Magnesium energy storage devices that take advantage of magnesium-based anodes while maintaining practical energy densities can be useful for large-scale energy storage as well as other applications. One such device can include a negative electrode having magnesium and a positive electrode material that can flow in a batch or continuous manner. The flowable positive electrode material can result in an increased practical energy density because the fresh active material can be flowed to the positive electrode, and as a result can be theoretically infinite in size. The positive electrode can include a cathode suspension contacting a positive current collector and having particulates of a cathode magnesium intercalation compound, a cathode magnesium conversion compound, a redox active species, or combinations thereof.