduced due to the deactivation of.

The production method of the present lithium-titanium composite oxide is not particularly limited, but a lithium compound serving as a lithium source and titanium oxide serving as a titanium source are mixed, and the mixture is easily synthesized by firing. be able to. As the lithium compound, Li ₂ CO ₃ , Li
(OH) or the like can be used. The firing is performed in an oxygen stream or in the atmosphere. The mixing ratio of each raw material may be a ratio according to the composition of the lithium titanium composite oxide to be synthesized. If the temperature is too low, the content of the titanium oxide phase (TiO ₂ phase) generated as an auxiliary phase increases, and the firing temperature is 700 to 16
Preferably, the temperature is set to 00 ° C. In consideration of firing efficiency such as fuel efficiency, the temperature is more preferably set to 800 to 1100 ° C.

It is difficult to completely eliminate the titanium oxide phase generated as a sub phase. Since this titanium oxide phase is generated in a mixed crystal state with the main phase of the lithium-titanium composite oxide, if present in a small amount, the charge / discharge characteristics as an active material and the cycle characteristics are extremely deteriorated. Not be. Therefore, the present lithium-titanium composite oxide may contain this titanium oxide in a mixed crystal state, and in the present specification, “lithium-titanium composite oxide” means containing the same. . The present lithium-titanium composite oxide includes various lithium-titanium composite oxides depending on the composition, and one of them may be used alone, or two or more thereof may be used in combination.

<Structure and Production of Electrode> The negative electrode for a lithium secondary battery of the present invention comprises the above-mentioned carbon material as a main active material and the above-mentioned lithium-titanium composite oxide as an auxiliary active material. As That is, both are mixed to form an active material.

The abundance ratio of the carbon material as the active material and the lithium-titanium composite oxide in the negative electrode determines the characteristics of the lithium secondary battery using the negative electrode of the present invention. In the negative electrode for a lithium secondary battery of the present invention, since the lithium-titanium composite oxide is used in an auxiliary manner, its existence ratio is somewhat smaller than that of the carbon material. More specifically, it is desirable that the ratio of the carbon material and the lithium-titanium composite oxide in the negative electrode be 99: 1 to 94: 6 by weight. In other words, when the total of the carbon material and the lithium-titanium composite oxide is 100 wt%, it is desirable to mix the lithium-titanium composite oxide at a ratio of 1 wt% to 6 wt%.

When the amount of the lithium-titanium composite oxide is less than the above range, the effect of the addition is not remarkable. Conversely, when the amount of lithium-titanium composite oxide is too large beyond the above range, the capacity of the lithium-titanium composite oxide is small, so that the energy density of the negative electrode is too low. Also, if the potential of the negative electrode itself rises too much and it is attempted to discharge the same capacity,
This is because the potential of the facing positive electrode is increased, and excess lithium is desorbed from the positive electrode, which may cause the crystal structure of the positive electrode active material to collapse and cause a reduction in discharge capacity due to the positive electrode. .

The negative electrode for a lithium secondary battery according to the present invention may follow the configuration of a generally used negative electrode using only a carbon material as an active material, except for an active material. I just need to follow. For example, first, the carbon material and the lithium-titanium composite oxide are mixed at a desired ratio, and further, a binder for binding them is mixed, and if necessary, an appropriate amount of a solvent (dispersion medium) is added. By adding and kneading the mixture sufficiently, a paste-like negative electrode mixture is prepared. Next, this negative electrode mixture is applied to the surface of a current collector made of a metal foil such as copper, dried, and then, if necessary, the density of the active material in the negative electrode is increased by pressing or the like to produce a sheet-shaped negative electrode. be able to.

The binder is not particularly limited.
A known device may be used. For example, fluorine-containing resins such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, and thermoplastic resins such as polypropylene and polyethylene can be used. Also, in the solvent,
An organic solvent such as N-methyl-2-pyrrolidone can be used. The prepared sheet-shaped negative electrode may be cut into a size corresponding to the specification of the lithium secondary battery and used.

