The high carbon source is continuous and ongoing every minute of the day.
The lithium (mostly mined in Australia with minimal environmental impacts) needs about 12Kg for a 500Kg battery (300mile EV size) as a one off.
The battery lasts about 400,000 miles.
The batteries are all reused or recycled at the end of their life as a rule.
So the Lithium compared to the high carbon source for the lifetime of the vehicle is negligible.