Redox mechanism in the NiP2 electrode for Li-ion batteries: A DFT study coupled with local chemical bond analyses

Abstract

Li reactivity of the monoclinic NiP2 electrode is investigated through first-principles density functional theory calculations and local chemical bond analysis. The stability of various LixNiP2 is studied with respect to the conversion reaction \({\text{NiP}}_{\text{2}} + {\text{6Li}} \to {\text{2Li}}_{\text{3}} + {\text{Ni}}^\circ \). The T = 0K LixNiP2 phase stability diagram, as obtained, reveals that several ternary phases of lithium composition Li2NiP2 can be electrochemically achieved upon reduction. They correspond to monoclinic or tetragonal structures in which Ni adopts a square-planar (D4h-Li2NiP2) or a pseudo-tetrahedral (Td-Li2NiP2) environment. A local chemical bond analysis suggests that D4h–Li2NiP2 would result from an interlayer P–P bond breaking induced by a two-phase (P redox) process, while Td-Li2NiP2 would result from a Jahn–Teller distortion associated with a single-phase (Ni–P redox) process.