Abstract
Sn-based conversion–alloying anodes store Na through combined conversion and alloying reactions, but the reversibility of these reactions depends strongly on the electrolyte. SnO, SnS and SnSe electrodes were compared in ether- and carbonate-based electrolytes using electrochemical measurements, synchrotron X-ray characterisation, and electron microscopy. All three retained substantially higher capacities in ether, while rapid capacity decay occurred in carbonate. Separating the reaction contributions showed that conversion was broadly similar in both electrolytes, whereas alloying was strongly restricted and progressively lost in carbonate. Initial capacity deficit in carbonate-based electrolyte was found be due to incomplete alloying caused by greater Na counter-electrode polarisation. After cycling, carbonate-based electrodes also showed greater accumulation of F-containing interphase products and less ordered Sn-rich domains. Together, these observations identify sustained alloying and interfacial stability as the main reasons for the superior performance in ether.