Sodium-halogen secondary cell

A sodium-halogen secondary cell, comprising:

• a negative electrode compartment comprising a negative electrode that comprises metallic sodium in molten state, wherein the negative electrode electrochemically oxidizes to release sodium ions during discharge and electrochemically reduces sodium ions to form sodium metal during recharge;
• a positive electrode compartment consisting of:
• a NaI positive electrode disposed in a mixed molten positive electrolyte consisting of comprising one or more NaAlX4 salts, wherein X is the same or different element selected from Cl, Br, and I;
• additional NaI is present in a molar ratio of NaI to the mixed molten positive electrolyte of 1:1 to 3:1;
• and
• a current collector;
• and
• a sodium ion conductive solid electrolyte membrane that separates the negative electrode compartment from the positive electrode compartment;
• and
• wherein the overall battery chemistry of the sodium-halogen secondary cell is 2Na+I2 ↔2Na++2I−;
• and
• the additional NaI is present in a solid phase at operating temperature of the sodium-halogen secondary cell.

A sodium-halogen secondary cell, comprising:

• a negative electrode compartment comprising a negative electrode that comprises metallic sodium in molten or solid state, wherein the negative electrode electrochemically oxidizes to release sodium ions during discharge and electrochemically reduces sodium ions to form sodium metal during recharge;
• a positive electrode compartment consisting of:
• a NaX positive electrode disposed in a mixed molten positive electrolyte consisting of at least two different NaAlX4 salts and is represented by the general formula NaAlX′4−δX″δ, wherein 0<δ<4, and X′ and X″ are different element selected from Cl, Br, and I;
• additional NaX or a mixture of NaX compounds present in a molar ratio of the additional NaX or mixture of NaX compounds to the mixed molten positive electrolyte ranging from greater than 1:1 to 3:1 of NaX:NaAlX′4−δX″δ;
• and
• a current collector;
• and
• a sodium ion conductive solid electrolyte membrane that separates the negative electrode compartment from the positive electrode compartment;
• and
• wherein:
• the overall battery chemistry of the sodium-halogen secondary cell is 2Na+X2↔2Na++2X−;
• and
• the additional NaX or mixture of NaX compounds are present in a solid phase within the positive electrode at operating temperature of the sodium-halogen secondary cell.