Chemical Composition of Lithiated Nitrodonickelates Li3-xyNixN: Evidence of the Intermediate Valence of Nickel Ions from Ion Beam Analysis and Ab Initio Calculations.

Lamellar lithiated nitridonickelates have been investigated from both experimental and theoretical points of view in a wide range of compositions. In this study, we show that the nickel ion in lamellar lithiated nitridonickelates adopts an intermediate valence close to +1.5. This solid solution can therefore be written Li3-1.5xNixN with 0 ≤ x ≤ 0.68. Attempts to introduce more nickel into these phases systematically lead to the presence of the endmember of the solid solution, Li1.97Ni0.68N, with metallic nickel as an impurity. The LiNiN phase has never been observed, and first-principles calculations suggested that all the structural configurations tested were mechanically unstable.

Li2.0Ni0.67N, a Promising Negative Electrode Material for Li-Ion Batteries with a Soft Structural Response.

The layered lithium nitridonickelate Li2.0(1)Ni0.67(2)N has been investigated as a negative electrode in the 0.02-1.25 V vs Li+/Li potential window. Its structural and electrochemical properties are reported. Operando XRD experiments upon three successive cycles clearly demonstrate a single-phase behavior in line with the discharge-charge profiles. The reversible breathing of the hexagonal structure, implying a supercell, is fully explained. The Ni2+/Ni+ redox couple is involved, and the electron transfer is combined with the reversible accommodation of Li+ ions in the cationic vacancies. The structural response is fully reversible and minimal, with a maximum volume variation of 2%. As a consequence, a high capacity of 200 mAh g-1 at C/10 is obtained with an excellent capacity retention, close to 100% even after 100 cycles, which makes Li2.0(1)Ni0.67(2)N a promising negative electrode material for Li-ion batteries.