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Just doing a little math here.
The Avalon's battery is made of 34 7.2V (nominal) modules which in turn are made of 6 1.2v (nominal) NiMH cells. At full charge (1.4V per cell) the total voltage would be 285.6V. If you charge Li ion batteries up to 4.2V per cell that would be the exact equivalent of 68 Li ion cells. (Of course keeping it down to a more reasonable 4.1 or 4.0V max per cell wouldn't be hard to do the bigger the battery is.) Coincidentally the minimum and nominal voltages are exactly the same too.
However, NiMH batteries can be charged at a higher voltage than their max voltage (due to battery resistance, kind of like charging a lead acid battery to 14V and taking it off the charger and seeing 12.8V). So in reality the Synergy drive system could possibly put out up to 326V! That would fry a Li ion battery of 68 cells connected in parallel, expecially if it were near full charge (like in a plug-in hybrid)! Some sort of battery management system would have to protect the Li ion battery under regen near full charge.
According to Car and Driver, the Avalon's battery is capable of delivering some 141hp. That puts us at around 500A. Of course the Li ion and NiMH batteries would be helping each other. But it would be best to match the current since the Li ion battery may have less resistance than the NiMH battery.
So for a parallel LI ION battery, we need 68 in series capable of delivering close to 500A. Maybe 1,700 of 20A rated 18650's? It seems like I've seen some 1300mAh 20A cells on eBay before for some $150 per 100 box. That would put me at some $2,550 for just the cells (if I could find them for that price) but would add nearly 8kWh. I need to check out the Chevy Volt battery specs too.
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