I am running a 40Ah GBS LFMP pack in my Prius for the aux battery:
http://elitepowersolutions.com/products ... ucts_id=90
It serves a similar purpose, basically boot up the car, act as a 12 volt reservoir, power accessories when the car is in accessory mode and act as an emergency brake booster power supply.
The OEM lead 12 volt has strange 2/3 size automotive posts so you just about have to go get a replacement from Toyota which is in the $150 range on the low end. I ended up not spending too much more for the lithium pack. I am running individual cell balancers for an added level of safety and longevity. The car under charges the pack slightly giving it about 3.5 volts per cell (3.6 is the target charge voltage). The under charge should extend the battery life actually. It's been in there for six months now and is working well. I plan to pull it at about a year and run a capacity test on it to see how it's really holding up.
The only down side is there is no forgiveness for over discharging. If you were to leave an interior light on and it drains the battery to 0 volts the battery is dead, it does not come back. A typical application in an EV has a battery management system which has interlocks to prevent over discharging or over charging.
When the lead battery dies in my Leaf it's being replaced with lithium as well.
*Disclaimer* I work at Elite Power Solutions