Electrolyte gating with ionic liquids is a powerful tool for inducing novel conducting phases in correlated insulators. An archetypal correlated material is vanadium dioxide (VO(2)), which is insulating only at temperatures below a characteristic phase transition temperature. We show that electrolyte gating of epitaxial thin films of VO(2) suppresses the metal-to-insulator transition and stabilizes the metallic phase to temperatures below 5 kelvin, even after the ionic liquid is completely removed. We found that electrolyte gating of VO(2) leads not to electrostatically induced carriers but instead to the electric field-induced creation of oxygen vacancies, with consequent migration of oxygen from the oxide film into the ionic liquid. This mechanism should be taken into account in the interpretation of ionic liquid gating experiments.

The lithium-air (Li-O 2 ) battery has received enormous attention as a possible alternative to current state-of-the-art rechargeable Li-ion batteries given their high theoretical specific energy.However, the maximum discharge capacity in shapes and sizes in ethereal electrolytes and we suggest that this is likely due to varying levels of water contamination in the cells. Figure S1 in the SI shows a diminishment in toroid particle diameter with current at a fixed H 2 O content, with no toroids present at currents > 1 mA for 4000 ppm H 2 O concentration. At low H 2 O content, although toroids are still observed at very low currents, they disappear at currents much lower than 1 mA, where no toroid formation is apparent, an increase in discharge capacity is observed with water present (Fig. S10b). We argue that the improvement in capacity at both currents arises due to a solution-mediated mechanism for Li 2 O 2 formation (discussed later) that overcomes charge transport limitations inherent in surface growth of Li 2 O 2 . While trace H 2 O has a positive impact on capacity, it is also critical to understand its effect on the battery chemistry and rechargeability. Figure 3a shows X-ray diffractograms (XRD) near the Li 2 O 2 (100) and (101) peaks from Avcarb P50 paper cathodes extracted from batteries otherwise similar to those studied in Fig 1. The only additional H 2 O-induced XRD feature is a small peak at 30.65 degrees that has tentatively been identified as Li 2 NH (Fig. S6). These results confirm that the majority of the crystalline discharge product is Li 2 O 2 , regardless of electrolyte water content. Notably, no crystalline LiOH is observed in the XRD of the cathodes. The Li 2 O 2 diffractograms clearly show a decreasing peak width as a function of increasing water content in the electrolyte solution, implying that the Li 2 O 2 crystallite size increases, in

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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 (VO 2 ) 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 VO 2 . 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 ∼3% near room temperature, in contrast to the 10 times smaller (∼0.3%) 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 VO 2 crystal facet, which we relate to the ease of electric-field-induced motion of oxygen ions along chains of edge-sharing VO 6 octahedra that exist along the (rutile) c axis.T he use of electric fields to influence the transport properties of various materials by electrostatic injection of charge at an interface is the foundation of much of modern day electronics (1). Using a three-terminal field-effect transistor geometry, the magnitude of the electric fields provided by conventional gate dielectrics is limited by their dielectric properties. Much higher electric fields are possible by replacing the conventional gate material with an ionic liquid. Consequently, much higher electrostatically induced charge densities are possible, leading to the control or creation of novel metallic (2-3) and superconducting phases (4-7). Materials that are insulating by virtue of strong electron-electron correlations, namely Mott-Hubbard and charge-transfer insulators (8), are anticipated to be particularly sensitive to the injection of small numbers of carriers that could result in their metallization (9-11). Often these materials exhibit a thermally driven insulator to metal transition: one of these, VO 2 , exhibits such a transition near room temperature (12, 13) and, for this reason, has been extensively studied (14-16). In VO 2 the metal to insulator transition (MIT) is accompanied by a structural phase transition (SPT) in which the monoclinic insulating phase transforms to a rutile metallic phase (17). Recently, both Nakano et al. (18) and Jeong et al. (19) showed that ionic liquid (IL) gating of thin films of VO 2 results in the suppression of the MIT to temperatures below ∼10 K and, moreover, that the entire film becomes metallic even though gating takes place at the top surface of the film in contact with the IL. However, whereas Nakano et al. (18) claimed the metallization phenomenon was a direct ...

The use of metallic lithium anodes enables higher energy density and higher specific capacity Li‐based batteries. However, it is essential to suppress lithium dendrite growth during electrodeposition. Li‐ion‐conducting ceramics (LICC) can mechanically suppress dendritic growth but are too fragile and also have low Li‐ion conductivity. Here, a simple, versatile, and scalable procedure for fabricating flexible Li‐ion‐conducting composite membranes composed of a single layer of LICC particles firmly embedded in a polymer matrix with their top and bottom surfaces exposed to allow for ionic transport is described. The membranes are thin (<100 μm) and possess high Li‐ion conductance at thicknesses where LICC disks are mechanically unstable. It is demonstrated that these membranes suppress Li dendrite growth even when the shear modulus of the matrix is lower than that of lithium. It is anticipated that these membranes enable the use of metallic lithium anodes in conventional and solid‐state Li‐ion batteries as well as in future LiS and LiO2 batteries.

We use polarization-and temperature-dependent x-ray absorption spectroscopy, in combination with photoelectron microscopy, x-ray diffraction and electronic transport measurements, to study the driving force behind the insulator-metal transition in VO 2 . We show that both the collapse of the insulating gap and the concomitant change in crystal symmetry in homogeneously strained single-crystalline VO 2 films are preceded by the purely-electronic softening of Coulomb correlations within V-V singlet dimers. This process starts 7 K (±0.3 K) below the transition temperature, as conventionally defined by electronic transport and x-ray diffraction measurements, and sets the energy scale for driving the near-roomtemperature insulator-metal transition in this technologically-promising material.

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.

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