Cobalt Carbodiimide as Negative Electrode for Li-Ion Batteries: Electrochemical Mechanism and Performance

Cobalt carbodiimide, CoNCN, shows outstanding performance as negative electrode material for Li-ion batteries, maintaining a reversible capacity of 530 mAh g(-1) over 140 cycles at a current density of 540 mA g(-1). The electrochemical lithiation/delithiation mechanism of cobalt carbodiimide was investigated using complementary in situ X-ray diffraction and X-ray absorption spectroscopy. Upon lithiation, CoNCN undergoes a reversible conversion reaction, forming Li2NCN and fcc Co metal nanoparticles, which are transformed back into CoNCN upon delithiation. However, the CoNCN obtained electrochemically after delithiation do not recover the local structure of the pristine phase, and might contain the NCN2- ligand in the cyanamide isomer form (N-C equivalent to N2-). It would be the first time that a transition metal cyanamide isomer is obtained at ambient conditions.