Reaction mechanism of tin nitride (de)lithiation reaction studied by means of 119Sn Mössbauer spectroscopy

Tin nitride thin films have been reported as promising negative electrode materials for lithium-ion solid-state microbatteries. However, the reaction mechanism of this material is not yet fully understood. Results on thin film electrodes pointed out that the conversion mechanism of tin nitride most likely differs from the conversion mechanism usually observed for other oxide and nitride conversion electrode materials. The electrochemical data showed that more than six Li per Sn atom can be reversibly exchanged by this material while about four are expected. In order to investigate in more detail the reaction mechanism of tin nitride, thick film electrodes of two compositions (1:1 and 3:4) have been studied. The as-prepared materials were characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy and 119Sn Mössbauer spectroscopy. Moreover, films (de)lithiated to various extents were analyzed ex situ with Mössbauer spectroscopy. The corresponding results indicate that a more complex reaction mechanism than that generally accepted takes place. During Li-ion insertion, the disappearance of Sn4+ environments is correlated with the formation of Li–Sn phases, and most likely also of Li3N. In the case of the SnNx 1:1 composition films, the formation of various Li–Sn phases is evidenced while only the signature of ‘Li22Sn5’ is clearly measured for the 3:4 composition. Upon Li-ion extraction, the Li–Sn phases and Li3N recombine to form octahedrally and tetrahedrally coordinated Sn4+. The extraction is not fully reversible and the end product consists of a mixture of a tin nitride structure plus a LiySn product having the same isomer shift as LiSn but a much higher quadrupole splitting, and most likely some Li3N.