Single-Step Synthesis of FeSO₄F_1–<i>y</i>OH_<i>y</i> (0 ≤ <i>y</i> ≤ 1) Positive Electrodes for Li-Based Batteries

The recent discovery of electrochemical activity at 3.6 V vs Li⁺/Li⁰ in LiFeSO₄F has generated widespread research activity in this new family of fluorosulfate electrode materials aiming at either increasing the Fe³⁺/Fe²⁺ redox potential, searching for new active members, or extending this family to hydroxyl-fluorosulfates. Here we present a new low temperature single step synthesis of FeSO₄F_1–<i>y</i>OH_<i>y</i> phases using FeF₃ and Fe₂(SO₄)₃·<i>n</i>H₂O as precursors. Using thorough chemical analytical techniques to test for F^– content in conjunction with Mössbauer measurements, we demonstrate the existence of a limited solid solution (0.35 < y<1) within this system. Members pertaining to this solid solution have a redox activity ranging from 3.2 to 3.6 V vs Li⁺/Li⁰ and show sustained reversible capacity retention of 130 mAh/g which makes them potentially interesting for Li-based polymer batteries. We demonstrate that the Li-insertion-deinsertion mechanism depends markedly on the sample F^–content by using joint in situ XRD and Mössbauer spectroscopy. Moreover, we show the versatility of our synthetic approach by extending it to the elaboration of Fe_1–<i>z</i><i>M</i>_<i>z</i>SO₄F_1–<i>y</i>OH_<i>y</i> phases with <i>M</i> = Ti and V