ABSTRACT
Aprotic metal-oxygen batteries, such as Li-O2 and Na-O2 batteries, are of topical research interest as high specific energy alternatives to state-of-the-art Li-ion batteries. In particular, Na-O2 batteries with NaO2 as the discharge product offer higher practical specific energy with better rechargeability and round-trip energy efficiency when compared to Li-O2 batteries. In this work, we show that the electrochemical deposition and dissolution of NaO2 in Na-O2 batteries is unperturbed by trace water impurities in Na-O2 battery electrolytes, which is desirable for practical battery applications. We find no evidence for the formation of other discharge products such as Na2O2·H2O. Furthermore, the electrochemical efficiency during charge remains near ideal in the presence of trace water in electrolytes. Although sodium anodes react with trace water leading to the formation of a high-impedance solid electrolyte interphase, the increase in discharge overpotential is only â¼100 mV when compared to cells employing nominally anhydrous electrolytes.
ABSTRACT
Reversible metallization of band and Mott insulators by ionic-liquid gating is accompanied by significant structural changes. A change in conductivity of seven orders of magnitude at room temperature is found in epitaxial films of WO3 with an associated monoclinic-to-cubic structural reorganization. The migration of oxygen ions along open volume channels is the underlying mechanism.
ABSTRACT
The use of electric fields to alter the conductivity of correlated electron oxides is a powerful tool to probe their fundamental nature as well as for the possibility of developing novel electronic devices. Vanadium dioxide (VO2) is an archetypical correlated electron system that displays a temperature-controlled insulating to metal phase transition near room temperature. Recently, ionic liquid gating, which allows for very high electric fields, has been shown to induce a metallic state to low temperatures in the insulating phase of epitaxially grown thin films of VO2. Surprisingly, the entire film becomes electrically conducting. Here, we show, from in situ synchrotron X-ray diffraction and absorption experiments, that the whole film undergoes giant, structural changes on gating in which the lattice expands by up to â¼3% near room temperature, in contrast to the 10 times smaller (â¼0.3%) contraction when the system is thermally metallized. Remarkably, these structural changes are fully reversible on reverse gating. Moreover, we find these structural changes and the concomitant metallization are highly dependent on the VO2 crystal facet, which we relate to the ease of electric-field-induced motion of oxygen ions along chains of edge-sharing VO6 octahedra that exist along the (rutile) c axis.
ABSTRACT
Given their high theoretical specific energy, lithium-oxygen batteries have received enormous attention as possible alternatives to current state-of-the-art rechargeable Li-ion batteries. However, the maximum discharge capacity in non-aqueous lithium-oxygen batteries is limited to a small fraction of its theoretical value due to the build-up of insulating lithium peroxide (Li2O2), the battery's primary discharge product. The discharge capacity can be increased if Li2O2 forms as large toroidal particles rather than as a thin conformal layer. Here, we show that trace amounts of electrolyte additives, such as H2O, enhance the formation of Li2O2 toroids and result in significant improvements in capacity. Our experimental observations and a growth model show that the solvating properties of the additives prompt a solution-based mechanism that is responsible for the growth of Li2O2 toroids. We present a general formalism describing an additive's tendency to trigger the solution process, providing a rational design route for electrolytes that afford larger lithium-oxygen battery capacities.
ABSTRACT
The electric-field-induced metallization of insulating oxides is a powerful means of exploring and creating exotic electronic states. Here we show by the use of ionic liquid gating that two distinct facets of rutile TiO2, namely, (101) and (001), show clear evidence of metallization, with a disorder-induced metal-insulator transition at low temperatures, whereas two other facets, (110) and (100), show no substantial effects. This facet-dependent metallization can be correlated with the surface energy of the respective crystal facet and, thus, is consistent with oxygen vacancy formation and diffusion that results from the electric fields generated within the electric double layers at the ionic liquid/TiO2 interface. These effects take place at even relatively modest gate voltages.
