Lithium secondary battery

Details Lithium secondary battery Lithium secondary battery

2000-09-15

2002-04-02

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S188000, C429S303000, C429S307000, C429S322000, C429S324000

Reexamination Certificate

active

06365300

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a lithium secondary battery that has large capacity and remarkable safety, especially to a lithium secondary battery that can suppress short circuits caused by the generation of dendrites from the negative electrode, that has high energy density, and that is excellent in charge and discharge-cycle performance.
BACKGROUND ART
Lithium secondary batteries with an organic electrolysis solution have been widely used. Their attractive feature being their high energy output per unit volume or unit weight in comparison with other batteries. In exploiting this advantage, researchers and engineers have been advancing the development and practical applications of the batteries as a power source for mobile communication devices, notebook-type personal computers, and electric vehicles.
The types of lithium secondary batteries include an organic electrolysis solution type, in which an organic electrolysis solution is impregnated in a porous polymer separator, and a gel polymer type, in which a gelatinous polymer. containing a large amount of electrolysis solution is used.
Both the organic electrolysis solution type and the gel polymer type have a substantial amount of electrolysis solution, thus posing problems. The problems include a poor withstanding property against voltage, instability against the electrode material (especially against carbon which is usually used as the negative electrode), and gas evolution. In addition, these organic electrolysis solutions are intrinsically inflammable substances, and therefore a short circuit resulting from a temperature rise or shock for any reason may cause an explosion.
Moreover, organic electrolysis solution-type and gel polymer-type batteries have been required to increase the energy density as their important technical challenge. Their limit at the present is about 300 Wh/l, and it is strongly required to increase this limit to 400 Wh/l or more. Researchers and engineers have been studying the use of metallic lithium as the negative electrode in order to solve the foregoing problems and improve the properties effectively.
However, when a lithium-containing material is used as the negative electrode, the electrolytic layer is affected by the change in the thickness of the metallic lithium utilized in charging and discharging and by the change in the shape of the negative electrode at the time of charge and discharge. It is particularly notable at increased cycles, such as several hundred cycles or more. Metallic lithium has a strong tendency to react with water vapor in the air, so that it is necessary to provide a device to block the air in the filming process.
Moreover, with a lithium secondary battery containing an organic electrolysis solution, repetition of charge and discharge causes dendritic metallic lithium to grow on the surface of the metallic lithium. This may cause an internal short circuit between the electrodes, triggering explosion and other abnormalities.
To eliminate this possibility, the following techniques have been proposed:
1. Formation of a compound layer by treating the surface of the metallic lithium to be used as the negative electrode. The types of compound layers include a polymer layer, a fluoride layer, a carbonic compound layer, and an oxide layer.
2. Production of an entirely solid battery containing no electrolysis solution that may cause explosion. For example, an organic polymer or inorganic crystals can be used as the electrolyte.
The foregoing techniques, however, have the following problems:
1-3. It is known that the techniques for the surface treatment of the metallic lithium include the following:
(a) The surface treatment is performed before forming the battery.
(b) A compound layer is formed by spontaneously reacting the metallic lithium with the compounds in the electrolysis solution and in the material for the positive electrode when the battery is formed.
1-2. In (a) above, it is understood that the acid treatment or plasma treatment forms a lithium fluoride, a lithium carbonate, or a lithium oxide layer, each of which is effective in suppressing the growth of the lithium dendrite at the time of charge and discharge. With this technique, however, repetition of charge and discharge poses problems such as the formation of voids at the interface, separation of the compound layer, and concentrated growth of metallic lithium in cracks and pinholes in the compound layer.
1-3. In (b) above, because substances that form a compound layer by reacting with metallic lithium are added in the organic electrolysis solution, a compound layer is formed continually at the interface on condition that the metallic lithium comes into contact with the electrolysis solution. As a result, although problems such as separation can be avoided with high possibility, impurities inevitably contained in the organic electrolysis solution cause the compound layer formed on the surface of the metallic lithium to be nonuniform. This reduces the effectiveness in suppressing the growth of the dendrites of metallic lithium.
2-1. The entirely solid type has a solid electrolyte. This poses a problem in the contact between the electrode and the electrolyte. The resultant reduction in the contact area increases the contact resistance, preventing the extraction of a large amount of the electric current.
2-2. The difficulty in handling the solid electrolyte restricts application forms. The types of materials as the solid electrolyte include a sulfide family, an oxide family, a nitride family, and a mixture of these, such as an oxynitride family and an oxysulfide family. However, although a compound containing sulfide has high lithium-ion conductivity, it has drawbacks such as high hygroscopic property and high hydrolytic property at the same time. These drawbacks cause the electrolytic layer to be difficult to handle after it is formed. More specifically, the electrolytic layer requires to be sealed in an inert gas atmosphere during the formation of the battery and transportation. It also requires provision of a glove box. These requirements pose problems in productivity and cost.
2-3. Lithium ion-conductive solid electrolytes mainly have been studied as a bulk-shaped sintered body or a powder for practical use. This restricts application forms, reduces total ionic conductivity, and lowers battery performance. On the other hand, when a thin-film electrolyte is used, it is difficult to suppress the formation of pinholes and cracks. In particular, when the positive electrode contains an organic electrolysis solution, the electrolysis solution effusing from the positive electrode penetrates through the pinholes and cracks to reach the surface of the negative electrode. Then, the electrolysis solution reacts with the negative electrode to cause concentrated growth of dendrites at the pinholes and cracks. This may cause a problem of short circuiting between the electrodes. In addition, when the current capacity per unit area is increased, the stress caused by the volume change of the negative electrode at the time of charge and discharge may fracture the electrolytic layer.
Consequently, the main object of the present invention is to offer a lithium secondary battery that can suppress short circuits caused by the generation of dendrites from the negative electrode, that has high energy density, and that is excellent in charge and discharge-cycle performance.
DISCLOSURE OF THE INVENTION
The present invention achieves the foregoing object by providing an electrolytic layer that is formed by an inorganic solid electrolyte and a positive electrode that contains an organic high polymer. This constitution prevents the growth of dendrites on the metallic lithium at the time of charge and discharge, suppresses the reaction between the organic electrolysis solution and the negative electrode, and suppresses the temperature rise in the battery even at the time of over charging. In short, an explosion can be avoided. The following is a detailed explanation of the electrolytic layer, positive electrode, negati

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