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 cathode 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). Our neutron diffraction data shows P63/mcm space group. Regarding the battery performance, Na₂Ni_2-xCo_xTeO₆ (x = 0.1) achieved an initial charge capacity exceeding the performance of the reference material Na₂Ni₂TeO₆. Cyclic voltammetry tests revealed the storage mechanism is more capacitive than diffusive. DFT calculations carried out 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.