Well, lithium is good, but for budget devices, they're all designed with 1.2v-1.35v in mind...
So I can't just stick normal lithium cells in there.
You wouldn't get one of those weirdo 1.5v lithiums.. they're not well regarded.. and many times, if you put them in there, they won't charge properly because the circuit is designed to charge nicd or nimh..
Lithium requires more voltage to charge, so if its detecting a higher starting volt, it won't even charge right..
And because lithium REQUIRES that last bit of voltage which accounts for 10-20 % of it's capacity, a swap from nicd//nimh into lithium is mostly not possible.