X‐Ray Absorption and X‐Ray Emission Spectroscopy Study of a Molybdenum Disulfide Anode in a Sodium‐Ion Half‐Cell

Abstract

The development of efficient anodes for sodium‐ion batteries requires a deep understanding of the electrochemical processes involved. Herein, X‐ray spectroscopy is used to study changes in the electronic structure of MoS2 nanosheets coupled with few‐layered graphene at early stages of battery life. Working electrode is combined with a sodium plate and a solution of NaClO4 in ethyl carbonate/dimethyl carbonate in a homemade cell having an X‐ray transparent window. X‐ray absorption spectra at the S K‐edge and Mo L3‐edge are recorded using a synchrotron source, X‐ray emission S Kα and Mo Lα spectra are obtained on a laboratory spectrometer for initial powder and electrode material at different potentials of the cell. Analysis of the spectra shows the occupation of the initially empty orbitals of MoS2 by sodium electrons as a result of the interaction of the components of the sodium‐ion half‐cell. Ex situ X‐ray photoelectron spectroscopy measurements of a fully discharged electrode material reveal partial transformation of hexagonal MoS2 to disordered tetragonal MoS2 coated with a surface electrolyte interphase layer. The obtained results indicate that coupling MoS2 to graphene prevents the breaking of Mo–S bonds when Na+ ions are first introduced, which may promote stable battery performance.