Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2002-03-08
2004-11-30
Yuan, Dah-Wei (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231400, C429S231800, C429S231900
Reexamination Certificate
active
06824923
ABSTRACT:
The present invention relates to a secondary power source having a large discharge capacity and excellent charge and discharge cycle reliability at a large current.
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 3.0 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 3.0 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 4.0 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, can be operated at a high voltage and has a large capacity as compared with a conventional electric double layer capacitor which employs activated carbon for both positive electrode and negative electrode.
Further, a lithium ion secondary cell which employs a lithium-containing transition oxide for a positive electrode and a carbon material for a negative electrode is available as a secondary power source with high performance 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 high voltage and has a large 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.
Both the secondary power source other than the above electric double layer capacitor and the lithium ion secondary cell have an upper limit voltage of at least 4.0 V, however, a useful capacity can be obtained within a voltage range of from the upper limit voltage to the vicinity of 2.7 V. Substantially no capacity can be obtained at a voltage of at most 2.7 V, and accordingly the secondary power source other than the above electric double layer capacitor and the lithium ion secondary cell can not be applied to a use with an operation voltage of at most 2.7 V.
Under these circumstances, it is an object of the present invention to provide a secondary power source which has quick charge and discharge capability and has a high energy density and which has 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 Li
4
Ti
5
O
12
and an organic electrolyte containing a lithium salt.
The present invention further provides a secondary power source, which comprises a positive electrode containing activated carbon, a negative electrode containing Li
4
Ti
5
O
12
and a carbon material capable of doping and undoping lithium ions, and an organic electrolyte containing a lithium salt.
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 Li
4
Ti
5
O
12
capable of doping and undoping lithium ions. Likewise, a positive electrode assembly is one obtained by bonding and integrating a current collector and a positive electrode containing activated carbon. 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 Li
4
Ti
5
O
12
and/or a carbon material capable of doping and undoping lithium ions, will be referred to simply as a secondary power source.
With respect to lithium titanate represented by Li
4
Ti
5
O
12
having a spinel crystal structure, the charge and discharge potential is in the vicinity of 1.5 V relative to Li
+
/Li potential, whereas with respect to activated carbon of the positive electrode, polarization up to from 4.0 to 4.6 V is possible relative to Li
+
/Li potential. Accordingly, the upper limit voltage of a new secondary power source obtained by combining a positive electrode containing activated carbon and a negative electrode containing Li
4
Ti
5
O
12
is from 2.5 to 3.1 V, and the lower limit is 1.5 V.
The doping and undoping reaction of lithium ions to the carbon material takes place mainly within a range of from 1.0 to 0 V relative to the Li
+
/Li potential, and if the potential is at least 1.0 V, no capacity can be substantially obtained. Accordingly, in a case where a positive electrode containing activated carbon and a negative electrode containing a carbon material alone are combined, the effective operation voltage is within a range of from 4.6 V to 2.5 V. Namely, by incorporating Li
4
Ti
5
O
12
into the negative electrode, the lower limit of the operation voltage can be lowered. Further, a new secondary power source obtained by combining a positive electrode containing activated carbon and a negative electrode containing both carbon material and Li
4
Ti
5
O
12
, operation can be carried out with a broad voltage range of from 4.6 V to 1.5 V.
Further, Li
4
Ti
5
O
12
has a spinel crystal structure wherein change in structure is small due to doping and undoping of lithium ions at the time of charging and discharging, and accordingly a secondary power source employing Li
4
Ti
5
O
12
as an active material of the negative electrode is stable and excellent in durability. On the other hand, in a case where a carbon material capable of doping and undoping lithium ions is used for the negative electrode, at the time of charging and discharging, a change in dimension takes place in a C axis direction of carbon crystals along with doping and undoping of lithium ions into between layers of the carbon material of the negative electrode. This change in dimension causes defects such that a coating on the carbon surface is broken or the bonding between carbon particles loosens, thus leading to decrease in capacity of the cell or to increase in the resistance.
According to further studies by the present inventors, it was found that by using as a negative electrode a mixed system of a carbon material and Li
4
Ti
5
O
12
, deterioration by the negative electrode can be minimized, and charge and discharge cycle performance improve as compared with a negative electrode containing either the carbon material or Li
4
Ti
5
O
12
alone. The mechanism is not clearly understood yet, but it is considered that the potential at which the doping and undoping reaction of
Che Yong
Morimoto Takeshi
Tsushima Manabu
Asahi Glass Company Limited
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Yuan Dah-Wei
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