Layered lithium cobalt nitrides: A new class of lithium intercalation compounds

Lithium cobalt nitrides Li_3−2xCo_xN (0.1 ≤ x ≤ 0.44) have been prepared and investigated as negative electrode in the 1/0.02 V potential window. The evolution of the unit cell parameters and unit cell volume with the Co content show a solid solution behaviour. Whatever the Co content, all these nitrides are electroactive with a single step around 0.6 V/0.7 V for the discharge and charge processes, respectively. The electrochemical behaviour observed is typical of a Li intercalation compound and involves the Co²⁺/Co⁺ redox couple in the interlayer plane combined with the reversible accommodation of Li⁺ ions in the cation vacancies located in Li₂N⁻ layers. XRD experiments performed after discharge, charge and cycling tests clearly indicate the hexagonal layered structure of the host lattice is maintained. This intercalation process explains the excellent capacity retention found after 50 cycles. A specific capacity of 180 mAh g⁻¹ at C/20 and 130 mAh g⁻¹ at C/5 rate (100 mA cm⁻²) is achieved for Li_2.23Co_0.39N. ac impedance measurements have allowed to characterize the kinetics of the reaction.