Nickel-cadmium was the default emergency lighting cell for decades. It is robust, tolerant of overcharge, and cheap. It is also being designed out of new products, and for good reasons.
Cycle life
A Ni-Cd cell in emergency service typically manages 500 to 1000 charge cycles before capacity falls below the point where it can sustain rated duration. LiFePO4 reaches 2000 cycles and beyond. In practice this moves battery replacement from a four-year interval to something closer to the life of the luminaire.
Temperature
Both chemistries lose capacity at the extremes, but Ni-Cd degrades faster under sustained heat, which is exactly the condition inside a sealed ceiling luminaire. LiFePO4 holds its capacity better across the 0 to 40 C operating band that most installations sit in.
Memory and self-discharge
The memory effect that plagued Ni-Cd in partial-discharge service does not apply to LiFePO4, and self-discharge is lower, so a unit that has been in storage reaches usable capacity faster after commissioning.
End of life
Cadmium is a restricted heavy metal, and disposal is regulated accordingly. LiFePO4 contains no cobalt and no cadmium, which simplifies both the disposal route and the compliance paperwork for a facility manager.
The trade-off
LiFePO4 costs more per cell and needs a proper protection circuit against overcharge and deep discharge, which is why every Polaryx board carries one. Across a ten-year building life, the lower replacement count more than covers the difference.