Numerical Simulation of the Heat and Mass Transfer in a Sodium Sulfur Cell

A mathematical model is built for the heat and mass transfer during charge and discharge in a sodium sulfur cell by coupling the electrochemical equations with the equations of species transport and heat transfer. Numerical simulation is performed for the two-dimensional axisymmetric domain of a single cell. The simulated charge-discharge characteristics agree well with the experimental data of a 650 Ah Na/S cell. The transient non-uniform distributions of the electric potential, the current density, the sodium polysulfide composition and the temperature during charge and discharge are obtained. The results show that the non-uniform distribution of the sodium polysulfide composition and current density may deteriorate the degradation of the ceramic electrolyte and the corrosion of the metal container, thus may shorten the cell life. The graphite fibers in the sulfur electrode matrix are preferably radially oriented, which is advantageous for reducing the cell resistance, for improving the rechargeability and for extending the cell life. The simulation results of the transient temperature fields provide useful guidance for the optimized thermal design so as to enhance the energy efficiency of the battery system.