JPH10208741A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery of high energy density applicable to driving power sources and power sources for power storage, etc., of portable equipment such as a portable telephone and a notebook type personal computer and an electric car by using a compound made of silicon and plural kinds of elements selected among group III a of the periodic table as an active material of a negative electrode. SOLUTION: As an active material of a negative electrode, a compound whose chemical composition is shown by SiAlx My (M is either one of P, Co, Ni, Mn and Fe; 0.001<=x<=1; 0.001<=y<=3) is used. The electric conductivity of the active material of this negative electrode is 0.1Ω<-1> cm<-1> or more and is excellent in charge/discharge cycle characteristics. In a battery, a positive electrode 20 is made of Li, CoO2 , graphite powder of conductive auxiliary and polyvinylidene fluoride of binder and a negative electrode 22 is made of SiAl0.5 Co0.5 , graphite powder and polyvinylidene. The positive electrode 20, a separator 21 and the negative electrode 22 are laminated in this order, and a battery cover 24 and a battery can 25 are calked via a gasket 23 and are tightly closed and sealed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-energy-density lithium secondary battery suitable for use in portable equipment such as portable telephones and notebook personal computers, as a driving power supply for electric vehicles, and as a power storage power supply.

[0002]

2. Description of the Related Art A lithium secondary battery capable of realizing a high energy density has been actively researched and developed recently as an alternative battery to a conventional lead storage battery or nickel cadmium battery. As a negative electrode material of a lithium secondary battery, lithium metal is considered to have the highest energy density. However, when charge and discharge are repeated, dendritic lithium is deposited on the negative electrode, and the dendritic lithium is deposited. Lithium was extended to the positive electrode, causing an internal short circuit, and there was a serious problem in terms of safety. Also, attempts have been made to suppress dendrite precipitation of lithium by using a lithium alloy. However, lithium alloy has a problem that cycle life is poor.

On the other hand, in recent years, it has been studied to use a carbon material capable of inserting and extracting lithium ions as a negative electrode active material. Since the negative electrode reaction during charging and discharging is a reaction of inserting and extracting lithium ions between carbon layers, it is difficult for lithium ions to precipitate on the negative electrode in the form of metal, and the cycle characteristics are improved. Being avoided.

[0004]

The charge / discharge capacity of a negative electrode prepared by using a carbon material as a negative electrode active material and adding a binder thereto is about 300 to 400 Ah / l per volume. In such batteries, energy densities in the range of 240 to 280 Wh / l per volume have been achieved.

However, since portable devices have been remarkably reduced in size and weight, development of secondary batteries having higher energy density than lithium secondary batteries is expected.

An object of the present invention is to provide a high-energy lithium secondary battery suitable for use as a portable power source such as a portable telephone or a notebook personal computer, a driving power source of an electric vehicle, and a power storage power source. .

[0007]

In order to achieve the above object, the present invention provides a lithium secondary battery comprising a positive electrode and a negative electrode which reversibly store and release lithium ions, and a lithium secondary battery comprising an electrolyte containing the lithium ions. In the secondary battery, the negative electrode active material contains at least silicon and a compound consisting of a plurality of elements selected from Group IIIa of the periodic table, and the NMR signal of lithium absorbed in the negative electrode active material during charging is 5% with respect to the lithium chloride standard. It occurs in the range of 範 囲 40 ppm. The electric conductivity of the negative electrode active material is 0.1 Ω to ¹ cm to ¹ or more.

Further, the negative electrode active material has a chemical composition of Si.
Al _x M _y (wherein M is at P, Co, Ni, Mn, at least one of Fe, 0.001 ≦ x ≦ 1 in the range of 0.0
01 ≦ y ≦ 3).

The positive electrode active material has a chemical formula of LiCo.
_{O 2, LiNiO 2, LiCo a} Ni 1-a O 2, LiMn a Ni
_{1-a O 2, LiB a} Ni 1-a O 2, LiAl a Ni 1-a O 2, Li
It is desirable to include a compound represented by Mn ₂ O ₄ or LiMnO ₂ (where a is in the range of 0.001 ≦ a ≦ 0.5).

