New O3-Type Layer-Structured Na_0.80[Fe_0.40Co_0.40Ti_0.20]O₂ Cathode Material for Rechargeable Sodium-Ion Batteries

Herein, we formulated a new O3-type layered Na_0.80[Fe_0.40Co_0.40Ti_0.20]O₂ (NFCTO) cathode material for sodium-ion batteries (SIBs) using a double-substitution concept of Co in the parent NaFe_0.5Co_0.5O₂, having the general formula Na_1-x[Fe_0.5-x/2Co_0.5-x/2M⁴⁺_x]O₂ (M⁴⁺ = tetravalent ions). The NFCTO electrode delivers a first discharge capacity of 108 mAhg^-1 with 80% discharge capacity retention after 50 cycles. Notably, the first charge-discharge profile shows asymmetric yet reversible redox reactions. Such asymmetric redox reactions and electrochemical properties of the NFCTO electrode were correlated with the phase transition behavior and charge compensation reaction using synchrotron-based in situ XRD and ex situ X-ray absorption spectroscopy. This study provides an exciting opportunity to explore the interplay between the rich chemistry of Na_1-x[Fe_0.5-x/2Co_0.5-x/2M⁴⁺_x]O₂ and sodium storage properties, which may lead to the development of new cathode materials for SIBs.