In Situ XAFS Study of the Capacity Fading Mechanisms in ZnO Anodes for Lithium-Ion Batteries

Correlation of electrochemical performance and in situ X-ray absorption fine structure (XAFS) spectroscopy measurements on ZnO anodes for lithium-ion batteries has enabled a detailed examination of the capacity fading mechanisms in this material. ZnO electrodes were galvanostatically charged/discharged in situ for several cycles while XAFS spectra at the Zn K-edge were taken. X-ray absorption near edge structure (XANES) spectroscopy provided information on the oxidation state of Zn atoms in each charged and discharged state. Modeling of extended X-ray absorption fine structure (EXAFS) provided detailed information on the Zn–O, Zn–Zn and even Zn–Li coordination numbers and atomic distances for each charged and discharged electrode states. Based on the changes in atomic arrangement deduced from EXAFS fitting results, it is suggested that metallic Zn nanoparticles larger than 10 nm in diameter and bulk-like properties are created during the first few cycles. In the first discharged state, a small fraction of metallic Zn is oxidized back to ZnO, but such re-oxidation is only observed in the first discharged state. On subsequent cycling, the local Zn environment is unchanged, indicating that majority of zinc is no longer participating in any electrochemical reaction. The observed rapid capacity fade is correlated to the irreversible conversion of ZnO to metallic Zn and segregation of Zn atoms into the large metallic zinc nanoparticles during the first charge, which is essentially conversion of the high capacity ZnO electrode to a poorly performing metallic Zn anode.