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.