Sodium-containing layered oxides for solid-state sodium-ion batteries

Discovering alternatives to replace critical elements like lithium (Li) and cobalt (Co) which are currently routinely used in solid-state rechargeable lithium-ion batteries has experienced rapid expansion and emerged as a prominent focus of research. It is an economically and environmentally viable solution to use sodium (Na) or other alkalis to replace the critical elements. In this project, we explored the family of P2-type layered honeycomb compounds A2M2TeO6 (A = alkali (Na and Ga), M = transition metal (Ni, Co)) to develop positive electrodes for rechargeable Na-ion batteries. The motivation for the chemical tuning is the close connection between crystal and defect structures and ionic conductivity in the proposed compounds. Our results include successful solid-state synthesis of several Na2Ni2TeO6-derived compounds with varying Na content (cationic vacancies) and substitutions (Ga replacing Na, Co replacing Ni) and particle size ranging from 500 nm to 1 µm (deduced by scanning electron microscopy (SEM)). Our neutron diffraction data shows P63/mcm space group, where Na+ ions occupy three different crystallographic positions Na1-Na3, and a two-dimensional network of Ni honeycombs with edge-sharing (Ni/Te)O6 octahedra with a Na monolayer sandwiched between them. Regarding the battery performance in half-cell, Na2Ni2-xCoxTeO6 (x = 0.1) achieved an initial charge capacity exceeding the performance of the reference material Na2Ni2TeO6. Moreover, cyclic voltammetry tests revealed that when Co is present, the storage mechanism is more capacitive than diffusive. DFT calculations carried out on Na2Ni2TeO6 using Vienna Ab initio Simulation Package (VASP) suggested that structures with the sodium atoms at the Na1 site are more energetically favorable than the Na2 and Na3. The defect and substitution routes hold promise to develop Na-ion based cathode materials with enhanced ionic conductivity.

Key Words: P2-type layered honeycomb compound, positive electrode materials, sodium-ion conductivity, cyclic voltammetry, Na-ion batteries.