The big challenge facing true solid-state cells, however, is the long climb to engineering validation. Galyen cites five “golden rules” of batteries–safety, performance, life, cost, and environmental–which must be met for solid-state cells to achieve industry-wide adoption.

“I haven’t seen cost numbers that are even close to competing with liquid-state, lithium-ion batteries.“

https://spectrum.ieee.org/solid-state-battery-production-challenges