Charge Layer Formation Research Articles

Issues Concerning Interfaces with Inorganic Solid Electrolytes in All-Solid-State Lithium Metal Batteries

All-solid-state batteries have attracted wide attention for high-performance and safe batteries. The combination of solid electrolytes and lithium metal anodes makes high-energy batteries practical for next-generation high-performance devices. However, when a solid electrolyte replaces the liquid electrolyte, many different interface/interphase issues have arisen from the contact with electrodes. Poor wettability and unstable chemical/electrochemical reaction at the interfaces with lithium metal anodes will lead to poor lithium diffusion kinetics and combustion of fresh lithium and active materials in the electrolyte. Element cross-diffusion and charge layer formation at the interfaces with cathodes also impede the lithium ionic conductivity and increase the charge transfer resistance. The abovementioned interface issues hinder the electrochemical performance of all-solid-state lithium metal batteries. This review demonstrates the formation and mechanism of these interface issues between solid electrolytes and anodes/cathodes. Aiming to address the problems, we review and propose modification strategies to weaken interface resistance and improve the electrochemical performance of all-solid-state lithium metal batteries.

Interfacial challenges in all-solid-state lithium batteries

This mini-review provides a current opinion on interfacial challenges in solid-state batteries (SSBs). We focus our discussion on the effects of mechanical instability, chemical instability, electrochemical instability, and space charge layer formation on the interfacial resistance of SSBs. We highlight that the resulting interfacial resistance of SSBs arises from the coupling of all these effects. Quantifying the exact contribution of each effect to the overall interfacial resistance can be very difficult, if not impossible, because all these effects occur simultaneously during battery operation, but the dominant source, which varies with different electrode/electrolyte combinations, will need to be determined for future development of SSBs.

Weak curvature asymptotics for Debye layers as electrohydrodynamic discontinuities.

Important microfluidic phenomena, such as droplet deformation and cell motion, are impacted by the formation of Debye layers at charged interfaces. Previous studies examined interface problems with leaky dielectrics or the formation of diffuse charge layers. In most cases, the results are derived for weakly curved spherical geometries. Moreover, many studies of streaming-potential phenomena at fluid-solid interfaces lack a macroscale description of effects that are higher than first order. An asymptotic methodology capturing both complex surface geometries and an accurate description of higher-order phenomena is presented in this study. For this purpose, we consider a generic streaming-potential problem. As a result, the complex three-dimensional electrohydrodynamics in the Debye layer are entailed in two-dimensional discontinuity conditions. The latter contain a free parameter, the layer thickness, which mathematically represents the discontinuity position within the Debye layer. It can be used to derive an alternative definition of the Debye thickness capturing the influence of the ζ potential. We introduce a virtual particle whose outer boundary envelopes the solid particle plus a fraction of the Debye layer. It interacts with the macroscopic flow while incorporating the detailed electrohydrodynamics inside the layer.

Properties of the Space Charge Layers Formed in Li-Ion Conducting Glass Ceramics.

For years, the space charge layer formation in Li-conducting solid electrolytes and its relevance to so-called all solid-state batteries have been controversially discussed from experimental and theoretical perspectives. In this work, we observe the phenomenon of space charge layer formation using impedance spectroscopy at different electrode polarizations. We analyze the properties of these space charge layers using a physical equivalent circuit describing the response of the solid electrolytes and solid/solid electrified interfaces under blocking conditions. The elements corresponding to the interfacial layers are identified and analyzed with different electrode metals and applied biases. The effective thickness of the space charge layer was calculated to be ∼60 nm at a bias potential of 1 V. In addition, it was possible to estimate the relative permittivity of the electrolytes, specific resistance of the space charge layer, and the effective thickness of the electric double layer (∼0.7 nm).

Charge and Electrical Double Layer Formation in a Nonpolar Solvent Using a Nonionic Surfactant.

In this work, we study the charge formation and the characteristics of the electrical double layer in a nonpolar medium using electrical impedance spectroscopy. To stabilize the free ionic species, a nonionic surfactant is added to the system. The conductivity and permittivity of the medium are obtained from high- to medium-frequency impedance data. Based on the correlation between (viscosity-adjusted) conductivity and surfactant concentration, we conclude that charge formation occurs due to a disproportionation mechanism. We accordingly estimate the concentration of the charge carriers in the sample and the Debye length of the diffuse double layer. The capacitance of the electrical double layer can be extracted from the low-frequency impedance data. We use this data to calculate the electrode distance of an equivalent parallel-plate capacitor. It is found that this distance is on the order of magnitude of Angstroms, indicating that the measured electrical double-layer capacitance is in fact the Stern layer capacitance.

