Electrochemical and in-Situ Investigation of Quaternary Lithium Transition Metal Fluorides in Li-Ion Batteries

During the last decade Lithium metal oxides or polyanionic compounds containing two or more different transition metal ions (e.g. LiNi_0.5Mn_1.5O₄ or LiNi_1-x-yCo_xMn_yO₂) are in the focus of research and commercial interest as positive electrodes for Li-ion batteries. Lithium transition metal fluorides are very promising materials compared to common oxide materials with corresponding electrochemically active cations because the more electronegative fluorine atoms increase the redox potential leading to a higher specific energy. However, no reports are given about the electrochemical properties of quaternary lithium transition metal fluorides as positive electrode. In this study a novel sol-gel synthesis route was applied to synthesize several quaternary lithium transition metal fluorides LiMFeF₆ (M²⁺ = 3d transition metal) without toxic chemicals like HF, LiF or F_2(gas). After the structural and morphological characterization the fundamental electrochemical properties are characterized. It could be shown that up to 1 eq. Lithium can be inserted fully reversible into the LiMFeF₆ host structure with a notable cycling stability and a remarkable rate performance. Furthermore an in-situ x-ray powder diffraction and x-ray absorption investigation reveals that the host structure is an insertion material. The electrochemical active redox couple Fe^3+/2+ was confirmed by Mößbauer analysis.