JP3135613B2 - Lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having an improved negative electrode.

[0002]

2. Description of the Related Art In recent years, non-aqueous electrolyte batteries using lithium as a negative electrode active material have attracted attention as high energy density batteries, and manganese dioxide (MnO ₂ ) has been used as a positive electrode active material.
Fluorocarbon [(CF _n )], thionyl chloride (SOC
Primary batteries using l ₂ ) and the like are already frequently used as backup batteries for power supplies of calculators and watches and memories. Furthermore,
In recent years, with the miniaturization and weight reduction of various electronic devices such as VTRs and communication devices, the demand for secondary batteries with high energy density has increased as a power source for them, and research on lithium secondary batteries using lithium as a negative electrode active material has been conducted. Is being actively conducted.

A lithium secondary battery uses lithium as a negative electrode and propylene carbonate (PC), 1,2-dimethoxyethane (DME), γ-butyrolactone (γ-BL), tetrahydrofuran (TH) as a lithium ion conductive electrolyte.
F) LiClO ₄ , LiBF ₄ , L
It is composed of a non-aqueous electrolyte solution in which a lithium salt such as iAsF ₆ or LiPF ₆ is dissolved, or a lithium ion conductive solid electrolyte. As a positive electrode active material, mainly TiS ₂ , MoS ₂ , V ₂
O _5, compounds of topochemical reaction with the lithium, such as V ₆ O ₁₃ have been studied.

However, the above-mentioned secondary battery is currently
It has not been put to practical use yet. The main reason for this is that the charge / discharge efficiency is low and the number of charge / discharge (cycle) life is short. It is considered that this is largely due to the deterioration of lithium due to the reaction between the negative electrode lithium and the non-aqueous electrolyte. That is, lithium dissolved in the non-aqueous electrolyte as lithium ions at the time of discharging reacts with the solvent at the time of deposition at the time of charging, and its surface is partially inactivated. For this reason,
When charge and discharge are repeated, phenomena such as generation of dendritic (dendritic) lithium, precipitation in small spheres, and elimination of lithium from the current collector occur.

[0005] For these reasons, carbonaceous materials and chalcogen compounds that occlude and release lithium have been studied as negative electrodes incorporated in lithium secondary batteries. As the carbonaceous material, for example, by using coke, resin fired body, carbon fiber, pyrolysis gas phase carbon body, and the like, it is proposed to improve the negative electrode degradation due to the reaction between lithium and the non-aqueous electrolyte and the dendrite deposition. ing. However, since such an anode has a small amount of absorbing and releasing lithium ions, the anode has a specific capacity (mAh / cm ^3). ) Is small, and the amount of absorbed lithium ions is increased (increase the charging capacity)
Then, for example, the structure of the carbonaceous material is deteriorated or the solvent in the non-aqueous electrolyte is decomposed. Furthermore, when the charging current density is increased, the amount of occluded lithium ions decreases, and there is a problem that lithium metal is deposited. This is because the potential for absorbing lithium ions is 0 V (VS, Li / Li ⁺ )
As a result, the lithium secondary battery incorporating the negative electrode has a problem that it is difficult to improve the cycle life.

On the other hand, among the chalcogen compounds into which lithium ions are inserted and desorbed, WO ₂ and MoO _{2 having} low electromotive force are used.
_2, FeOCl, been proposed to use NbSe _3, etc. as the negative electrode, especially a battery comprising a negative electrode of WO ₂ / LiCoO ₂ that the average voltage becomes 3.2 V J. Elect
rochem. Soc. , 134, 638 (1987).
It has been announced at.

However, the negative electrode composed of the chalcogen compound has problems such that the electrolyte is easily decomposed when lithium ions are inserted (during charging), the charge / discharge efficiency is low, and the conductivity is low. For this reason, lithium secondary batteries incorporating the negative electrode have not been satisfactory in terms of capacity and life. In order to improve the conductivity of the negative electrode composed of the chalcogen compound, it has been attempted to mix carbon such as graphite and acetylene black as a conductive agent.However, the chalcogen compound reductively decomposes the electrolytic solution on the carbon. There was a problem of doing.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and an object thereof is to provide a lithium secondary battery having a high capacity and an excellent cycle life.

[0009]

SUMMARY OF THE INVENTION A lithium secondary battery according to the present invention is a lithium secondary battery containing a positive electrode, a negative electrode, and a lithium ion conductive electrolyte in a container.

The negative electrode (a) has lithium ions inserted therein.
The reaction to be eliminated has an average potential of 2 V (VS, Li / Li ⁺ ) A mixture of a chalcogen compound or a lithium ion-containing chalcogen compound having the same properties as described below, and (b) a carbonaceous substance capable of inserting and extracting lithium ions.

