Could liquid alkali metal fuels be used for ramjet/scramjet propulsion?
How long does the lithium take to evaporate and then to react with oxygen? Air in a scramjet is moving extremely fast (supersonic by definition!), so there is very little time left for mixing and combustion if you desire to create thrust from the the combustion heat energy.

Your lithium engine will most likely trail a bright contrail of burning lithium but develop rather little thrust. The high flame speed of a mixture of hydrogen and oxygen makes hydrogen the preferred scramjet fuel.