AC conductivity, electrochemical and magnetic studies of CoWO4/PbWO4 nanocomposites

Structural, AC conductivity, electrochemical and magnetic properties of CoWO4 and CoWO4/PbWO4 nanocomposites are reported in the present manuscript. Structural characterization and HRTEM images of the nanocomposites confirm interface formation. Room temperature (RT) DC conductivity (σdc) calculated from Jonscher power law, carrier concentration (N) and carrier mobility (μ) of the interfaced sample with maximum agglomeration (P2) are 6.7 times, 3 orders and 4 orders higher than that of CoWO4 sample respectively. Enhanced AC conductivity is also observed for P2 sample. These enhanced properties of P2 sample are due to the space charge layer (SCL) formation at the interface. Conduction models for all the samples are explained in detail. The specific capacitance (calculated from GCD curve) of the CoWO4 sample is 2.1 times larger than the CoWO4/PbWO4 nanocomposite for 1 A/g of current density. Paramagnetic behaviour at (RT) and antiferromagnetic nature below the transition temperature (TN) are observed for all the samples.

Materials Informatics Screening of Li‐Rich Layered Oxide Cathode Materials with Enhanced Characteristics Using Synthesis Data

AbstractLithium‐ion batteries (LIBs) are the objects of active research and attract interest as important elements of near‐term energy storage technologies. The ever‐growing requirements for cathode materials of next‐generation LIBs impel the need to screen the materials with high energy and power densities, cycling stability, rate capability, safety and compatibility with other battery elements. This study is focused on Li‐rich layered oxide cathode materials as the materials candidates that are characterized by high energy density and large capacity and are therefore attractive as the potential solution for next‐generation LIBs whereas moderate structural stability hinders their commercialization. Machine learning‐assisted analysis of the collected experimental data has been performed for assessing the contribution of lattice doping, composition of compounds, the method and details of synthesis in the electrochemical characteristics. The possibility to relate parameters with the formation of a certain structure type (composite or solid solution), conceivable phase transformations and space charge layer formation are discussed. From this analysis, the features most probably amenable for electrochemical characteristics enhancement are distinguished and the focus area for further research is defined.

Reaction and Space Charge Layer Formation at the LiCoO2–LiPON Interface: Insights on Defect Formation and Ion Energy Level Alignment by a Combined Surface Science–Simulation Approach

In this contribution, we investigate the formation and evolution of LiCoO2–LiPON interfaces upon annealing using photoelectron spectroscopy. We identify interlayer compounds related to the deposition process and study the chemical reactions leading to interlayer formation. Based on the structure of the pristine interface as well as on its evolution upon annealing, we relate reaction layer and space charge layer formation to chemical potential differences between the two materials. The results are discussed in terms of a combined Li-ion and electron interface energy level scheme providing insights into fundamental charge transfer processes. In constructing the energy level alignment, we take into account calculated defect formation energies of lithium in the cathode and solid electrolyte.

Energy level offsets and space charge layer formation at electrode-electrolyte interfaces: X-ray photoelectron spectroscopy analysis of Li-ion model electrodes

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 LiCoO2 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.

