We would be very happy to get 300k out of ours. We know that BMW will warrant the battery for 8 years to 70% of its capacity. Not sure what will happen beyond that. The EV world is changing so quickly. Our 2015 has a 60 aH battery and the new ones come with a 120 aH battery.
There is a company who made a 700km range battery that they put into an i3. I think that the current batteries in older i3's will be open to many upgrades options down the road.
https://www.express.co.uk/life-style...s-battery-pack