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

Abstract

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.Lithium-ion batteries (LIBs) are the primary power source for portable electronic applications. Graphite is the most common anode material in commercially available products due to its excellent cycle life and specific capacity (theoretical capacity of 372 mAh g −1 ). 1,2 However, for LIBs to be viable options for large scale applications such as electric vehicles, significant improvements in energy and power densities, along with cycle life of next generation LIBs are needed. Metal oxides are often discussed as reasonable alternatives to traditional carbon anode materials as they exhibit theoretical capacities upwards of three times higher than graphitic anodes. 3−5 ZnO is one of the potential alternatives as it has a theoretical capacity of 978 mAh g −1 , 3 however it has been reported to suffer from severe capacity loss in the first few cycles, even at slow charging rates. 6−11 Attempts have been made to control the capacity fading by using nano-scale ZnO particles, 3,12−14 coating of ZnO with carbon, 15 nickel, 16 copper, 17 tin, 10,18 and even replacing oxygen with nitrogen 5 and phosphorous 19,20 resulting in modest improvements. In order to successfully stabilize cycling behavior of ZnO the overall lithation/delithiation mechanism must be understood at the atomic level to properly engineer better performing anodes. The proposed reduction-oxidation reactions are 4,5,9where ZnO is converted to Zn and Li 2 O on initial charging and, upon further chargin...

Elucidating the Impact of Cobalt Doping on the Lithium Storage Mechanism in Conversion/Alloying‐Type Zinc Oxide Anodes

“…In contrast, the peaks in region d) related to the dealloying processes remain relatively unchanged, although they reveal a slightly decreasing intensity. In fact, these results reveal the often‐reported irreversibility of Li 2 O degradation and the limited reversibility of the Li‐alloying process of Zn –…”

“…A careful analysis of the different XRD patterns also reveals the appearance of a very broad and yet tiny reflection at about 43.0° (marked by a red arrow). Given the indicated reduction of ZnO accompanied by the formation of amorphous Li 2 O, it appears reasonable to assign this feature to the main reflection of metallic zinc (JCPDS card file no. 00‐004‐0831), which is also in good agreement with the onset of the following voltage plateau (Figure a) assigned to the occurrence of an equilibrium between different but simultaneously present phases.…”

“…This finding is, in fact, also in good agreement with the recorded specific capacity (Figure a) because the alloying contribution for ZnO theoretically corresponds to 329 mAh g −1 and our assignment of the different peaks in the CV profile (Figure a and b). The subsequent decrease in intensity observed for the two reflections at around 38.8 and 43.0° may be ascribed to the partial reoxidation of metallic zinc at potentials higher than 0.7 V . This is further supported by the appearance of a very broad reflection in the range of 31.0 to 35.0° and the limited decrease in intensity of the reflection at about 36.2° because it overlaps with the main reflection of the wurtzite‐structured oxide.…”

“…In fact, although a theoretical specific capacity of about 988 mAh g −1 (if the reaction ZnO+3 Li + +3 e − ↔LiZn+Li 2 O is considered to be fully reversible) certainly arouses interest, apart from a few outstanding exceptions most electrochemical studies revealed poor performance and rapid capacity fading . To explain the origin of this inferior electrochemical performance of ZnO‐based electrodes, Pelliccione and co‐workers very recently performed an in situ X‐ray absorption fine structure (XAFS) spectroscopy study. It was found that this substantial capacity decay mainly originates from the formation of relatively large metallic zinc particles upon continuous de‐/lithiation.…”

“…In addition, the large volume changes upon alloying–dealloying lead to pulverization of the active material, ongoing electrolyte decomposition, and finally the loss of electrical contact within the composite electrode . Additionally, the decrease in electronic conductivity within the electrode owing to the insulating nature of Li 2 O presumably contributes to the observed rapid capacity fading . We have recently reported that these challenges and, in particular, the reversibility of the conversion reaction can be substantially enhanced by introducing a transition‐metal dopant into the metal oxide structure .…”