Rechargeable electrochemical cell with a negative electrode comprising a hydrogen absorbing alloy including rare earth component

DETAILED DESCRIPTION The object of the present invention is to provide a long-life rechargeable electrochemical cell with a negative electrode comprising a hydrogen absorbing alloy which can be manufactured at low cost and whose electrochemical capacity does not decrease even after a number of charging and discharging cycles.
According to the invention, this object is achieved by means of a rechargeable electrochemical cell comprising a positive electrode, an alkaline electrolyte, a separator and a negative electrode constituted mainly by the hydrogen absorbing alloy containing a mixture of at least two rare earth elements including Ce as a rare earth component in which the Ce content is less than 40 wt % relative to the total amount of rare earth elements.
It is particularly suitable to use a mixture of rare earth elements prepared from ordinary mischmetal from which Ce has been partially removed to make the Ce content less than 40 wt % relative to the total amount of the rare earth elements as the rare earth component of a hydrogen absorbing alloy, since it permits easy manufacture at low cost.
The Ce content in this case is preferably 0.
1-12 wt % relative to the total amount of the rare earth elements, since making it this value improves the cell characteristics.
The optimum range is 0.
1-8 wt %.
The rare earth component of the hydrogen absorbing alloy constituting the material of the negative electrode in the cell according to the invention can be easily manufactured by, for example, effecting removal of a certain amount (partial removal) of Ce from the raw materials during the process of produce of commercial mischmetal (containing 40-50 wt % of Ce).
In more detail, on roasting of naturally produced, refined bastnaesite or monazite, etc.
, only Ce can become a quadrivalent oxide, while the other rare earth elements become trivalent oxides, and so hydrochloric acid extraction of the oxides results in precipitation only of Ce, which can easily be separated by filtering.
Rare earth elements which include a small amount of Ce contained in the resulting solution are precipitated as hydroxides by NH