The positive electrode is made of various oxides, for example, manganese dioxide, lithium manganese composite oxide, lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing nickel-cobalt oxide, and amorphous vanadium pentoxide containing lithium. And 2V (VS, Li / Li ^{+) of} titanium disulfide, molybdenum disulfide, etc. And chalcogen compounds having the above electromotive force. Among them,
Lithium cobalt oxide (LiCoO ₂ ) with high electromotive force
It is preferable to use

As the chalcogen compound or the lithium-containing chalcogen compound which is the component (a) constituting the negative electrode, a lithium ion has an average potential of 2 V (VS, Li /
Li ⁺ ) WO ₂ and MoS which cause insertion / desorption reactions below
₂ , VSe ₂ , VS ₂ , LiVSe ₂ , LiTiS ₂ ,
Fe ₂ O _{3 and the} like can be mentioned. Among them, WO ₂ is useful because the battery voltage can be further increased.

The carbonaceous material as the component (b) constituting the negative electrode has a graphite structure and a turbostratic structure, and has a (002) plane spacing (d ₀₀₂ ) obtained by X-ray diffraction defining the graphite structure. ) Is desirably 0.340 nm or more, and the crystallite size (Lc) in the C-axis direction is desirably 20 nm or less. If the values of d ₀₀₂ and Lc deviate from the above ranges, the negative electrode containing the carbonaceous material as the component (b) may have a.
5V (VS, Li / Li ⁺ ) At the above potential, decrease of lithium ion occlusion / release amount, deterioration of graphite structure, gas generation due to reductive decomposition of solvent in non-aqueous electrolyte, etc., may cause reduction of secondary battery capacity and cycle life. There is. More preferably, d ₀₀₂ and Lc are each 0.1.
The range is from 345 to 0.370 nm and from 1 to 10 nm.

As a measure of the ratio between the graphite structure and the turbostratic structure constituting the carbonaceous material, an argon laser (wavelength: 51
(4.5 nm) as a light source. The measured Raman spectrum is 1
A peak derived from a turbostratic structure appearing near 360 cm ⁻¹ ,
There is a peak derived from the graphite structure appearing around 1580 cm ^-1 , and the peak intensity ratio (for example, the intensity when the Raman intensity derived from the turbostratic structure is R ₁ and the Raman intensity derived from the graphite structure is R ₂₎ It is effective to use the ratio (R ₁ / R ₂ ) or the area ratio. The ratio of the graphite structure to the turbostratic structure in the carbonaceous material suitable for the component (b) constituting the negative electrode is as follows:
It is desirable to set R ₁ / R ₂ to be larger than 0.8. When the intensity ratio is 0.8 or less,
0.5 V (VS, Li / Li ⁺ At the above potential, the amount of lithium ion occlusion / release is reduced. More preferably the intensity ratio (R ₁ / R ₂₎ is in the range of 1.0 to 1.7.

The ratio of residual hydrogen due to ungraphitization in the carbonaceous material is defined by the atomic ratio of hydrogen / carbon (H / C). The H / C of the carbonaceous material suitable as the negative electrode material is desirably 0.15 or less. If the H / C exceeds 0.15, it is not only difficult to increase the amount of lithium ions absorbed and released by the negative electrode, but also the charge / discharge efficiency may be reduced. More preferred H / C is 0.0
4 or less. As the carbonaceous material, coke, pitch-based carbon fiber, spherical carbonaceous material, pyrolytic gas-phase carbonaceous material, or the like can be used as long as it has the above-mentioned properties.

The mixing ratio (b / a) of (a) the chalcogen compound or the lithium ion-containing chalcogen compound constituting the negative electrode and (b) the carbonaceous material is determined by the conductivity and specific capacity (mAh / cm ³ ) of the negative electrode. ) Is desirably set based on the relationship of
It is preferable to set the range to 0.5. This is because if the mixing ratio is less than 0.04, the capacity may decrease due to an increase in the resistance of the negative electrode and a decrease in the utilization factor. This is because the capacity may be reduced. The more preferable mixing ratio (b / a) is in the range of 0.07 to 0.35.

Examples of the lithium ion conductive electrolyte include ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, sulfolane, acetonitrile, 1,2-dimethoxyethane,
Lithium perchlorate (LiClO ₄ ) and lithium hexafluorophosphate (LiPF) are used in at least one non-aqueous solvent selected from 3-dimethoxypropane, dimethyl ether, tetrahydrofuran and 2-methyltetrahydrofuran.
₆ ), non-aqueous electrolytes in which lithium salts (electrolytes) such as lithium borofluoride (LiBF ₄ ), lithium arsenic hexafluoride (LiAsF ₆ ), and lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ) are dissolved. it can. The amount of the electrolyte dissolved in the nonaqueous solvent is 0.5 to 1.
It is desirably 5 mol / l. In addition, a lithium ion conductive solid electrolyte can be used. For example,
A polymer solid electrolyte in which a polymer compound is compounded with a lithium salt can be given.

