Electrochemical Evaluation of Pb, Ag, and Zn Cyanamides/Carbodiimides

PbNCN, Ag2NCN, and ZnNCN were tested as negative electrode materials for Li-ion batteries. A thorough analysis of the electrochemical mechanism by X-ray diffraction and X-ray absorption spectroscopy showed that, unlike transition metal carbodiimides, these compounds react with lithium via a two-step reaction, starting with conversion followed by alloying. The conversion reaction is highly irreversible for the three compounds, whereas the reversibility of the alloying reaction depends on the metal, that is, highly irreversible for PbNCN and Ag2NCN which contain the cyanamide group (N≡C–N2–) and more reversible for ZnNCN containing carbodiimide (−N═C═N–). In the case of the more covalent, cyanamide-type PbNCN and Ag2NCN, the conversion reaction occurs at a higher voltage compared to the more ionic, carbodiimide-type ZnNCN, correlated with the nature of bonding in the NCN group and in the phases themselves. Compared to transition metal carbodiimides, these materials show rather low performance, with no improvement in capacity as it would have been expected from the combination of conversion and alloying.