High temperature sodium battery with high energy efficiency

A molten sodium secondary cell that charges at a high temperature and discharges at a relatively lower temperature, comprising:

• a sodium anode that undergoes electrochemical oxidation during discharge and electrochemical reduction during charge;
• a cathode that undergoes electrochemical reduction during discharge and electrochemical oxidation during charge, wherein the cathode comprises a catholyte composition that is thermally stable at cell operating conditions;
• a sodium ion conductive solid membrane separating the cathode and catholyte composition from the sodium anode that selectively transports sodium ions and is chemically and thermally stable at cell operating conditions;
• and
• a solar energy source comprising:
• a photovoltaic system to provide an electric charging potential to the sodium anode and the cathode;
• and
• a solar thermal concentrator to provide heat to the cathode and catholyte composition to cause the molten sodium secondary cell to charge at a temperature in the range from about 300 to 800° C.;
• wherein the cell has a charge temperature and a charge voltage and a discharge temperature and a discharge voltage, wherein the charge temperature is substantially higher than the discharge temperature and the charge voltage is lower than the discharge voltage.