Amorphous and highly nonstoichiometric titania (TiOx) thin films close to metal-like conductivity

Oxygen-deficient titanium oxide films (TiOx) have been prepared by pulsed laser deposition at room temperature. Samples in their as-deposited state have an average composition of TiO1.6, are optically absorbing and show electronic conductivities in the range of 10 S cm−1. The films are metastable and consist of grains of cubic titanium monoxide (γ-TiO) embedded in an amorphous TiO1.77 matrix. Upon annealing in an argon atmosphere the electrical conductivity of the films increases and comes close to metal-like conductivity (1000 S cm−1) at about 450 °C whereas the local structure is changed: nanocrystalline grains of metallic Ti are formed in the amorphous matrix due to an internal solid state disproportionation. The highly conductive state can be frozen by quenching. During heat treatment in an argon atmosphere a stoichiometric rutile TiO2 surface layer forms due to oxidation by residual oxygen. The combination of a highly conductive TiOx film with such an approximately 20 nm thick rutile cover layer leads to a surprisingly high efficiency for the water-splitting reaction without the application of an external potential

The Formation of the Solid-/Liquid Electrolyte Interphase (SLEI) on NASICON-Type Glass Ceramics and LiPON

Most electrochemical energy storages (battery cells) consist of solid electrodes separated by a liquid electrolyte (LE). If electrode materials are – at least partially – soluble in the electrolyte, detrimental mass transport between both electrodes (electrode cross-talk) occurs. The shuttle mechanism in lithium-sulfur batteries or leaching of Mn in high voltage cathode materials are important examples. Implementing a solid electrolyte (SE) membrane between the electrodes is a comprehensible approach to suppress undesired mass transport but additional resistances arise due to charge transport across the SE and charge transfer through the solid/liquid electrolyte interfaces. The latter contribution is often overlooked as its determination is challenging, however, these interface properties are crucial for practical application. In previous work a resistive solid-/liquid-electrolyte interphase “SLEI” was found at the interface between the SE lithium aluminum germanium phosphate (LAGP) in contact with a liquid ether-based electrolyte. Here we aim for deeper insight into this interphase formation, referring to a lithium ion conducting glass ceramic (NASICON-type) and the commonly used thin film ion conductor “LiPON” (lithium phosphorous oxide nitride). The growth of the SLEI is monitored by a combination of electrochemical characterization, XPS (x-ray photoelectron spectroscopy) and time-of flight secondary ion mass spectrometry (ToF-SIMS)

Applying Capacitive Energy Storage for In Situ Manipulation of Magnetization in Ordered Mesoporous Perovskite-Type LSMO Thin Films

Mesostructured nonsilicate materials, particularly mixed-metal oxides, are receiving much attention in recent years because of their potential for numerous applications. Via the polymer-templating method, perovskite-type lanthanum strontium manganese oxide (La_1–<i>x</i>Sr_<i>x</i>MnO₃, LSMO, with <i>x</i> ≈ 0.15 to 0.30) with a continuous 3D cubic network of 23 nm pores is prepared in thin-film form for the first time. Characterization results from grazing incidence X-ray scattering, X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and electron microscopy and tomography show that the dip-coated sol–gel-derived films are of high quality in terms of both composition and morphology and that they are stable to over 700 °C. Magnetic and magnetotransport measurements demonstrate that the material with the highest strontium concentration is ferromagnetic at room temperature and exhibits metallic resistivity behavior below 270 K. Besides, it behaves differently from epitaxial layers (e.g., enhanced low-field magnetoresistance effect). It is also shown that carriers (electrons and holes) can be induced into the polymer-templated mesostructured LSMO films via capacitive double-layer charging. This kind of electrostatic doping utilizing ionic liquid gating causes large relative changes in magnetic susceptibility at room temperature and is a viable technique to tune the magnetic phase diagram in situ

Interfacial Reactivity Benchmarking of the Sodium Ion Conductors Na₃PS₄ and Sodium β‑Alumina for Protected Sodium Metal Anodes and Sodium All-Solid-State Batteries

The interfacial stability of solid electrolytes at the electrodes is crucial for an application of all-solid-state batteries and protected electrodes. For instance, undesired reactions between sodium metal electrodes and the solid electrolyte form charge transfer hindering interphases. Due to the resulting large interfacial resistance, the charge transfer kinetics are altered and the overvoltage increases, making the interfacial stability of electrolytes the limiting factor in these systems. Driven by the promising ionic conductivities of Na₃PS₄, here we explore the stability and viability of Na₃PS₄ as a solid electrolyte against metallic Na and compare it to that of Na-β″-Al₂O₃ (sodium β-alumina). As expected, Na-β″-Al₂O₃ is stable against sodium, whereas Na₃PS₄ decomposes with an increasing overall resistance, making Na-β″-Al₂O₃ the electrolyte of choice for protected sodium anodes and all-solid-state batteries

Degradation of NASICON-Type Materials in Contact with Lithium Metal: Formation of Mixed Conducting Interphases (MCI) on Solid Electrolytes

We report on the transport properties of lithium ion conducting glass ceramics represented by the general composition Li_{1+<i>x</i>–<i>y</i>}Al_<i>x</i>³⁺M_<i>y</i>⁵⁺M_{2–<i>x</i>–<i>y</i>}⁴⁺(PO₄)₃ with NASICON-type structure and their stability in contact with lithium metal. In particular, solid electrolyte phases with M = Ge, M = Ti, Ge, and M = Ti, Ta were investigated. AC impedance spectroscopy and DC polarization measurements were applied to determine the conductivity as a function of temperature, and to extract the partial electronic conductivity. The maximum total conductivity at room temperature was found to be about 4 × 10^–4 S/cm for the solely Ge containing sample. We demonstrate that the combination of vacuum-based lithium thin film deposition and X-ray photoelectron spectroscopy (XPS) is well suited to study the reactivity of the solid electrolyte membranes in contact with lithium. As a major result, we show that none of the materials investigated is stable in contact with lithium metal, and we discuss the reactive interaction between solid electrolytes and Li metal in terms of the formation of a mixed (ionic/electronic) conducting interphase (MCI) following the well-known SEI concept in liquid electrolytes

Hierarchical Carbon with High Nitrogen Doping Level: A Versatile Anode and Cathode Host Material for Long-Life Lithium-Ion and Lithium–Sulfur Batteries

Nitrogen-rich carbon with both a turbostratic microstructure and meso/macroporosity was prepared by hard templating through pyrolysis of a tricyanomethanide-based ionic liquid in the voids of a silica monolith template. This multifunctional carbon not only is a promising anode candidate for long-life lithium-ion batteries but also shows favorable properties as anode and cathode host material owing to a high nitrogen content (>8% after carbonization at 900 °C). To demonstrate the latter, the hierarchical carbon was melt-infiltrated with sulfur as well as coated by atomic layer deposition (ALD) of anatase TiO₂, both of which led to high-quality nanocomposites. TiO₂ ALD increased the specific capacity of the carbon while maintaining high Coulombic efficiency and cycle life: the composite exhibited stable performance in lithium half-cells, with excellent recovery of low rate capacities after thousands of cycles at 5C. Lithium–sulfur batteries using the sulfur/carbon composite also showed good cyclability, with reversible capacities of ∼700 mA·h·g^–1 at C/5 and without obvious decay over several hundred cycles. The present results demonstrate that nitrogen-rich carbon with an interconnected multimodal pore structure is very versatile and can be used as both active and inactive electrode material in high-performance lithium-based batteries