Batteries for cordless power tool are mostly Lithium-ion batteries these days. This is often represented by manufacturers as Lithium-ion, Lithium or Li-ion.
The advantages of a li-ion cell of battery such as a higher energy density, a much lower self discharge rate over the Ni-Cad batteries and no requirement to pre-charge them before use or run them flat over their lifetime to preserve the battery’s life, as you would do with an Ni-Cad battery. On the downside however, they do suffer from ageing, so can wear out. They are also more expensive to make (and consequently cost more to buy) than the N-Cad batteries.
RYOBI
Ryobi 18V 1.3AH Lithium Battery RB18L13
Ryobi 18V 1.5AH Lithium+ Battery RB18L15
Ryobi 18V 2.0AH Lithium+ Battery RB18L20
Ryobi 18V 2.5AH Lithium+ Battery RB18L25
Ryobi 18V 5.0AH Lithium+ Battery RB18L50