1. Field of the Invention
The present invention relates to a non-aqueous inorganic electrolyte cell using oxyhalides as solvents for the electrolyte and active cathode materials, and using alkali metals as active anode materials. More particularly, the present invention relates to provision of a non-aqueous inorganic electrolyte which is not affected by charging at a voltage as high as 5 V as caused due to erroneous operations or accidents and which has a good discharging performance.
2. Description of the Related Art
Recently, non-aqueous inorganic electrolyte cells using oxyhalides such as thionyl chloride and the like as solvents for electrolytes and active cathode materials, and using alkali metals as active anode materials have been received particular attention as having a discharge voltage not less than 3 V, a higher energy density, and excellent characteristics at lower temperatures as compared with other non-aqueous electrolyte cells.
FIG. 1 is a cross-sectional view of a typical non-aqueous inorganic electrolyte cell using an oxyhalide as a solvent for electrolyte and an active cathode material. Referring to the Figure, the reference number 1 is an anode of lithium, 2 is a separator of non-woven glass fiber fabrics, and 3 is a porous cathode of carbon such as artificial graphite or carbon black. 4 is a cathode collector, and 5 is a cell container made of stainless steel and also acting as an anode collector. 6 is a sealing plate made of stainless steel, the perimeter of which is welded onto the opening of the cell container 5. Inside the central hole of the sealing plate 6 is formed a glass seal 7, through which a metal pipe 9 has been disposed and welded thereto. After injecting an electrolyte through the metal pipe 9, a cathode collector 4 is inserted through the pipe into the porous cathode 3 along the central axis thereof. The cathode collector 4 and the metal pipe 9 are welded to each other to seal the inside of cell. An example of electrolytes to be used in this type of cell is, for example, a mixture of thionyl chloride containing 1.5 mol/l of CiAlCl.sub.4 dissolved therein, which thionyl chloride functions concurrently as cathode active material. 10 is an upper spacer composed of the similar materials to those of the separator, and 8 is a space in the cell where free thionyl chloride is present. 12 is a bottom spacer.
In the above structure, the separator 2 will have an insufficient mechanical strength if it is made of glass fibers alone. Therefore, a binder such as polyvinyl alcohols, polyvinyl acetates, polyvinyl chlorides, acrylic resins, or other synthetic resins should be used therewith, as disclosed in Japanese Patent KOKAI (Laid-open) Nos. Sho 59-14260 and 61-16465.
A content of the binder should be in a minimum amount sufficient to impart a mechanical strength to the separator. If the content is too high, the uniform distribution of electrolyte is prevented, the electric resistance of the separator is increased, and a reaction of the binder with oxyhalides during storage is caused to evolve hydrogen chloride and the like, which will result in an increase in the pressure within the cell and deterioration of the discharging performance thereof, as reported in the art. In this regard, for example, Japanese Patent KOKAI (Laid-open) No. Sho 59-14260 describes the use of a separator having a content of glass of 85 to 95% by weight, and Japanese Patent KOKAI (Laid-open) No. Sho 62-254357 describes the use of a separator containing no binder.
In the above structure, the thionyl chloride containing CiAlCl.sub.4 is present in the porous cathode 3, separator 2, and the spacer 10 as impregnated therewith, and further on the spacer as liberated thionyl chloride 11. The thionyl chloride impregnated in the porous cathode 3 is consumed with discharge so that the thionyl chloride existing in other parts must be transferred to the porous cathode 3 in order to allow continuous discharging. Moreover, the thionyl chloride containing LiAlCl.sub.4 acts also as electrolyte. Therefore, the velocity of migration of the thionyl chloride containing LiAlCl.sub.4 in the separator and the spacer has a great influence on the discharging capacity and the characteristics under a high load of the cell.
An extensive study has been made on the relationship between the separator or the spacer and the discharge characteristics and preservability, as described above.
On the other hand, the liquid thionyl chloride used in the aforementioned arrangement is highly volatile and corrosive, and harmful to health. For the reason, the cells have been sealed by welding with laser beam. In another aspect, however, it is this complete sealing to provide a risk of blast or explosion of the cells due to erroneous operations such as charging and short circuit. This type of cell has been widely used in memory backup circuits, in which it is connected in parallel with a 5 V power supply with a diode being disposed in series therebetween for the purpose of preventing charging. Notwithstanding, there has been a high risk of the explosion when the diode is broken down from any possible causes.
As measures of imparting a safety against charging to the separator or spacer, the aforementioned Japanese Patent KOKAI (Laid-open) No. Sho 59-14260 requires glass fibers having a specific diameter and length and a specified content of glass. As to the separator having a glass content in a lower range, however, any study on the safety to resist charging has not been made into details yet by those skilled in the art including us.
As to the migration velocity of the thionyl chloride in the separator and spacer, we have already proposed for improvement in discharge characteristics of the cells standing upside down that the separator should be projected partly into the space so as to contact the electrolyte therein, and that the separator having a water absorbance of 20 mm/10 minutes as measured according to the JIS standard 2111 should be used. The results of research show that there is always no close relationship between the water absorbance and the thionyl chloride absorbance. For example, a low water absorbance does not always mean giving a low thionyl chloride absorbance, like the case where the thionyl chloride absorbance was 9.5 mm/10 minutes, or where the thionyl chloride absorbance was 23.0 mm/10 minutes where the water absorbance was 7.5 mm/10 minutes. Furthermore, it has been found that there occurs such a problem of safety as mentioned above, when the thionyl chloride absorbance is too high.