Reconstruction of the LiCoO2 Near-Surface Region upon Overgrowth of Lipon

Recently, all-solid-state batteries have been actively studied for a next-generation secondary battery. All-solid-state batteries have an advantage in safety compared with the current flammable liquid electrolyte-based lithium ion batteries because of its excellent chemical stability. In addition, since this improved stability will eliminate the need for safety apparatus, the energy density of the all-solid-state battery module is also expected to be improved. One of the major issues of all-solid-state batteries towards their practical application is high interfacial resistance between the solid electrolyte and electrode active materials. While it has been speculated that space charge layer formation at the interface is a major factor for the high interfacial resistance, the interfacial phenomena still remains unclear due mainly to the difficulty in analysis of the interface in bulk battery form. In this study, we prepared thin-film battery with a well-defined pristine interface using thin film technique, and studied the changes in the crystal structure and electronic state in the vicinity of the interface between the cathode active material and solid electrolyte upon charging and discharging. In this study, we used LiCoO2 as an positive-electrode active material, and LiPON as a solid electrolyte. LiCoO2 was epitaxially grown in (104) orientation after forming the SrRuO3 as a current collector on SrTiO3(100) substrate by Pulsed laser deposition (PLD). LiPON was deposited on LiCoO2 by RF sputtering method using Li3PO4 as a target under N2 atmosphere. Li metal was deposited by vapor deposition as a negative electrode. In all the film formation processes, the samples were kept in vacuum to prevent the surfaces from being exposed to air in order to maintain clean and pristine interfaces. Cross sectional transmission electron microscope (TEM) images of the as-deposited film revealed the presence of the interfacial layer between LiCoO2 and LiPON which corresponds to neither LiCoO2 nor LiPON. The thickness of the interfacial layer was about 15 nm. From the results of TEM-EDS and electron diffraction, the interfacial layer is composed of elements of Li, Co and O, and its structure is similar to LiCoO2 but with larger lattice constant than LiCoO2. In addition, from TEM-EELS measurement results, Li-K edge of the interface layer was found to be shifted to the low-energy side compared with that of LiCoO2. This indicates that the Li in the interfacial layer is bounded more weakly than that of LiCoO2. By charging and discharging the above thin film battery at a current density of 4.33 μA/cm2(1/20C), the discharge capacity of 140 mAh/g was obtained. The characterisitic plateau associated with the oxidation-reduction of LiCoO2 was observed, which confirms excellent charge and discharge characteristics of the thin film battery. Taking the cross sectional TEM images after charging and discharging, the interface layer disappeared after charging while the interface layer was found to appear again after discharge. From this result, it can be inferred that the interface layer was formed when the Li concentration of LiCoO2 in the vicinity of the interface was high. The effect of interfacial layer on the electrochemical properties are to be discussed at the presentation.

Surface Analysis of Li-Ion Electrode-Electrolyte Interfaces: Insights By a Surface Science Approach

Electrode-electrolyte interfaces play a key role for the performance and degradation of electrochemical devices such as lithium-ion cells. Side reactions at cathode-electrolyte interfaces result in the formation of a solid-electrolyte interface (SEI) layer, resulting in loss or performance due to lithium loss and increase in surface layer resistance. Over the past years, the cathodic SEI layer has been subject to extensive investigation, focused on the correlation between composition and battery cycling parameters. Yet, open questions remain with regard to formation mechanisms and properties. Most investigations on SEI layers have been performed by post mortem analysis of composite electrode surfaces by photoelectron spectroscopy (XPS). XPS gives information on the chemical structure by way of core level electron binding energies. Despite of the standard use of XPS for SEI analysis, unambiguous identification of the different phases is often not straightforward as binding energy shifts are encountered. Moreover, the post mortem analysis of composite materials is not ideally suited to investigate the SEI on the active material, as emissions from the other materials in the electrode (carbon, binder) add complexity. Nevertheless, such data on the SEI composition of composite electrodes are often used to propose reaction mechanisms on SEI-formation and side reactions (electrolyte oxidation) occurring at the surface of the active materials. Improved insights into SEI-composition and side reactions require experiments that address binding energies and interaction of single material surfaces with components of the electrolyte. We prepare model interfaces of LCO (LiCoO2) thin film electrodes with components of the SEI, artificial SEI-layers, as well as with solvents, and analyze them with surface analysis methods [1]. In such experiments, the SEI or electrolyte-related components are deposited as top phase either by thin film deposition or low temperature adsorption, and analysis is performed by photoelectron spectroscopy (XPS, UPS, SXPS) and high resolution electron energy loss spectroscopy (HREELS). Usually, the preparation is performed stepwise with intermediate analysis in order to investigate the interface formation in detail with respect to chemical structure, electronic structure and space charge effects. In this contribution, we present our results with respect to the deposition of several solid compounds such as lithium oxide and LiPON, as well as the exposure to several solvents (water, diethyl carbonate, etc.). In most cases, the formation of interface and/or gradient layers is observed. For the substrate-related binding energies we usually note a shift to higher values which is attributed to space charge layer formation in the LiCoO2 (band bending downwards). Also, we find indications that binding energies from nominally identical overlayer compounds and those related to the substrate are differently positioned against each other depending on the preparation method. Possible causes for this are charging effects, differences in the overlayer compound´s Fermi-level, or presence of interfacial dipoles. Mechanistic insights regarding the formation of cathode-electrolyte interfaces and of ionic interfaces in general are discussed [2][3]. [1] R. Hausbrand, D. Becker, and W. Jaegermann, Progress in Solid State Chemistry 42 (2014) [2] A. Schwöbel, W. Jaegermann, and R. Hausbrand, Solid State Ionics, in press. [3] D. Becker, et al., Journal of Physical Chemistry C, 118 (2014)

