Excellent air storage stability of Na-based transition metal oxide cathodes benefiting from enhanced Na−O binding energy

Abstract

O3-type layered oxides NaTmO2 are promising cathodes for Na-ion batteries. However, their practical applications have been greatly hindered by the high air affinity of lattice Na, originating from the reaction of NaTmO2 with H2O/CO2 in air accompanying the production of Na1-xTmO2 and NaOH/Na2CO3, which leads to serious loss of active Na and performance degradation. Here we propose that the air storage stability can be significantly improved by introducing a weak orbital hybridization between transition metal and oxygen layers. The weakened hybridization can facilitate more charge transfer from sodium to oxygen to form a stronger Na−O binding energy, suppressing the unfavorable active Na loss. With the tailored electronic structure in transition metal layer and more active Na being reserved in Na layers, the optimized material exhibits a smooth electrochemical profile, a high capacity of 137.5 mAh g−1, and an improved active Na retention of 90.4% after air exposure, in sharp contrast with plateau curve, 101.1 mAh g−1 and 58.0% of unmodified sample. This research provides a new insight to design air stable layered oxide materials to push their mass production and practical applications.