1. Field of the Invention
The present invention relates to a lithium battery, and more particularly to a lithium primary battery having coiled electrode members for use in supplying high current.
2. Description of the Prior Art
In a conventional coiled type lithium battery (FIG. 1), an electrode body of a cylindrical shape of the battery is formed in such a manner that a negative electrode plate 1 made of a strip sheet of lithium or lithium alloy and a positive electrode 2 made of metal charge collecting mesh holding a positive electrode agent are laminated through a separator 3 made of a non woven cloth, such as polypropylene fibers, and are coiled in a roll.
Moreover, in order to avoid collapse of the roll of the electrode members, the outer end part of the separator 3, wound around the outer cylindrical surface of the electrode body, is adapted to contact with the inner cylindrical surface of a negative electrode can except for a portion of an adhesive tape wound around the central portion of the outer cylindrical surface of the roll of the electrode body. At the inner central portion of the roll of the electrode body, namely at the beginning of the winding of the electrode body, a rolled portion 3a of only the separator 3 is formed and the stripe-like positive electrode 2 is disposed outside of the rolled portion 3a and the strip-like negative electrode 1 is disposed outside of positive electrode 2 through the separator 3.
The inner end 1a of the strip-like negative electrode plate 1 ends beyond the inner end 2a of the positive electrode plate 2 toward the inner portion of the roll of the electrode body in order to prevent slip of the respective members of the electrode body and to facilitate winding of the electrode body when the electrode body is wound by a winding machine by clamping the inner end of the separator 3 by the rotation spindle of the machine. Accordingly, as shown by 3a, only the separator 3 is rolled along some length (normally 90.degree. to 180.degree.) from the inner end la of the strip-like negative electrode plate 1.
As the collecting mesh of the positive electrode, there is used an expanded metal plate made of stainless steel which metal plate is cut by a number of slits directed in one direction and the metal plate is expanded in a direction perpendicular to the slit to open the slits. Conventionally, the stripe-like positive electrode plate is formed in such a manner that the expanded metal is sandwiched by positive electrode agent sheets made of a positive active agent, electrically conductive powder and a binder with the positive electrode agent sheets bonded to the expanded metal by a rolling machine.
However, according to recent developments in the wide field of use of the battery, there are increasing cases of wrong use of the battery of this kind such that the battery, despite the fact that the battery is a primary battery, is erroneously subjected to charging or forced discharging by an external power source. An example of forced discharging, which occurs many times, is that one or more old batteries, which are already used and the discharging capacity thereof is depleted or small, are connected with one or more new batteries of which the discharging capacity is plentiful. In this case, if the battery of which the discharging capacity is depleted is forced to be discharged, the battery is loaded with a minus voltage by the current supplied from the other healthy battery.
In the conventional coiled type lithium battery, if such error occurs, the battery is apt to generate heat due to an internal short circuit and there are many cases of the batteries catching fire.
The holding of the positive electrode agent in the positive electrode is done mainly by bonding of the positive electrode mix, carried on the expanded metal on both sides of the slit. However, in case of expanded metal, if the slit is opened too wide, the expanded metal is apt to be cut off. Therefore, in general, the opening rate of the expanded metal is small, about less than 70%, whereby the bonding areas are insufficient, the sufficient holding force can not be obtained and therefore there is a problem that the positive electrode mix can be easily separated from the expanded metal at the time of formation of the coiled electrode body.
In case the pressing force of the positive electrode agent sheet is increased for increasing the bonding force of the positive electrode agent and electric collecting ability, there is such a problem that rigid deformation of and cut-off in the expanded metal sheet easily occur. There is a further disadvantage that since the edge of the opened slit and the cut off edge are acute in the expanded metal sheet, the separator is apt to be damaged.
It has also been discovered that when the lithium batteries are subjected to work using relatively large current, the batteries generate a lot of heat and the temperature of the battery becomes higher at the central portion, and the insulation layer 3a is molten, whereby an internal short circuit occurs at the inner end 1a of the negative electrode plate 1 and the inner end 2a of the positive electrode 2 and a further large amount of current flows there with the generation of further heat, causing the lithium and lithium alloy to become molten resulting in burning or an explosion of the battery.