Fig. 139: a) Galvanostatic cycling of electrodes based on ZnO, (carbon-coated)
Zn0.9Fe0.1O and Zn0.9Co0.1O. b,d) Ex-situ recorded Zn K-edge XANES
spectra measured on metallic zinc (black dashed line) and on anodes (orange and
green) recovered at selected potentials along the first de-/lithiation.
c,e) Results of the LCF of the ex-situ Zn K-edge XANES data, showing the evolution
of the fraction of oxidised Zn2+ (blue) and metallic zinc (Zn0 + LixZn) (purple)
following Li uptake and release (adapted from principal publication with permission
S T R U C T U R E O F M A T E R I A L S
S C I E N T I F I C H I G H L I G H T S
1 5 6 H I G H L I G H T S 2 0 2 1 I
with metals such as Fe or Co has been recently proven to be a viable strategy to improve performance , enabling long-term stable cycling of such materials at specific capacities equivalent or close to the theoretical maximum of about and above 1000 mAhg-1 (Figure 139a). The precise mechanism of the conversion-alloying reaction, allowing for the long-term stable cycling of such materials, however, remains to be elucidated specifically, the impact of the chemical nature of the dopant and its initial oxidation state. To address these questions, an in-depth X-ray absorption spectroscopy (XAS) investigation of Fe- doped and Co-doped ZnO was performed at beamline BM08.
XAS spectra were collected ex situ and operando along the first discharge (lithiation) and charge (de-lithiation) of electrodes with (carbon-coated) Zn0.9Fe0.1O and Zn0.9Co0.1O as the active material. An earlier analysis of the pristine materials had revealed that both dopants substitute Zn in the wurtzite crystal structure and the incorporated iron is almost purely (95%) trivalent, while cobalt is purely divalent . For both materials, the Zn K-edge XANES spectra collected on anodes cycled until selected potentials (Figures 139b, 139d) clearly show the expected reduction of zinc oxide to metallic Zn. Moreover, the edge position of the two fully lithiated electrodes (at 0.01 V) shifted to slightly lower values than for Zn metal,