Medium-Temperature Molten Sodium Batteries with Aqueous Bromine and Iodine Cathodes

Since their development five decades ago high temperature batteries, such as sodium/nickel chloride and sodium sulphur, have always used molten sodium as the anode. This type of batteries uses sodium-βʺ-alumina as a solid electrolyte and must be operated at around 300^°C since the cathodes used require such temperatures so as to be in a molten state and also to main a sufficiently high ionic conductivity of the solid electrolyte. These systems portray high energy density, combined with reasonable safety and long cycle life. However, at these high temperatures, thermal losses are quite significant and tend to reduce the overall efficiency of these systems or limit their use to very large units in the megawatt-hour range for stationary applications. Measures to lower the operating temperatures in a bid to reduce the thermal losses have proven difficult due to the insufficient wetting of the ceramic material with molten sodium and decreasing sodium-ion conductivity. NaSICON, a sodium-ion ceramic conductor of high ionic conductivity, has been presented here as a potential alternative solid electrolyte, together with a cathode that can be used at temperatures around 100 °C

Michael Holzapfel and colleagues at Fraunhofer Institute for Chemical Technology in Germany, evaluated the applicability of the sodium-ion conductive NaSICON as a solid electrolyte in the preparation of rechargeable medium-temperature sodium-bromine and sodium-iodine battery systems. They aimed at providing stability data of NaSICON in aqueous sodium halogenide solution and sodium halogenide solutions containing bromine or iodine at operating temperature so as to confirm their choice of NaSICON as solid electrolyte. Their research work is now published in Electrochimica Acta.

The researchers commenced their empirical procedure by preparing the NaSICON solid electrolyte to be used. They then assembled the cell where they ensured the materials selected for the cell body do not react with molten sodium and the halogen-containing cathode, respectively, at high temperatures. Later, they under took physiochemical characterization by conducting X-ray diffraction measurements. Eventually, electrochemical tests were conducted at temperatures around 100⁰C at constant current mode using different current densities.

The research team observed that the NaSICON was more stable in aqueous solution than sodium- βʺ -alumina. The team also noted that sodium-poor samples exhibited higher stability in aqueous media. They also deduced that high concentrations of Na⁺ in the solution favor Na⁺/Na⁺ exchange, to the disadvantage of Na⁺/H⁺ exchange and formation of hydronium NaSICON. The presence of bromine was also seen to decrease the pH of the solution thereby decreasing the stability and formation of surface hydronium NaSICON. Considerably, the iodine-based solution was noted to attack NaSICON less as compared to the bromine-based solution.

The stability data presented in this paper positively affirms the use of NaSICON solid electrolyte in the preparation of medium-temperature molten sodium batteries with aqueous bromine and iodine cathodes. The sodium-halogen batteries can be designed in static and flow-cell setup and permit a high energy density which depends on the concentration of the halogen in the charged catholyte. Such batteries could have applications as low-cost alternative for stationary energy storage.

“Medium-temperature sodium battery systems get into reach. The use of an aqueous iodine cathode permits a reversible and low-cost battery with specific energies of around 200 Wh/kg on active material level. The possible redox-flow setup is an additional feature compared to Na/NiCl₂ or Na/S batteries. It permits to separate the storage tank from the power unit, both units can be scaled independently.” said Michael Holzapfel, the first author.

 Note: NaSICON – Na₃Zr₂Si_2.3P_0.7O_11.85