A molten sodium secondary cell charges at a high temperature and discharges at a relatively lower temperature. The cell includes a sodium anode and a cathode. A sodium ion conductive solid membrane separates the cathode from the sodium anode and selectively transports sodium ions. A solar energy source includes 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. The cell has a charge temperature and a charge voltage and a discharge temperature and a discharge voltage. The charge temperature is substantially higher than the discharge temperature, and the charge voltage is lower than the discharge voltage.