Studies of li and mn-rich Li_x[MnNiCo]O₂ electrodes: Electrochemical performance, structure, and the effect of the aluminum fluoride coating

We report herein on the study of Li andMn rich Li_x[MnNiCo]O ₂ cathode materials with an emphasis on the effect of AlF₃ coating on their electrochemical performance. The initial stoichiometry of these materials was xLi₂MnO₃ .(1-x)LiMnyNizCowO ₂ where x is in the range 0.4-0.5 and the y:z:w ratio was as we previously reported. Their structure was considered on the basis of two-components model, namely monoclinic Li₂MnO₃ (C2/m) and rhombohedral LiMO₂ (R-3m) (M = Mn, Ni, Co) that are structurally compatible and closely integrated phases. Based on TEM studies we concluded that the coating had a crystalline tetragonal structure t-AlF₃ (P4nmm symmetry) and AlF₃ nano-crystals were regularly distributed over the particles surface. Amorphous clusters of AlF3 and/or other Al-containing species, like AlF_xO_y, Al[FOH], etc. may also present, as it follows from solid-state NMR measurements. It was shown that electrodes comprising the AlF3-coated material exhibited higher reversible capacities of ?250 mAh/g at a C/5 rate, more stable cycling behavior, higher lithium storage capability at 60?C, and lower impedance measured during Li-deinteraclation comparing to electrodes prepared from the uncoated material. An important finding is that Li_x[MnNiCo]O₂ /AlF₃ materials revealed much higher thermal stability both in the pristine (lithiated) and cycled (delithiated) states than their uncoated counterparts.