[0018]

According to the present invention, (a) the reaction for inserting and removing lithium ions is performed at an average potential of 2 V (VS, Li / Li ⁺ )
By forming the negative electrode from a mixture of the following chalcogen compound or a lithium ion-containing chalcogen compound having the same properties and (b) a carbonaceous substance capable of occluding and releasing lithium ions, the negative electrode is composed of only the component (a). The conductivity, charge / discharge capacity, and cycle life of the negative electrode can be increased as compared with the negative electrode. This is because the carbonaceous material capable of occluding and releasing the lithium ion as the component (b) does not cause decomposition of the lithium ion conductive non-aqueous electrolyte housed in the container together with the negative electrode, and the negative electrode active material and the conductive material do not decompose. This is because it acts as a material. In particular, the carbonaceous material has a graphite structure and a turbostratic structure, and the plane spacing (d ₀₀₂ ) of the (002) plane in the graphite structure is 0.340 nm or more, and the crystallite size (Lc) in the C-axis direction is 20 nm or less. And 15 in the argon laser Raman spectrum.
When the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 80 cm ^-1 is used greater than 0.8, the capacity of occluding lithium ions 0.5V (VS, Li / L
i ⁺ ) Since it increases in the above potential range, it can contribute to an increase in the negative electrode capacity.

Further, the mixing ratio (b / a) of (a) the chalcogen compound or the lithium ion-containing chalcogen compound constituting the negative electrode and (b) the carbonaceous material is 0.04 to 0.5 by weight. Within this range, the specific capacity of the negative electrode (mAh / cm ³ ) And the conductivity can be set to optimal values, and the charge / discharge capacity can be increased.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a cylindrical lithium secondary battery will be described in detail with reference to FIG. Example 1

Reference numeral 1 in the drawing denotes a bottomed cylindrical stainless steel container having an insulator 2 disposed at the bottom. In this container 1, an electrode group 3 is housed. The electrode group 3 has a structure in which a band formed by laminating a positive electrode 4, a separator 5 and a negative electrode 6 in this order is spirally wound so that the negative electrode 6 is located outside.

The positive electrode 4 is composed of 80% by weight of lithium cobalt oxide (Li _x CoO ₂ ) powder, 15% by weight of acetylene black and 5% by weight of polytetrafluoroethylene powder.
It is mixed with a weight%, formed into a sheet, and pressed into contact with an expanded metal current collector. The separator 5
Is formed from a polypropylene porous film.

The negative electrode 6 has 17% by weight of spherical carbonaceous material particles (component (b)) having an average particle diameter of 10 μm obtained by carbonizing the mesophase spheres heat-treated and separated from the pitch, and WO ₂ [(a ) Ingredient] 80% by weight and 3% by weight of ethylene propylene copolymer were mixed, and this was applied to a nickel foil as a current collector at 60 mg / cm ^2. It was applied in the amount of The carbonaceous material particles had various parameters obtained by X-ray diffraction, d ₀₀₂ = 0.3508 nm and Lc = 2.5.
0 nm, measured using an argon laser as a light source.
60cm ratio of the Raman intensity R ₂ of the Raman intensity R ₁ and 1580 cm ^-1 of ^-1 (R ₁ / R ₂₎ is 1.1. The carbonaceous material particles have an atomic ratio of hydrogen / carbon of 0.003.
The weight ratio (a / b) of WO ₂ to carbonaceous material is 0.21
25.

In the vessel 1, lithium hexafluorophosphate (LiPF ₆ ) is added to a mixed solvent of ethylene carbonate, propylene carbonate and 1,2-dimethoxyethane (mixing volume ratio 25:25:50) in a volume of 1.0%. An electrolytic solution having a composition of mol / l dissolved therein is contained. On the electrode group 3, an insulating paper 7 having a central opening is placed.
Further, an insulating sealing plate 8 is provided at the upper opening of the container 1 in a liquid-tight manner by caulking the container 1 or the like.
A positive electrode terminal 9 is fitted in the center of the insulating sealing plate 8. The positive terminal 9 is connected to the positive electrode 4 of the electrode group 3 via a positive electrode lead 10. In addition, the negative electrode 6 of the electrode group 3 is connected to the container 1 as a negative electrode terminal via a negative lead (not shown). Example 2

The spherical carbonaceous material particles as the component (b) constituting the negative electrode have various parameters determined by X-ray diffraction: d ₀₀₂ = 0.3452 nm, Lc = 2.50 nm, and R ₁ / R ₂ is 1.0. A lithium secondary battery having the same configuration as in Example 1 was assembled, except that the above-described lithium secondary battery was used. Example 3

