1. Field of the Invention
The present invention relates to an improvement in a negative electrode plate for a lead storage battery.
2. Description of the Related Art
Lead storage batteries are used for a lot of purposes widely from starting/lighting batteries for cars and small-capacity consumer batteries to large-capacity stationary batteries. Further, recently, the lead storage battery has attracted public attention also as an electric source for an electric car from the point of view of environmental problems.
The battery for an electric vehicle requires a large current and repetition of deep charging and discharging. Further, because the battery is arranged in a narrow space in order to maximize the interior space of a car, release of heat becomes insufficient. As a result, the battery is exposed to a high temperature. Accordingly, in the battery for an electric car, the high-rate discharging characteristic of the negative electrode may be lowered in a relatively early stage.
Generally, an organic expander, an inorganic expander (barium sulfate) and carbon are added to a negative electrode plate for a lead storage battery so that they contribute to improvement in various performances of the negative electrode plate for a lead storage battery. Of these added materials, the organic expander is generally constituted by a by-product called lignin (or lignin derivatives) obtained at the time of production of pulp, so that the organic expander suppresses the contraction of the active material and finely divide the active material to thereby prevent the lowering of the discharging capacity of the negative electrode plate, especially, the lowering of the high-rate discharging capacity of the negative electrode plate with the charging/discharging of the battery.
In the conventional negative electrode plate, it was difficult to obtain life performance satisfactory for use under a high temperature, for example, in application to a lead storage battery for an electric vehicle as described above. This suggests that the amount of lignin decreases due to decomposition or dissolution to an electrolyte when lignin is exposed to a high temperature.
As one of measures to solve this problem, increase of the amount of lignin added was considered. When the amount of lignin was increased, however, there arose a problem that the capacity was lowered rather in an early stage because charge acceptance after discharging was lowered to bring about a failure in charging so that lead sulfate accumulated in the negative electrode active material.
It is an object of the present invention to provide a negative electrode plate for a lead storage battery capable of preventing the lowering of the life performance even under a high temperature.
In the present invention, graphite powder having a mean particle size equal to or not larger than 30 xcexcm is added into the negative electrode plate of the lead storage battery. Further, the mean particle size of the graphite powder is equal to or not smaller than 0.5 xcexcm in the present invention, because if it is less than 0.5 xcexcm, the conductivity of the graphite can not sufficiently obtained. Moreover, the mean particle size of the graphite powder is preferably in the range of 5 to 30 xcexcm. The amount of graphite powder added is preferably in the range of 0.3 to 2% by weight, preferably, 0.3 to 0.8% by weight, and preferably crystalline graphite powder is used as the graphite powder. Incidentally, crystalline graphite includes flake graphite and vein graphite. Vein graphite is more preferable. More preferably, 0.3% by weight or more of an organic expander is added thereto.
According to the present invention, not only the performance of the battery can be improved but also the lowering of the high-rate discharging capacity under a high temperature can be prevented.