Materiales con estructura tipo K2NiF4 como cátodos para pilas de combustible de óxido sólido de temperatura intermedia

Tesis doctoral inédita de la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Teórica y Materia Condensada. Fecha de lectura:15-12-200

Understanding and Engineering Interfacial Adhesion in Solid-State Batteries with Metallic Anodes

Funding Information: The authors acknowledge funding for this work from the Engineering and Physical Sciences Research Council (EP/R002010/1, EP/R024006/1 and EP/P003532/1), Shell Global Solutions International B.V., the Spanish government (TED2021‐129254B‐C22) and Horizon Europe HORIZON‐CL5‐2021‐D2‐01 “SEATBELT” 101069726.Peer reviewedPublisher PD

Oxygen reduction reaction at La_xCa_1-xMnO₃ nanostructures: interplay between A-site segregation and B-site valency

The mean activity of surface Mn sites at LaxCa1-xMnO3 nanostructures towards the oxygen reduction reaction (ORR) in alkaline solution is assessed as a function of the oxide composition. Highly active oxide nano-particles were synthesised by an ionic liquid-based route, yielding phase-pure nanoparticles, across the entire range of compositions, with sizes between 20 and 35 nm. The bulk vs. surface composition and structure are investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). These techniques allow quantification of not only changes in the mean oxidation state of Mn as a function of x, but also the extent of A-site surface segregation. Both trends manifest themselves in the electrochemical responses associated with surface Mn sites in 0.1 M KOH solution. The characteristic redox signatures of Mn sites are used to estimate their effective surface number density. This parameter allows comparing, for the first time, the mean electrocatalytic activity of surface Mn sites as a function of the LaxCa1-xMnO3 composition. The ensemble of experimental data provides a consistent picture in which increasing electron density at the Mn sites leads to an increase in the ORR activity. We also demonstrate that normalisation of electrochemical activity by mass or specific surface area may result in inaccurate structure–activity correlations

Toward an Understanding of SEI Formation and Lithium Plating on Copper in Anode-Free Batteries.

Funder: Blavatnik Family Foundation"Anode-free" batteries present a significant advantage due to their substantially higher energy density and ease of assembly in a dry air atmosphere. However, issues involving lithium dendrite growth and low cycling Coulombic efficiencies during operation remain to be solved. Solid electrolyte interphase (SEI) formation on Cu and its effect on Li plating are studied here to understand the interplay between the Cu current collector surface chemistry and plated Li morphology. A native interphase layer (N-SEI) on the Cu current collector was observed with solid-state nuclear magnetic resonance spectroscopy (ssNMR) and electrochemical impedance spectroscopy (EIS). Cyclic voltammetry (CV) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) studies showed that the nature of the N-SEI is affected by the copper interface composition. An X-ray photoelectron spectroscopy (XPS) study identified a relationship between the applied voltage and SEI composition. In addition to the typical SEI components, the SEI contains copper oxides (Cu x O) and their reduction reaction products. Parasitic electrochemical reactions were observed via in situ NMR measurements of Li plating efficiency. Scanning electron microscopy (SEM) studies revealed a correlation between the morphology of the plated Li and the SEI homogeneity, current density, and rest time in the electrolyte before plating. Via ToF-SIMS, we found that the preferential plating of Li on Cu is governed by the distribution of ionically conducting rather than electronic conducting compounds. The results together suggest strategies for mitigating dendrite formation by current collector pretreatment and controlled SEI formation during the first battery charge

Establishing Ultralow Activation Energies for Lithium Transport in Garnet Electrolytes.

Garnet-type structured lithium ion conducting ceramics represent a promising alternative to liquid-based electrolytes for all-solid-state batteries. However, their performance is limited by their polycrystalline nature and inherent inhomogeneous current distribution due to different ion dynamics at grains, grain boundaries, and interfaces. In this study, we use a combination of electrochemical impedance spectroscopy, distribution of relaxation time analysis, and solid-state nuclear magnetic resonance (NMR), in order to understand the role that bulk, grain boundary, and interfacial processes play in the ionic transport and electrochemical performance of garnet-based cells. Variable temperature impedance analysis reveals the lowest activation energy for Li transport in the bulk of the garnet electrolyte (0.15 eV), consistent with pulsed field gradient NMR spectroscopy measurements (0.14 eV). We also show a decrease in grain boundary activation energy at temperatures below 0 °C, that is followed by the total conductivity, suggesting that the bottleneck to ionic transport resides in the grain boundaries. We reveal that the grain boundary activation energy is heavily affected by its composition that, in turn, is mainly affected by the segregation of dopants and Li. We suggest that by controlling the grain boundary composition, it would be possible to pave the way toward targeted engineering of garnet-type electrolytes and ameliorate their electrochemical performance in order to enable their use in commercial devices

Electrocatalytic Site Activity Enhancement via Orbital Overlap in A ₂MnRuO ₇(A = Dy ³⁺, Ho ³⁺, and Er ³⁺) Pyrochlore Nanostructures

Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of the bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure-activity relationship of A2RuMnO7 (A = Dy3+, Ho3+, and Er3+) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states at the appropriate energy range for oxygen electrocatalysis. The bulk structure and surface composition of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS). The materials exhibit high phase purity (cubic fcc with a space group Fd3\uaf m) in which variations in M-O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition was slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 toward the 4-electron oxygen reduction reaction in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by density functional theory calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where the Ru/Mn ratio is closer to 1 in comparison with the Ho3+ and Er3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions

Revealing Strain Effects on the Chemical Composition of Perovskite Oxide Thin Films Surface, Bulk, and Interfaces

Understanding the effects of lattice strain on oxygen surface and diffusion kinetics in oxides is a controversial subject that is critical for developing efficient energy storage and conversion materials. In this work, high-quality epitaxial thin films of the model perovskite LaSrMnCoO (LSMC), under compressive or tensile strain, are characterized with a combination of in situ and ex situ bulk and surface-sensitive techniques. The results demonstrate a nonlinear correlation of mechanical and chemical properties as a function of the operation conditions. It is observed that the effect of strain on reducibility is dependent on the "effective strain" induced on the chemical bonds. In-plain strain, and in particular the relative BO length bond, is the key factor controlling which of the B-site cation can be reduced preferentially. Furthermore, the need to use a set of complimentary techniques to isolate different chemically induced strain effects is proven. With this, it is confirmed that tensile strain favors the stabilization of a more reduced lattice, accompanied by greater segregation of strontium secondary phases and a decrease of oxygen exchange kinetics on LSMC thin films

Understanding improved capacity retention at 4.3 V in modified single crystal Ni-rich NMC//graphite pouch cells at elevated temperature

The capacity retention of commercially-sourced pouch cells with single crystal Al surface-doped Ni-rich cathodes (LiNi0.834Mn0.095Co0.071O2) is examined. The degradation-induced capacity fade becomes more pronounced as the upper-cut-off voltage (UCV) increases from 4.2 V to 4.3 V (vs. graphite) at a fixed cycling temperature (either 25 or 40 °C). However, cycles with 4.3 V UCV (slightly below the oxygen loss onset) show better capacity retention upon increasing the cycling temperature from 25 °C to 40 °C. Namely, after 500 cycles at 4.3 V UCV, cycling temperature at 40 °C retains 85.5% of the initial capacity while cycling at 25 °C shows 75.0% capacity retention. By employing a suite of electrochemical, X-ray spectroscopy and secondary ion mass spectrometry techniques, we attribute the temperature-induced improvement of the capacity retention at high UCV to the combined effects of Al surface-dopants, electrochemically resilient single crystal Ni-rich particles, and thermally-improved Li kinetics translating into better electrochemical performance. If cycling remains below the lattice oxygen loss onset, improved capacity retention in industrial cells should be achieved in single crystal Ni-rich cathodes with the appropriate choice of cycling parameter, particle quality, and particle surface dopants

Suppressing void formation in all-solid-state batteries : The role of interfacial adhesion on alkali metal vacancy transport

Funding Information: I. D. S. and A. A. acknowledge funding for their research from EPSRC Platform Grant EP/R002010/1. I. D. S. acknowledges the Imperial College Research Computing Service (10.14469/hpc/ 2232), and associated support services used during this work. We are grateful to the UK Materials and Molecular Modelling Hub for computational resources, which is partially funded by EPSRC (EP/P020194/1, EP/T022213/1, EP/P003532/1 and ICSF EP/R024006/1). I. D. S. and A. A. would like to thank Professor Michael Finnis, Professor Graeme Henkelman, Edouard Quérel and Dr Rowena Brugge for valuable discussions during the initial stages of this project. Funding Information: I. D. S. and A. A. acknowledge funding for their research from EPSRC Platform Grant EP/R002010/1. I. D. S. acknowledges the Imperial College Research Computing Service (10.14469/hpc/2232), and associated support services used during this work. We are grateful to the UK Materials and Molecular Modelling Hub for computational resources, which is partially funded by EPSRC (EP/P020194/1, EP/T022213/1, EP/P003532/1 and ICSF EP/R024006/1). I. D. S. and A. A. would like to thank Professor Michael Finnis, Professor Graeme Henkelman, Edouard Quérel and Dr Rowena Brugge for valuable discussions during the initial stages of this project. Publisher Copyright: © The Royal Society of Chemistry 2021.Peer reviewedPublisher PD

Low activation energies for interstitial oxygen conduction in the layered perovskites La1+xSr1−xInO4+δ

K2NiF4-type La1+xSr1−xInO4+δ (x = 0.1, 0.2) oxides have been prepared and investigated as possible solid electrolytes for solid-oxide fuel cells (SOFC). These materials were synthesized using a citrate–nitrate soft-chemistry technique followed by annealing in air at a temperature of 1000 °C. Preliminary characterization by X-ray diffraction (XRD) indicated that these layered perovskites crystallize in an orthorhombic structure with the space group Pbca. The crystal structural features were explored at RT by neutron powder diffraction (NPD). Their capability to incorporate interstitial oxygen atoms (δ) and their effect on the crystal structure were identified by difference Fourier maps in the NaCl layers of the K2NiF4 structure; subsequent Rietveld refinements yielded excess oxygen values, δ = 0.07(1) and 0.11(2) for x = 0.1 and 0.2, respectively. The electrical properties were studied by impedance spectroscopy (IS) in the temperature range of 500–900 °C and compared with those of the parent compound LaSrInO4. A better conductivity was observed for La1.2Sr0.8InO4.11, which is consistent with the higher quantity of interstitial oxygen found from NPD data. The extremely low activation energy of only 0.51 eV for the conduction mechanism via interstitials at low temperatures (T < 650 °C) is significantly smaller than that of other electrolytes working with a vacancy mechanism, typically of 1 eV, as theoretically predicted. The present result endorses the validity of this design procedure and supports K2NiF4-related compounds as promising candidates for solid-oxide electrolytes.We are grateful to the Spanish Ministry of Economy and Competitivity for granting the project MAT2013-41099-R, and ILL and PSI for making all facilities available for the neutron diffraction experiments. L.T. thanks the financial support of CONICYT for “Becas de Postdoctorado en el Extranjero BECAS CHILE”. AA wants to thank the European Commission for a Marie Curie IE fellowship