The electrolytic solution is at least one of propylene carbonate, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, and dimethoxyethane. With LiPF ₆ , LiB
It is desirable that at least one of F ₄ , LiClO ₄ , and LiCF ₃ SO ₃ be contained as an electrolyte. The lithium secondary battery of the present invention can also be obtained by using a gel-like film in which a resin containing at least one of polyethylene oxide, polyacrylonitrile, polymethyl methacrylate, and polyvinylidene fluoride contains the above electrolyte. realizable. Further, the above-described lithium secondary battery of the present invention can realize a high energy density, and is most suitable for use in portable equipment such as a mobile phone and a notebook personal computer, and a drive power supply and a power storage power supply of an electric vehicle. is there.

As described above, the volume energy density of a lithium secondary battery using a carbon material as a negative electrode active material is as small as about 240 to 280 Wh / l. This is because the carbon material has a low specific gravity, so that the negative electrode mixture becomes bulky, the amount of the negative electrode that can be filled in a predetermined volume is limited, and a high energy density lithium battery cannot be formed. Further, the theoretical energy density per weight of the carbon material is limited to 372 mAh / g.

[0012] In contrast, in the present invention, SiAl _x M _y (wherein M is used as the negative electrode P, Co, Ni, Mn, Fe
At least one of 0.001 ≦ x ≦ 1 and 0.001 ≦ y ≦ 3), the compound is capable of reversibly inserting and extracting lithium ions, and has a specific gravity of carbon material of 2 or less. Compared with 0.2 g / cm ³ , the negative electrode material has a large specific gravity of 2.5 to 7 g / cm ³ and can be packed at a high density, so that a lithium secondary battery with a high energy density can be realized. Further, it is characterized in that the capacity per active material weight is 500 mAh / g or more, which is larger than that of the carbon material.

Further, since the negative electrode material of the present invention has better charge / discharge cycle characteristics than the conventional alloy-based negative electrode, the charge / discharge reaction was examined. When the state of lithium in the negative electrode active material during charging was analyzed by NMR, a signal was generated in the range of 5 to 40 ppm based on lithium chloride.
It is considered that a signal is generated in the vicinity of 270 ppm in the lithium in the metal state, which suggests that lithium is occluded in the ionic state in the negative electrode active material. Therefore, it is considered that the charge / discharge reaction of the negative electrode active material is not caused by the alloying reaction, and the most significant feature of the present invention is that the charge / discharge cycle characteristics are excellent.

Further, the electric conductivity of the negative electrode active material was examined. As a result, it was found that the electric conductivity was as large as 0.1 Ω to ¹ cm to ¹ or more, and this was one of the causes of the excellent charge / discharge cycle characteristics.

The positive electrode active material of the lithium secondary battery of the present invention has a chemical formula of LiCoO ₂ , LiNiO ₂ , LiCo _a Ni.
_{1-a O 2, LiMn a} Ni 1-a O 2, LiB a Ni 1-a O 2,
In particular, high energy density is required to include at least one of the compounds represented by LiAl _a Ni _1-a O ₂ , LiMn ₂ O ₄ , and LiMnO ₂ (where a is in the range of 0.001 ≦ a ≦ 0.5). Cycle characteristics are also excellent and most desirable.

On the other hand, the electrolyte solution of the lithium secondary battery according to the present invention comprises prolene carbonate, ethylene carbonate, propylene carbonate, dimethyl carbonate,
Diethyl carbonate, methyl ethyl carbonate, γ
-Butyrolactone, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, dimethoxyethane as a solvent, LiPF ₆ , LiBF ₄ , LiCl
It is desirable to use O ₄ and LiCF ₃ SO ₃ as the electrolyte. The lithium secondary battery of the present invention can also be realized by using a gel-like film in which a resin containing at least one of polyethylene oxide, polyacrylonitrile, polymethyl methacrylate, and polyvinylidene fluoride contains an electrolytic solution. . When a gel film is used, heat resistance is increased and safety is improved as compared with the case where an electrolytic solution is used.

