Enhanced high-rate performance of Zr-doped P2-Na0.67Ni0.33Mn0.67O2 cathode for sodium-ion batteries

Abstract

Layered P2-type oxide compounds are an essential class of cathode materials for Na-ion batteries because of their superior capacity, average working potential, enhanced diffusivity of Na⁺ ions, and air stability compared to the O3-type oxides. Among the P2-type oxides, Na_0.67Ni_0.33Mn_0.67O₂ (NNMO) is one of the most explored materials because of its superior electrochemical performance. The inherent problem of low capacity retention because of phase changes during high-voltage cycling and Na⁺/vacancy ordering is still a considerable challenge for P2-type oxide cathodes. In this work, we have doped NNMO with Zr⁴⁺ ions at the Ni site to improve the compound's cycle stability. Partial substitution of Ni²⁺ ions with Zr⁴⁺ breaks the Na⁺/vacancy ordering and increases the interslab distance in the lattice, allowing easy movement of Na⁺ ions. These effects boost the cycle stability and the rate kinetics at higher rates. Herein, we report Na_0.67Ni_0.29Zr_0.02Mn_0.67O₂, which delivers 80 mAh g⁻¹ at 1 C rate and retains 90.1% of it after 500 cycles. At 5 C, it delivers 62 mAh g⁻¹ after 700 cycles, showing an outstanding retention of 75.6%. Interestingly, a full cell made with commercial hard carbon anode delivers 74 mAh g⁻¹ in the initial cycle and retains 65.7% after 50 cycles at 1 C, demonstrating an energy density of 229 Wh kg⁻¹.