Design of Li-O2 cells and study of the electrodes reactivity by means of a multi-technique approach

The PhD thesis work has been focused on aprotic Li-O2 cells assembled with different cathode materials in combination with the ether-based LiTFSI/TEGDME electrolyte. The performance of these promising energy storage systems have been investigated and related to the electrochemical and chemical processes at the triple O2/cathode/electrolyte interface and to the peculiar influence of each type of cathode material. These goals have been achieved by a powerful combination of techniques such as X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). The remarkable role of the TFSI- anion in the degradation process of a carbon-based cathode has been discussed in Chapter 4 on the basis of a post-mortem characterization of the positive electrode. Moreover, the fade of long-term stability of the cells has been related to the accumulation on the cathode surface of both insulating reaction products and byproducts, leading to the rise of overvoltages upon discharge and charge. A full electrochemical and chemical characterization of novel carbon-free Mn+-doped (Mn+=Cr3+ and Zn2+) NiCo2O4 nanomaterials grown on Ni foam (NCCr@Ni and NCZn@Ni) has been reported in Chapters 5 and 6. For the first time in the literature these materials have been tested as cathodes in Li-O2 cells and the beneficial effect of Cr(III)- and Zn(II)-doping of the NiCo2O4 spinel to boost the kinetics of both the ORR and the OER has been demonstrated. By means of in-house and synchrotron measurements the dopants role in the increase of the performance respect to the undoped material has been investigated. The preparation and characterization of novel composite Pd/PdO@NCCr@Ni cathodes made of Pd nanoparticles deposited on a nanostructured Cr(III)doped NiCo2O4 on Ni foam has been described in Chapter 7. The synergistic co-catalytic effect between the Pd NP and the doped nickel cobaltite has been proved to reduce both discharge and charge overvoltages and to improve the cell calendar life. Overall, the general objectives of the PhD activity have been met, since three novel materials have been produced and demonstrated to be performing cathodes for Li-O2 cells. Moreover, this study allowed the acquisition of skills concerning laboratory practice and widely applicable electrochemical and chemical characterization techniques.