First-Principles Investigation of a Phase Transition in NaxC6O6 as an Organic Cathode Material for Na-ion Batteries: Role of Intermolecule Bonding of C6O6

Abstract

The structural and electronic properties of NaxC6O6 (2 ≤ x ≤ 4) as an organic cathode material for Na-ion batteries are investigated by first-principles calculations. The mechanism of a structural phase transition caused by Na insertion into Na2C6O6 with the space group Fddd is discussed on the basis of electronic structure analyses. The structure with space group C2/m is one of the candidate phases after the Na insertion. In the C2/m phase, C6O6 molecules are arranged pairwise, leading to the stabilization of (C6O6)3− and (C6O6)4− owing to the intermolecule bonding of C6O6. Bonding states between C6O6 pair molecules play a crucial role in the mechanism of the phase transition in NaxC6O6.