In Situ X‑ray Absorption Spectroscopy Study
of the Capacity Fading Mechanism in Hybrid Sn<sub>3</sub>O<sub>2</sub>(OH)<sub>2</sub>/Graphite Battery Anode Nanomaterials
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Abstract
In
situ X-ray absorption spectroscopy (XAS) of an electrode material
under electrochemical control has enabled a detailed examination of
the capacity fading mechanism during charge–discharge cycling
in a hybrid nanomaterial, Sn<sub>3</sub>O<sub>2</sub>(OH)<sub>2</sub>/graphite, that is considered for use as a high-capacity lithium-ion
battery anode. By the use of an original one-pot solvothermal synthesis
technique, Sn<sub>3</sub>O<sub>2</sub>(OH)<sub>2</sub> nanoparticles
were directly deposited on the surface of nanothin graphite and were
charged/discharged in situ for several cycles while XAS spectra at
the Sn K-edge were taken. Modeling of the collected extended X-ray
absorption fine structure (EXAFS) spectra provides detailed information
on the Sn–O, Sn–Sn, and Sn–Li coordination numbers
and atomic distances for each charged and discharged electrode state.
On the basis of electrochemical data and the changes in atomic arrangement
deduced from the EXAFS fitting results, including the first unambiguous
observation of Sn–Li near neighbors, a capacity fading mechanism
is proposed that is different from widely accepted volume expansion
for tin metal and tin oxides. Our experimental results suggest that
atomic clusters of metallic tin surrounded by highly disordered Li<sub>2</sub>O shells are formed on first charge. The metallic tin clusters
participate in lithiation and delithiation on the following charge/discharge
cycles; however, because of continued segregation of tin and Li<sub>2</sub>O phases, the tin clusters eventually lose electrical contact
with the rest of the electrode and become excluded from further participation
in electrochemical reactions, resulting in reduced capacity of this
anode material