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]O2 (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 AlF3 coating were confirmed to be well crystallized in the layered structure with a space group R3¯m of the hexagonal α-NaFeO2 structure with additional peaks between 20–25° 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 Li1.134Ni0.3Mn0.556O2 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.