The role of M@Ni6 superstructure units in honeycomb-ordered layered oxides for Li/Na ion batteries

Abstract Honeycomb-ordered layered transition metal (TM) oxides, which are characteristic of a honeycomb network, have recently emerged as a novel class of cathode materials with high voltage and superior long-term cycling stability. Here, we provide a systematic first-principles study of the structural and electrochemical properties of honeycomb-ordered ANi2/3M1/3O2 (A = Li and Na; M = As, Sb and Bi) aimed at disentangling the role of the M5+ species. Our results show that M endorses strong bonding with O and can give rise to superior thermodynamic stability of the compound as compared to ANiO2 counterparts. Upon alkali deintercalation of ANi2/3M1/3O2, there is a driving force for disproportionation of Ni3+ ions, which originates from the high-symmetric MO6 octahedron and leads to the activation of both Ni2+/Ni3+ and Ni3+/Ni4+ redox couples. The distortion of NiO6 octahedron can be modulated by the size of M, and a clear correlation is revealed between the distortion from octahedral symmetry and the electronic structures of the compounds, including the energy position of eg orbitals and the temporary stabilization of Ni3+ ions. Both properties are linked to the voltage and polarization of the cathodes. This work provides a basis for further development of cathode materials based on honeycomb-ordered superstructure.