Structure of iron- and cobalt- doped ZnO as anode material for Li-ion batteries

Transition-metal-doped zinc oxide has gathered significant interest in the scientific com-munity because of its potential for application in spintronics devices. This same material has been recently demonstrated to be also a very promising alternative anode material for Li-ion batteries (Bresser et al., 2013). Remarkably, the doping of ZnO nanoparticles by iron and co-balt substantially enhances the electrochemical performance of the un-doped oxide. Herein, we report a complete structural study of the pristine anode material by X-ray dif-fraction (XRD) and X-ray absorption spectroscopy (XANES and EXAFS) to investigate the influence of the dopant on the crystalline wurtzite structure and to elucidate the major differ-ences between Co- and Fe-doped ZnO nanoparticles regarding their oxidation state and struc-ture around the dopant site (Giuli et al., 2015). Preliminary results from further studies per-formed on Fe-doped ZnO cycled anodes will be also presented. ZnO nanoparticles having the general formula Zn0.9(TM)0.1O (where TM = Fe or Co) were prepared through sucrose assisted wet chemical synthesis method (Bresser et al., 2013). Struc-tural refinement of powder XRD data revealed the absence of reflections related to spurious oxide phases and showed small changes in the ZnO unit-cell parameters associated with dop-ing. The full width at half maximum (FWHM) of the XRD reflections was found to increase in the order ZnO, Zn0.9Co0.1O, carbon coated Zn0.9Fe0.1O/C, and Zn0.9Fe0.1O revealing that differ-ent dopants or synthesis conditions remarkably affect the average crystallites size. XAS data were collected at the Italian BM08 beamline of the ESRF at the Fe, Co and Zn K-edges and showed that both Co and Fe substitute Zn in the host wurtzite structure. From the analysis of the XANES spectra, we found that Co is divalent, whereas Fe is trivalent in Zn0.9Fe0.1O, and 95% trivalent in Zn0.9Fe0.1O/C. The Co-doped sample displays similar EXAFS features as that of pure ZnO (Figure 1). On the other hand, the EXAFS signals measured at the Fe K-edge of both Zn0.9Fe0.1O and Zn0.9Fe0.1O/C show oscillations strongly damped compared to those measured at Zn and Co K-edges in pure ZnO and Co0.9Fe0.1O. This was attributed to the occurrence of structural defects (such as cationic vacancies and/or interstitial oxygen atoms) in the immediate environment of Fe3+ in tetrahedrally coordinated sites, in agreement with both the results on the crystallinity as obtained by XRD on the Fe oxidation state as derived from XANES. Indeed, the presence of defects is compatible with the aliovalent substitution of Fe3+ for Zn2+.