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It is common belief that conventional layered oxide battery cathode materials operate on a cationic redox mechanism, and that oxygen oxidation triggered by charge compensation results in oxygen loss and limited practical capacity. In this work, thin films of sodium cobaltate demonstrate a mixed redox behavior with strong oxygen participation well within the reversible intercalation range. This unexpected behavior is attributed to an increasing degree of covalency of the transition metal-oxygen (TM-O) bonds during deintercalation in combination with a high structural flexibility of the material. These insights are based on electronic structure data obtained using an operando solid state cell, allowing analysis with unpreceded control, and are supported by density functional theory (DFT) calculations. They demonstrate that the spectrum of possible charge compensation mechanisms is broader than commonly believed, and indicate that tuning TM-O covalency is a key factor for increasing the capacity also of conventional layered materials.

Most Li+/Na+-conducting solid electrolytes are unstable in moisture, and the formed hydroxides and carbonates on their surfaces result in the increase of the interfacial resistance between solid el...

The sodium (Na) anode suffers severe interfacial resistance and dendrite issues in a classic NASICON-type Na3Zr2Si2PO12 (NZSP) electrolyte, resulting in poor electrochemical performance for solid-state Na metal batteries. There has been little success in the reduction of interfacial resistance in recent years. The exact mechanism of this resistance has not been fully understood because of little information about the interface. In this work, we effectively address the large interfacial resistance issue and the metal dendrite problem between the Na anode and NZSP by introducing a TiO2 film as an active interphase. We employ quasiinsitu X-ray photoelectron spectroscopy (XPS) to uncover the interphase formation mechanism at the Na/TiO2–NZSP electrolyte interface. Our quasiinsitu XPS results confirm the formation of a sodiated-TiO2 interphase upon stepwise Na evaporation on the surface of the NZSP electrolyte. Further investigation by molten Na contact angle measurements, impedance spectroscopy and DFT calculations demonstrates that the sodiated-TiO2 interphase promotes Na ion transport between the Na anode and NZSP electrolyte. Moreover, the electrostatic potential formed at the NZSP/NaxTiO2 interface can effectively reduce electronic conductivity at the interface and hence prevent the growth of sodium dendrites. A representative paradigm for interphase design is provided to address the interface contact for developing stable solid-state batteries with high performance.

All-solid-state sodium batteries are attractive due to the abundance of sodium and advantageous for safe battery operation by avoiding flammable organics and liquids and suppressed dendrite formation. Currently, the lack of a chemically stable sodium solid electrolyte with high ion conductivity at room temperature is one of the challenges for future development of sodium batteries. Herein, we present a NaxCoO2/Nasicon/Na thin-film model sodium solid-state battery using a Sc-substituted Nasicon solid electrolyte with a high ionic conductivity of 4 × 10−3 S cm−1. The battery shows a high specific capacity of 150 mAh g−1 at room-temperature and discharge rates of up to 6C. Excellent chemical stability of this solid electrolyte at high voltages of up to 4.2 V increases the accessible sodium (de)intercalation range and battery capacity. Direct extraction of the interface resistances between the electrode materials of the thin-film model cell using electrochemical impedance spectroscopy gives a unique opportunity of correlation the electrochemical performance with properties of electrode materials and their interfaces.

A common way to improve the electrochemical performance of the NaxCoO2 thin-film cathodes is to increase their crystallinity. Here we present our study of the electrochemical performance of all-solid-state sodium ion batteries with NaxCoO2 thin-film cathodes having two different degrees of crystallinity tuned by their post-deposition annealing at 700°C. The NaxCoO2 cathode thin-films were grown by pulsed laser deposition onto a bulk Na3.4Sc0.4Zr1.6(SiO4)2(PO4) (Nasicon) solid electrolyte substrates and assembled with sodium metal into a Swagelok battery cells. Cells with the low-crystalline NaxCoO2 cathodes show discharge capacities of up to 124 mAh g−1 over 800 charge/discharged cycles. However, cells with highly crystalline NaxCoO2 cathodes revealed a significant capacity loss down to 9 mAh g−1 and a pronounced increase of the overpotential from 100 to 890 mV during the 200 cycles. The observed loss in capacity can be attributed to a strong increase of the interface resistance between the highly crystalline annealed NaxCoO2 films and Nasicon during cycling.

On the way to sodium-based all-solid-state batteries, interface analysis is a key issue. In this work, the interface between two typical solid sodium-ion conductors (Na-β″-Al2O3 and NASICON) and the layered oxide material NaCoO2 is investigated. The formation of the interface between the electrolyte material and the NaCoO2 coating is studied using an in-situ X-ray photoemission spectroscopy (XPS) surface science approach. Only a slight reduction of zirconium and scandium in NASICON was observed in proximity to the cathode side interface, while severe reactions, including the formation of a CaO Al2O3 mixed phase, were detected between Na-β″-Al2O3 and NaCoO2.

