Transition-Metal Carbodiimides as Molecular Negative Electrode Materials for Lithium- and Sodium-Ion Batteries with Excellent Cycling Properties

We report evidence for the electrochemical activity of transition-metal carbodiimides versus lithium and sodium. In particular, iron carbodiimide, FeNCN, can be efficiently used as negative electrode material for alkali-metal-ion batteries, similar to its oxide analogue FeO. Based on 57Fe Mossbauer and infrared spectroscopy (IR) data, the electrochemical reaction mechanism can be explained by the reversiblevtransformation of the FeNCN into Li/NaNCN bonds during discharge and charge. These new electrode materials exhibit higher capacity compared to well-established negative electrode references such as graphite or hard carbon. Contrary
to its oxide analogue, iron carbodiimide does not require heavy treatments (such as nanoscale tailoring, sophisticated textures, or coating) to obtain long cycle life with current density as high
as 9 Ag-1 for hundreds of charge–discharge cycles. Similar to the iron compound, several other transition-metal carbodiimides Mx(NCN)y with M=Mn, Cr, Zn can cycle successfully versus lithium and sodium. Their electrochemical activity and performance open the way to the design of a novel family of anode materials.