Reversible High Capacity and Reaction Mechanism of Cr2(NCN)3 Negative Electrodes for Li-Ion Batteries

Abstract

A detailed study of the electrochemical reaction mechanism between lithium and the trivalent transition-metal carbodiimide Cr2(NCN)3, which shows excellent performance as a negative electrode material in Li-ion batteries, is conducted combining complementary operando analyses and state-of-the-art density functional theory (DFT) calculations. As predicted by DFT, and evidenced by operando X-ray diffraction and Cr K-edge absorption spectroscopy, a two-step reaction pathway involving two redox couples (Cr3+/Cr2+ and Cr2+/Cr0) and a concomitant formation of Cr metal nanoparticles is apparent, thus indicating that the conversion reaction of this carbodiimide upon lithiation occurs only after a preliminary intercalation step involving two Li per unit formula. This mechanism, evidenced for the first time in transition-metal carbodiimides, is likely behind its outstanding electrochemical performance as Cr2(NCN)3 can maintain more than 600 mAh g−1 for 900 cycles at a high rate of 2 C.