Interfacial energy level alignment and energy level diagrams for all-solid Li-ion cells: Impact of Li-ion transfer and double layer formation

Thin film batteries (TFB) are discussed to be one of the future power sources for integrated circuits and miniaturized autonomous electrical devices. The open circuit voltage of a TFB is determined by the difference of the Fermi level positions of the two electrodes, which are electronically separated by an ionically conductive solid electrolyte. The stepwise deposition of one material on top of another combined with in-situ X-ray photoemission spectroscopy (XPS) offers the possibility to measure energy band alignments and band bending due to space charge layer formation at the electrode/electrolyte interfaces, allowing insights into the formation and the properties of the (electrochemical) interface. In this contribution we show results on in-situ XPS measurements of the interface formation between the cathodic interface between LiCoO2/LiPON and on the anodic interface between LiPON/Li. From these measurements an electronic energy band diagram of the full battery cell is determined. The contributions of dipole- and space charge layers to the formation of the electrified interfaces as well as the ionic contribution to the cell voltage is discussed.

Precipitation of CH3NH3PbCl3 in CH3NH3PbI3 and Its Impact on Modulated Charge Separation

A precipitation of CH3NH3PbCl3 (MAPbCl3) in CH3NH3PbI3 (MAPbI3) has been observed for deposition from solutions containing CH3NH3I (MAI), PbCl2 and small amounts of ammoniumvaleric acid iodide (HOOC(CH2)4NH3I or AVAI) in N,N-dimethylformamide. The influence of the precipitation of MAPbCl3 on the electronic properties of the free perovskite layer and of the nanoporous TiO2/perovskite nanocomposite has been investigated by modulated surface photovoltage spectroscopy. The lowest characteristic energy of exponentially distributed defect states below the band gap was found for the highest amount of AVAI in the solution (2.6% for the given experiments). Stabilization of MACl by AVA and the role of coadsorption of ions on TiO2 surfaces for charge separation and layer formation are discussed on the basis of interaction and adsorption energies obtained by molecular modeling.

The study of the amount of heat release in the sinter charge layer

Based on the data on the chemical composition of charge at the sinter plant MK Zaporizhstal (Ukraine), dependencies, allowing to determine the amount of released and absorbed heat energy per unit volume of the sintered charge by the layer height from the average diameter of the material particles were obtained in the paper. Equations, allowing to calculate the amount of heat energy, released and absorbed per unit volume of the charge as a result of the combustion of the fuel particles and exo- and endothermic reactions, proceeding therein are presented. The results allow a close approach to the problem of the optimization of the thermal regime of the agglocharge sintering process on the author's model, based on finite element method, which takes into account the internal heat release per unit volume, as well as determination of polydisperse charge layer formation laws for rational distribution of fuels and chemical components by the height of the agglomerated layer.

Parallel-sheets model analysis of space charge layer formation at metal/ionic conductor interfaces

The space charge formation at metal/yttria-stabilized zirconia interface is investigated by combining defect formation energies from first principles with a parallel-sheets model. In this model, the crystal orientation and interlayer distances between crystal planes are taken into account explicitly. This is in contrast to previous space charge models, which utilized a continuum approximation. The crystal-plane-resolved picture of the space charge is discussed in detail. It is found that the results from the parallel-sheets model resemble closely those of the continuum model. We find this to be the case even for the polar (111) orientation of cubic zirconia, despite the fact that continuum approximations implicitly assume a homogeneous system. We also examine the effect of yttrium dopant segregation on the space charge utilizing the parallel-sheets model. It is found that dopant segregation tends to decrease the width of the oxygen vacancy depletion layer that occurs in oxidizing atmosphere, and that it tends to increase the accumulation of vacancies in reducing atmosphere.

Theoretical analysis of space charge layer formation at metal/ionic conductor interfaces

We examine the space charge layer formation at metal/ionic conductor interfaces by solving Poisson's equation using defect energetics calculated from first principles. The scheme is applied to zirconia, and we find that oxidizing atmosphere and high valence band offset result in negative space charge accumulation at the interface, while reducing atmosphere and low valence band offset result in positive charge accumulation. This is explained in terms of the alignment of the Fermi levels in the metal and the ionic conductor.

