Electrochemical Performance of AlF3-Coated LiNi0.5Mn1.5O4 5-V Cathodes at Elevated Temperature

Abstract

In the past decade, rechargeable lithium-ion batteries (LIBs) are acknowledged as one of the most suitable energy storage devices due to their high energy density, excellent cyclic performance, high power density, environmentally friendly and high operation voltage conditions. However, the development of LIBs with high energy density, high rate capability and reasonable lifetime is of great importance to meet their growing applications from portable gadgets to electric vehicles (EVs), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicle (PHEV). Up to date, LiNi0.5Mn1.5O4 (LNMO) has been regarded as one of promising cathode materials due to its high and flat working potential at around 4.7 V Vs Li/Li. The high working potential of LNMO corresponds to the high energy density and it exhibits the 658 Wh Kg-1 which is significantly higher than that of the that of LiMn2O4 (440 Wh kg-1) or LiFePO4 (591 Wh kg-1). However, the main issue of high-voltage spinel LNMO is its limited cycle life, especially at a high temperature, which is triggered by decomposition of electrolyte or dissolution of Mn/Ni at high operating potential. In the present investigation, the surface modification of LNMO cathode materials was conducted by coating with thin layer to suppress the electrolyte decomposition and transition metal dissolution at high voltage. The surface morphology and crystal structure of the AlF3-coated LNMO were systemically investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM) equipped with an energy-dispersive X-ray spectroscope (EDS). According to the series of electrochemical studies, it is found that the introduction of AlF3 conductive coating layer significantly enhanced the rate and cycle life of the LNMO spinel cathode than the pristine LNMO cathode, especially at elevated temperature. The optimum loading of AlF3 layer on LNMO surface was found to be 1 wt.%. The 1 wt.% AlF3-coated LNMO can maintain a stable discharge capacity of 100 mAh g-1 even after consecutive charging/discharging for 50 cycles at 0.1 C at 55oC.