What Could Be the Next Major Energy Source Beyond Lithium Batteries?

As a battery engineer and materials science student, I spend a lot of time thinking about the future of energy storage. Lithium-ion batteries are currently dominant, but every technology has limits in terms of energy density, cost, and raw materials.

This makes me wonder: what could be the next major energy source or storage technology for humanity in the long term?

Some possibilities people often discuss include solid-state batteries, sodium-ion systems, hydrogen energy, or entirely new materials and electrochemical systems that are still in early research.

In your opinion, which future energy technology has the greatest potential, and what are the main obstacles that must be solved before it becomes practical?