^ Quite untrue.
Flooded lead acid is the most cost effective way to make a battery with acceptable performance and lifespan.
I don't want to pay more to reinvent the wheel nor to have a more complex charge circuit, more prone to failure and more difficult to troubleshoot, as well as far more expensive to repair.
If 50lbs seems important to you, okay, but realize that it will change fuel economy by such a tiny amount that you need to be mesmerized by numbers to care. Consider a smaller vehicle (where weight savings % is higher so it makes more of a difference):
50lb / 2800lb vehicle = 1.8% weight reduction.
EPA testing finds that small vehicles average a 2.1% fuel economy increase from a 5% weight reduction, so that is only a 0.75% fuel economy increase for a small vehicle and it goes down as vehicle size increases. For a small vehicle achieving, say 30MPG, at 12K mi/yr that's (given current fuel prices) little under $8 saved per year, except that for a small vehicle, you wouldn't save as much as 50lbs but let's ignore that.
It is unlikely that a Li-Ion starter battery is going to last more than 10 years (some sources even claim a ridiculously low 2-3 year lifespan, but let's say 10yr), and let's say flooded lead acid lasts 5 since you can get them with a 5 year prorated warranty. 5 additional years at $8 per year is $40 from a battery cost perspective, but even if you count total lifespan that's only $80. If the Li-Ion battery costs more than $40/$80 more than a lead acid, you never break even. I think we can concede that it costs hundreds more for the battery and more complex charging circuit, let alone repair later.
In short, it is bad engineering to put a Li-Ion starter battery in anything that does not place weight as the most critical factor for racing performance. If you really care about modern engine emissions from 3 gallons per year of fuel, you'd make life plans to drive less than 12K mi/yr... or plant a tree, or both.
Not only do trees and other vegetation scrub CO2, they also convert solar energy into cellulose and eventually oil (if we don't find a more expedient way to extract that energy before then). It's really kind of funny that in the bigger picture, planting crops to make alcohol and 'sink CO2 is more green and sustainable than EVs. Plants necessarily 'sink all the CO2 that burning the fuel produced from them, gives off. The remaining need is obviously to get politics out of it and move forward with more efficient bio-solar cells to produce fuel.