Multi-layer heterostructure oxide semiconductors employing a layer-by-layer deposition of alternating indium oxide and zinc oxide thin films generated via atomic layer deposition (ALD) are investigated for their feasibility into high performance thin film transistors (TFT). The successful deposition of uniform film thickness across the alternating indium oxide and zinc oxide deposition at 200 °C is achieved using trimethyl indium (TMI), diethyl zinc (DEZ) and water as oxidizing agent. The as-prepared polycrystalline material shows a conductive behaviour which upon additional mild annealing between 250–300 °C demonstrates a high TFT device performance. In addition, insights into the dependency of the defect passivation gradient within the multilayer upon thermal annealing of the oxide stack are presented. Studies towards an optimised film thickness result in a high device performance in enhancement mode with a saturation field-effect mobility (μsat.) of 6.5 cm2 V−1 s−1 and an on/off ratio (Ion/off) of 4.6 × 107 using a deliberately large width to length channel ratio (W/L = 500). The TFT performance turned out to be dependent on the position of the individual oxide layers within the stack and the number of heterostructure stacks. These findings on the influence of semiconductor stack formation allow for a better understanding on the formation of the active semiconductor channel and serve towards the applicability of ALD based heterostructure metal oxide semiconductors in next generation electronics.

Energy level offsets and related space charge layer formation are crucial for the kinetic properties of functional electric interfaces. In this contribution, we give a concise overview of our results on model Li-ion cathode interfaces with different overlayer phases obtained by surface science methodology using X-ray photoelectron spectroscopy. In particular, we present electronic energy level alignment, space charge layer formation, and Li1s core level binding energy differences at thin film LiCoO 2 surfaces covered by thin layers of solid state electrolyte, other ionic compounds and organic solvents. The data suggest a correlation between the binding energy of the Li1s emission and the extent of the space charge layer formation, which can be rationalized using the concept of ionic energy levels. The data also indicate that interface dipole potentials might be relevant for the formation of some of the investigated interfaces, and suggest that large electronic bulk energy level offsets are insufficient indicators for interface stability, as is usually assumed for electrode-electrolyte interfaces.

Commercial NASICON-type electrolyte plates are coated with a thin film of LiPON to obtain a composite electrolyte system with high conductivity at room temperature that can be used in contact with metallic Lithium. The formation of the interface between the NASICON substrate and the LiPON coating is studied using an in-situ X-ray photoemission spectroscopy (XPS) surface science approach. The process of LiPON deposition induces changes in the surface chemical structure of the NASICON substrates as observed by XPS, including the partial reduction of Ti and the incorporation of N into the NASICON. The practical impact of the interface formation is studied by impedance spectroscopy, revealing a substantial increase of resistance for LiPON coated samples.

Amorphous zinc tin oxide (ZTO) thin films are accessible by a molecular precursor approach using mononuclear zinc(II) and tin(II) compounds with methoxyiminopropionic acid ligands. Solution processing of two precursor solutions containing a mixture of zinc and tin(II)–methoxyiminopropinato complexes results in the formation of smooth homogeneous thin films, which upon calcination are converted into the desired semiconducting amorphous ZTO thin films. ZTO films integrated within a field-effect transistor (FET) device exhibit an active semiconducting behavior in the temperature range between 250 and 400 °C, giving an increased performance, with mobility values between μ = 0.03 and 5.5 cm2/V s, with on/off ratios increasing from 105 to 108 when going from 250 to 400 °C. Herein, our main emphasis, however, was on an improved understanding of the material transformation pathway from weak to high performance of the semiconductor in a solution-processed FET as a function of the processing temperature. We have co...

Active brazing is a commonly used method for joining ceramic materials. In the present study, the wetting behavior of four Ti-rich ternary Ni–Ti–Zr alloys was investigated through sessile drop experiments on alumina disks of 96 and 99.9 % purity. The microstructure at the metal/alumina interface was analyzed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. Three of the analyzed alloys exhibited reactive wetting with final contact angles between 40° and 70°. The reaction phases at the metal/alumina interface had a thickness of about 1 µm and were of a similar composition for all alloys. Dilatometer measurements showed thermal expansion coefficients between 13.2 and 15.8 × 10−6 °C−1. The lowest wetting angle of 40° was achieved with the alloy 61Ti–20Zr–19Ni at temperatures above 980 °C.

A model all-solid-state battery cell with a thin film Na

β″-alumina is a promising solid electrolyte candidate for room-temperature all-solid-state sodium-ion batteries due to its high sodium ionic conductivity of up to 10−2 S cm−1 at room temperature. However, calcium impurities are known to be present in the Al2O3 precursors used to produce commercial β″-alumina. Here we present a study on the impact of the calcium impurities which are diffusing in a temperature controlled process from the β″-alumina substrates into the NaxCoO2 layered oxide cathode. The increasing calcium content in the NaxCoO2 cathode leads to a degradation due to the formation of the calcium-rich Ca3Co4O9 phase. The electrochemical performance of sodium-ion batteries is investigated with NaxCoO2/β″-alumina/Na solid-state model batteries. The thin-film cathodes were grown onto commercial β″-alumina ceramic substrates by pulsed laser deposition and cycled between 2.0 and 3.8 V with metallic sodium anodes.

A model all-solid-state battery cell with a thin film NaxCoO2 cathode was assembled under ultra-high vacuum conditions and cycled inside the vacuum chamber, using a dedicated sample holder. We present in-operando x-ray photoelectron spectroscopy measurements of a NaxCoO2 cathode at different charging states. During battery operation, the change in sodium content, the change in cobalt oxidation state, and the evolution of the O1s and VB emissions could be monitored. Comparison with a conventional post-mortem analysis technique showed that the new measurement technique produces comparable results regarding the oxidation state of the transition metal, but sodium and oxygen results show differences due to cathode/electrolyte interfacial reactions for conventional analysis. By using surface layer-free samples in the presented techniques, we could circumvent such reactions and obtain reliable spectra for the pure bulk-like active cathode material.