Y-doped P2-type Na0.67Ni0.33Mn0.67O2: A sodium-ion battery cathode with fast charging and enhanced cyclic performance

P2-type Na0.67Ni0.33Mn0.67O2 has a high energy density and thus is considered a promising cathode material for sodium-ion batteries (SIBs). However, it has the critical disadvantage of rapid electrochemical performance reduction at high C rates. In this study, it was confirmed that Y doping enhanced the rate capability and cycle retention of the P2-type Na0.67Ni0.33Mn0.67O2 cathode material. Based on X-ray diffraction data and density functional theory calculation results, it was demonstrated that Y doping changed the c lattice. This is because of two reasons: the large ionic radius of the Y dopant and changes in the atomic charge due to Y doping. Na0.67Ni0.31Mn0.67Y0.02O2 has an expanded Na layer, which facilitates Na ion diffusion. As a result, the Y-doped material had a greater rate capability (63.4%, 2 C/0.05 C) than the pristine material (35.0%). The Y-doped material has a strong Y‒O bond, forming a stable structure, and is surrounded by Y2O3, which acts as a protective layer. The expanded Na layer also causes the Na ion to be inserted not only on the surface, but also in the bulk. As a result, the Y-doped material had a higher capacity retention of 98.1% after 60 cycles than the pristine material (76.7%).