<Lithium secondary battery> A lithium secondary battery using the negative electrode of the present invention is composed of the negative electrode and a higher oxidation / removal battery.
What is necessary is just to comprise so that it may oppose the positive electrode which has a reduction potential, and should just follow the structure of the already known lithium secondary battery except a negative electrode. For example, if the positive electrode facing the negative electrode of the present invention uses a lithium transition metal composite oxide such as a layered rock salt structure LiCoO ₂ , LiNiO ₂ , and spinel structure LiMn ₂ O ₄ as an active material, a battery voltage of 4 V class is obtained. A high lithium secondary battery can be constructed. The configuration and manufacturing method of this positive electrode may be in accordance with a general configuration and manufacturing method.

In the lithium secondary battery using the negative electrode of the present invention, similarly to a general lithium secondary battery, in addition to the positive electrode and the negative electrode, a separator sandwiched between the positive electrode and the negative electrode, a non-aqueous electrolyte, and the like are used. Main components. The separator separates the positive electrode and the negative electrode and holds the electrolyte, and a thin microporous film of polyethylene, polypropylene, or the like can be used. The non-aqueous electrolyte is a solution in which a lithium salt as an electrolyte is dissolved in an organic solvent.As the organic solvent, an aprotic organic solvent, for example, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, acetonitrile, 1,2-dimethoxyethane,
One kind of tetrahydrofuran, dioxolane, methylene chloride or the like, or a mixture of two or more kinds thereof can be used. As the electrolyte to be dissolved, LiI, LiI
ClO ₄ , LiAsF ₆ , LiBF ₄ , LiPF ₆ , LiN
A lithium salt such as (CF ₃ SO ₂ ) ₂ can be used.

The lithium secondary battery using the negative electrode of the present invention configured as described above can have various shapes such as a cylindrical type, a laminated type, a coin type and the like. In any case, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and a connection between each electrode and the positive electrode terminal and the negative electrode terminal communicating to the outside is made using a current collecting lead or the like. The battery can be completed by sealing the electrode body together with the non-aqueous electrolyte in a battery case.

The embodiment of the negative electrode for a lithium secondary battery according to the present invention has been described above. However, the above-described embodiment is merely an embodiment, and the negative electrode for a lithium secondary battery according to the present invention is:
The present invention can be implemented in various forms including various modifications and improvements based on the knowledge of those skilled in the art, including the above embodiment.

[0031]

EXAMPLE Based on the above embodiment, a negative electrode for a lithium secondary battery using a carbon material as a main active material and a lithium titanium composite oxide as an auxiliary active material was produced. Then, a lithium secondary battery using these negative electrodes was manufactured,
By evaluating the characteristics of these secondary batteries, the superiority of the negative electrode for a lithium secondary battery of the present invention was confirmed. Hereinafter, these will be described.

<Preparation of Negative Electrode for Lithium Secondary Battery> In the present negative electrode, graphitized mesocarbon microbeads (MCMB25-28: manufactured by Osaka Gas) (hereinafter abbreviated as “MCMB”) are used as the main active material carbon material. Was used. The lithium-titanium composite oxide serving as an auxiliary active material is L
Compositional formula Li _1.33 T obtained by calcining a mixture of iOH · H ₂ O and TiO _{2 at} a weight ratio of 4: 5 at 900 ° C.
A lithium-titanium composite oxide represented by i _1.67 O ₄ was used. The lithium-titanium composite oxide (hereinafter referred to as “L
X) was confirmed by X-ray diffraction analysis to be substantially a single phase having a spinel structure.

