Metal fluorides/oxides (MF(x)/M(x)O(y)) are promising electrodes for lithium-ion batteries that operate through conversion reactions. These reactions are associated with much higher energy densities than intercalation reactions. The fluorides/oxides also exhibit additional reversible capacity beyond their theoretical capacity through mechanisms that are still poorly understood, in part owing to the difficulty in characterizing structure at the nanoscale, particularly at buried interfaces. This study employs high-resolution multinuclear/multidimensional solid-state NMR techniques, with in situ synchrotron-based techniques, to study the prototype conversion material RuO2. The experiments, together with theoretical calculations, show that a major contribution to the extra capacity in this system is due to the generation of LiOH and its subsequent reversible reaction with Li to form Li2O and LiH. The research demonstrates a protocol for studying the structure and spatial proximities of nanostructures formed in this system, including the amorphous solid electrolyte interphase that grows on battery electrodes.

Na 3 V 2 (PO 4 ) 2 F 3 is a novel electrode material that can be used in both Li ion and Na ion batteries (LIBs and NIBs). The long-and short-range structural changes and ionic and electronic mobility of Na 3 V 2 (PO 4 ) 2 F 3 as a positive electrode in a NIB have been investigated with electrochemical analysis, X-ray diffraction (XRD), and high-resolution 23 Na and 31 P solid-state nuclear magnetic resonance (NMR). The 23 Na NMR spectra and XRD refinements show that the Na ions are removed nonselectively from the two distinct Na sites, the fully occupied Na1 site and the partially occupied Na2 site, at least at the beginning of charge. Anisotropic changes in lattice parameters of the cycled Na 3 V 2 (PO 4 ) 2 F 3 electrode upon charge have been observed, where a (= b) continues to increase and c decreases, indicative of solid-solution processes. A noticeable decrease in the cell volume between 0.6 Na and 1 Na is observed along with a discontinuity in the 23 Na hyperfine shift between 0.9 and 1.0 Na extraction, which we suggest is due to a rearrangement of unpaired electrons within the vanadium t 2g orbitals. The Na ion mobility increases steadily on charging as more Na vacancies are formed, and coalescence of the resonances from the two Na sites is observed when 0.9 Na is removed, indicating a Na1−Na2 hopping (two-site exchange) rate of ≥4.6 kHz. This rapid Na motion must in part be responsible for the good rate performance of this electrode material. The 31 P NMR spectra are complex, the shifts of the two crystallograpically distinct sites being sensitive to both local Na cation ordering on the Na2 site in the as-synthesized material, the presence of oxidized (V 4+ ) defects in the structure, and the changes of cation and electronic mobility on Na extraction. This study shows how NMR spectroscopy complemented by XRD can be used to provide insight into the mechanism of Na extraction from Na 3 V 2 (PO 4 ) 2 F 3 when used in a NIB.

Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO2 emissions. Solid materials capable of reversibly absorbing CO2 have been the focus of intense research for the past two decades, with promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this review, we explore the fundamental aspects underpinning solid CO2 sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines, including materials discovery, synthesis, and in situ characterization, to present a coherent understanding of the mechanisms of CO2 absorption both at surfaces and within solid materials.

With the advent of large structural databases containing both optimised crystallographic structures and their ground state energies, the goal of rationally designing novel functional materials for a variety of applications can be realised. Through selection of relevant, property-specific parameter(s), screening criteria can then be applied to thousands of potential candidates in silico, efficiently selecting the most promising materials for subsequent experimental testing. Here we describe our work developing screening methodologies for the design of novel materials for carbon capture and storage (CCS) and ionic conductivity. Our holistic approach combines theoretical screening with experimental validation to better understand the trends underpinning the performance of the selected materials. We have made use of the Materials Project database (www.materialsproject.org), which not only contains an extensive variety of calculated structures, but is also constructed in such a way to be amenable to high throughput screening. The first application we consider is CO2 absorption looping for CCS, which requires oxide materials that are able to reversibly absorb CO2 at high temperatures. CaO is the prototypical material for this application, but unfortunately suffers irreversible capacity loss and sintering upon continuous carbonation-regeneration cycles, necessitating materials with improved performance. With large scale screening, we were able to simulate the carbonation equilibria for 640 prospective sorbents and then select a number of candidates based on (i) minimising the energy cost associated with their use and (ii) maximising their theoretical CO2 capture capacity [1]. The accuracy of the screening was validated using structural and thermogravimetric analysis, and the process led to a number of design rules for optimising materials performance, including focussing on ternary oxides and materials containing magnesium and calcium. The second part of this work concerns another CCS technology, chemical looping combustion, which utilises materials that can spontaneously release gaseous O2 in order to efficiently burn fuel in a N2-free environment and create pure CO2 for subsequent capture. This is in contrast to the former process, which separates CO2 post-combustion. Binary metal oxides with multiple oxidation states such as CuO and MnO2 can be reduced under reactor conditions, but materials that reduce at lower temperatures and cycle more stably are desirable in order to minimise energy costs involved with combustion at higher temperatures. Our screening found over 2200 materials that were able to undergo redox reactions under the specific process conditions [2], with further experimental studies revealing a number of promising materials that could be stably cycled between their perovskite and brownmillerite phases. The final part details the use of local structural similarity algorithms to efficiently screen materials for oxygen ionic conductivity. Building on a previous methodology using Voronoi tessellations to ...

Novel lithium-based materials for carbon capture and storage (CCS) applications have emerged as a promising class of materials for use in CO looping, where the material re- 2 acts reversibly with CO2 to form Li2CO3 amongst other phases depending on the parent phase. Much work has been done to try and understand the origin of the continued reactivity of the process even after a layer of Li2CO3 has covered the sorbent particles. In this work, we have studied the lithium and oxygen ion dynamics in Li2CO3 over the temperature range of 293– 973 K in order to elucidate the link between dynamics and reactivity in this system. We have used a combination of powder X-ray diffraction, solid-state NMR spectroscopy and theoretical calculations to chart the temperature dependence of both structural changes and ion dynamics in the sample. These methods together allowed us to determine the activation energy for both lithium ion hopping processes and carbonate ion rotations in Li2CO3. Importantly, we have shown that these processes may be coupled in this material, with the initial carbonate ion ro- tations aiding the subsequent hopping of lithium ions within the structure. Additionally, this study shows that it is possible to measure dynamic processes in powder or crystalline materials indirectly through a combination of NMR spectroscopy and theoretical calculations

For magnesium ion batteries (MIBs) to be used commercially, new cathodes must be developed that show stable reversible Mg intercalation. VS 4 is one such promising material, with vanadium and disulfide anions [S 2 ] 2− forming onedimensional linear chains, with a large interchain spacing (5.83 Å) enabling reversible Mg insertion. However, little is known about the details of the redox processes and structural transformations that occur upon Mg intercalation and deintercalation. Here, employing a suite of local structure characterization methods including X-ray photoelectron spectroscopy (XPS), V and S X-ray absorption nearedge spectroscopy (XANES), and 51 V Hahn echo and magic-angle turning with phase-adjusted sideband separation (MATPASS) NMR, we show that the reaction proceeds via internal electron transfer from V 4+ to [S 2 ] 2− , resulting in the simultaneous and coupled oxidation of V 4+ to V 5+ and reduction of [S 2 ] 2− to S 2− . We report the formation of a previously unknown intermediate in the Mg−V−S compositional space, Mg 3 V 2 S 8 , comprising [VS 4 ] 3− tetrahedral units, identified by using density functional theory coupled with an evolutionary structure-predicting algorithm. The structure is verified experimentally via X-ray pair distribution function analysis. The voltage associated with the competing conversion reaction to form MgS plus V metal directly is similar to that of intermediate formation, resulting in two competing reaction pathways. Partial reversibility is seen to re-form the V 5+ and S 2− containing intermediate on charging instead of VS 4 . This work showcases the possibility of developing a family of transition metal polychalcogenides functioning via coupled cationic−anionic redox processes as a potential way of achieving higher capacities for MIBs.

A combined computational and experimental methodology is developed to predict new materials that should have desirable properties for CCS looping, and then select promising candidates to experimentally validate these predictions.

While solid-state NMR spectroscopic techniques have helped clarify the local structure and dynamics of ionic conductors, similar studies of mixed ionic–electronic conductors (MIECs) have been hampered by the paramagnetic behavior of these systems. Here we report high-resolution 17O (I = 5/2) solid-state NMR spectra of the mixed-conducting solid oxide fuel cell (SOFC) cathode material La2NiO4+δ, a paramagnetic transition-metal oxide. Three distinct oxygen environments (equatorial, axial, and interstitial) can be assigned on the basis of hyperfine (Fermi contact) shifts and quadrupolar nutation behavior, aided by results from periodic DFT calculations. Distinct structural distortions among the axial sites, arising from the nonstoichiometric incorporation of interstitial oxygen, can be resolved by advanced magic angle turning and phase-adjusted sideband separation (MATPASS) NMR experiments. Finally, variable-temperature spectra reveal the onset of rapid interstitial oxide motion and exchange with axial sites at ∼130 °C, associated with the reported orthorhombic-to-tetragonal phase transition of La2NiO4+δ. From the variable-temperature spectra, we develop a model of oxide-ion dynamics on the spectral time scale that accounts for motional differences of all distinct oxygen sites. Though we treat La2NiO4+δ as a model system for a combined paramagnetic 17O NMR and DFT methodology, the approach presented herein should prove applicable to MIECs and other functionally important paramagnetic oxides.