A combination of various technologies that are still in early development could make submarines that relies on batteries only much more attractive. A lot of research is ongoing to increase battery energy density to 1,000 Wh/l and above. However in addition some interesting development is ongoing into high temperature superconducting electric engines:
Toshiba Asia Pacific | Toshiba develops a lightweight, compact, high-power superconducting motor prototype for mobility applications – contributes to carbon neutrality in industries and transportation.Toshiba announced in June last year on developing a prototype for a compact, high-speed superconducting motor with a maximum output of 2 MW, bringing together its manufacturing technology for high-speed rotating machines and superconductivity technology. The newly developed superconducting motor weighs only a few hundred kilograms, has an outer diameter of about 50 cm, and an overall length of about 70 cm (excluding the shaft), which is “less than one-tenth” of a motor with the same level of power output.