Single-Step Synthesis of FeSO4F1-yOHy (0 ≤ y ≤ 1) Positive Electrodes for Li-Based Batteries

The recent discovery of electrochemical activity at 3.6 V vs Li+/Li-0 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 FeSO4F1-yOHy phases using FeF3 and Fe-2(SO4)(3)center dot nH(2)O as precursors. Using thorough chemical analytical techniques to test for F- content in conjunction with Mossbauer 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+/Li-0 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 Mossbauer spectroscopy. Moreover, we show the versatility of our synthetic approach by extending it to the elaboration of Fe1-zMzSO4F1-yOHy phases with M = Ti and V.