[0017]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a coin-type model battery in which the capacity of a negative electrode active material according to the present invention is examined. The lithium metal 10, the separator 11, and the test electrode 12 are stacked in this order, and the battery lid 14 and the battery can 15
Sealed and sealed by swaging. Hereinafter, an example of a specifically manufactured model battery will be described.

(Example 1) A compound having a chemical composition of SiAl _0.5 P _0.5 as a negative electrode active material, graphite powder as a conductive aid,
Polyvinylidene fluoride (PVDF) was used as a binder, blended at a weight ratio of 80%, 15%, and 5%, respectively, and N-methyl-2-pyrrolidone (NM) was used as a solvent.
P) was added and mixed well to prepare a negative electrode mixture. This negative electrode mixture was applied to one side of a Cu foil having a thickness of 20 μm,
After drying the NMP, it was formed by a roll press to prepare a negative electrode sheet. This negative electrode sheet was punched out to a size of 16 mm in diameter to produce a negative electrode.

The separator has a thickness of 25 μm and a diameter of 18
A microporous membrane made of mm mm polyethylene was used.

The electrolyte is a mixed solvent of ethylene carbonate and diethyl carbonate having a volume ratio of 1: 1 and LiP
Concentration was prepared by electrolyte F ₆ was used a solution of 1 mol / l.

A lithium battery, a separator, and a test electrode using a negative electrode active material are stacked in this order and impregnated with an electrolyte, and then caulked with a battery lid and a battery can to produce a coin-type model battery shown in FIG. did.

Using this model battery, a charge / discharge current of 3 m
A, the end voltage of lithium storage and release are 0V and 2V, respectively.
And lithium insertion and extraction were repeated.

FIG. 2 shows the potential change of the test electrode using the negative electrode active material during insertion and extraction of lithium. Since the occlusion / release reaction of lithium occurs in a flat region of the potential, that is, in the range of 0 to 1 V, it was found that it can be applied as a negative electrode material for a lithium secondary battery. In addition, it was found that the amount of inserted and extracted lithium at this time was 870 mAh / g at maximum per active material weight, which was larger than that of the carbon material.

Further, as a result of examining the cycle characteristics of lithium insertion and extraction, good cycle characteristics were obtained as shown in FIG. Then, in order to investigate the charge / discharge reaction, when the state of lithium in the negative electrode active material was analyzed by NMR, as shown in FIG.
A signal was generated in the range. This suggests that lithium is occluded in the negative electrode active material in an ion state. Therefore, it was considered that the charge / discharge reaction of the negative electrode active material was not due to the alloying reaction, and it was found that the charge / discharge cycle characteristics were excellent.

Further, when the electric conductivity of the negative electrode active material was examined, it was found that the electric conductivity was as large as 10 Ω to ¹ cm to ^1, which was one of the reasons why the charge / discharge cycle characteristics were excellent.

FIG. 5 is a view showing a coin-type lithium secondary battery according to one embodiment of the present invention. The positive electrode 20, the separator 21, and the negative electrode 22 are stacked in this order, and are caulked with a battery lid 24 and a battery can 25 via a gasket 23, and hermetically sealed.
It is sealed.

Examples of the lithium secondary battery specifically manufactured according to the present invention will be described below.

