Secondary power source

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 negative electrode has a density of from 0.6 to 1.2 g/cm3.

The present invention relates to a secondary power source having a high upper limit voltage, a large capacity and a high reliability for large current 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 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 an electrode composed mainly of activated carbon as a positive electrode and 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.

A secondary power source which employs activated carbon for a positive electrode and a carbon material capable of doping and undoping lithium ions for a negative electrode, provides a higher upper limit voltage and a larger capacity than a conventional electric double layer capacitor which employs activated carbon for both positive and negative electrodes. Particularly when a graphite type carbon material with which the potential for doping and undoping lithium ions is low is used for the negative electrode of the secondary power source, a larger capacity can be obtained.

Further, a lithium ion secondary cell is available as a high-performance secondary power source other than the electric double layer capacitor and the above secondary power source. The lithium ion secondary cell has characteristics such that it can be operated at a higher voltage and it provides a larger 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.

A secondary power source which employs activated carbon for a positive electrode and a carbon material capable of doping and undoping lithium ions for a negative electrode is excellent in durability against quick charge and discharge cycles as compared with a lithium ion secondary cell, however, it has an inadequate durability against quick charge and discharge cycles as compared with an electric double layer capacitor. This is considered to be attributable to a negative electrode having a construction different from the electric double layer capacitor and a difference in an electrode reaction at the negative electrode.

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 large capacity and has a high energy density and which has a high charge and discharge cycle reliability, particularly by studies on a negative electrode.

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 negative electrode has an electrode density of from 0.6 to 1.2 g/cm3.

As a carbon material capable of doping and undoping lithium ions, hard (non graphitizable) carbon having a lattice spacing of [002] face of 0.373 nm and vapor grown carbon fibers having a lattice spacing of [002] face of 0.336 nm were mixed in a mass ratio of 8:1, and the obtained mixture was dispersed in a solution having polyvinylidene fluoride dissolved in NMP. This dispersion was coated on a current collector made of cupper and dried to form a negative electrode on the current collector. This assembly was further pressed by a roll pressing machine so that the electrode density was 0.8 g/cm3, the area of the negative electrode was 1 cm×1 cm and the thickness was from 15 to 30 μm, and a heat treatment was carried out under reduced pressure at 150° C. for 10 hours to obtain a negative electrode assembly. Here, in the negative electrode, the mass ratio of the carbon material capable of doping and undoping lithium ions and polyvinylidene fluoride was made to be 9:1.

Then, a mixture obtained by mixing activated carbon having a specific surface area of 2,000 m2/g obtained by steam-activation using a phenol resin as a starting material, conductive carbon black and polytetrafluoroethylene as a binder in a mass ratio of 8:1:1, was added to ethanol, followed by kneading, rolling and drying in vacuum at 200° C. for 2 hours to obtain an electrode sheet having a thickness of 150 μm. From this electrode sheet, an electrode of 1 cm×1 cm was obtained and was bonded to a current collector made of an aluminum foil by means of a conductive adhesive using polyamideimide as a binder, followed by heat treatment under reduced pressure at 260° C. for 10 hours to obtain a positive electrode assembly.

The positive electrode assembly and the negative electrode assembly thus obtained were disposed to face each other with a polypropylene separator interposed therebetween, and were thoroughly impregnated with the electrolyte, to obtain a secondary power source. A solution having LiBF4in a concentration of 1 mol/l dissolved in a mixed solvent of ethylene carbonate and ethylmethyl carbonate (mass ratio 1:1) was used as an electrolyte, and the above secondary power source was thoroughly impregnated with the electrolyte, whereupon the initial capacity was measured within a range of from 4.2 V to 2.75 V. Thereafter, a charge and discharge cycle test was carried out at a charge and discharge current of 10 mA/cm2within a range of from 4.0 V to 2.75 V, and the capacity after 2,000 cycles was measured, whereupon the change in capacity was calculated. The results are shown in Table 1.

A secondary power source was obtained in the same manner as in Example 1 except that the density of the negative electrode was made to be 0.95 g/cm3by changing the pressure at the time of rolling, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

A secondary power source was obtained in the same manner as in Example 1 except that the density of the negative electrode was made to be 0.65 g/cm3by changing the pressure at the time of rolling, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

A secondary power source was obtained in the same manner as in Example 1 except that the density of the negative electrode was made to be 1.1 g/cm3by changing he pressure at the time of rolling, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

A secondary power source was obtained in the same manner as in Example 1 except that a solution having 0.9 mol/l of LiN(SO2C2F5)2and 0.1 mol/l of LiClO4dissolved in a mixed solvent of ethylene carbonate and ethylmethyl carbonate (mass ratio 1:1) was used as the electrolyte, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

A secondary power source was obtained in the same manner as in Example 1 except that the density of the negative electrode was made to be 0.55 g/cm3by changing the pressure at the time of rolling, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

A secondary power source was obtained in the same manner as in Example 1 except that the density of the negative electrode was made to be 1.25 g/cm3by changing the pressure at the time of rolling, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

Here,FIG. 1is a diagram illustrating the relation of the density of the negative electrode with the initial capacity and the change in capacity in Examples, from the results of Examples 1 to 4 and 6 and 7.

According to the present invention, a secondary power source having a large capacity, a high upper limit voltage and high rapid charge and discharge cycle reliability, can be provided.

The entire disclosure of Japanese Patent Application No. 2001-63627 filed on Mar. 7, 2001 including specification, claims, drawings and summary are incorporated herein by reference in its entirety.