Aliovalent-doped sodium chromium oxide (Na0.9Cr0.9Sn0.1O2 and Na0.8Cr0.9Sb0.1O2) for sodium-ion battery cathodes with high-voltage characteristics

Abstract

NaCrO₂ 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_0.9Sn_0.1]O₂ (NCSnO) and O3–Na_0.8[Cr_0.9Sb_0.1]O₂ (NCSbO), both of which show high-voltage characteristics that translate to an increase in energy density. In contrast to NaCrO₂, 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_0.9Sn_0.1]O₂ and Na_0.3[Cr_0.9Sb_0.1]O₂ is not associated with the suppression of Cr⁶⁺ formation. Both compounds show concentrations of Cr⁶⁺ that are higher than that of Na_0.3CrO₂, with an absence of O3′ phases. This implies that aliovalent-doping contributes to a suppression of the Cr⁶⁺ migration into tetrahedral sites in the interslab space, which reduces the possibility of irreversible comproportionation. NCSnO and NCSbO deliver capacities comparable to that of NaCrO₂, but show a higher average discharge voltage (2.94 V for NaCrO₂; 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.