Example 2 The lithium secondary battery of the example shown in FIG. 5 was manufactured as follows. LiCoO ₂ , LiCo _0.2 Ni _0.8 O ₂ , LiMn as positive electrode active material
Three types of _0.2 Ni _0.8 O ₂ , graphite powder as a conductive aid and polyvinylidene fluoride (PVDF) as a binder were blended at a weight ratio of 88%, 7% and 5%, respectively, and N was used as a solvent. -Methyl-2-pyrrolidone (NMP) was added and mixed well to prepare a positive electrode mixture. This positive electrode mixture was applied to one side of an Al foil having a thickness of 20 μm, and NMP was dried and then formed by a roll press to prepare a positive electrode sheet. This positive electrode sheet was punched out to a size of 15 mm in diameter to produce a positive electrode. As in the preparation of the positive electrode, the chemical composition of the negative electrode active material is SiAl _0.5 P _0.5 , SiAl _0.5 Co _0.5 , Si
Al _0.5 Ni _0.5 , SiAl _0.5 Mn _0.5 , SiAl _0.5 F
e Five kinds of compounds of _0.5 , graphite powder as a conductive aid, polyvinylidene fluoride (PVDF) as a binder,
80%, 15%, and 5% by weight, respectively, and N-methyl-2-pyrrolidone (NMP) is used as a solvent.
Was added and mixed well to prepare a negative electrode mixture. This negative electrode mixture was applied to one surface of a Cu foil having a thickness of 20 μm, and NM
After drying P, it was formed by a roll press to prepare a negative electrode sheet. This negative electrode sheet was punched out to a size of 16 mm in diameter to produce a negative electrode.

The separator has a thickness of 25 μm and a diameter of 18
A microporous membrane made of mm mm polyethylene was used.

The electrolytic solution is a mixed solvent of ethylene carbonate and diethyl carbonate having a volume ratio of 1: 1 and LiP
Concentration was prepared by electrolyte F ₆ was used a solution of 1 mol / l.

After the positive electrode, the separator, and the negative electrode were laminated in this order and impregnated with the electrolytic solution, they were caulked with a battery lid and a battery can to produce a lithium battery shown in FIG.

Using this lithium secondary battery, the charge / discharge current is 3 mA, the charge end voltage is 4.2 V, and the discharge end voltage is 1.0.
The charging and discharging were performed at 5 V.

(Example 3) LiCo _0.2 Ni in Example 2
_0.8 O ₂ was used as a positive electrode and a negative electrode active material.
iAl _0.5 P _0.001 , SiAl _0.5 P _0.1 , SiAl
A lithium battery shown in FIG. 5 was produced using the negative electrode produced in the same manner as in Example 2 using _0.5 P and SiAl _0.5 P ₃ .

Using this lithium secondary battery, the charge / discharge current was 3 mA, the charge end voltage was 4.2 V, and the discharge end voltage was 1.0.
The charging and discharging were performed at 5 V.

(Example 4) In Example 2, LiCo _0.2 Ni
_0.8 O ₂ was used as a positive electrode and a negative electrode active material.
iAl _0.001 P _0.5 , SiAl _0.1 P _0.5 , SiAl
A lithium battery shown in FIG. 5 was produced using the negative electrode produced in the same manner as in Example 2 using P _0.5 and SiAl ₃ P _0.5 .

Using this lithium secondary battery, the charge / discharge current was 3 mA, the charge end voltage was 4.2 V, and the discharge end voltage was 1.
The charging and discharging were performed at 5 V.

Comparative Example 1 LiCoO as a positive electrode active material
With ₂ to prepare a positive electrode in the same manner as in Example 2. On the other hand, graphite powder was used as the negative electrode active material, and polyvinylidene fluoride (PVDF) was used as the binder.
%, 10%, and N-methyl-
2-Pyrrolidone (NMP) was added and mixed well to prepare a negative electrode mixture. Thereafter, a negative electrode was produced in the same manner as in Example 2, and a lithium secondary battery shown in FIG. 5 was produced.

Using this lithium secondary battery, the charge / discharge current was 3 mA, the charge end voltage was 4.2 V, and the discharge end voltage was 2.2.
Charge and discharge were performed at 8 V.

(Comparative Example 2)
_As positive and negative electrode active materials prepared using _0.2 Ni _0.8 O ₂ , SiP _0.5 , SiAl ₄ P _0.5 , SiAl _0.5 , and SiA
A lithium battery shown in FIG. 5 was produced using l _0.5 P ₄ and a negative electrode produced in the same manner as in Example 2.

