Other use cases would be where a car has some sodium batteries and some lithium based ones. The sodium batteries could serve most of the “wear and tear” of short trips, with a second smaller lithium back available to augment overall range which is not used as often.
This is a neat idea, but wouldn't solid-state lithium be far, far better for that purpose?
Not really — portable batteries are already on the market and aren't that much more than conventional Li-ion.