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

Highlights: • Pentavalent main-group elements contribute to the high thermodynamic stability of honeycomb-ordered layered oxides. • Disproportionation of Ni³⁺ ions leads to the activation of both Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ redox couples. • The size of main group elements influences the voltage and polarization of the cathodes by distorting NiO₆ octahedron. 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 ANi_2/3M_1/3O₂ (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 ANi_2/3M_1/3O₂, 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 e_g 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.