Selective sodium-ion diffusion channels in Na_2-xFe₃(PO₄)₃ positive electrode for Na-ion batteries

The electrochemical community around the world is focusing heavily on sodium-ion batteries R&D due to the huge abundance of sodium and need for large scale deployment of safe and inexpensive batteries for the electrical grid energy storage. We report on the characterization of model parameters such as ionic diffusivity and interfacial kinetics, for a potential sodium-ion battery cathode material, Na_2-xFe₃(PO₄)₃, using experimental and computational investigations. The Na_2-xFe₃(PO₄)₃ structure is characterized by two distinct one-dimensional (1-D) Na diffusion channels. Density Functional Theory (DFT) calculation reveals that Na is first fully removed from one of the channels followed by the subsequent removal of Na from the second channel. The experimental investigation reveals that, in the beginning of Na removal at 0 ≤ x ≤ 0.5; the sodium ion diffusivity is of the order of ~10⁻¹¹ cm²s⁻¹ and then slightly decreases. This is further confirmed by comparing the calculated sodium diffusion barriers along the individual 1-D channels as well as between the channels. Nevertheless, the exchange current density slowly increases up to x = 1.0 and remains quite constant thereafter. The magnitude of exchange current density is very low suggesting that the interfacial kinetics are the rate limiting factor. The obtained results suggest that Na_2-xFe₃(PO₄)₃ could achieve better rate performance with long cycling stability through engineering of the particle morphology and microstructure.