The spherical carbonaceous material particles as the component (b) constituting the negative electrode have various parameters determined by X-ray diffraction: d ₀₀₂ = 0.3410 nm, Lc = 5.00 nm, and R ₁ / R ₂ is 0.75. , The atomic ratio of hydrogen / carbon is 0.0
A lithium secondary battery having the same configuration as that of Example 1 was assembled, except that an average particle diameter of 01 and 20 μm was used. Example 4

27% by weight of spherical carbonaceous material particles (component (b)) having an average particle diameter of 10 μm obtained by carbonizing the mesophase small spheres heat-treated and separated from the pitch, and WO
₂ [Component (a)] A mixture of 70% by weight of ethylene propylene copolymer and 3% by weight of ethylene propylene copolymer was added to a nickel foil as a current collector at 40 mg / cm ^2. A lithium secondary battery having the same configuration as that of Example 1 was assembled except that the negative electrode coated in the amount described above was used. Example 5

7% by weight of spherical carbonaceous material particles (component (b)) having an average particle size of 10 μm obtained by carbonizing the mesophase small spheres heat-treated and separated from the pitch and WO
₂ [Component (a)] A mixture of 90% by weight and 3% by weight of an ethylene propylene copolymer was added to a nickel foil as a current collector at 70 mg / cm ^2. A lithium secondary battery having the same configuration as that of Example 1 was assembled except that the negative electrode coated in the amount described above was used. Comparative Example 1

WO_Two97% by weight of ethylene propylene
3% by weight of a polymer were mixed, and this was used as a current collector.
85mg / cm for keel foil^Two Using the negative electrode coated in the amount of
Other than that, assemble the lithium secondary battery having the same configuration as in Example 1.
Was. Comparative Example 2

A mixture of 98% by weight of spherical carbonaceous material particles obtained by carbonizing mesophase small spheres and 2% by weight of an ethylene-propylene copolymer was added to a nickel foil as a current collector.
0 mg / cm ² A lithium secondary battery having the same configuration as that of Example 1 was assembled except that the negative electrode coated in the amount described above was used. Comparative Example 3

Mixing 17% by weight of graphite, 80% by weight of WO ₂ and 3% by weight of ethylene propylene copolymer,
This was applied to a nickel foil as a current collector at 60 mg / cm ^2. A lithium secondary battery having the same configuration as that of Example 1 was assembled except that the negative electrode coated in the amount described above was used. For the graphite, various parameters obtained by X-ray diffraction were as follows: d ₀₀₂ = 0.338
nm, Lc = 50nm, the R ₁ / R ₂ is 0.1.

Thus, Examples 1 to 5 and Comparative Examples 1 to
2. For the lithium secondary battery of No. 3, at a charging current of 50 mA.
Charge and discharge by charging to 4 V and discharging to 2.0 V at a current of 50 mA (however, the battery of Comparative Example 2 has a charging current of 50 m
A to 4.2 V and discharge to 3.0 V at a current of 50 mA) were repeated, and the discharge capacity and cycle life of each battery were measured. The result is shown in FIG.

As is apparent from FIG. 2, the lithium secondary batteries of Examples 1 to 5 have an increased capacity and a remarkably improved cycle life as compared with the batteries of Comparative Examples 1 to 3.

[0034]

As described above, according to the present invention, a lithium secondary battery having a high capacity and an excellent cycle life can be provided.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional view showing a cylindrical lithium secondary battery in Embodiment 1 of the present invention.

FIG. 2 is a characteristic diagram showing a relationship between a charge / discharge cycle and a discharge capacity in the lithium secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 3.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Stainless steel container, 3 ... Electrode group, 4 ... Positive electrode, 5 ... Separator, 6 ... Negative electrode, 8 ... Sealing plate, 9 ... Positive electrode terminal.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-37968 (JP, A) JP-A-63-124372 (JP, A) French Patent Application Publication 2677175 (FR, A1) (58) Search Fields (Int.Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/58 H01M 10/40

Claims (2)

(57) [Claims] 1. A lithium secondary battery containing a positive electrode, a negative electrode and a lithium ion conductive electrolyte in a container, wherein the negative electrode has (a) a reaction for inserting and removing lithium ions in which an average potential is 2 V (VS, Li / Li ⁺ A) a lithium secondary battery comprising a mixture of the following chalcogen compound or a lithium ion-containing chalcogen compound having the same properties and (b) a carbonaceous substance capable of inserting and extracting lithium ions. 2. The carbonaceous material has a graphite structure and a turbostratic structure, and the (002) plane spacing (d) in the graphite structure.
₀₀₂₎ is more than 0.340 nm, and the size of the C-axis direction of the crystallite (Lc) is 20nm or less, is greater than 0.8 the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon laser Raman spectra The lithium secondary battery according to claim 1, wherein: