can you charge a lithium ion battery 18 volt in a nicad battery charger

No, and it would be dangerous to do so.
Li-ion batteries are prone to overheating if charged too fast and in the best case scenario the battery itself will be permanently damaged. In the worst case scenario the overheating will lead to expansion, case rupture and combustion. Outside of the battery case lithium reacts violently with the water in the air and bursts in flames. I'm pretty fortunate to have had a school chemistry teacher great many years ago who was not afraid to show the reaction in the lab. As awesome as it looked, I would not want that to happen anywhere in my house, not the least without me standing there with an extinguisher.
Anyhow, because of that threat, all Li-ion battery packs are equipped with a special fail-safe circuitry that checks on the charging current and the temperature of cells, and the charger should be intelligent enough to understand the shutdown signal from it and respond accordingly.
A charger designed for NiCad or NiMH batteries would not know what to do when the Li-ion battery starts the dreaded thermal runaway.  Do not use NiCad charger with Li-ion batteries.
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