An alkaline battery gives 1.5 volts, while a lithium battery is rated at 1.75. Voltage tells us how powerful the force is. So if we compare two flashlights where all else is equal, the light with the lithium battery will give a brighter beam. The capacity of a battery tells us how long the force lasts. Both alkaline and lithium batteries have a high capacity.
Both batteries have a slow discharge rate, meaning they have a long shelf life. An alkaline battery has a shelf life of up to five years. The lithium battery also has a long shelf life in everyday circumstances, and clearly outshines the alkaline battery in sub-freezing temperatures.
In ease of disposability, the alkaline battery wins hands down. It can be thrown in the regular trash, whereas a lithium battery is toxic and needs special disposal.
The lithium battery is expensive. An alkaline battery is cheap, and easily available--an important consideration for a flashlight battery.
A lithium battery in a standard, household size like AA or AAA is not rechargeable. Neither is the standard alkaline battery. There is now a rechargeable alkaline alternative available, which is well suited to a periodically used device such as a flashlight. A new brand, launched in February 2009 can be recharged safely up to 60 times. It can also be disposed of in the regular fashion.