Layered NaxCrxTi1–xO2 as Bifunctional Electrode Materials for Rechargeable Sodium Batteries

A binary system of NaCrO2–TiO2, NaxCrxTi1–xO2 (1.0 ≥ x ≥ 0.5), is systematically examined, and the electrodes' performance is tested in Na cells. Different layered phases, O3, Na-deficient O3, P2, and P3, are found in this system, as confirmed by X-ray and neutron diffraction techniques. Among them, P2-type Na2/3Cr2/3Ti1/3O2 and P3-type Na0.58Cr0.58Ti0.42O2 show excellent electrode performance as positive and negative electrode materials, respectively. P2 Na2/3Cr2/3Ti1/3O2 shows excellent rate capability as a positive electrode, and average voltage based on Cr3+/Cr4+ redox in a Na cell is increased compared with that of O3-type NaCrO2. The operating voltage of Cr3+/Cr4+ is also enhanced because of an inductive effect of Ti4+ substitution for Cr3+. However, the reversible range is limited to x < 1/3 in Na2/3–xCr2/3Ti1/3O2 associated with partial Cr oxidation to Cr6+ and migration into tetrahedral sites upon charging, as found by X-ray diffraction and X-ray absorption spectroscopy. P3 Na0.58Cr0.58Ti0.42O2 delivers ∼110 mAh g–1 of reversible capacity as a negative electrode with excellent capacity retention and rate capability. From these results, the possibility of NaxCrxTi1–xO2 as bifunctional electrode materials for rechargeable sodium batteries is also discussed.