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2017-07-13

Full picture discovery for mixed-fluorine anion effects on high-voltage spinel lithium nickel manganese oxide cathodes




Experimental and computational studies on the fluorine anion distribution inside the spinel frame work and the crystals of LiNi0.5Mn1.5O4-xFx (LNMOF). All possible O/F arrangements were considered to determine the most stable 2, 13 and 9 different O/F arrangements, which were calculated for x=0.125, 0.25 and 0.375, respectively: (a) x=0.125, (b) x=0.25 and (c) x=0.375 in LiNi0.5Mn1.5O4-xFx. (d) X-ray diffraction (XRD) patterns of LNMOF crystals with various amounts of fluorine anion incorporation. XRD patterns taken from all of the prepared samples are in good agreement with the reference data of ICDD PDF 70-8650 (LiNi0.5Mn1.5O4) with a small shift from fluorine addition. (e) The lattice parameters of LNMOF determined from the XRD patterns and density function theory (DFT) calculations. Series of F1s X-ray photoelectron core-level spectra: (f) LNMOF0.1, (g) LNMOF0.2 and (h) LNMOF0.3 with varying Ar spattering etching thicknesses (black: as prepared; red: 20?nm; blue: 40?nm; and pink: 60?nm).

Small amounts of fluorine substituting for oxygen deficiencies could reduce Mn dissolution, enhancing the cyclability in spinel-type lithium nickel manganese oxides (LiNi0.5Mn1.5O4). Fluorine anion incorporation simultaneously enhances the C-rate capability and specific capacity fading. We used experimental and theoretical approaches to obtain a full picture of the mixed-anion effects for LiNi0.5Mn1.5O4-xFx cathode materials. The fluorine anion reduced the activation barrier for lithium-ion hopping along the most energetically preferable 8a-16c-8a route, enhancing the C-rate capability. Simultaneously, the coordination bond of the linear F-Mn3+F- (Mn@2F diagonal) arrangement increased the oxidation potential to 5.1?V (vs Li+/Li). This hampered full extraction of Li+ from the spinel lattice, which was triggered by the oxidation of Mn3+ below the cutoff voltage (3.54.8?V (vs Li+/Li)), leading to a capacity loss.

(full paper)