Aliovalent-doped sodium chromium oxide (NaCrSnO and NaCrSbO) for sodium-ion battery cathodes with high-voltage characteristics

NaCrO 2 with high rate-capability is an attractive cathode material for sodium-ion batteries (NIBs). However, the amount of reversibly extractable Na + ions is restricted by half, which results in relatively low energy density for practical NIB cathodes. Herein, we describe aliovalent-doped O3-Na 0.9 [Cr 0.9 Sn 0.1 ]O 2 (NCSnO) and O3-Na 0.8 [Cr 0.9 Sb 0.1 ]O 2 (NCSbO), both of which show high-voltage characteristics that translate to an increase in energy density. In contrast to NaCrO 2 , NCSnO and NCSbO can be reversibly charged to 3.80 and 3.95 V, respectively, delivering 0.5 Na + along with Cr 3+/4+ redox alone. The reversible chargeability to Na 0.4 [Cr 0.9 Sn 0.1 ]O 2 and Na 0.3 [Cr 0.9 Sb 0.1 ]O 2 is not associated with the suppression of Cr 6+ formation. Both compounds show concentrations of Cr 6+ that are higher than that of Na 0.3 CrO 2 , with an absence of O3′ phases. This implies that aliovalent-doping contributes to a suppression of the Cr 6+ migration into tetrahedral sites in the interslab space, which reduces the possibility of irreversible comproportionation. NCSnO and NCSbO deliver capacities comparable to that of NaCrO 2 , but show a higher average discharge voltage (2.94 V for NaCrO 2 ; 3.14 V for NCSnO; 3.21 V for NCSbO), which leads to a noticeable increase in energy densities. The high-voltage characteristics of NCSnO and NCSbO are also validated via density-functional-theory calculations. In contrast to O3-NaCrO 2 , O3-Na 0.8 [Cr 0.9 Sb 0.1 ]O 2 shows reversibility when charged to 3.95 V and high-voltage charge/discharge features, which results in improvement of energy density.