Elucidating the effect of Nb doping on the electrochemical performance of Fe–Mn based Li-rich cathode materials

Fe–Mn based Li-rich cathode materials are regarded as one of the most promising cathode materials for their low cost and high energy density but possess drawbacks such as high oxygen release, low conductivity, and slow kinetics for lithium-ion transport. To address these issues, herein, we proposed a high-valance cations Nb5+ doping strategy and synthesized a series of Li1.26[Fe0.22Mn0.52]1−xNb0.74xO2 (x = 0, 0.02, 0.03, 0.04) cathode materials. Using a combination of experimental approaches and density functional theory (DFT) calculations, we found that the doped samples presented expanded interlayer spacing for Li-ion migration and better structural stability due to the large ionic radius of Nb5+ and the strengthened Nb–O bond. Moreover, Nb doping could not only decrease the band gap and the density of states of the O 2p band but also contribute extra electrons to surrounding O atoms, thereby enhancing electronic conductivity and stabilizing lattice oxygen. Benefit from the underlying mechanisms mentioned above, the Nb0.022 sample possessed a higher Coulombic efficiency (71.1 vs 65.0%) at 0.2 C, showing more stable cycling performance (90.4 vs 67.7 mAh g−1) at 0.5 C and rate capability. These favorable results can assist in the development of Fe–Mn based Li-rich cathode materials with excellent electrochemical properties.