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 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.
•Compilation of energy level offsets at LiCoO2 interfaces•Influence of electronic and ionic structure on interface properties•Differences in ionic structure as origin for space charge layer formation
This open access book presents a thorough look at tortuosity and microstructure effects in porous materials. The book delivers a comprehensive review of the subject, summarizing all key results in ...the field with respect to the underlying theories, empirical data available in the literature, modern methodologies and calculation approaches, and quantitative relationships between microscopic and macroscopic properties. It thoroughly discusses up to 20 different types of tortuosity and introduces a new classification scheme and nomenclature based on direct geometric tortuosities, indirect physics-based tortuosities, and mixed tortuosities (geometric and physics-based). The book also covers recent progress in 3D imaging and image modeling for studying novel aspects of tortuosity and associated transport properties in materials, while providing a comprehensive list of available software packages for practitioners in the community. This book is a must-read for researchers and students in materials science and engineering interested in a deeper understanding of microstructure–property relationships in porous materials. For energy materials in particular, such as lithium-ion batteries, tortuosity is a key microstructural parameter that can greatly impact long-term material performance. Thus, the information laid out in this book will also greatly benefit researchers interested in computational modeling and design of next-generation materials, especially those for sustainability and energy applications.
Porosity is frequently specified as only a value to describe the microstructure of a battery electrode. However, porosity is a key parameter for the battery electrode performance and mechanical ...properties such as adhesion and structural electrode integrity during charge/discharge cycling. This study illustrates the importance of using more than one method to describe the electrode microstructure of LiNi0.6Mn0.2Co0.2O2 (NMC622)-based positive electrodes. A correlative approach, from simple thickness measurements to tomography and segmentation, allowed deciphering the true porous electrode structure and to comprehend the advantages and inaccuracies of each of the analytical techniques. Herein, positive electrodes were calendered from a porosity of 44–18% to cover a wide range of electrode microstructures in state-of-the-art lithium-ion batteries. Especially highly densified electrodes cannot simply be described by a close packing of active and inactive material components, since a considerable amount of active material particles crack due to the intense calendering process. Therefore, a digital 3D model was created based on tomography data and simulation of the inactive material, which allowed the investigation of the complete pore network. For lithium-ion batteries, the results of the mercury intrusion experiments in combination with gas physisorption/pycnometry experiments provide comprehensive insight into the microstructure of positive electrodes.
The interaction of water with a magnetron-sputtered nickel oxide thin film on an n-type silicon photoanode is investigated in perspective to oxygen evolution. The substrate was exposed in-situ ...stepwise to gas phase water up to 10 L at liquid N2 temperature and analyzed via X-ray and UV photoelectron spectroscopy in the so called frozen electrolyte approach. Photoemission of the pristine NiOx layer shows the presence of stoichiometric NiO and Ni2O3 as well as of non-stoichiometric phases. In the monolayer range, molecular and dissociative adsorption is detected assigned to the NiO respective Ni2O3 phase. Initially, the emission of the molecular adsorbed water species interacting with NiO is found at 0.8 eV lower binding energies as compared to water related emission for higher coverages with binding energies commonly assigned to H2O-H2O interaction. In addition to the chemical analysis, the electronic structure of the n-Si/SiOx/NiOx/H2O photoanode is measured and discussed.
The chemical instability of the glassy solid electrolyte LiPON against metallic lithium and the occurrence of side reactions at their interface is investigated by combining a surface science approach ...and quantum-mechanical calculations. Using an evolutionary structure search followed by a melt-quenching protocol, a model for the disordered structure of LiPON is generated and put into contact with lithium. Even the static optimization of a simple model interface suggests that the diffusion of lithium into LiPON is driven by a considerable driving force that could easily take place under experimental conditions. Calculated reaction energies indicate that the reduction and decomposition of LiPON is thermodynamically favorable. By monitoring the evolution of the LiPON core levels as a function of lithium exposure, the disruption of the LiPON network alongside the occurrence of new phases is observed. The direct comparison between UV photoelectron spectroscopy measurements and calculated electronic densities of states for increasing stages of lithiation univocally identifies the new phases as Li2O, Li3P and Li3N. These products are stable against Li metal and form a passivation layer which shields the electrolyte from further decomposition while allowing for the diffusion of Li ions.
•The interface between glassy electrolyte LiPON and Li metal is investigated.•Spectroscopic measurements observe side reactions and electrolyte decomposition.•Simulations independently predict electrolyte interfacial instability.•Products are identified by comparison of simulated and measured spectra.•The interphase acts as a passivation layer.
The chemical stability between Li metal and garnet-type solid electrolytes is currently under debate, mainly catalyzed by theoretical studies. Here, we investigate the stability of Li5La3Ta2O12 ...(LLTaO) towards lithium experimentally. Using a surface science approach, lithium is stepwise evaporated on an LLTaO thin film grown by CO2-laser assisted chemical vapor deposition. By annealing of the LLTaO thin film, the Li2CO3 surface layer can be removed, leaving only small traces of Li2CO3, Li2O2 and Li2O behind. The interface formation of LLTaO towards lithium is then monitored by means of X-ray and ultraviolet photoelectron spectroscopy. Neither reaction products related to decomposition nor structural changes in the matrix of the Ta-based garnet-type solid-electrolyte can be detected, indicating that LLTaO exhibits chemical stability under equilibrium conditions. Furthermore, a model for the energy level alignment at the LLTaO/Li interface is discussed.
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•In situ electrolyte-electrode interface experiment between Li5La3Ta2O12 and lithium.•Evidence for the chemical stability of Li5La3Ta2O12 towards lithium.•Energy level alignment at the Li5La3Ta2O12/lithium interface.•Electronic Structure of the garnet-type solid electrolyte Li5La3Ta2O12via XPS & UPS.
•Combined XPS/UPS/XAS analysis of electronic structure of layered cathode materials and their interfaces.•Non-rigid behavior of electronic structure as cause for oxygen loss and degradation.•Stepwise ...interface formation and analysis by means of emersion and adsorption experiments.•Extensive characterization of interface properties: chemical structure, band bending, energy level alignment, and surface chemistry.•Interface and surface layer formation in view of ion/electron transfer and covalent interaction processes.•Electrode surface – solvent interaction as reason for electrolyte decomposition.
Lithium-Ion batteries are important devices for present and future energy storage, offering high energy density and durability. Positive electrode (cathode) materials are predominantly transition metal oxides containing lithium, such as layered oxides or oxides with spinel structure. The electronic/ionic structure of these materials in the bulk as well as at the phase boundary of the solid-electrolyte interface is a key for their properties, such as electrode potential, degradation and reactivity.
This contribution gives examples of surface science investigations on battery materials, addressing especially electronic structure of layered-oxide cathode materials and cathode-electrolyte interface formation. In a surface science approach, well-defined surfaces/interfaces are prepared by thin film deposition techniques and analyzed with surface sensitive analytical methods such as photoelectron spectroscopy (XPS, UPS) and X-ray absorption spectroscopy (XAS) before and after polarization in liquid electrolyte or contact with electrolyte components. Surface science offers the possibility to analyze the electronic and chemical structure of surfaces and interfaces, allowing conclusions on the electronic and ionic structure of the bulk, on reactivity with other phases, and on electrochemical interface formation.
After an introduction into the methodology and fundamental phenomena, the contribution focuses on the role of the electronic structure and electric double layer for electrode degradation and surface layer formation. Charge transfer (electrons and ions) and defect formation is discussed on basis of energy level diagrams extracted from experimental data.
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