The conversion of the global energy supply from carbon emitting to renewable sources requires a large amount of energy storage systems being efficient at high energy- and power densities. Many requirements are fulfilled best by Lithium-ion batteries which are prone to thermal runaways leading to fire and severe damage after unusual usage. For some applications, like large stationary battery systems for independent energy supply or grid stabilization, Sodium-ion batteries (NIBs) would be a saver alternative. The ongoing improvement of anode- and cathode materials, together with adapted electrolytes led to excellent advances in power- and energy densities, competitive to Lithium iron phosphate (LFP) cells. In comparison to Lithium ion-based systems the long-term cycling behavior of commercial NIBs is not yet investigated sufficiently. The increasing relevance of NIBs in several fields of application underlines the importance of testing their long-term cell performance (e.g. capacity fades, coulombic efficiencies, AC-and DC-impedances, C-rate scanning) in order to adapt cycling conditions and expand the lifetime.

In this study, long term cycling tests (T = 25, 40 °C; 1 C charging, 3.5 C discharging) of commercial 1.3 Ah-18650-cells with hard carbon anode, layered sodium transition metal oxide cathode and NaPF6-based electrolyte were performed. In parallel, commercial LFP-cells (1.5 Ah) were cycled under the exact same conditions, in order to compare capacity fade and coulombic efficiency with Lithium based cells. After defined cycling steps (10, 250, 500, 750 cycles) the recording of electrochemical impedance spectra should monitor cell ageing in both cases. In a second step, impedance spectra were recorded to get deeper insight to cell ageing. Afterwards, the cells were opened under inert gas atmosphere and post-mortem analysis (SEM/EDX, GC-MS) took place to identify main reasons for ageing. The results give insights into the long-term cycling behavior of the latest NIBs generation. The study should reveal the main ageing issues and provide new approaches for improving cycle lifetime in terms of cell design or cycling conditions.