Synthesis, Electrochemical Properties, and Phase Stability of Li₂NiO₂ with the Immm Structure

The electrochemical properties and phase stability of the orthorhombic Immm structure of composition Li 2 NiO 2 are studied experimentally and with first principles calculations. The material shows a high specific charge capacity of about 320 mAh/g and discharge capacity of about 240 mAh/g at the first cycle. The experimental results and first principles calculations all indicate that the orthorhombic Immm structure is rather prone to phase transformation to a close-packed layered structure during the electrochemical cycling. The possibility of stabilizing the orthorhombic Immm structure during the electrochemical cycling by partial substitution of Ni is also evaluated. A detailed analysis of the crystal field energy difference between octahedral and square-planar coordinated Ni 2+ indicates that crystal field effects may not be large enough to stabilize Ni 2+ in a square planar environment when the cost of electron pairing is taken into account. Rather, we attribute the stability of Li 2 NiO 2 in the Immm structure to the more favorable Li arrangement as compared to a possible Li 2 NiO 2 structure with octahedral Ni.