Using this lithium secondary battery, the charge / discharge current was 3 mA, the charge end voltage was 4.2 V, and the discharge end voltage was 1.0.
The charging and discharging were performed at 5 V.

The following is a comparison study between Examples 2 to 4 and Comparative Examples 1 and 2 in which the lithium secondary battery manufactured according to the present invention was specifically charged and discharged. Table 1 summarizes the maximum discharge capacity of the lithium secondary batteries of Examples 2 to 4 and Comparative Examples 1 and 2, and the number of cycles when the capacity is reduced to 70% of the maximum capacity.

[0043]

[Table 1]

As shown in Table 1, it has been found that the lithium secondary battery of the present invention has a larger discharge capacity than the lithium secondary battery of the comparative example, and therefore can have a high energy density. It was also found that the charge / discharge cycle characteristics were comparable to those of the conventional lithium secondary battery.

[0045]

As described above, since the negative electrode can be filled with high capacity and high density, a lithium secondary battery having high energy density can be realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a model secondary battery according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a potential change of a test electrode using a negative electrode active material of one example according to the present invention.

FIG. 3 is a cycle characteristic diagram of lithium occlusion and release of a test electrode using a negative electrode active material of one example according to the present invention.

FIG. 4 is a characteristic diagram showing a ⁷ Li NMR signal of a test electrode using the negative electrode active material of one example according to the present invention.

FIG. 5 is a sectional view showing a lithium secondary battery according to one embodiment of the present invention.

[Explanation of symbols]

20 ... Positive electrode, 21 ... Separator, 22 ... Negative electrode, 23 ... Gasket, 24 ... Battery lid, 25 ... Battery can.

Claims (7)

[Claims] 1. A lithium secondary battery comprising a positive electrode capable of reversibly occluding and releasing lithium, a negative electrode, and an electrolyte containing lithium ions, wherein a plurality of types of silicon selected from the group consisting of silicon and Group IIIa of the periodic table are used as the active material of the negative electrode. And a nuclear magnetic resonance signal of lithium occluded in the active material of the negative electrode is 5 to 4 with respect to lithium chloride.
A lithium secondary battery produced in the range of 0 ppm. 2. The negative electrode active material has an electric conductivity of 0.1 Ω or more.
^2. The lithium secondary battery according to claim 1, which is ¹ cm to ¹ or more. 3. The negative electrode active material has a chemical composition of SiAl.
_x M _y (wherein M is either P, Co, Ni, Mn, of Fe, a range of 0.001 ≦ x ≦ 1, 0.001 ≦ y ≦ 3
The lithium secondary battery according to claim 1, comprising a compound represented by the following formula: 4. The chemical formula of the active material of the positive electrode is LiCo.
_{O 2, LiNiO 2, LiCo a} Ni 1-a O 2, LiMn a Ni
_{1-a O 2, LiB a} Ni 1-a O 2, LiAl a Ni 1-a O 2, Li
Mn ₂ O ₄ , LiMnO ₂ (where a is 0.001 ≦ a ≦ 0.5
A compound represented by the formula (1):
4. The lithium secondary battery according to 2 or 3. 5. The method according to claim 1, wherein the electrolyte is propylene carbonate,
Ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate,
One or more of dimethoxyethane as a solvent, LiP
_{4. An} electrolyte comprising at least one of F ₆ , LiBF ₄ , LiClO ₄ and LiCF ₃ SO ₃ as an electrolyte.
Or the lithium secondary battery according to 4. 6. The method according to claim 1, wherein the electrolyte is polyethylene oxide,
The resin containing at least one of polyacrylonitrile, polymethyl methacrylate, and polyvinylidene fluoride is a gel film containing the electrolytic solution according to claim 5. Lithium secondary battery. 7. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is used as a drive power source for a portable telephone, a portable information terminal device, a personal computer, and a portable audio device.