Energy storage device and method

An energy storage device, comprising:

• a separator having a first surface that defines at least a portion of a first chamber, and a second surface that defines a second chamber, and the first chamber is in ionic communication with the second chamber through the separator;
• and
• a plurality of cathodic materials comprising at least a first cathodic material and a second cathodic material, both cathodic materials being in electrical communication with the separator and both cathodic materials having a reduced form and an unreduced form, the unreduced form comprising a metal halide,
• and both cathodic materials having:
• a reduction exhausted state, in which further reduction of the energy storage device produces no increase in the portion of the cathodic material in the reduced form;
• a transition state, in which further reduction of the energy storage device produces an increase in the portion of the cathodic material in the reduced form and further oxidation of the energy storage device produces an increase in the portion of the cathodic material in the unreduced form;
• and
• an oxidation exhausted state, in which further oxidation of the energy storage device produces no increase in the portion of the cathodic material in the unreduced form;
• wherein the first cathodic material has a first electric potential that differs from a second electric potential of the second cathodic material,
• such that when the first cathodic material is in a transition state the energy storage device potential is the same as the first electric potential, and when the first cathodic material is in a reduction exhausted state or in an oxidation exhausted state and the second cathodic material is in a transition state the energy storage device potential is the same as the second electric potential,
• with the proviso that if either of the first cathodic material or the second cathodic material is a transition metal, then the other of the first cathodic material and the second cathodic material is not iron, arsenic, or antimony.