Synthesis of Li-1.1(Ni0.425Mn0.425Co0.15)(0.9)O1.8F0.2 Materials by Different Routes: Is There Fluorine Substitution for Oxygen?

A one-step synthesis method was used, with LiF or NiF2 as fluorine precursor, to prepare Li-1.1(Ni0.425Mn0.425Co0.15)(0.9)O1.8F0.2 materials. Li-7 and F-19 magic angle spinning NMR analyses revealed the presence of fluorine as LiF at the surface of the Li(Ni0.425Mn0.425Co0.15)O-2 particles, rejecting the formation of fluorine-substituted Li-1.1(Ni0.425Mn0.425Co0.15)(0.9)O1.8F0.2 materials. These results highlighted that change in cell parameters with increasing fluorine content is not by itself proof for effective fluorine substitution for oxygen in layered oxides and that heterogeneity in the transition metal and fluoride-ion distribution at the crystallite scale can be at the origin of these modifications. LiF was shown to be present as small particles in some grain boundaries but not as a continuous layer covering the particles surface. Improved cycling stability was observed for these LiF-coated materials, showing that effective fluorine substitution for oxygen is not required for improvement of the cyclability of these layered oxides; a surface modification can be sufficient and can also have a huge impact. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3080659] All rights reserved.