Aging of commercial sodium-ion batteries with layered oxides: how to measure and analyze it?

Abstract

Improving the understanding of the aging of sodium-ion batteries (SIBs) is vital for their subsequent improvement and commercialization. Nevertheless, the electrode-resolved aging of SIBs has not been thoroughly investigated to date. Here, we use commercial SIBs that were cyclically aged for almost two years and show how to use three-electrode cells to measure the changes of both kinetics and balancing of the electrodes over the aging. The area-normalized comparison between the three-electrode cells and the 18 650 cells shows that both state-of-charge resolved pulse resistance and capacity are transferable. Using a modified setup with a tin-wire reference electrode, we demonstrate how to use the three-electrode cells to measure the electrode-resolved impedance. In addition, we use the open-circuit potentials from three-electrode cells to perform degradation mode analysis on the 18 650 cells. The results of the degradation mode analysis are validated both by the low difference between measured and reconstructed full-cell voltage over the aging and by post-mortem analysis. Furthermore, we verify that the fundamental assumption of the degradation mode analysis, i.e., that the shapes of the electrode open-circuit potentials do not change during aging, is valid for the investigated SIBs.