Effects of Relaxation on Conversion Negative Electrode Materials for Li-Ion Batteries: A Study of TiSnSb Using 119Sn Mössbauer and 7Li MAS NMR Spectroscopies

Abstract

Conversion materials were recently considered as plausible alternatives to conventional insertion negative electrode materials in lithium-ion batteries due to their large gravimetric and volumetric energy densities. The ternary alloy TiSnSb was recently proposed as a suitable negative electrode material due to its large capacity (550 mA h g–1) and rate capability over many cycles. TiSnSb has been investigated at the end of lithiation (discharge) using 119Sn Mossbauer and 7Li magic-angle spinning (MAS) NMR spectroscopies to determine the species formed, their relative stabilities and their behavior during relaxation. During discharge, TiSnSb undergoes a conversion reaction to produce a mixture of phases believed to consist of lithium antimonides, lithium stannides, and titanium metal. In situ 119Sn Mossbauer spectroscopy indicates the presence of Li7Sn2 at the end of discharge, while 7Li NMR experiments suggest the formation of two distinct Sn-containing species (tentatively assigned to Li7Sn2 and Li7Sn3), in addition to two Sb-containing species (tentatively assigned as Li3Sb and a non-stoichiometric phase of Li2Sb, Li2–xSb). To gain insight into the relative stabilities of the species formed, experiments have been completed under open circuit voltage conditions. A new Sn-based species has been identified via 119Sn Mossbauer spectroscopy at the end of relaxation. Similar changes are observed in the 7Li NMR spectra obtained during relaxation. The species created at the end of discharge are extremely unstable and spontaneously evolve towards delithiated phases. Surprisingly, it is possible to resume electrochemical cycling after relaxation. It is likely that this behavior can be extended to this family of electrode materials that undergo the conversion reaction.