Single-Step Synthesis of FeSO₄F_1–yOH_y (0 ≤ y ≤ 1) Positive Electrodes for Li-Based Batteries

The recent discovery of electrochemical activity at 3.6 V vs Li + /Li 0 in LiFeSO 4 F has generated widespread research activity in this new family of fluorosulfate electrode materials aiming at either increasing the Fe 3+ /Fe 2+ 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 FeSO 4 F 1– y OH y phases using FeF 3 and Fe 2 (SO 4 ) 3 · n H 2 O 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 + /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 Mössbauer spectroscopy. Moreover, we show the versatility of our synthetic approach by extending it to the elaboration of Fe 1– z M z SO 4 F 1– y OH y phases with M = Ti and V.