These are available in cylindrical and button formats. Zn/HgO have the highest volumetric energy density of conventional cells, a flat discharge curve and a good shelf life, but they are expensive. They are mostly used for special applications such as hearing aids, cameras, intruder alarms, pacemakers, sensors, detectors, and radio transmitters. However, they are being phased out in favor of lithium cells because of the toxicity of mercury and poor performance below 0oC (the performance of spirally wound cells is extended to -20oC).
Cd/HgO cells have a better storage life than Zn/HgO, because of the lower solubility of Cd in the electrolyte. Although the cell voltage is only 0.9V, the cells operate very efficiently over a large temperature range, from -55oC to 80oC. Unfortunately, the Cd/HgO cells are even more expensive than the Zn/HgO ones and cadmium is also toxic; therefore, these cells are only used in very special applications.
It is important to be aware that the percentage of Mercuric batteries is decreasing as more countries are adopting legislation to prohibit their sale. The batteries are shredded; the electrolytes are neutralized; and the heavy are recovered through a controlled temperature process. The remainder of the battery is non-hazardous and is interned into a double-lined environmental protection facility.