@achtungbai
The factory battery is pretty dam big and already in the boot
if you want yet more capacity, I'd go with a LIPO4 battery and a good quality DC-DC charger
The Alt on the XC90 isn't the biggest in the world so if DC-DC charger over 30 amps I would make sure you can manually set it down to 30amps or under
Battery https://au.renogy.com/renogy-smart-lithium-iron-phosphate-battery-12-volt-100ah/
DC-DC Charger https://au.renogy.com/renogy-50a-dc-dc-on-board-battery-charger-with-mppt/
For the DC-DC charger you will need to run a wire to an ignition controlled source like the front 12v socket so that it only runs when the car is running