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.

We examine the formation of the solid electrolyte interface (SEI) on anodes made of carbon encapsulated zinc ferrite (ZnFe 2 O 4 ) nanoparticles (50 nm ZFO-C) as a standard metal oxide electrode prototype. The SEI formation and phase evolution are studied by two soft X-ray absorption techniques with different probing depths in the 10−100 nm range and by surface-sensitive X-ray photoemission spectroscopy at several specific capacities of the ZFO-C anodes. These techniques are shown to be able to provide information about the nature and extension of the individual chemical species within the SEI with a typical spatial resolution of 1−5 nm. A peculiar footprint of the interphase formations is obtained by comparing the chemical history of the reactive element sites in the anodes. The progressive development of the SEI in the first cycle and the variety of compositional transformations prior to stabilization are elucidated. Formation of a reversible alkyl carbonate layer, with maximum thickness of 7 nm, is detected at the SEI topmost region. On the basis of these results, we have obtained a map of suitable spatial resolution of the evolution of the different components of the interface layer.

Conversion/alloying materials (CAMs) provide substantially higher specific capacities than graphite, the state‐of‐the‐art lithium‐ion battery anode material. The ability to host much more lithium per unit weight and volume is, however, accompanied by significant volume changes, which challenges the realization of a stable solid electrolyte interphase (SEI). Herein, the comprehensive characterization of the composition and evolution of the SEI on transition metal (TM) doped zinc oxide as CAM model compound, is reported, with a particular focus on the impact of the TM dopant (Fe or Co). The results unveil that the presence of iron specifically triggers the electrolyte decomposition. However, this detrimental effect can be avoided by stabilizing the interface with the electrolyte by a carbonaceous coating. These findings provide a great leap forward toward the enhanced understanding of such doped materials and (transition) metal oxide active materials in general.

Superconducting and normal state properties of Niobium nanofilms have been systematically investigated as a function of film thickness, on different substrates. The width of the superconducting-to-normal transition for all films is remarkably narrow, confirming their high quality. The superconducting critical current density exhibits a pronounced maximum for thickness around 25 nm, marking the 3D-to-2D crossover. The magnetic penetration depth shows a sizeable enhancement for the thinnest films. Additional amplification effects of the superconducting properties have been obtained with sapphire substrates or squeezing the lateral size of the nanofilms. For thickness close to 20 nm we measured a doubled perpendicular critical magnetic field compared to its large thickness value, indicating shortening of the correlation length and the formation of small Cooper pairs. Our data analysis indicates an exciting interplay between quantum-size and proximity effects together with strong-coupling effects and the importance of disorder in the thinnest films, placing these nanofilms close to the BCS-BEC crossover regime.

A parallel approach for fabricating nanocrystal-based semiconductor-insulator-metal tunnel diodes is presented. The devices consisted of a Au electrode, a monolayer of 38 Å CdSe nanocrystals, an insulating bilayer of eicosanoic acid (C 19 H 39 CO 2 H), and an Al electrode. Each device was approximately 100 m 2 . Conductance measurements at 77 K reveal strong diode behavior and evidence of Coulomb blockade and staircase structure. A single barrier model was found to reproduce the electronic characteristics of these devices. © 1999 American Institute of Physics. ͓S0003-6951͑99͒04502-7͔The unique chemical and size-dependent properties of nanocrystals ͑NCs͒ have made them attractive candidates as electronic and photonic materials. Relatively straightforward fabrication procedures have been reported for certain NCbased devices, such as light-emitting diodes, 1 photovoltaics, 2 and capacitors. 3 Other devices, however, such as singleelectron transistors 4 and single-electron tunnel junctions, 5 require two electrical contacts to a single particle, and are hard to fabricate. 6 The difficulty arises not just because of the length scales involved, but also because chemically fabricated NCs are unstable toward many electronic materials processing technologies. In this letter, we present a parallel fabrication technique for the construction of CdSe nanocrystal-based metal-insulator-semiconductor ͑MIS͒ tunnel diodes. The key component that allows for top and bottom contacts to the nanocrystal ͑mono͒ layer is a molecularly thin insulating layer of eicosanoic acid, prepared as a Langmuir-Blodgett ͑LB͒ film. Strong diode behavior and a series of single-electron charging states, characterize the conductance of the devices. A standard model for MIS devices, 7 modified to include the finite-size characteristics of the NCs, successfully reproduced the measured conductance of the diodes. Figure 1͑a͒ shows a cross sectional view of the device. It consists of a Au electrode, NC monolayer, molecular insulator, and a top Al electrode. The structure resembles a normal metal-insulator-semiconductor diode, in which a CdSe NC monolayer 8 replaces the semiconductor, and a molecular ͑LB͒ film replaces what is, typically, an oxide tunnel barrier.Other groups have investigated the insulating behavior of LB films of organic amphiphiles in sandwich-structure devices. 9,10 Because of defects in the LB film, it is necessary to transfer several monolayers to fabricate what are, effectively, single-monolayer devices. We found that, for ϳ100 m 2 devices, 6 LB monolayers of eicosanoic acid gave an ''effective'' insulating layer of 2-3 nm ͑about 1 ML͒, and a device resistance of ϳ10 M⍀.38Ϯ4 Å CdSe NCs were synthesized according to literature recipes, 11 ligand-exchanged with hexanethiol, and characterized by UV/vis absorption spectroscopy and x-ray powder diffraction. A particle/HCCl 3 solution was spread on the a͒ Corresponding author; electronic mail: heath@chem.ucla.edu FIG. 1. Cross-sectional diagram ͑a͒ and energy-level diagram ͑b͒ of the CdSe na...

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The metal assisted etching mechanism for Si nanowire fabrication, triggered by doping type and level and coupled with choice of metal catalyst, is still very poorly understood. We explain the different etching rates and porosities of wires we observe based on extensive experimental data, using a new empirical model we have developed. We establish as a key parameter, the tunneling through the space charge region (SCR) which is the result of the reduction of the SCR width by level of the Si wafer doping in the presence of the opposite biases of the p- and n-type wafers. This improved understanding should permit the fabrication of high quality wires with predesigned structural characteristics, which hitherto has not been possible.

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