Effect of Y-doping on high voltage stability of NaNi1/3Fe1/3Mn1/3O2 as a cathode material for sodium-ion batteries

Layered NaNi1/3Fe1/3Mn1/3O2 is of particular interest for sodium-ion batteries due to its satisfactory properties and low production cost. However, it commonly suffers from deterioration in air, side reactions with electrolyte and structural instability during delithiation/lithiation (especially at high voltage). In this work, Y-doping is applied to NaNi1/3Fe1/3Mn1/3O2, and the crystallographic structure, morphology and electrochemical properties are characterized. An appropriate amount of Y-dopant significantly improves the structural stability of the cathode when charged over 4 V, resulting in higher capacity and more importantly, remarkably improved cycling stability. The optimized cathode delivers an initial discharge capacity of 156.33 mAh g−1 at 0.1 C (1 C = 150 mA g−1) in a Na-metal half cell, with an initial Coulombic efficiency of 86.94 %. After being cycled at 0.5 C for 300 cycles, it exhibits a capacity retention of 68.42 %, whereas the undoped sample only maintains 28.40 % of its initial capacity under the same test conditions. When tested in a full cell with a hard carbon anode, the Y-doped cathode also shows a promising cycle life that a capacity retention of 83.12 % after 200 cycles is obtained. X-ray diffraction measurement after long-term cycling reveals that the Y-doping enhances the structural stability of the cathode. •Y-doping is applied to NaNi1/3Fe1/3Mn1/3O2 cathode by a facile solid-state method.•The specific capacity is enhanced by Y-doping.•The cycling stability and rate capability are improved by Y-doping.•The phase transitions above 4 V become more reversible after Y-doping.