Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-06-02
2001-07-24
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218100
Reexamination Certificate
active
06265109
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a battery, especially to a non-aqueous electrolyte secondary battery. More especially, the present invention relates to an improvement in the negative electrode thereof.
The recent years, with the rapid development of portable or cordless electric appliances, demands are growing greater for such batteries having a high energy density that can drive these appliances for a long period of time.
For these demands, lithium ion secondary batteries, for instance, are attracting much attention. The lithium ion secondary batteries can obtain a high energy density by employing Li as the negative electrode thereof. However, Li is very expensive because the resources thereof are limited mainly to seawater and rock salt that contain only a scarce percentage of Li. Therefore, there is no promising possibility of reducing the price of the Li ion secondary batteries in future even by conducting a large-scale manufacturing.
For those reasons, an investigation directed to a secondary battery having a further high capacity is briskly conducted in recent years on a non-aqueous electrolyte battery employing a metal such as Mg or Al that produces polyvalent cations such as Mg
2+
or Al
3+
as a negative electrode active material. For instance, in a battery system employing Mg
2+
, two electrons are transported by a reaction per one atom of Mg. Therefore, Mg battery has a higher energy density than the Li battery.
In addition, Mg is abundant in natural resources. For that reason, Mg takes up a very large expectation as a negative electrode material. The secondary battery system employing the metal that produces a polyvalent cation such as Mg
2+
is proposed in, for instance. Japanese Unexamined Patent Publications Sho 62-211861, Hei 1-95469, Hei 4-28172, respectively.
In the field of aqueous electrolyte secondary battery employing the polyvalent cation, lead-acid storage batteries, nickel-iron secondary batteries and nickel-zinc secondary batteries have conventionally developed as a low cost secondary battery. However, these aqueous electrolyte secondary batteries are difficult to raise the cell voltage because of the decomposition voltage of water. Further, in such nickel-iron secondary batteries and nickel-zinc secondary batteries, since decomposition of water in the electrolyte and a dry up of the aqueous electrolyte occur due to a high voltage during the charging stage, there is a need for supplementing water.
In the non-aqueous electrolyte secondary batteries which employ the polyvalent cation, an insulating layer may be formed on the surface of the negative electrode. Once the insulting layer is formed, the negative electrode becomes non-active and the overpotential becomes large. Therefore, the output current characteristic, the discharge capacity, voltage and cycle life characteristic of the battery are deteriorated. For these reason, utilization rate of the Mg negative electrode has been limited to as small as 10% to 20% of the theoretical capacity.
In addition, there are number of counter ions around the polyvalent cations such as Mg
2+
and they hinder the immigration of the polyvalent cations. The non-aqueous electrolyte itself which has an enough ionic conductivity is the key factor to realize the higher energy density batteries. The electrolyte solution for the Li ion secondary battery employs a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC), &ggr;-butyrolactam (&ggr;-BL) or the like in which an electrolyte salt such as LiPF
6
or LiBF
4
are dissolved. Such electrolyte has a considerably high ionic conductivity in the case of monovalent ion of Li
+
, but does not have a good conductivity in the case of the polyvalent cation such as Mg
2 +
.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to solve the above-mentioned problems associated with the prior art. The present invention provides a high capacity battery which employs Mg for the negative electrode.
The present invention employs a negative electrode active material in a powder state having a large specific surface area, thereby improving the utilization rate of the Mg negative electrode.
The battery of the present invention comprises a negative electrode active material including fine particles of a magnesium alloy.
The fine particles are preferably small, and most suitably the average particle size of the fine particles is not larger the 70 &mgr;m.
The magnesium alloy contains at least one element selected from the group consisting of In, Ga, Sn, Pb, Cd, Mn, Co, Zn and Tl. In order to secure a satisfactory discharge capacity, it is desirable that the magnesium alloy contains magnesium at not less the 70 atomic %.
It is further effective for the improvement in the cycle characteristic if Ni or Cu is added to the surfaces of the active material fine particles. Such coating of Ni or Cu is also applicable to a magnesium metal fine particle. It is preferable that the amount of the added Ni or Cu is not more that 10 wt % of the fine particles. The Ni or Cu is added to the surfaces of the fine particles by means of, for instance, mixing, mechano-fusion process, or plating.
Incidentally, the non-aqueous battery employing the fine particles prepared by jet-milling (fluid energy milling) or mechanical pulverization in wet state as the negative electrode active material can not exhibit satisfactory performance. This is because the oxide or hydroxide firm coating which is an electrically insulation is formed on the surface in powdering step. Therefore, it is preferable to employ as a negative electrode active material a Mg alloy prepared by a process selected form the group consisting of gas-atomizing process, high-frequency melting process, ball-mill process, planetary ball-mill process and thermal diffusion process.
In another preferred mode of the present invention, the battery includes a non-aqueous electrolyte.
The preferred electrolyte contains a solvent including an acid amide and an electrolyte salt dissolved therein, thereby to ionically dissociate the salt into a polyvalent cation and an anion.
In a case of using a metal that produces the polyvalent cation for the negative electrode, it is effective to use such a solvent that has a large donor number or acceptor number. Further, in order to secure a high ionic conductivity, it is desirable to use a solvent having a high specific dielectric constant. The acid amides is suitable for this requirement. Although they have a slightly narrow potential window as compared with polypropylene carbonate (hereinafter referred to as “PC”) or the like, they are almost within the range of the dissociation potential, for a metal other that Li.
Preferable acid amide is, for instance, N-methylformamide (hereinafter referred to as “NMF”) or N,N-dimethylformamide (hereinafter referred to as “DMF”). In particle, NMF has a very high dielectric constant of about 186. In contrast, DMF has a feature that the specific dielectric constant becomes high when it dissolves an electrolytic salt therein. By employing an electrolyte further containing at least one other solvent selected form the group consisting of dimethyl acetoamide (hereinafter referred to as “DMAA”), acetonitrile (hereinafter referred to as “AN”), ethylene carbonate (hereinafter referred to as “EC”), propylene carbonate (hereinafter referred to as “PC”) and &ggr;-butyrolactam (hereinafter referred to as “&ggr;-BL”), it is possible to widen the electric potential window and to increase the donor number and the acceptor number.
It is further preferable if the polyvalent cation contained in the electrolyte in Mg
2+
. Further, the electrolyte salt is preferably a halogenide or a perchlorate of the polyvalent cation.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction w
Ito Shuji
Kanbara Teruhisa
Yamamoto Osamu
Chaney Carol
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
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