An electrochemical energy generation system can include a sealed vessel that contains inside (i) at least one electrochemical cell, which has two electrodes and a reaction zone between them; (ii) a liquefied halogen reactant, such as a liquefied molecular chlorine; (iii) at least one metal halide electrolyte; and (iv) a flow circuit that can be used for delivering the halogen reactant and the electrolyte to the at least one cell. The sealed vessel can maintain an inside pressure above a liquefication pressure for the halogen reactant. Also disclosed are methods of using and methods of making for electrochemical energy generation systems.