In Situ X‑ray Absorption Spectroscopy Study of the Capacity Fading Mechanism in Hybrid Sn₃O₂(OH)₂/Graphite Battery Anode Nanomaterials

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, Sn₃O₂(OH)₂/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, Sn₃O₂(OH)₂ 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 Li₂O 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 Li₂O 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