There are 3 main Lithium based battery solutions currently avail:
Li-ion = Lithium Ion - been around longest and widest deployed (avail since mid 1990's at least) - Good density; somewhat unstable (can catch fire); moderate price
Li-Poly = Lithium Polymer - been around since early 2000's - Best density; more unstable; most expensive
LiFePO4 = Lithium Iron Phosphate - been around since early 2010's - Less density; stable; least expensive -> almost all SLA replacement batteries are these - you can typically use SLA chargers to charge and have voltages around same as typical SLA's