Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-03-12
2003-05-06
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231100
Reexamination Certificate
active
06558846
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a secondary power source having a low resistance, a high upper limit voltage and a large capacity. More particularly, the present invention relates to a secondary power source having a high reliability for quick charge and discharge cycles.
DISCUSSION OF THE BACKGROUND
As electrodes for a conventional electric double layer capacitor, polarizable electrodes composed mainly of activated carbon are used for both the positive electrode and the negative electrode. The upper limit voltage of an electric double layer capacitor is 1.2 V when an aqueous electrolyte is used, or from 2.5 to 3.3 V when an organic electrolyte is used. The energy of the electric double layer capacitor is proportional to the square of the upper limit voltage. Accordingly, an organic electrolyte having a high upper limit voltage provides a high energy as compared with an aqueous electrolyte. However, even with an electric double layer capacitor employing an organic electrolyte, the energy density is as low as at most 1/10 of a secondary cell such as a lead storage battery, and further improvement of the energy density is required.
Whereas, JP-A-64-14882 proposes a secondary power source for an upper limit voltage of 3 V, which employs, as a negative electrode, an electrode having lithium ions preliminarily doped in a carbon material having a spacing of [002] face of from 0.338 to 0.356 nm as measured by X-ray diffraction. Further, JP-A-8-107048 proposes a battery which employs, for a negative electrode, a carbon material having lithium ions preliminarily doped by a chemical method or by an electrochemical method in a carbon material capable of doping and undoping lithium ions. Still further, JP-A-9-55342 proposes a secondary power source for an upper limit voltage of 4 V, which has a negative electrode having a carbon material capable of doping and undoping lithium ions supported on a porous current collector which does not form an alloy with lithium. However, these secondary power sources have had a problem from the viewpoint of the process for their production which requires preliminary doping of lithium ions.
Further, a lithium ion secondary cell is available as a power source capable of heavy current charge and discharge other than the electric double layer capacitor. The lithium ion secondary cell has characteristics such that it provides a high voltage and a high capacity as compared with the electric double layer capacitor. However, it has had problems such that the resistance is high, and the useful life due to quick charge and discharge cycles is very short as compared with the electric double layer capacitor.
Under these circumstances, it is an object of the present invention to provide a secondary power source which has quick charge and discharge capability, provides a high upper limit voltage and a high capacity and has a high energy density and which has a high charge and discharge cycle reliability.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a secondary power source, which comprises a positive electrode containing activated carbon and a lithium-containing transition metal oxide, a negative electrode containing a carbon material capable of doping and undoping lithium ions, and an organic electrolyte containing a lithium salt.
DETAILED DESCRIPTION OF THE INVENTION
Now, the present invention will be described in detail with reference to the preferred embodiments.
In this specification, a negative electrode assembly is one obtained by bonding and integrating a current collector and a negative electrode composed mainly of a carbon material capable of doping and undoping lithium ions. Likewise, a positive electrode assembly is one obtained by bonding and integrating a current collector and the positive electrode. A secondary cell as well as an electric double layer capacitor is a kind of a secondary power source. However, in this specification, a secondary power source of a specific construction wherein the positive electrode contains activated carbon and the negative electrode contains a carbon material capable of doping and undoping lithium ions, will be referred to simply as a secondary power source.
With the secondary power source of the present invention, at the time of charging, at the positive electrode, in addition to the adsorption of anions of the lithium salt to the activated carbon, undoping of lithium ions from the lithium-containing transition metal oxide takes place. At the negative electrode, lithium ions will be doped in the carbon material. Here, the lithium ions to be doped in the carbon material are those from the lithium salt in the electrolyte and those undoped from the lithium-containing transition metal oxide.
Accordingly, with the secondary power source of the present invention, lithium ions can adequately be doped in the carbon material of the negative electrode, as compared with a conventional secondary power source employing a positive electrode composed mainly of activated carbon without containing a lithium-containing transition metal oxide. Therefore, an adequate amount of lithium ions can be doped in the negative electrode by charging after the positive electrode and the negative electrode facing each other with a separator interposed therebetween, are impregnated with the electrolyte, without necessity to preliminarily occlude lithium ions to the negative electrode as required in the above-mentioned conventional secondary power sources. And, the potential of the negative electrode becomes low enough, so that the voltage of the secondary power source can be made high.
With a lithium ion secondary cell wherein the positive electrode is made of an electrode composed mainly of a lithium-containing transition metal oxide, and the negative electrode is made of an electrode composed mainly of a carbon material capable of doping and undoping lithium ions, deterioration is remarkable when quick charge and discharge cycles are carried out as compared with a case where mild charge and discharge cycles are carried out. One of the main causes is deterioration by an oxidation-reduction reaction due to the charge and discharge of the lithium-containing transition metal oxide as the active material of the positive electrode.
Whereas, with the secondary power source of the present invention, when an adequate amount of the lithium-containing transition metal oxide is contained in the positive electrode, the activated carbon will be involved in the case of quick charge and discharge with a heavy current, and the lithium-containing transition metal oxide will be involved in the case of charge and discharge with a relatively small current. Accordingly, the lithium-containing transition metal oxide of the positive electrode is less burdened, whereby the deterioration due to charge and discharge cycles can be reduced to a low level, and it is possible to obtain a secondary power source having a high voltage, a high capacity and a long useful life for charge and discharge cycles.
Further, if the amount of the lithium-containing transition metal oxide contained in the positive electrode in the secondary power source of the present invention is reduced, only the activated carbon will substantially be involved in the charge and discharge irrespective of the current level. In such a case, the substantial role of the lithium-containing transition metal oxide will be to provide lithium ions to be doped in the carbon material of the negative electrode during the initial charge, and to supplement lithium ions when lithium ions in the electrolyte have decreased by the use of the secondary power source. Accordingly, the deterioration in,the capacity due to the charge and discharge cycles will be particularly small, although the capacity may be small as compared with a case where the content of the lithium-containing transition metal oxide is/large.
The amount of the lithium-containing transition metal oxide in the positive electrode is preferably from 0.1 to 80 wt %. If it is less than 0.1 wt %, the amount of lit
Che Yong
Morimoto Takeshi
Tsushima Manabu
Asahi Glass Company Ltd.
Chaney Carol
Dove Tracy
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