The battery configuration and position is very different compared to standard cars:

http://insideevs.com/wp-content/uploads/2016/02/Concept_one_battery_01.jpg

I believe it is encased in a carbon fibre shell before installation.

Detailed overview of rimac one:
http://insideevs.com/rimac-to-unveil-production-version-of-concept_one-in-geneva-only-eight-will-be-produced/

Rimac will be producing the batteries for the new electric Aston Martin.