Layered Na_xCr_xTi_1–xO₂ as Bifunctional Electrode Materials for Rechargeable Sodium Batteries

A binary system of NaCrO 2 –TiO 2, Na x Cr x Ti 1– x O 2 (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 Na 2/3 Cr 2/3 Ti 1/3 O 2 and P3-type Na 0.58 Cr 0.58 Ti 0.42 O 2 show excellent electrode performance as positive and negative electrode materials, respectively. P2 Na 2/3 Cr 2/3 Ti 1/3 O 2 shows excellent rate capability as a positive electrode, and average voltage based on Cr 3+ /Cr 4+ redox in a Na cell is increased compared with that of O3-type NaCrO 2 . The operating voltage of Cr 3+ /Cr 4+ is also enhanced because of an inductive effect of Ti 4+ substitution for Cr 3+ . However, the reversible range is limited to x < 1/3 in Na 2/3– x Cr 2/3 Ti 1/3 O 2 associated with partial Cr oxidation to Cr 6+ and migration into tetrahedral sites upon charging, as found by X-ray diffraction and X-ray absorption spectroscopy. P3 Na 0.58 Cr 0.58 Ti 0.42 O 2 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 Na x Cr x Ti 1– x O 2 as bifunctional electrode materials for rechargeable sodium batteries is also discussed.