Synthesis of “Li[sub 1.1](Ni[sub 0.425]Mn[sub 0.425]Co[sub 0.15])[sub 0.9]O[sub 1.8]F[sub 0.2]” Materials by Different Routes: Is There Fluorine Substitution for Oxygen?

Abstract

A<sup> </sup>one-step synthesis method was used, with LiF or NiF<sub>2</sub> as<sup> </sup>fluorine precursor, to prepare Li<sub>1.1</sub>(Ni<sub>0.425</sub>Mn<sub>0.425</sub>Co<sub>0.15</sub>)<sub>0.9</sub>O<sub>1.8</sub>F<sub>0.2</sub> materials. <sup>7</sup>Li and <sup>19</sup>F magic<sup> </sup>angle spinning NMR analyses revealed the presence of fluorine as<sup> </sup>LiF at the surface of the Li(Ni<sub>0.425</sub>Mn<sub>0.425</sub>Co<sub>0.15</sub>)O<sub>2</sub> particles, rejecting the<sup> </sup>formation of fluorine-substituted Li<sub>1.1</sub>(Ni<sub>0.425</sub>Mn<sub>0.425</sub>Co<sub>0.15</sub>)<sub>0.9</sub>O<sub>1.8</sub>F<sub>0.2</sub> materials. These results highlighted that change<sup> </sup>in cell parameters with increasing fluorine content is not by<sup> </sup>itself proof for effective fluorine substitution for oxygen in layered<sup> </sup>oxides and that heterogeneity in the transition metal and fluoride-ion<sup> </sup>distribution at the crystallite scale can be at the origin<sup> </sup>of these modifications. LiF was shown to be present as<sup> </sup>small particles in some grain boundaries but not as a<sup> </sup>continuous layer covering the particles surface. Improved cycling stability was<sup> </sup>observed for these LiF-coated materials, showing that effective fluorine substitution<sup> </sup>for oxygen is not required for improvement of the cyclability<sup> </sup>of these layered oxides; a surface modification can be sufficient<sup> </sup>and can also have a huge impact.