Unlocking the Full Capacity of Iron Oxychloride as a Sustainable Lithium-ion Cathode Material

Layered iron oxychloride (FeOCl) has long been envisioned as an energy-dense and more sustainable alternative to layered transition metal oxides and lithium iron phosphate as intercalation material for Li-ion batteries. However, achieving full capacity has thus far remained elusive owing to material degradation, typically attributed to solvent co-intercalation in dilute liquid electrolytes and ionic liquids in which less than 0.5 Li+/f.u. was intercalated. In this work, we alleviate this limitation by developing a suitable electrolyte engineering approach. Using a highly dissociating salt dissolved in organic solvents with varying dielectric constants and binding energies with Li, we demonstrate that solvent co-intercalation can be suppressed in high concentration electrolytes using solvents with low Li-solvent binding energy independent of the dielectric constant. Reversible intercalation of 1 Li+ per FeOCl for a specific capacity of 250 mAh/g was achieved using 5 M lithium bis(fluorosulfonyl)imide (LiFSI) in dimethylcarbonate, providing a material specific energy of 590 Wh/kg. 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 reveals a reversible cationic ordering event when half of the interlayer Li+ sites are filled; this ordering is associated with a high activation energy and slow phase-front diffusion. Interestingly, phase transitions do not prevent good cycling ability for FeOCl even at relatively high C-rate. Overall, our work demonstrates that energy dense, sustainable, and cheap intercalation materials previously discarded for their instability can be stabilized by adopting an electrolyte engineering approach, calling for revisiting structures and compositions previously discarded.