They need a charger built for lithium-ion cells, which stops at 4.2 volts per cell; a NiMH or alkaline charger is not safe for them. Never leave lithium cells charging unattended. Before using this RIX charger with 18650s, ask us to confirm it supports that size, since the listing doesn't say yet.
About 4.2 volts. A standard lithium-ion 18650 has a nominal voltage of 3.6 to 3.7 volts and sits near 3.0 volts when it is drained. If a cell reads well below that or won't hold a charge, retire it instead of trying to force it back up.