1. Field of the Invention
This invention relates to a battery having a solid ion conductive polymer electrolyte, more particularly to a rechargeable battery (secondary battery) having a solid ion conductive polymer electrolyte.
2. Description of the Background Art
As the electrolytes of rechargeable batteries, there have mainly been used liquid substances such as water, propylene carbonate, tetrahydrofuran and the like. Since a liquid electrolyte is apt to leak, however, a hermetically sealed container has to be used to ensure its long-term stability. Because of this, electrical and electronic devices using liquid electrolytes are heavy and require complex manufacturing processes.
In contrast, electrolytes consisting of ion-conductive solids involve almost no possibility of leakage, simplify manufacture and enable reduction of product weight. Owing to these advantages, they are being vigorously researched.
Solid ion conductive electrolytes can be divided into inorganic and organic material types. Organic solid ion conductive electrolytes are superior to inorganic solid ion conductive electrolytes in the points of weight, formability and flexibility.
Organic solid ion conductive electrolytes are generally formed of a matrix polymer and an ion conductive metallic salt which is a low molecular weight compound. The matrix polymer is the most important constituent of an organic solid ion conductive electrolyte because it is responsible both for solidifying the electrolyte and for serving as a solvent for dissolving the ion conductive metallic salt.
In 1978, M. B. Armand et al., working at the University of Grenoble in France, discovered that lithium perchlorate dissolves in polyethylene oxide and reported that this system exhibits ionic conductivity of 10.sup.-7 S/cm. Since then, similar research has been conducted regarding analogous polymers, including polypropylene oxide, polyethyleneimine, polyurethane, polyester and a wide range of other polymeric substances.
Application of organic polymers to solid electrolytes for rechargeable batteries is being pushed forward for taking advantage of their various merits, which include excellent film formability, flexibility and high energy characteristics when used in batteries.
As the polymer employed in the solid ion conductive electrolyte of a battery having a solid ion conductive polymer electrolyte, it is preferable to use one that ensures good interactivity in combination with the ion conductive metallic salt used in the solid ion conductive polymer electrolyte and, from this viewpoint, the preferred characteristics of the polymer are that it:
1) Exhibit interactivity with and be capable of dissolving the ion conductive metallic salt, PA1 2) Have a donor type structure, PA1 3) Have amorphous regions and exhibit a low glass transition temperature, PA1 4) Not crystallize after dissolving the ion conductive metallic salt, and PA1 5) Not react with electrochemically active substances. PA1 Isocyanate reacts easily with moisture and is therefore difficult to manage from the points of storage and reactivity. PA1 The urethane crosslinking reaction between the polyoxyalkylene derivative of glycerin and the polyisocyanate compound is affected by the ion conductive metallic salt and solvent components. As a result, the reactivity may be reduced or the reaction be accelerated. Because of this, the method of synthesizing the polymer matrix first and then impregnating it with the ion conductive metallic salt together with an appropriate solvent is generally used, despite its poor industrial productivity. PA1 Widely used general-purpose aromatic isocyanate is susceptible to electrochemical degradation, while the reactivity of aliphatic isocyanate is low. PA1 Formation into film requires a long period of reaction under heating. PA1 a monoester compound containing a polyoxyalkylene component having the molecular structure defined by the formula ##STR2## (wherein R.sub.4, R.sub.5, R.sub.6 each represents H or C.sub.1 -C.sub.5 alkyl, preferably C.sub.1 -C.sub.3 alkyl and may be the same or different, and A and B satisfy the condition of A+B.ltoreq.50, A.gtoreq.1 and B.gtoreq.0 or the condition of A+B.ltoreq.50, A.gtoreq.0 and B.gtoreq.1). However, it is not limited to these.
Not many polymers meet all of these conditions. Polyethylene oxide, the most thoroughly researched polymer, satisfies conditions 1) and 2) to a high degree but, being a semicrystalline polymer, it forms a quasi-crosslinked structure that increases its crystallinity even further when a large amount of metallic salt is dissolved therein. It is therefore very unsatisfactory in terms of condition
4) and, as a result, exhibits conductivity that is considerably lower than might be expected.
To secure high ionic conductivity at room temperature, it is important to ensure the presence of many amorphous regions in which the ionic conductors can migrate and to use a polymer design which lowers the glass transition temperature of the polymer.
A method of introducing a branched structure into polyethylene oxide attempted for this purpose led to the synthesis of a polyethylene oxide derivative which exhibited high conductivity (about 10.sup.-4 S/cm at room temperature) as a solid ion conductive polymer electrolyte (Naoya Ogata et al., Sen'i Gakkaishi (Journal of the Society of Fiber Science and Technology, Japan) Vol 46, No 2, p52-57, 1990). Owing to the complexity of the polymer synthesis method, however, the method has not been commercialized.
Another reported method for securing high ionic conductivity is that of imparting a three-dimensional network structure to a matrix polymer so as to prevent its crystallization. In one such method, for example, a solid ion conductive polymer electrolyte is obtained by crosslinking a polyoxyalkylene derivative of glycerin with polyisocyanate compound.
Owing to the following unsolved problems, however, this method has not led to the development of a practical battery having solid polymer electrolyte: