Characterization of the Interfaces in LiFePO4/PEO-LiTFSI Composite Cathodes and to the Adjacent Layers

Interface resistances between the different components of battery cells limit their fast charge and discharge capability which is required for different applications such as electromobility. To decrease interface resistances, it is necessary to understand which individual interface they arise at and how they can be controlled. Electrochemical impedance spectroscopy is a well-established technique for the distinction of different contributions to the internal cell resistance and allows the characterization of interface resistances. Especially the use of suitable cell setups allows one to attribute the measured resistances to specific interfaces. In this contribution, we investigate the impedance of dry polymer full cells containing a lithium iron phosphate/ poly(ethylene oxide)-lithium bis(trifluoromethanesulfonyl)imide composite cathode, a solid polymer electrolyte separator and a lithium-metal anode. Based on the results on different cell setups, we are able to reliably determine the planar resistances between the components as well as the charge transfer resistance inside the composite cathode. For unoptimized systems, we find high planar resistances, which can be significantly reduced by coating and processing strategies. For the charge transfer resistance, we find a dependence on the SOC as well as on the charging direction. Possible mechanisms for the evolution of interface resistances are discussed also based on chemical analysis performed by photoelectron spectroscopy (XPS)

On the Surface Modification of LLZTO with LiF via a Gas-Phase Approach and the Characterization of the Interfaces of LiF with LLZTO as Well as PEO+LiTFSI

In this study we present gas-phase fluorination as a method to create a thin LiF layer on Li₆.₅La₃Zr₁.₅Ta₀.₅O₁₂ (LLZTO). We compared these fluorinated films with LiF films produced by RF-magnetron sputtering, where we investigated the interface between the LLZTO and the deposited LiF showing no formation of a reaction layer. Furthermore, we investigated the ability of this LiF layer as a protection layer against Li₂CO₃ formation in ambient air. By this, we show that Li₂CO₃ formation is absent at the LLZTO surface after 24 h in ambient air, supporting the protective character of the formed LiF films, and hence potentially enhancing the handling of LLZTO in air for battery production. With respect to the use within hybrid electrolytes consisting of LLZTO and a mixture of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), we also investigated the interface between the formed LiF films and a mixture of PEO+LiTFSI by X-ray photoelectron spectroscopy (XPS), showing decomposition of the LiTFSI at the interface

Surface science of intercalation materials and solid electrolytes: a view on electron and ion transfer at Li-ion electrodes based on energy level concepts

This book shares essential insights into the formation and properties of ionic interfaces based on the energy level structures of their interfaces obtained using a surface science approach. It covers both interfaces with liquid and solid electrolyte contacts, and includes different material classes, such as oxides and phosphates. The specific material properties result in particular effects observed at interfaces, which are often not yet, or not sufficiently, taken into account in battery development and technologies. Discussing fundamental issues concerning the properties of intercalation electrodes and electrode–solid electrolyte interfaces, the book investigates the factors that determine voltage, kinetics and reactivity. It presents experimental results on interface formation, and relates them to electron and ion energy levels in the materials and at their interfaces. It explores these topics integrating electrochemistry, solid-state ionics and semiconductor physics, and accordingly will appeal not only to battery scientists, but also to a broader scientific community, including material scientists and electrochemists

On the use of energy level diagrams for semiconducting ionic electrodes

General treatment of charge transfer at electrochemical interfaces is based on charge carrier concentration and activation enthalpy of the transfer step. For the description of electron transfer at semiconductor electrodes, energy-level diagrams have proved useful, giving a detailed picture of the nature of the interface. In the case of semiconducting ionic electrodes, however, energy-level diagrams are currently not being utilized. The reason is insufficient knowledge regarding ion energy levels and nature of double layer. This contribution briefly reviews the fundamentals of charge transfer at semiconducting electrodes and discusses the establishment of energy-level diagrams for semiconducting ionic electrodes

Process related effects upon formation of composite electrolyte interfaces: Nitridation and reduction of NASICON-type electrolytes by deposition of LiPON

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

Interface reactions between LiPON and lithium studied by in-situ X-ray photoemission

Abstract Lithium phosphorus oxynitride (LiPON) is an amorphous solid lithium ion conductor commonly used in all solid state thin film batteries (TFBs) with LiCoO2 as cathode- and lithium as anode-material. {TFBs} exceed conventional Li ion batteries with respect to lifetime and safety but may suffer from high ion transfer resistances and interface reactions between the electrodes and the electrolyte. In this contribution we study interface layer formation between LiPON and metallic lithium using an in-situ X-ray photoemission spectroscopy (XPS) surface science approach

XPS-Surface Analysis of SEI Layers on Li-Ion Cathodes: Part I. Investigation of Initial Surface Chemistry

In this contribution, we investigate the initial surface chemistry on fully lithiated LiCoO2 thin film model electrodes in the electrolyte solvent diethyl carbonate (DEC) and the LiPF6-electrolyte by means of soaking experiments. The interfacial layer composition is analyzed by X-ray photoelectron spectroscopy (XPS), and possible layer morphologies and spontaneous formation mechanisms are discussed in detail. Upon decomposition of DEC a layered system of surface-bound semi-organic components (inner layer) and cross-linked organic moieties (outer layer) is formed, while a change of the Co3+ oxidation state and thus a surface corrosion of LiCoO2 was not observed. In contrast, the solid electrolyte interface (SEI) film of the LiPF6-electrolyte soaked electrode showed an inner layer, containing predominantly corroded LiCoO2, i.e. Co(II,III)xOy(OH)z and LiF as well as aliphatic fluoroorganic species. The outer SEI layer consists mainly of a poly-organic network and randomly distributed LixPOyFz domains. The thickness of the deposit on the electrolyte soaked electrode surface was only half as thick due to the significantly lower amount of organic and semi-organic compounds. Our investigation indicates that the solvent decomposition is related to the catalytically active LiCoO2 surface, which is passivated by reaction products such as LiF originating from HF induced processes