“s that it could extend the driving range of electric vehicles due to the
fact that 25% more energy per kg means 25% less weight per unit of
storage capacity, e”

Not true. 25% more energy capacity is a 5/4 increase. Meaning you need 4/5ths as much mass, a 20% decrease.

Just like how a 100% increase in battery is 2/1 and you would therefore need 1/2 the battery mass. Not 0, like your maths gives!