To cope with the trend towards downsizing or solidification in the field of ionics, a solid electrolyte material has been proposed as a new ion conductive material in place of conventional electrolytic solutions. Investigators have aggressively attempted to apply the electrolyte material to solid primary or secondary batteries, electrolytic capacitors, electrical double layer capacitors, photoelectric cells, solar cells, fuel cells, electrochromic elements, various sensors and antistatic film.
Conventional batteries using an electrolytic solution employ a porous thin film separator impregnated with an electrolytic solution. In this case, the production and processing cost of the porous film is high which in turn increases the cost of conventional batteries. Furthermore, the film is not capable of holding the electrolytic solution. This causes the solution to leak from the battery or causes the electrode substance to elute, thereby giving rise to problems with respect to long-term reliability and safety of the battery.
On the other hand, products using a solid electrolyte material are generally free from the above-described problems and are furthermore capable of providing a product having a reduced thickness. Additionally, the solid electrolyte has excellent heat resistance and is advantageously employed in the production of products such as batteries. In particular, batteries employing a solid electrolyte material containing a polymer as a constituent component have better flexibility as compared with those using an inorganic material, and are advantageous in that they can be formed into various shapes.
As an example of a solid electrolyte material containing a polymer as a constituent component (hereinafter also referred to as a "solid polymer electrolyte" or "polymer gel electrolyte"), Br. Polym. J., Vol. 319, page 137 (1975) describes a composite of a polyethylene oxide with an inorganic alkali metal salt. However, the ion conductivity thereof at room temperature is as low as 10.sup.-7 S/cm.
In recent years, a comb structure polymer has been reported having introduced into the side chain thereof an oligooxyethylene which elevates the thermal motility of the oxyethylene chain bearing ion conductivity, to thereby improve the ion conductivity of the polymer. For example, J. Phys. Chem., Vol. 89, page 987 (1984) describes a polymethacrylic acid having added to the side chain thereof an oligooxyethylene compounded with an alkali metal salt. Furthermore, J. Am. Chem. Soc., Vol. 106, page 6,854(1984) describes a polyphosphazene having an oligooxyethylene side chain compounded with an alkali metal salt.
U.S. Pat. No. 4,357,401 describes a solid polymer electrolyte having an ion conductivity at 50.degree. C. of approximately from 10.sup.-4 to 10.sup.-5 S/cm which can be obtained by compounding a metal salt with a cross-linked polymer having reduced crystallinity. U.S. Pat. No. 4,792,504 proposes to improve the ion conductivity by using a cross-linked solid polymer electrolyte impregnated with an electrolytic solution comprising a metal salt and an aprotic solvent in polyethylene oxide having a continuous network.
Furthermore, in recent years, an electrical double layer capacitor has been used, for example, as a memory backup power source, where a carbon material having a large specific surface area, such as activated carbon and carbon black, is used as a polarizable electrode, and an ion conductive solution is deposited between such electrodes. For example, Kino Zairyo (Functional Materials), page 33 (February, 1989) describes a capacitor employing a carbon-base polarizable electrode and an organic electrolytic solution, and 173th Electrochemical Society Meeting, Atlanta, Georgia, No. 18 (May, 1988) describes an electrical double layer capacitor using an aqueous sulfuric acid solution. Furthermore, Japanese Unexamined Patent Publication (kokai) No. 63-244570 discloses a capacitor employing Rb.sub.2 Cu.sub.3 I.sub.3 Cl.sub.7 which has a high electrical conductivity as an inorganic solid electrolyte.
However, electrical double layer capacitors using a known electrolytic solution are bound to create problems with respect to long-term use or reliability. This is because the solution readily leaks from the capacitor under severe conditions such as when the capacitor is used for a long period of time or when a high voltage is applied thereto. On the other hand, electrical double layer capacitors using a conventional inorganic ion conductive substance are disadvantageous in that the decomposition voltage of the ion conductive substance is low and the output voltage is low.
Japanese Unexamined Patent Publication (kokai) No.4-253771 proposes to use a polyphosphazene-base polymer as an ion conductive substance for batteries or electrical double layer capacitors. When a solid ion conductive substance mainly comprising the above-described polymer is used, the resulting advantages are that the output voltage is relatively high as compared with that obtained when an inorganic ion conductive substance is used, the device can be formed into various shapes, and sealing is easy.
The solid polymer electrolyte under general investigation has an improved ion conductivity of approximately from 10.sup.-4 to 10.sup.-5 S/cm. However, this is still in a low level that is two order of magnitude or more lower than the ion conductivity of a liquid ion conductive material. Furthermore, the ion conductivity considerably decreases at a temperature of 0.degree. C. and below.
In order to improve ion conductivity, the polymers for use in the solid polymer electrolyte have a low glass transition temperature. If the glass transition temperature is lowered, a problem arises in that the polymer has reduced mechanical strength which causes difficulties in industrial handling. Furthermore, when a solvent is added to further improve the ion conductivity, the mechanical strength disadvantageously is further reduced.
The polymers for use in the solid polymer electrolyte usually absorb water, and water absorptivity is a problem when used in non-aqueous electrochemical elements such as lithium (ion) batteries or electrical double layer capacitors.
In addition to the above described batteries and capacitors, the ion conductive material is an important constituent material of electrochemical devices such as electrochromic displays and power generating apparatuses such as photoelectric cells and solar cells, and as an electrochemical element for use in assembling these devices such as electrochemical power generating elements, electrochemical coloring elements and electrochemical light-emitting elements. The ion conductive material is an important constituent of antistatic materials which are capable of eliminating undesirable electrostatic effects and can also be used as a sensor material. However, the above described problems of conventional ion conductive materials with respect to batteries or capacitors are also encountered in the production of products for these additional uses. Accordingly, there is a need to overcome the above problems of the prior art, to develop solid polymer electrolyte materials having excellent ion conductivity, and to develop ion conductive materials which can be easily integrated into an electrochemical element or electrochemical apparatus as a solid polymer electrolyte.