Secondary power source

Details Secondary power source Secondary power source

1999-07-14

2001-09-25

Dunn, Tom (Department: 1725)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S162000, C429S215000, C429S231950

Reexamination Certificate

active

06294292

ABSTRACT:

The present invention relates to a secondary power source having a high upper limit voltage, a large capacity and a high reliability for quick charge and discharge cycles.
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 {fraction (1/10)} of a secondary cell such as a lead-acid 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 lattice 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 a problem from the viewpoint of the process for their production which requires preliminary doping of lithium ions in the carbon material for the negative electrode.
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 a high charge and discharge cycle reliability.
The present invention provides a secondary power source, which comprises a positive electrode containing activated carbon, a negative electrode containing a carbon material capable of doping and undoping lithium ions, and an organic electrolyte containing a lithium salt, wherein the ratio of the positive electrode capacity (C
+
) to the negative electrode capacity (C
−
), i.e. C
+
/C
−
, is from 0.1 to 1.2.
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 containing a carbon material capable of doping and undoping lithium ions (hereinafter referred to as carbon material for the negative electrode). 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 carbon material for the negative electrode, will be referred to simply as a secondary power source.
In the present invention, the positive electrode capacity (C
+
) is a capacity measured at a current density of 0.25 mA/cm
2
within a range of from 4.5 V to electrostatic potential (3 to 3.3 V) applied between a lithium reference electrode and the positive electrode impregnated in the electrolyte. The negative electrode capacity (C
−
) is a capacity measured at a current density of 0.25 mA/cm
2
within a range of from 0.005 V to electrostatic potential (3 to 3.3 V) applied between a lithium reference electrode and the negative electrode impregnated in the electrolyte.
When the secondary power source of the present invention, wherein the positive electrode and the negative electrode are faced each other with a separator interposed therebetween, is charged, anions in the electrolyte are adsorbed on the activated carbon of the positive electrode, and lithium ions in the electrolyte are doped in the carbon material of the negative electrode. In such a case, if C
+
/C
−
exceeds 1.2, metal lithium is likely to deposit on the negative electrode. On the other hand, if C
+
/C
−
is less than 0.1, the negative electrode capacity is too large as compared with the positive electrode capacity, whereby charging can not adequately be carried out. In the present invention, C
+
/C
−
is from 0.1 to 1.2, preferably from 0.5 to 0.8. When C
+
/C
−
is within a range of from 0.1 to 1.2, the negative electrode potential can be made adequately low at the time of charging, and a change in the lattice spacing of [002] face of the carbon material for the negative electrode is small during the charge and discharge cycles, whereby the negative electrode is less likely to deteriorate.
In the secondary power source of the present invention, each of the positive electrode and the negative electrode is preferably an electrode in a form of a layer formed on a current collector. In such a case, if the thicknesses of the positive electrode and the negative electrode are about the same, as the negative electrode capacity is larger than the positive electrode capacity, the potential of the negative electrode does not become low enough when the secondary power source having such positive and negative electrodes thoroughly impregnated with the electrolyte, is charged, and a secondary power source having a high voltage can not be obtained. Accordingly, in the present invention, it is preferred to adjust the balance of the thicknesses of the positive electrode and the negative electrode, to make the negative electrode thin as compared with the positive electrode, and to adjust C
+
/C
−
to be within a range of from 0.1 to 1.2.
Specifically, in the case where the positive electrode is composed mainly of activated carbon and contains no lithium transition metal oxide as mentioned hereinafter, the thickness of the negative electrode which faces the positive electrode with a separator interposed therebetween, is preferably from 7 to 60%, more preferably from 10 to 40%, to the thickness of the positive electrode. By adjusting the ratio in the thickness of the negative electrode to the positive electrode to be within such a range, the capacity of the positive electrode and the capacity of the negative electrode can be well balanced, and a secondary power source having a high upper limit voltage can be constituted.
Further, in such a case, the thickness of the positive electrode is preferably from 80 to 250 &mgr;m, particularly preferably from 100 to 220 &mgr;m. If it is less than 80 &mgr;m, the capacity of the secondary power source can not be made large. Further, if it exceeds 250 &mgr;m, the resistance tends to increase at the time of charging and discharging, whereby quick charging and discharging can not practically be carried out.
On the other hand, the thickness of the negative electrode is

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