Solutal convection in Na-Zn liquid metal batteries and its impact on self-discharge

Abstract

In the field of electrical energy storage, liquid metal batteries (LMBs) are a promising technology. A newly invented cell concept based on sodium and zinc is currently being developed. Solutal convection is understood to be a highly significant flow phenomenon in those cells, causing substantial effects on mass transfer-induced self-discharge. The latter is a great challenge for Na-Zn cells. A diaphragm has often been proposed as a mitigation measure, but its mode of action and efficacy have not been reported yet, as this is both experimentally challenging and a multiphysical problem which is nontrivial for numerical modeling. The present study models solutal convection in the electrolyte of an LMB for the first time and includes porous zones to assess the influence of the diaphragm. A charge-discharge asymmetry during cycling is identified. Consequently, a constant conflict between reducing concentration gradients and promoting self-discharge is faced: convective flow is always present in one electrolyte compartment while the other compartment is stably stratified. Moreover, cycling performance is influenced by the occurrence of previous cycles. In particular, the first charge period exhibits unique behavior as the electrolyte is initially homogeneously mixed. Thereafter, stably stratified layers as well as locally mixed regions determine the subsequent flow behavior. While the simulations generally confirm the importance of a flow barrier, the diaphragm is not an ion-selective membrane and cannot fully mitigate self-discharge: species transport via migration and diffusion is still possible, necessitating optimization of operational current density and charging time to limit self-discharge.