Mg-doped Na[Ni1/3Fe1/3Mn1/3]O-2 with enhanced cycle stability as a cathode material for sodium-ion batteries

Abstract

O3-type Na[Ni1/3Fe1/3Mn1/3]O-2 (NaNFM) is considered as a promising cathode material for sodium-ion batteries; however, its poor cycling stability is still a concern. In this study, we discuss the structural, surface and electrochemical properties of Mg-doped NaMgx[Ni1/3Fe1/3Mn1/3](1-x)O-2 materials and their enhanced cycling performance. The variations of the lattice parameters by substitution of Mg ion and its uniform distribution on the particles are confirmed using X-ray diffraction and transmission electron microscopy. The optimized NaMg0.05[Ni1/3Fe1/3Mn1/3](0.95)O-2 delivers a discharge capacity of similar to 120 mAh g(-1) and has a diffusion coefficient of Na ranging from 6.5 x 10(-13) to 2.7 x 10(-10) cm(2) s(-1). In particular, it shows a relatively high discharge capacity of 42 mAh g(-1) even at a high current density of 1200 mA g(-1) and exhibits considerably enhanced cycling stability (77% capacity retention after 50 cycles), compared with that of the undoped NaNFM (40%). Based on structural and electrochemical analyses, it is suggested that Mg doping can effectively suppress the irreversible structural degradation and induce more reversible phase transitions; this results in a more stable cycling performance of the Mg-doped NaNFM than that of undoped NaNFM.