A battery comprises a negative electrode including a negative electroactive material, a positive electrode, an electrolyte located between the negative electrode and the positive electrode, and a protection layer, the protection layer separating the electrolyte and the negative electroactive material. Example batteries include lithium batteries and lithium-sulfur batteries having a molten salt electrolyte. The negative electroactive material may be a lithium-containing material such as lithium metal, lithium alloy, or other lithium compound. In other battery technologies, the negative electroactive material may comprise another alkali metal, alkaline earth metal, or other material. The protection layer may comprise a solid polymer electrolyte or other ion-transmissive material.