At least at the moment, it's usually more efficient to recycle the good cells from an old pack into a new pack for automotive use. The chemistry between automotive and residential use diverges significantly because they are optimizing for different things: Car batteries minimize weight and maximize power input/output. For residential, weight barely matters at all, but # of discharge cycles and thermal stability matter more*. In 10 years, who knows what the economics will turn out to be though.
You can already DIY battery packs from used EVs in order to build a residential whole-house backup.