Aliovalent-doped sodium chromium oxide (Na(0.9)Cr(0.9)Sn(0.1)O(2) and Na(0.8)Cr(0.9)Sb(0.1)O(2)) 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.