title: Is the Solid Electrolyte Interphase an Extra-Charge Reservoir in Li-Ion Batteries?
abstract: Advanced metal oxide electrodes in Li-ion batteries usually show reversible capacities exceeding the theoretically expected ones. Despite many studies and tentative interpretations, the origin of this extra-capacity is not assessed yet. Lithium storage can be increased through different chemical processes developing in the electrodes during charging cycles. The solid electrolyte interface (SEI), formed already during the first lithium uptake, is usually considered to be a passivation layer preventing the oxidation of the electrodes while not participating in the lithium storage process. In this work, we combine high resolution soft X-ray absorption spectroscopy with tunable probing depth and photoemission spectroscopy to obtain profiles of the surface evolution of a well-known prototype conversion-alloying type mixed metal oxide (carbon coated ZnFe2O4) electrode. We show that a partially reversible layer of alkyl lithium carbonates is formed (∼5-7 nm) at the SEI surface when reaching higher Li storage levels. This layer acts as a Li reservoir and seems to give a significant contribution to the extra-capacity of the electrodes. This result further extends the role of the SEI layer in the functionality of Li-ion batteries.
authors.affiliation: Physics Division, School of Science and Technology, University of Camerino , 62032 Camerino (MC), Italy.
authors.forename: S Javad
authors.identifier: http://orcid.org/0000-0002-6771-170X
authors.initials: SJ
authors.lastname: Rezvani
authors.affiliation: Physics Division, School of Science and Technology, University of Camerino , 62032 Camerino (MC), Italy.
authors.forename: Roberto
authors.initials: R
authors.lastname: Gunnella
authors.affiliation: Department of Solid State Physics, Gdańsk University of Technology , 80-309, Gdansk, Poland.
authors.forename: Agnieszka
authors.initials: A
authors.lastname: Witkowska
authors.affiliation: Helmholtz Institute Ulm (HIU) , Albert-Einstein-Allee 11, 89081 Ulm, Germany.|Karlsruhe Institute of Technology (KIT) , 76131 Karlsruhe, Germany.|Institute of Physical Chemistry, University of Muenster , Corrensstrasse 28/30, 48149 Muenster, Germany.
authors.forename: Franziska
authors.initials: F
authors.lastname: Mueller
authors.affiliation: Chemistry Division, School of Science and Technology, University of Camerino , 62032 Camerino (MC), Italy.
authors.forename: Marta
authors.initials: M
authors.lastname: Pasqualini
authors.affiliation: Chemistry Division, School of Science and Technology, University of Camerino , 62032 Camerino (MC), Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Nobili
authors.affiliation: Helmholtz Institute Ulm (HIU) , Albert-Einstein-Allee 11, 89081 Ulm, Germany.
authors.forename: Stefano
authors.initials: S
authors.lastname: Passerini
authors.affiliation: Physics Division, School of Science and Technology, University of Camerino , 62032 Camerino (MC), Italy.
authors.forename: Andrea Di
authors.initials: AD
authors.lastname: Cicco
keywords: Li-ion battery; X-ray absorption spectroscopy; X-ray photoemission spectroscopy; extra-capacity; metal alloying oxide anodes; solid electrolyte interphase