Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1997-08-21
2001-08-14
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231500, C429S232000
Reexamination Certificate
active
06274271
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improvement in performance of a non-aqueous electrolyte lithium secondary battery comprising a negative electrode of a lithium-titanium oxide.
With the recent rapid development of technology in the field of electronics, electronic appliances have been miniaturized remarkably, and the demands for batteries of smaller size and lighter weight with a high energy density are increasing accordingly. As such batteries, lithium secondary batteries configured with a lithium negative electrode are attracting attention in this field of art, and researches and developments of these batteries are now briskly conducted in global scale. However, if metallic lithium is utilized in the negative electrode, the lithium may sometimes be pulverized and arborescent crystals called lithium dendrites may sometimes be formed on the lithium negative electrode with repetition of charge and discharge. In such cases, the shape of the lithium negative electrode may be damaged and a satisfactory charge/discharge cycle life cannot be obtained, and in an extreme case, the lithium dendrites pierce through a separator and cause an inner short-circuiting of the battery.
As a means for solving the above-mentioned inconvenience, an investigation is now conducted for a negative electrode which comprises a lithium-aluminum alloy, a lead-containing alloy capable of intercalating and deintercalating lithium, various carbon materials, an oxide of transition metal such as niobium pentoxide, an anatase-type titanium oxide, tungsten dioxide, or a ferrous or ferric oxide, each doped with lithium. For preventing against the lithium dendrite formation, carbon or an oxide of transition metal is excellent in particular, and it has been found that the charge/discharge cycle life of the battery can greatly be improved by employing these materials in the negative electrode.
Recently, lithium secondary batteries are used in various appliances as a direct-current power source for a mobile unit, a power source for memory back-up and the like. In order to fully satisfy the required performances, it is extremely important to fulfill not only sufficient energy density or charge/discharge cycle life characteristic, but also high reliability of the battery, in particular, withstanding overcharge and overdischarge characteristics of the battery. However, a combination of the above-specified negative electrode and a conventional positive electrode may not necessarily give a battery that has the withstanding overcharge and overdischarge characteristics.
BRIEF SUMMARY OF THE INVENTION
As a result of experiments of various metal compounds for configuring the negative electrode, the present inventors have found that a lithium-titanium oxide of spinel-type structure which is disclosed in Japanese Laid-Open Patent Hei 6-275263 shows excellent charge/discharge performance, and that a negative electrode comprising this material is excellent in stability in a wide potential range which cannot be obtained with any other negative electrode materials.
The primary object of the present invention is to provide a non-aqueous electrolyte secondary battery which has improved withstanding overcharge and overdischarge characteristics as the battery system, which was realized by focusing on these characteristics of the lithium-titanium oxide of a spinel-type structure.
The present invention provides a non-aqueous electrolyte lithium secondary battery comprising
a negative electrode which comprises at least one spinel-type lithium-titanium oxide,
a positive electrode which has a higher potential than that of the spinel-type lithium-titanium oxide, and
a non-aqueous electrolyte,
wherein rechargeable electric capacity of the negative electrode is smaller than that of the positive electrode.
In the conventional lithium secondary battery which utilizes metal oxides as positive and negative electrodes, it is very difficult to satisfy both withstanding overcharge and overdischarge characteristics at the same time, even if the electric capacity of the positive electrode is kept balanced to that of the negative electrode by some means. The reason for this is that although there exists a number of positive and negative electrodes which can function stably in a normal charge/discharge potential range, there are very scarce materials with a constant stable crystal structure in a very wide potential range over the overcharge and overdischarge potentials which greatly exceed the normal charge/discharge potential region.
The present inventors have now found that among a number of metal oxides, the lithium-titanium oxide having the spinel structure exists in the very stable state without being destroyed its crystal structure over a very wide potential range from 0 V to 4 V, despite the fact that a plateau potential during the charge/discharge processes is near 1.5 V with respect to metallic lithium (in the following description, “potential” is used by taking the potential of metallic lithium as reference).
Since intercalating and deintercalating reaction of lithium ions in and from the spinel-type lithium-titanium oxide crystal proceeds almost free from distortion of the crystal lattice of the oxide, it is believed that the spinel-type lithium-titanium oxide crystal not only demonstrates excellent charge/discharge performance but also very strong resistance against a potential stress.
The present invention is completed by focusing on this very stable characteristic of the spinel-type lithium-titanium oxide in a wide potential range. The electric capacity of the negative electrode is made smaller than the electric capacity of the positive electrode in the reversible region in order to best utilize such characteristic. By doing so, the potential of the positive electrode does not vary greatly even at the end of the discharging process and remains in the reversible region, and thus the crystal structure is maintained stable without being destroyed.
On the other hand, although the potential of the negative electrode is close to the potential of the positive electrode during the discharging process, the negative electrode is not essentially destroyed and exists stably even if the potential of the positive electrode remains as high as 3 V or 4 V, and thus the negative electrode can strongly withstand against the overdischarge.
And, even if the battery is stood still while holding the discharge circuit in a state close to a complete discharge for a long period of time, the performance of the electrode is not deteriorated.
In addition, in the charging process, since the electric capacity of the negative electrode is smaller than that of the positive electrode, the potential of the positive electrode does not vary greatly and the charging process is completed by lowering the potential of the negative electrode.
At that time, since the negative electrode can maintain its stable state until at least a potential of 0 V, the upper-limit within which the battery can be overcharged can be set higher by at least 0.15 V than the plateau voltage during the charging process, and the reliability for withstanding the overcharge is greatly improved.
In the description of the present invention, the term “electric capacity of the positive electrode or the negative electrode” means a reversible and chargeable or dischargeable capacity of the respective electrodes under standard conditions equivalent to an ambient temperature and charge/discharge current, as far as a battery configured with these positive electrode and negative electrode is utilized. Accordingly, the present invention establishes a relationship between the negative electrode with an electric capacity which is substantially equal to the chargeable and dischargeable electric capacity of the battery configured with this electrode, and the positive electrode with an electric capacity which is larger than the above-mentioned capacity. Since these electric capacities vary slightly depending on the employed measuring conditions, the values of the electric capacities of the pos
Asaka Emi
Chikayama Koichi
Koshiba Nobuharu
Sano Yoko
Chaney Carol
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
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