Origin of alkali-metal loss in high-temperature synthesis of rechargeable battery layered oxide cathodes

Synthesis of layered oxides for lithium and sodium ion batteries is typically performed using high-temperature routes with an excess of the respective alkali-metal. This is due to often observed deficiency of alkali metals in the final composition when precursors are mixed in a stoichiometric ratio. The loss of alkali-metals during high-temperature synthesis is a widely accepted fact based on synthetic work on lithium battery cathode systems, however, the root cause of this process is not fully understood. Therefore, we investigated the alkali-metal loss by inductively coupled plasma optical emission spectrometry (ICP-OES) for both, Li and Na, during hightemperature synthesis of a model layered oxide system Na 1 Li 1/3 Mn 2/3 O 2 , as it contains both alkali metals. We observed a temperature dependent Li-loss, independent of the Li-precursor (either Li 2 CO 3 or Li 2 O), confirming Lievaporation. Contrary to this, Na-loss is temperature independent, indicating that Na-evaporation does not contribute significantly to the Na-loss in the tested temperature window. Moreover, for pelletized samples we did not detect significant losses due to reaction with porous alumina crucibles, as suggested previously. We have found that apart from evaporation of Li, additional Li and Na metal loss is caused by violent chemical decomposition reactions of the precursors at temperatures below 300 • C. Our findings highlight the differences in the origin for Li and Na-based systems, and underlines importance of the precursors in this process.