The MCMB and the LTO were mixed at a weight ratio of 89: 1, 85: 5, 80:10, and 70:20, respectively, to prepare four types of mixed active material. Next, polyvinylidene fluoride (PVdF) as a binder is mixed at a ratio of 10 parts by weight to 90 parts by weight of these active material materials, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) is further added. Then, the mixture was sufficiently kneaded to prepare a paste-like negative electrode mixture. Then, these negative electrode mixture materials were added to 56
It was applied to both sides of a copper foil current collector having a size of 500 mm × 500 mm, and was vacuum-dried at 200 ° C. to produce a sheet-shaped negative electrode. For comparison, the TLO was not mixed and the M
A negative electrode having the same configuration was also prepared by the same manufacturing method using only CMB as an active material.
The negative electrode of Sample No. 1 was used as the negative electrode of Sample No. 1 and a mixed active material material obtained by mixing MCMB and LTO at a ratio of 89: 1 was used as the active material. 2 negative electrodes, and 8
Sample No. 5: 5 was used as the negative electrode. Three
And a negative electrode using a mixture mixed at 80:10 was sample No. Sample No. 4 and a negative electrode using a mixture of 70:20 were prepared. The negative electrode of No. 5 was used. (Less than,
“Sample No. 1” and the like are simply abbreviated as “No. 1” and the like. <Preparation of Lithium Secondary Battery> Lithium secondary batteries used as the above negative electrodes were prepared. The positive electrode to be opposed is Ni
The composition formula LiN obtained by calcining a mixture of (OH) ₂ and LiOH · H ₂ O at a molar ratio of 1: 1 at 900 ° C.
A lithium nickel composite oxide having a layered rock salt structure represented by iO ₂ was used as an active material. To 85 parts by weight of the lithium nickel composite oxide, 1 carbon black was added as a conductive additive.
0 parts by weight and 5 parts by weight of polyvinylidene fluoride as a binder were mixed, an appropriate amount of N-methyl-2-pyrrolidone was added and kneaded sufficiently to prepare a paste-like positive electrode mixture. Next, this positive electrode mixture was applied to both surfaces of a 54 mm × 450 mm aluminum foil current collector, and this was vacuum-dried at 200 ° C. to produce a sheet-shaped positive electrode.

The positive electrode and the negative electrode were wound with a polypropylene separator sandwiched between them to form a roll-shaped electrode body. This electrode body was provided with a positive electrode and a negative electrode current collecting lead, and then 18650
A non-aqueous electrolyte was injected into the battery case, and then the battery case was sealed to produce a cylindrical lithium secondary battery. The non-aqueous electrolyte used was a solution obtained by dissolving LiPF ₆ at a concentration of 1 M in a mixed solvent obtained by mixing ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1. No. The lithium secondary battery using the negative electrode of No. 1 was No. 1.
No. 1 lithium secondary battery. 2
-No. The secondary battery using the electrode of No. 5 was No. 5. 2-No.
No. 5 lithium secondary battery.

<Charge / Discharge Test for Lithium Secondary Battery>
An initial charge / discharge test was performed on each of the above lithium secondary batteries. The conditions for the initial charge / discharge test were as follows: at an environmental temperature of 20 ° C., charging was performed at a constant current of 100 mA up to a charge end voltage of 4.1 V, and then 100 mA up to a discharge end voltage of 3.0 V
Discharge at a constant current. A charge / discharge curve was prepared for each lithium secondary battery, the charge capacity at that time was measured to be the initial charge capacity, and the discharge capacity was measured to be the initial discharge capacity. The percentage of the initial discharge capacity relative to the initial charge capacity was defined as the initial charge / discharge efficiency.

Next, a high-temperature charge / discharge cycle test was performed on each battery. High temperature charge / discharge cycle test
The operation is performed in a high-temperature environment of 60 ° C., which is regarded as the upper limit temperature at which the lithium secondary battery is actually used. The conditions of the charge and discharge cycle are as follows: charge at a constant current of 972 mA up to a charge end voltage of 4.1 V; 9722 up to discharge end voltage 3.0V
Discharging at a constant current of mA was defined as one cycle, and the cycle was performed up to 500 cycles. The percentage of the discharge capacity at the 500th cycle relative to the discharge capacity at the first cycle was defined as the capacity retention rate after 500 cycles.

Further, a high-temperature storage test was performed on each of them. First, the discharge capacity was measured according to the procedure of the above initial charge / discharge test, and this was defined as the capacity before storage. Next, the battery was charged at a constant current of 100 mA to a charge termination voltage of 4.1 V at a temperature of 20 ° C., and each charged secondary battery was stored in a thermostat at 60 ° C. for one month. After storage, the battery was discharged at a constant current of 100 mA up to a discharge end voltage of 3.0 V at a temperature of 20 ° C., and then the discharge capacity was measured according to the procedure of the above initial charge / discharge test. The percentage of the capacity after storage relative to the capacity before storage was defined as the capacity retention rate after high-temperature storage.

<Characteristic Evaluation of Lithium Secondary Battery> As a result of the above various charge / discharge tests, the composition of the negative electrode active material used for each lithium secondary battery, the initial charge capacity of each lithium secondary battery, The discharge capacity, initial charge / discharge efficiency, capacity retention after 500 cycles, and capacity retention after high-temperature storage are shown in Table 1 below. The initial charge capacity and the initial discharge capacity are expressed as the charge capacity and the discharge capacity per unit weight of the positive electrode active material. In addition, as an example of the charge / discharge curve in the initial charge / discharge, No. 3 is used. 1 to No. FIG. 1 shows a charge / discharge curve of the secondary battery of No. 3.

