Spot on. I worked it out some months ago, with the best supercaps we have right now, it was over a ton for a reasonably sized battery… and then you still have to add all the electronics to deal with the linear discharge curve 
Looks like LG has also pulled their NMC batteries due to the issues.
I came across a LiFePO4 18650 cell recently: https://za.rs-online.com/web/p/speciality-size-rechargeable-batteries/8183005
I wasn’t aware of these thinking they were all LiPO… 
geewiz also have similar LiFePO4, but their valtages and specs did not match so I did not want to take a chance ordering them
What I have gathered is that the LiPo batteries have a high energy density.
This prompted them to move to LiFePO4 which have lower density but are safer…