Promising Properties of 2D Layered Cobalt and Nickel Nitrides as Negative Electrodes for Li-Ion Batteries

2D materials largely dominate the field of Li-ion batteries with the transition metal oxides LCO (LiCoO2), NMC LiNixMnyCoz) and NCA (LiNixCoyAlz) used as positive electrode materials and carbon graphite as negative electrode. However graphite suffers from a low rate capability and from a limited specific capacity. Intensive research has been made to find alternative anode materials such as the LiAl, LixSn, LixSi..alloys or transition metal oxides as conversion materials. But in all cases, huge volumetric changes occur upon discharge-charge reaction inducing a fast capacity decline. In spite of numerous studies, this important drawback has never been solved as yet.<br/>We have decided to explore the group of lithiated transition metal nitrides as alternative negative electrode materials to overcome safety issues related to the use of graphite at high current. Some of these compounds were firstly investigated in the wide voltage window 1.4V-0V leading to high specific capacities but also to irreversible structure change with the irreversible oxidation of nitride anion framework. Hence poor electrochemical properties were achieved. Our key idea has been to rigorously control the upper cut-off voltage inside the stability domain of the nitride framework which is strongly dependent on the metal and metal content.<br/>The present work reports the chemical synthesis and the structural characterization of lithium nitridocobaltates Li3-2xCoxN and Li3-2xNixN nitridonickelates with 0.1 ≤ x ≤ 0.6. These 2D compounds with an hexagonal symmetry constitute unique examples of intercalation compounds based on the nitrogen framework in which lithium can be reversibly inserted and extracted due to numerous cationic vacancies. With a mean working voltage around 0.5V, these “zero strain” materials deliver high specific capacities in the range 200-320 mAh/g with a remarkable stability over at least 100 cycles. In addition to a complete investigation of their electrochemical features in the 1V-0.02V potential range, we have elucidated the electrochemical mechanism involving the Co and Ni redox species (I/II/III) in relation with the tiny structural changes revealed through operando XRD experiments. A detailed kinetic study of the charge-discharge process in the case of nickel-based compounds allows to give a complete picture of electrochemical performances of these 2D lithiated nitrides as negative electrode materials for Li-ion batteries.