The way I read it, the paper is more interested in what happens to EV batteries when they drop to 80% of the maximum storage capacity. If I understood them, they're saying that at this level, the battery is no longer suitable for transportation purposes. So what do you do with the old battery when you replace it?

They're suggesting using these extremely expensive but no-long brand-new batteries for additional grid storage. Take the battery you don't use anymore, and instead of paying someone to dispose of it, plug it into the grid and get paid for its use.