Airsoft Batteries are made up of a number of chemical compounds - Each type has its benefits and disadvantages. The most common types are NiMh (Nickel-metal Hydride), Lipo (Lithium Polymer), Li-Ion (Lithium Ion), LiFe (Lithium Iron Phosphate), NiCd (Nickel-Cadmium) and so many more. The most common types that can be found in Airsoft are LiPo, Li-Ion, Coin and NiMh batteries.
LiPo & Li-Ion Batteries
These are the top of the tree (along with LiFe) when it comes to the latest and greatest of rechargeable battery tech - These are not only used in Airsoft RIF's but can also be found in RC cars, Powertools, Mobile phones and camcorders. Whilst these batteries are very lightweight, compact, thrive with high discharge rates and have a high power density they do need to be handled with care. These batteries are very delicate and require special circuitry to "protect" the battery to prevent them from critically failing. Cells are 3.7v and can normally be found in configurations of 7.4v, 11.1v and 14.4v
PRO's - The lightest and most compact batteries available, High power, High Cell voltage, High capacity
CON's - Expensive to produce, delicate, can explode if misused!!
These are the grandfather of batteries as this tech has been around for a long time. They also come in some of the most widely used batteries configurations such as AA, AAA and C cell batteries used in a lot of our home appliances. They are not used as much now due to the NiMH batteries having a much larger power density, however as still on the market as they are very inexpensive to produce - You will tend to find them in lower end RIFs where performance is not as important as the price point. These batteries are 1.2v and often bundled in packs of 3 to make 3.6v.
PRO's - Inexpensive, rugged, available in standard sizes, easy to recharge
CON's - Low power density, can require a full discharge and then recharge to recover the batteries memory, contain toxic metals
Whilst these are still an older tech battery (newer than NiCd) they have become the industry standard for Airsoft. Lots of RIFs are supplied with these batteries and they offer a good level of performance when compared to their cost. They are a good option for new players as they are more rugged than LiPo's and Li-Ion's and do not require any special circuitry to prevent the battery from failing. The cell voltage is 1.25v so between Alkalines (1.5v) and NiCads (1.2v).
PRO's - A good starter and all-round battery, high power density, a "Gold Standard" in the Airsoft world, easy to recharge, rugged
CON's - More expensive than NiCd, the batteries "life" is not as long, self-discharge quickly over time
These batteries have a specific capacity that is larger than a standard Li-ion, however, the energy density is a lot less. This is because LiFe has a much lower operating voltage (this could also be an advantage)
PRO's - Lighter, very slow self-discharge rate, No "memory" effect
CON's - Start to degrade as soon as they leave the factory, sensitive to high temperature, can not run to very low voltages, can explode if charged/ discharged incorrectly