Enhanced Ionic Transport and Structural Stability of Nb-Doped O3-NaFe0.55Mn0.45-xNbxO2 Cathode Material for Long-Lasting Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are promising candidates for inexpensive and sustainable energy storage devices for the widespread utilization of intermittent renewable energy because of the natural abundance of sodium raw materials. However, since the ionic radius of Na+ is inherently larger than that of Li+, Na-based intercalation materials often suffer from poor stability and slow reaction kinetics. Regarding SIB cathodes, layered transition metals oxides (NaxTMO2) show promising theoretical capacities but low stability. In this work, a series of NaFe0.55Mn0.45-xNbxO2 (x = 0, 0.01, 0.02, and 0.03) compounds are synthesized and show superior stability and rate-capability compared with the pure oxide. For instance, the best-performing sample, NaFe0.55Mn0.45-xNbxO2, has a specific capacity of 127 mAh g(-1) and 80% capacity retention over 100 cycles at 0.1 C. Ex situ X-ray diffraction (XRD) result shows that the Nb incorporation could suppress TMO2 slip and reduce the energy barrier of the O3-P3 phases' transition. When coupled with a hard carbon (HC) anode in a full cell, the battery exhibits significant weight and volume energy and power densities. It is believed that Nb-doping enlarges lattice spacing of the oxide and partially reduces Mn's to Mil's, increasing the ionic conductivity of the cathodic materials.