BMW are now fitting Li-ion starter batteries in the place of AGM on some of their more prestigious marks. Li-ion is probably half the weight of the AGM and I would expect it to outlast the AGM. We at County Battery Services supply Lithium Battery for motorcycle starting which is useful for racing bikes weight, but I see no real advantage for the car battery especially as the majority will be supplied for road use. BMW are saying that the product will enable better starting in cold weather I don’t know what the price is I guess it will be £1000 or more to replace.
Another issue is the charging if you get a car with one of these batteries please make sure you have the correct charger it will kill the battery if you do not and could quite easily explode!