[0039]

[Table 1]

As is clear from Table 1 and FIG.
No. 1 using a negative electrode in which a lithium-titanium composite oxide was present at an appropriate ratio as an auxiliary active material. 2 and N
o. The lithium secondary battery of No. 3 used the negative electrode in which the active material was composed of only MCMB. It can be seen that the initial charge / discharge efficiency is higher than that of the lithium secondary battery of No. 1 and the retention is effectively reduced. Therefore, it can be confirmed that the lithium secondary battery using the negative electrode of the present invention is a lithium secondary battery having a large capacity, that is, a high energy density.

In the case of No. 3 using a negative electrode in which a larger amount of lithium-titanium composite oxide was present. 4 and no. The lithium secondary battery of No. 5 has low initial charge / discharge efficiency. This is because the potential of the negative electrode rises due to the increase in the lithium-titanium composite oxide, and the potential of the positive electrode also rises accordingly. As a result, the amount of Li desorbed from the lithium nickel composite oxide serving as the positive electrode active material increases, and the crystal structure Is considered to be caused by instability. By the way, No.
2 and No. The weight ratio of the carbon material and the lithium-titanium composite oxide in the negative electrode of No. 3 was 99: 1 to 9
4: 6.

As can be seen from Table 1, No. 2 and No. The lithium secondary battery of No. 3 also shows high values for the capacity retention after 500 cycles and the capacity retention after high-temperature storage. From this, it can be confirmed that the lithium secondary battery using the negative electrode of the present invention is a long-life lithium secondary battery having excellent high-temperature storage characteristics and cycle characteristics, particularly excellent high-temperature cycle characteristics and excellent durability. In addition,
No. 3 using the negative electrode in which more lithium-titanium composite oxide was present. 4 and no. The lithium secondary battery of No. 5 is
For the reasons described above, it is considered that the cycle characteristics and the high-temperature storage characteristics also deteriorate.

[0043]

The negative electrode for a lithium secondary battery according to the present invention is one in which lithium titanium oxide is supplementarily added to a carbon material to form a negative electrode active material, and this negative electrode active material is used to constitute a negative electrode. With such a configuration, the decomposition of the non-aqueous electrolyte is suppressed by increasing the negative electrode potential to some extent, and
Li contained in the lithium-titanium composite oxide can compensate for Li deactivated in the negative electrode, and a lithium secondary battery using the negative electrode of the present invention has a high energy density, excellent durability, and long life. Next battery.

[Brief description of the drawings]

FIG. 1 shows a negative electrode in which a carbon material is used as a main active material and a lithium-titanium composite oxide is used as an auxiliary active material, and a negative electrode in which an active material is composed of only a carbon material.
2 shows an initial charge / discharge curve of each of the lithium secondary batteries used.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yoshio Ukyo F-term in Toyota Central R & D Laboratories Co., Ltd. 1 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi F-term (reference) 4G047 CA06 CB04 CC03 CD03 CD07 5H050 AA07 AA08 BA17 CA08 CB03 DA03 FA17 HA01 HA02

Claims (3)

[Claims] 1. A carbon material comprising as a main active material, the composition formula the auxiliary active material _{Li x Ti y O 4 (0.5} ≦
and a lithium-titanium composite oxide represented by the following formula: x ≦ 3, 1 ≦ y ≦ 2.5) as an active material. 2. A weight ratio of the carbon material and the lithium-titanium composite oxide in the negative electrode is 99: 1 to 9: 9.
The negative electrode for a lithium secondary battery according to claim 1, wherein the ratio is 4: 6. 3. The lithium-titanium composite oxide has a composition of Li _0.8 Ti _2.2 O ₄ , Li _2.67 Ti _1.33 O ₄ , Li
Ti ₂ O ₄ , Li _1.33 Ti _1.67 O ₄ , Li _1.14 Ti _1.71 O ₄
And at least one member selected from the group consisting of:
Alternatively, the negative electrode for a lithium secondary battery according to claim 2.