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If you're going Lithium, you NEED LiFePO4. Lithium Titanate can work, but it's not optimal. Li ion is not at all optimal.
The thing is, you don't want or need to go over 15V with your charging system. 14V would be better.
LiFePO4 has a min voltage of around 2.8, a nominal voltage of 3.2, is fully charged around 3.6 and can handle up to 4V each. Put them in a pack of 4 in series and you get 11.2 min, 12.8 nom, 14.4 full, and 16V max, perfect for a 12V car system. Set your charge controller to around 14.5V and check the balance every month if you don't want to play around with a balancer.
But be aware, with the exception of Lithium Titanate, any Li battery will not like the cold or the heat.
As for Li Titanate, 5 cells isn't quite enough (11.5V nom/15V full), and 6 is too much (13.8V/18V full), although with 6 cells I guess you could keep them close to nominal voltage instead of full charged. But for a solar panel that tops off batteries so you can try to not use your alternator, you probably want to charge them up as much as possible. A fully charged LiFePO4 will stay at around 13V and fully charges with a normal Lead Acid battery charger that charges to 14.5V. Not only that, LiFePO4 is more energy dense. Li Titanate cells are close to Lead Acid in energy density. Having one that's half charged all the time would be like having a half sized Lead Acid battery.
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