At the Heart of a Conversion Reaction: An Operando X-ray Absorption Spectroscopy Investigation of NiSb2, a Negative Electrode Material for Li-Ion Batteries

The reaction mechanism of NiSb2 with lithium was studied in detail by operando Ni K-edge X-ray absorption spectroscopy (XAS). The redox activity of the Ni centers was followed during the first two lithiation/delithiation cycles, showing that a reversible conversion takes place, with the formation of Ni metal nanoparticles at the end of lithiation and the reversible formation of a NiSb phase close to pristine NiSb2 at the end of the delithiation process. The comparison of operando results with ex situ measurements on cycled NiSb2 materials emphasizes the importance of real-time investigation tools for the study of battery materials, in particular in the case conversion reactions. These results can be considered as a reference for the study of other compounds of this family of reactions, where the formation of very reactive and/or unstable species is observed during electrochemical cycling. In fact, such reactive species, which can be identified only by using operando techniques, are one of the keys of the good cycling properties of such materials together with an appropriate formulation strategy.