Lithium-ion batteries have a higher energy density as compared to NiMH batteries. Therefore, a Lithium-ion battery stores more energy than NiMH batteries.
Lithium-ion batteries operate at higher voltages, that is, 3.7 volts of lithium-ions, while NiMH operates at 1.2 volts. Hence, a single cell can be used in the place of multiple NiMH cells.
Lithium-ion batteries have lower self-discharge rates as compared to NiMH batteries. This enables the Lithium-ion batteries t to retain charge for a longer time as compared to NiMH.
When not in use, NiMH batteries still lose 1-5% of charge per day, whereas, Lithium-ion batteries retain most of its charge even if stored for months.
To sum up the above points, Lithium-ion batteries are smaller in size, lighter in weight, carry a higher voltage of power, and have the capacity to hold a charge for a much longer time as compared to NiMH batteries. Therefore, Lithium-ion batteries are better than NiMH batteries in every aspect.