Enhanced electrochemical performance of Na0.5Ni0.25Mn0.75O2 micro-sheets at 3.8 V for Na-ion batteries with nanosized-thin AlF3 coating

Abstract

Na_0.5Ni_0.25Mn_0.75O₂ (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.