In Situ Raman Microscopy of Metal Chloride Formation for Metal-Chloride Batteries

Recently there has been a revived interest in high temperature molten salt battery systems, particularly sodium-sulphur and sodium metal-chloride, SMC. In particular SMC batteries have high theoretical specific energies and open circuit potentials (787 Whkg^-1 and 2.58 V for Ni cell chemistry). They comprise a liquid Na anode, solid beta-alumina electrolyte and a metal chloride cathode which also contains sodium tetrachloroaluminate, NaAlCl₄ as a secondary electrolyte [1]. However, battery performance is constrained by cathode underperformance resulting in obtainable specific energies in the region of 120 Wh kg^-1 [2]. The best performing cathode formulations contain both Fe and Ni as active cathode materials, with an excess of Ni to prevent Cl₂ liberation on overcharge. The excess Ni and NaAlCl₄ increase the weight of the battery, while not contributing to the output, therefore reducing the specific energy. Additional reductions in performance are attributed to the cell chemistry occurring in the cathode. Alternative cathode formulations could enhance the performance of this battery technology and enable its application for grid storage. This should be achieved while maintaining the existing technologies high abundance of raw materials used in the manufacturing process.

This talk presents our research on the chemical processes occurring in the cathode for cell chemistries based on various transition metals, particularly Fe, Co, Ni, Zn and Cu, over the temperature range 200°C to 400°C. Cyclic voltammetry on polished coated wire tip electrodes have been used to determine the diffusion coefficients of the metal ions diffusing through their respective metal chloride layers. In situ Raman spectroscopy has been undertaken to develop a better understanding of the processes occurring to form the metal chlorides.

[1]          J. Coetzer, J. Power Sources (1986) 18 377

[2]          R.C. Galloway & S. Haslam, J. Power Sources (1999) 80 164