Probing the role of the so-called inactive transition metal in conversion reactions: Not so inactive!

Ternary alloys such as TiSnSb and NbSnNb have been proposed as suitable negative electrode materials for lithium-ion batteries due to their large capacities and rate capability over many cycles. During lithiation, TiSnSb undergoes a conversion reaction, leading to the formation of multiple, highly reactive species. Previous in situ 119Sn Mo<spacing diaeresis>ssbauer and 7Li magic-angle spinning (MAS) NMR spectroscopic studies suggested the phases Li3Sb, Li7Sn2, Li7Sn3 and Li2_ xSb are formed at the end of lithiation alongside Ti or Nb nanoparticles. However, their stability and overall contribution to the conversion reaction is not yet fully understood. A series of model Sn- and Sb-based mixtures and alloys (both binary and ternary) have been investigated at the end of lithiation using 7Li MAS NMR spectroscopy to determine both the phases formed and their contribution to the conversion reaction. In all cases, a mixture of reactive lithiated phases and metallic nanoparticles are formed at the end of lithiation. Changing the nature of the inactive element in binary and ternary alloys changes the local Li environment and the observed chemical shifts. Considerable differences in chemical shift are observed for alloys relative to less intimate mixtures. The synthetic conditions used, particularly the intimacy of mixing achieved during synthesis, is key in determining both the phases formed and how the reaction proceeds, i.e., via a conversion or alloying reaction. The data presented show that the so-called "inactive" element and its nature in fact plays a key role in the conversion mechanism and therefore influences the ability for this class of materials to be commercialised in the future.