Initial lithiation of carbon-coated zinc ferrite anodes studied by in-situ X-ray absorption spectroscopy

Spinel metal oxide nanoparticles are promising alternative anode materials for Li-ion batteries showing enhanced cycling performances and specific theoretical capacity resulting from a combination of alloying and conversion reactions during dis-/charge. In this work, we study the effect of the initial lithium insertion into carbon-coated ZnFe2O4 anodes by X-ray absorption spectroscopy. We are able to closely monitor the structural changes and the electronic structure in-situ and in-operando. For low lithium uptake (up to 0.3 Li+ per formula unit) the initial crystalline structure is not significantly modified. Further lithium insertion causes the migration of Zn atoms from tetrahedral 8a sites into vacant octahedral 16c sites, and Fe is gradually reduced from Fe3+ to Fe2+ upon lithium insertion. In the early stage of lithiation (up to 2 Li+ per formula unit) we find no evidence of Zn reduction. Also, a gradual damping of EXAFS signals, linked to the increasing structural disorder, is observed