Improved electrochemical performance of LiNi0.5Mn0.5O2 by Li-enrichment and AlF3 coating

LiNi0.5Mn0.5O2 (x = 0.5), and Li-rich Li1.134Ni0.3Mn0.566O2 (x = 0.3), Li1.2Ni0.2Mn0.6O2 (x = 0.2), in the series Li[NixLi1/3-2x/3Mn2/3-x/3]O-2 (0 <= x <= 0.5) were synthesized by sol-gel method using citric acid as chelating agent, and coated with a 2-5 nm thick AlF3 layer by chemical deposition method. The synthesized Li rich oxides before and after AlF(3 )coating were confirmed to be well crystallized in the layered structure with a space group R (3) over barm of the hexagonal alpha-NaFeO2 structure with additional peaks between 20-25 degrees related to a C2/m symmetry (monoclinic phase) by (XRD) analysis. Scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDX), high resolution transmission electron microscopy (HRTEM) showed that the particle sizes are in the range of 100-250 nm and decrease with increasing Li content. Raman spectra show that Li-rich oxides are a composite of Li2MnO3 and LiMO2 (M=Ni, Mn). Successive improvements in discharge capacity after lithium enrichment and coating process for Li-rich oxides was observed from electrochemical tests. AlF3-coated Li(1.134)Ni(0.3)Mn(0.556)O2 and Li1.2Ni0.2Mn0.6O2 delivered a capacity of 225 and 248 mAh g(-1), respectively, stable over the 55 cycles investigated. According to the analysis from electrochemical impedance spectra (EIS), the improvements of the electrochemical performance are mainly attributed to the pre-activation of the Li-rich layered oxide induced by the AlF3 coating.