That is an excellent question.
In purely scientific terms, Kristen Hall-Geisler of ‘How Stuff Works.com’ puts it this way:
“The most obvious difference between Li-ion and NiMH batteries is the material used to store power. Lithium-ion batteries are made of carbon and highly reactive lithium, which can store a lot of energy. Nickel metal hydride batteries use hydrogen to store energy, with nickel and another metal (such as titanium) keeping a lid on the hydrogen ions”.
Every sort has its qualities and drawbacks. For instance, the nickel-metal hydride (NiMH) is the cheaper of the two, but they’re much heavier and much more cumbersome. However, both kinds can store comparable amounts of charge, though normally Lithium-ion batteries (Li-ion) can hold more.
Generally, NiMH batteries usually last longer than their Li-ion counterparts as well, particularly when subjected to extremes of hot or cold. As Hall-Geisler points out:
“Some Li-ion batteries don’t last as long in extreme temperatures, particularly in very hot climates. But manufacturers are working to improve the chemistry to make the Li-ion batteries last as long as the vehicles they power”.
Having said all that, Li-ion is probably the battery sort with the most hopeful future, at the very least in terms of consumer gadgets. As one expert from the ‘Battery University’ blog writes,
“For many years, nickel-cadmium had been the only suitable battery for portable equipment from wireless communications to mobile computing. Nickel-metal-hydride and lithium-ion emerged In the early 1990s, fighting nose-to-nose to gain customer’s acceptance. Today, lithium-ion is the fastest growing and most promising battery chemistry”.
Lithium Ion is a relatively low maintenance technology and is mostly easier to work with than NiMH batteries. No individual memory or scheduled cycling are needed to keep it going for more time. However, Battery University is quick to note that:
“Aging is a concern with most lithium-ion batteries and many manufacturers remain silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. The battery frequently fails after two or three years”.
For its part, NiMH can be safer than Li-ion. Battery University has this to say,
“The advancements of NiMH are impressive. Since 1991, the specific energy has doubled and the life span extended. The hype of lithium-ion may have dampened the enthusiasm for NiMH a bit but not to the point to turn HEV makers away from this proven technology. Batteries for the electric powertrain in vehicles must meet some of the most demanding challenges, and NiMH has two major advantages over Li-ion here. These are price and safety. Makers of hybrid vehicles claim that NiMH costs one-third of an equivalent Li-ion system, and the relaxation on safety provisions contribute in part to this price reduction”.
You will find that Lithium-ion batteries (Li-ion) are slightly costlier than the nickel-metal hydride battery (NiMH) but the advantages clearly outweigh the added cost. They do last longer and are a lighter option, mobile phones use the Lithium-ion battery for a reason.