Y-doped P2-type Na_0.67Ni_0.33Mn_0.67O₂: A sodium-ion battery cathode with fast charging and enhanced cyclic performance

Highlights: • Y was selected as a promising dopant for increasing phase, oxygen stability and suppressing volume change. • The expanded Na layer of the Y-doped samples provides a broad Na-ion pathway that facilitates Na-ion mobility. • Y₂O₃ causes high resistance, which in turn adversely affects the electrochemical performance at high C rates. • The strong Y-‒O bond can form a stable structure and improve the cycling stability. • The expanded Na layer causes the Na ion to become inserted not only in the surface, but also in the bulk. -- Abstract: P2-type Na_0.67Ni_0.33Mn_0.67O₂ 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 Na_0.67Ni_0.33Mn_0.67O₂ 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. Na_0.67Ni_0.31Mn_0.67Y_0.02O₂ 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%).