The reason that manufacturers have switched to Li batteries (from NiCads) is because a NiCad of the same capacity (and therefore running time) would be considerably larger and heavier, perhaps as much as twice the weight.

Also, the voltages are different. NiCads are 1.2 volts per cell and Li-ion batteries are 3.7 volts per cell. If you have typical 7.4 volt setup (2 cell Li-ion battery), this would require 6 Nicad cells (7.2 volts nominal) to achieve similar voltage.

Of course, these are nominal voltages. The actual voltage varies over the discharge cycle. Most LEDs are not very sensitive to voltage variations, but some other types of equipment are.