Unlocking the Full Capacity of Iron Oxychloride as a Sustainable Mixed Anion Intercalation Material

Mixed anion compounds offer unique opportunities to tune the redox properties of intercalation compounds. Among them, layered iron oxychloride (FeOCl) has long been envisioned as an energy-dense and sustainable alternative to layered oxides for Li–ion batteries. However, achieving full capacity has thus far remained elusive owing to detrimental solvent cointercalation typically observed in dilute liquid electrolytes and ionic liquids. In this work, we alleviate this limitation by developing a suitable electrolyte engineering approach and demonstrate that 1 Li atom can be reversibly intercalated in FeOCl. We demonstrate that solvent cointercalation can be suppressed in high-concentration electrolytes using a highly dissociating salt, lithium bis(fluorosulfonyl)imide (LiFSI), dissolved in solvents with low Li-solvent binding energy, including dimethylcarbonate and acetonitrile, achieving a capacity of 235 mAh/g and a material specific energy of 590 Wh/kg in its lithiated form. Combining X-ray absorption measurements at iron, oxygen, and chloride K-edges, we observe that the intercalation proceeds with a transition from distorted high-spin Fe3+ to low-spin Fe2+, while operando X-ray diffraction shows three successive biphasic processes associated with changes in interlayer spacing. Entropic potential measurements coupled with temperature-dependent cycling reveal a reversible cationic ordering event when half of the interlayer Li+ sites are filled. This ordering is associated with high activation energy and a slow phase-front diffusion, which does not prevent good cycling ability at room temperature even at high C-rate. Our work calls for revisiting through an electrolyte engineering approach energy dense and sustainable mixed anion materials previously discarded for their instability.