Reversible multi-electron transfer of Cr^2.8+/Cr^4.4+ in O3-type layered Na_0.66Fe_1/3Cr_1/3Ti_1/3O₂ for sodium-ion batteries

Na-deficient O3-type Na_0.66Fe_1/3Cr_1/3Ti_1/3O₂ material is synthesized by a simple solid-state reaction and its electrochemical property as a cathode material for sodium ion battery is evaluated for the first time. It delivers an initial large discharge capacity of 135.5 mA h g⁻¹ in the voltage range of 1.5–4.1 V at 0.1C with a smooth discharge voltage profile. X-ray absorption technique indicates that the redox processes occur entirely at the chromium sites during the Na ion intercalation and deintercalation, while Ti⁴⁺ and Fe³⁺ remain as stabilizing spectators in layered Na_0.66Fe_1/3Cr_1/3Ti_1/3O_2. Our results have demonstrated the new pathway to realize multi-electron transfer of Cr²+∼3⁺/Cr^4.4+ redox in layered compounds.