Enhanced electrochemical performance of Na_0.5Ni_0.25Mn_0.75O₂ micro-sheets at 3.8 V for Na-ion batteries with nanosized-thin AlF₃ coating

Abstract

Na0.5Ni0.25Mn0.75O2 (NNMO) micro-sheets are novel and attractive cathode materials for Na-ion batteries because of the high capacity and a considerable operating voltage of 3.8 V versus Na/Na+, while the rate and cycling capabilities were not satisfactory. In this paper, AlF3 and Al2O3 were employed as surface modifying materials to enhance the electrochemical performance of NNMO micro-sheets by a simple wet chemical process. XRD, SEM and TEM analyses indicated that the AlF3 layer was amorphous and evenly coated on the surface of NNMO, while the Al2O3 layer was attached to the NNMO surface with the morphology of nanoparticles. Different coating modes led to different electrochemical results. AlF3 coated NNMO (NNMO@AlF3) delivered a better rate and cycling performance than the pristine NNMO, while the Al2O3 coated NNMO (NNMO@Al2O3) could not. The NNMO@AlF3 exhibited a 1C discharge capacity of 108 mA h g-1 and 0.2C capacity retention of 80% up to 100 cycles with the almost invariable high potential. The electrochemistry impedance spectroscopy analysis demonstrated that the AlF3 coated NNMO had the best structure stability and fastest Na-intercalation kinetics.