Core-Shell Structure and X-Doped (X = Li, Zr) Comodified O3-NaNi0.5Mn0.5O2: Excellent Electrochemical Performance as Cathode Materials of Sodium-Ion Batteries

O3-NaNi0.5Mn0.5O2 is one of the most promising materials for sodium-ion batteries, which holds advantages of high cost efficiency and environmental friendliness. However, poor cycle stability and inferior rate performance impede their further development because of complex phase transitions. Herein, the successful synthesis of O3-Na0.98X0.02Ni0.5Mn0.5O2@5%Na-Mn-O (X = Li, Zr) ensured excellent rate performance, superior cycle stability by a method of forming and comodifying a core-shell structure with elemental doping. First, a core-shell structure with high-nickel in the core, and high-manganese on the surface improve cycle stability. Second, doping Li and Zr into Na sites allow them to serve as pillars to suppress phase change according to ex situ X-ray diffraction (XRD) observations. Specifically, the capacity retention rates of Na0.98Li0.02Ni0.5Mn0.5O2@5%Na-Mn-O and Na0.98Zr0.02Ni0.5Mn0.5O2@5%Na-Mn-O samples are 61% and 67%, respectively, whereas the pristine (NaNi0.5Mn0.5O2) sample is 52% cycling at a high current density of 3 C. A double modification method is proposed to ensure excellent electrochemical performance of cathode materials.