Studies of Li and Mn-Rich Li_x[MnNiCo]O₂Electrodes: Electrochemical Performance, Structure, and the Effect of the Aluminum Fluoride Coating

We report herein on the study of Li and Mn rich Li x [MnNiCo]O 2 cathode materials with an emphasis on the effect of AlF 3 coating on their electrochemical performance. The initial stoichiometry of these materials was xLi 2 MnO 3 . (1-x)LiMn y Ni z Co w O 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 Li 2 MnO 3 ( C 2/ m ) and rhombohedral Li M O 2 ( 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-AlF 3 ( P4nmm symmetry) and AlF 3 nano-crystals were regularly distributed over the particles surface. Amorphous clusters of AlF 3 and/or other Al-containing species, like AlF x O y , Al[FOH], etc. may also present, as it follows from solid-state NMR measurements. It was shown that electrodes comprising the AlF 3 -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]O 2 /AlF 3 materials revealed much higher thermal stability both in the pristine (lithiated) and cycled (delithiated) states than their uncoated counterparts.