On the generation of charge layers in MCS stratiform regions

We suggest a quantitative one-dimensional model treating the formation of charge layers near the 0 °C isotherm in stratiform regions of mesoscale convective systems. A number of factors principal for the field generation have been taken into account: both non-inductive and inductive melting charging, light ions, a complicated profile of the vertical air velocity near the 0 °С isotherm, the boundary conditions proper for the horizontally extended systems in the global electric circuit. Non-inductive collisional charging near the 0 °C isotherm was not considered. It was found that both non-inductive and inductive melting mechanisms can contribute; the inductive melting charging of ice aggregates was found more preferable, while the contribution of non-inductive mechanisms might be significant depending on particular conditions. The role of light ions in the formation of the positive charge layer near the 0 °C isotherm may be important. If the advection from the convective region ensures charge inflow to the upper charged layers, the melting charging mechanisms are able to explain an observable electric field structure in the whole stratiform region. It is important that the mutual position of the zero point on the vertical air velocity profile and the point of maximum melting-charge-transfer determines the fine structure of the electric field in the vicinity of the 0 °C isotherm.

Combinatorial electrochemistry on Al–Fe alloys

Combinatorial material development was combined with high throughput microelectrochemistry to allow an efficient but comprehensive investigation of the interface chemistry of Al rich Al–Fe alloys as a function of their chemical composition. Composition spread thin films with a linear composition gradient were produced by thermal codeposition. A scanning droplet cell was used to determine the open circuit potential and to perform successive anodic oxide formation with intermittent impedance spectroscopy. The film formation factor, the relative permittivity of the oxides and the onset potential of oxide formation were determined quantitatively as function of the composition with a resolution of 0.5 at.%. An unexpected synergistic effect is found in a very narrow composition range between 9 and 12 at.% Fe. This effect, which shifts the onset potential by nearly 1 V, is discussed in terms of a local accumulation of Fe resulting in a redox stabilisation of space charge layer formation during high-field oxide growth. The results are supported by composition and depth dependent XPS measurements.

Formation of Charge Layers in the Planetary Atmospheres

This section focuses on the physical phenomena, leading to large-scale space-charge and electric field generation (electric dynamo) in the planetary atmospheres, and ways of their theoretical description. The main attention is paid to charge-layer formation in atmospheres. Under terrestrial conditions, a problem of charge-layer formation in the atmosphere is important from the viewpoint of both thunderstorm and fair weather electricity. It is important also for the problems of intense layer generation under perturbed ionization conditions, charge layer formation over deserts, high field generation in the mesosphere etc. On the other hand, charge-layer treatment allows verifying electrification theories being applied to more or less simple 1D conditions such as the electrode effect, cloud screening layers, long-term charge layers in mesoscale convective systems. The paper reviews the results of recent research in this field. General conditions of the electro-hydro-dynamic description and their applications to the planetary atmospheres are discussed in terms of the Debye length, mean free path length of charged particles, Langmuir frequency and electrical conductivity. In terms of electrostatic interaction energy, it is found that three phases for charge carriers co-exist in strongly electrified clouds in the Earth’s atmosphere. Crucial role of turbulent motion of conducting media for electric dynamo realization is revealed. The results of recent research in the modeling of the electrode effect, fog electrodynamics, screening layers in clouds and aerosol/dust structures, long-term charge layers in mesoscale convective systems are presented. Nonlinear solutions, demonstrating the formation of charge layers in planetary atmospheres, are examined.

Controlling carrier density and its effect on I–V characteristics of the anatase–TiO 2 thin films prepared by a sputter deposition method

TiO 2 thin films were prepared by the inductively coupled Ar +-plasma sputtering method. The effects of O 2 partial pressure on photoelectrochemical performance of the mainly anatase–TiO 2 electrodes were studied. O 2 partial pressure systematically affected the carrier density and the current–voltage characteristics of the electrodes. An electrode deposited at lowest O 2 partial pressure (11 mPa) showed almost 3 orders of magnitude greater donor density and 4 times larger photocurrent compared to the one deposited at highest O 2 partial pressure (52 mPa). The differences in current–voltage characteristics of the electrode samples are discussed on the basis of the space charge layer formation in the electrodes with different carrier densities.