Photoelektronenspektroskopische Analyse von Grenzflächen an Li+-Ionen-Modellzellen mit PEG und LiTFSI basierten organisch-anorganischen Hybridelektrolyten

Trockene Poly (Ethylen Oxid) basierte Polymermembrane bilden vielversprechende Elektrolyte, die Li+-Ionen-Batterien mit festen organisch-anorganischen Hybridelektrolyten ermöglichen könnten. Entscheidend für die Leistung sind die Oxidationsstabilität des PEOs an der Kathode und der innere Grenzflächenwiderstand zwischen PEO und der Li+-Ionen leitenden Keramik (Li7-xLa3Zr2-xTaxO12). In beiden Szenarien ist eine genaue Kenntnis der Grenzflächenbildung, d.h. von Reaktionen an den Grenzflächen und der Ausbildung elektrischer Doppelschichten, unumgänglich, die jedoch mit post mortem Methoden nicht messbar sind. Diese Arbeit bietet einen neuartigen Ansatz zur Studie von PEO + LiTFSI Grenzflächen zwischen LiCoO2 Kathoden, keramischen Elektrolyten der Granat-Klasse sowie zu Lithium Metallanoden auf Grundlage des oberflächenwissenschaftlichen Ansatzes. Durch ihre weiche Natur sind Polymere nicht dafür geeignet, mit Keramiken bei höheren Temperaturen oder im Plasma beschichtet zu werden, weshalb sie selbst als Dünnschicht auf die Keramikkathode aufgetragen werden müssen. Da Verfahren, wie Rotationsbeschichten, zu viele Verunreinigen aufweisen, wird das PEO in Form seines Oligomers PEG 2000 g/mol zusammen mit dem Leitsalz LiTFSI thermisch im Ultrahochvakuum direkt auf das Substrat aufgedampft und anschließend im Clustertool mit der Photoelektronenspektroskopie analysiert. Zusätzlich kommen ergänzende Methoden zur Charakterisierung wie die dynamische Differenzkalorimetrie, die thermogravimetrische Analyse, Massenspektroskopie und Rasterelektronenmikroskopie zum Einsatz, um die gleichen physikalischen Eigenschaften der PEG + LiTFSI Dünnschichten, wie für dicke PEO Membranen benötigt, zu garantieren. Darüber hinaus wird der Einfluss von LiF Beschichtungen auf dem LLZ(T)O berücksichtigt und deren Einfluss systematisch untersucht. In einem neuen Verfahren mit Hilfe einer Vibrationsbühne in der Abscheidekammer wird dann die planare 2D Beschichtung auf 3D Partikel Systeme übertragen. Auf Grundlage der XPS Ergebnisse werden alle relevanten Grenzflächen in der Modellzelle LiCoO2/PEG+LiTFSI/LLZ(T)O (+LiF)/Lithium anhand der auftretenden Reaktionsprodukte und der Ausbildung von Raumladungszonen und deren Ursprung diskutiert sowie deren Einfluss auf den Grenzflächenwiderstand eingeordnet. Ein zentraler Punkt der Diskussion sind die Möglichkeit von Elektronen und Li+-Ionen-Transfers über die Grenzfläche sowie der Einfluss von parasitären Zustanden in der Bandlücke und die daraus folgenden Oxidations- und Reduktionsreaktionen

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

Electrochemical Generation of Catalytically Active Edge Sites in C₂N‐Type Carbon Materials for Artificial Nitrogen Fixation

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH₃) is a potentially carbon‐neutral and decentralized supplement to the established Haber–Bosch process. Catalytic activation of the highly stable dinitrogen molecules remains a great challenge. Especially metal‐free nitrogen‐doped carbon catalysts do not often reach the desired selectivity and ammonia production rates due to their low concentration of NRR active sites and possible instability of heteroatoms under electrochemical potential, which can even contribute to false positive results. In this context, the electrochemical activation of nitrogen‐doped carbon electrocatalysts is an attractive, but not yet established method to create NRR catalytic sites. Herein, a metal‐free C₂N material (HAT‐700) is electrochemically etched prior to application in NRR to form active edge‐sites originating from the removal of terminal nitrile groups. Resulting activated metal‐free HAT‐700‐A shows remarkable catalytic activity in electrochemical nitrogen fixation with a maximum Faradaic efficiency of 11.4% and NH₃ yield of 5.86 µg mg⁻¹cat h⁻¹. Experimental results and theoretical calculations are combined, and it is proposed that carbon radicals formed during activation together with adjacent pyridinic nitrogen atoms play a crucial role in nitrogen adsorption and activation. The results demonstrate the possibility to create catalytically active sites on purpose by etching labile functional groups prior to NRR

Evidence of the chemical stability of the garnet-type solid electrolyte Li 5 La 3 Ta 2 O 12 towards lithium by a surface science approach

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

A Selective Copper Based Oxygen Reduction Catalyst for the Electrochemical Synthesis of H 2 O 2 at Neutral pH

H2O2 is a bulk chemical used as "green" alternative in a variety of applications, but has an energy and waste intensive production method. The electrochemical O2 reduction to H2O2 is viable alternative with examples of the direct production of up to 20% H2O2 solutions. In that respect, we found that the dinuclear complex Cu2(btmpa) (6,6'-bis[[bis(2-pyridylmethyl)amino]methyl]-2,2'-bipyridine) reduces O2 to H2O2 with a selectivity up to 90 % according to single linear sweep rotating ring disk electrode measurements. Microbalance experiments showed that complex reduction leads to surface adsorption thereby increasing the catalytic current. More importantly, we kept a high Faradaic efficiency for H2O2 between 60 and 70 % over the course of 2 h of amperometry by introducing high potential intervals to strip deposited copper (depCu). This is the first example of extensive studies into the long term electrochemical O2 to H2O2 reduction by a molecular complex which allowed to retain the high intrinsic selectivity of Cu2(btmpa) towards H2O2 production leading to relevant levels of H2O2

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

TiO₂ as second phase in Na₃Zr₂Si₂PO₁₂ to suppress dendrite growth in sodium metal solid-state batteries

Solid-state sodium–metal batteries will not achieve reasonable power density without electrolytes that solve the dendrite (filamentation) problem. Metal-filament formation during plating at ceramic/metal interfaces can cause electrical failure by internal short-circuit or mechanical failure by electrolyte fracture. Herein, an Na3Zr2Si2PO12 (NZSP) sodium-ion-conducting NASICON electrolyte in which TiO2 is incorporated as an additive is presented, leading to a two-phase composite NZSP(TiO2) with improved density, Young's modulus, hardness, grain structure, and bulk permittivity. These features of NZSP(TiO2) suppress dendrite growth along grain boundaries, microcracks, and micropores. As well as demonstrating ultralow ceramic/Na kinetic resistance with electrochemical measurements, X-ray photoelectron spectroscopy is performed to probe interfacial reaction mechanisms. The TiO2 phase forms within grain boundaries and along NZSP surfaces. This modifies the two-phase material's microstructure and improves its electrochemical performance, while also increasing the critical current density for dendrite formation. Design guidelines are discussed to mitigate microscopic defects and dendrites in two-phase ceramic electrolytes