An electrochemical impedance spectroscopy (EIS) study of three members of the family of compounds Li x Ni1 - y Co y O2 (y = 0, 0.2, and 1) has been performed by taking spectra at closely spaced bias potentials over the potential windows of utilization as cathodes in lithium-ion batteries. The spectra have been interpreted in terms of electronic and ionic transport properties. It has been found that, for x values greater than about 0.9, all the compounds show semiconductive properties with a specific resistance that increases with decreasing Ni content. For x close to about 0.9−0.8, the properties change into those of a metal-like material in which the ionic conductivity becomes the limiting factor. The transition between these two limiting conditions clearly appears in the impedance spectra.

Vanadium oxide gels are appealing cathode materials as they offer multiple electron redox processes leading to high cation‐storage capacities. Moreover, they are able to intercalate different ionic and molecular species. Apart from low electronic conductivity, one of the main factors hindering the use of highly porous V2O5 gels is the difficulty in preserving their unique morphology, made up of an entangled network of thin ribbons, during conventional laminated electrode preparation. In this study, we tune the V2O5 synthesis conditions and use an innovative and green binder system (polyacrylic acid and ethanol) to obtain electrodes with a morphology optimized for ion intercalation. The electrochemical performance of such electrodes, tested against lithium and sodium anodes, are shown to be excellent.

An AC impedance spectroscopic study of the Li x CoO 2 electrode in the temperature range of 0-30 °C is presented. The results are interpreted on the basis of an equivalent circuit that includes elements related to the electronic and ionic transport in addition to the charge transfer process. The evolution of the impedance spectra with the temperature shows that a thermally activated insulator to metal transition occurs at the beginning of the deintercalation process. At intermediate intercalation degrees, the effects of the finite electronic conductivity of the material and of the charge transfer process clearly appear as separate features in the spectra at low temperature.

Advanced metal oxide electrodes in Li-ion batteries usually show reversible capacities exceeding the theoretically expected ones. Despite many studies and tentative interpretations, the origin of this extra-capacity is not assessed yet. Lithium storage can be increased through different chemical processes developing in the electrodes during charging cycles. The solid electrolyte interface (SEI), formed already during the first lithium uptake, is usually considered to be a passivation layer preventing the oxidation of the electrodes while not participating in the lithium storage process. In this work, we combine high resolution soft X-ray absorption spectroscopy with tunable probing depth and photoemission spectroscopy to obtain profiles of the surface evolution of a well-known prototype conversion-alloying type mixed metal oxide (carbon coated ZnFeO) electrode. We show that a partially reversible layer of alkyl lithium carbonates is formed (∼5-7 nm) at the SEI surface when reaching higher Li storage levels. This layer acts as a Li reservoir and seems to give a significant contribution to the extra-capacity of the electrodes. This result further extends the role of the SEI layer in the functionality of Li-ion batteries.

No abstract