Probing Cation and Vacancy Ordering in the Dry and Hydrated Yttrium-Substituted BaSnO₃ Perovskite by NMR Spectroscopy and First Principles Calculations: Implications for Proton Mobility

Hydrated BaSn_1–<i>x</i>Y_<i>x</i>O_{3–<i>x</i>/2} is a protonic conductor that, unlike many other related perovskites, shows high conductivity even at high substitution levels. A joint multinuclear NMR spectroscopy and density functional theory (total energy and GIPAW NMR calculations) investigation of BaSn_1–<i>x</i>Y_<i>x</i>O_{3–<i>x</i>/2} (0.10 ≤ <i>x</i> ≤ 0.50) was performed to investigate cation ordering and the location of the oxygen vacancies in the dry material. The DFT energetics show that Y doping on the Sn site is favored over doping on the Ba site. The ¹¹⁹Sn chemical shifts are sensitive to the number of neighboring Sn and Y cations, an experimental observation that is supported by the GIPAW calculations and that allows clustering to be monitored: Y substitution on the Sn sublattice is close to random up to <i>x</i> = 0.20, while at higher substitution levels, Y–O–Y linkages are avoided, leading, at <i>x</i> = 0.50, to strict Y–O–Sn alternation of B-site cations. These results are confirmed by the absence of a “Y–O–Y” ¹⁷O resonance and supported by the ¹⁷O NMR shift calculations. Although resonances due to six-coordinate Y cations were observed by ⁸⁹Y NMR, the agreement between the experimental and calculated shifts was poor. Five-coordinate Sn and Y sites (i.e., sites next to the vacancy) were observed by ¹¹⁹Sn and ⁸⁹Y NMR, respectively, these sites disappearing on hydration. More five-coordinated Sn than five-coordinated Y sites are seen, even at <i>x</i> = 0.50, which is ascribed to the presence of residual Sn–O–Sn defects in the cation-ordered material and their ability to accommodate O vacancies. High-temperature ¹¹⁹Sn NMR reveals that the O ions are mobile above 400 °C, oxygen mobility being required to hydrate these materials. The high protonic mobility, even in the high Y-content materials, is ascribed to the Y–O–Sn cation ordering, which prevents proton trapping on the more basic Y–O–Y sites

Mapping Structural Changes in Electrode Materials: Application of the Hybrid Eigenvector-Following Density Functional Theory (DFT) Method to Layered Li_0.5MnO₂

The migration mechanism associated with the initial layered-to-spinel transformation of partially delithiated layered LiMnO₂ was studied using hybrid eigenvector-following coupled with density functional theory. The initial part of the transformation mechanism of Li_0.5MnO₂ involves the migration of Li into both octahedral and tetrahedral local minima within the layered structure. The next stage of the transformation process involves the migration of Mn and was found to occur through several local minima, including an intermediate square pyramidal MnO₅ configuration and an independent Mn³⁺ to Mn²⁺ charge-transfer process. The migration pathways were found to be significantly affected by the size of the supercell used and the inclusion of a Hubbard U parameter in the DFT functional. The transition state searching methodology described should be useful for studying the structural rearrangements that can occur in electrode materials during battery cycling, and more generally, ionic and electronic transport phenomena in a wide range of energy materials

Density Functional Theory-Based Bond Pathway Decompositions of Hyperfine Shifts: Equipping Solid-State NMR to Characterize Atomic Environments in Paramagnetic Materials

Solid-state nuclear magnetic resonance (NMR) of paramagnetic samples has the potential to provide a detailed insight into the environments and processes occurring in a wide range of technologically-relevant phases, but the acquisition and interpretation of spectra is typically not straightforward. Structural complexity and/or the occurrence of charge or orbital ordering further compound such difficulties. In response to such challenges, the present article outlines how the total Fermi contact (FC) shifts of NMR observed centers (OCs) may be decomposed into sets of pairwise metal–OC bond pathway contributions via solid-state hybrid density functional theory calculations. A generally applicable “spin flipping” approach is outlined wherein bond pathway contributions are obtained by the reversal of spin moments at selected metal sites. The applications of such pathway contributions in interpreting the NMR spectra of structurally and electronically complex phases are demonstrated in a range of paramagnetic Li-ion battery positive electrodes comprising layered LiNiO₂, LiNi_0.125Co_0.875O₂, and LiCr_0.125Co_0.875O₂ oxides; and olivine-type LiMPO₄ and MPO₄ (M = Mn, Fe, and Co) phosphates. The FC NMR shifts of all ^6/7Li and ³¹P sites are decomposed, providing unambiguous NMR-based proof of the existence of local Ni³⁺-centered Jahn–Teller distortions in LiNiO₂ and LiNi_0.125Co_0.875O₂, and showing that the presence of M²⁺/M³⁺ solid solutions and/or M/M′ isovalent transition metal (TM) mixtures in the olivine-type electrodes should lead to broad and potentially interpretable NMR spectra. Clear evidence for the presence of a dynamic Jahn–Teller distortion is obtained for LiNi_<i>x</i>Co_1–<i>x</i>O₂. The results emphasize the utility of solid-state NMR in application to TM-containing battery materials and to paramagnetic samples in general

Insights into the Nature and Evolution upon Electrochemical Cycling of Planar Defects in the β‑NaMnO₂ Na-Ion Battery Cathode: An NMR and First-Principles Density Functional Theory Approach

β-NaMnO₂ is a high-capacity Na-ion battery cathode, delivering ca. 190 mAh/g of reversible capacity when cycled at a rate of C/20. Yet, only 70% of the initial reversible capacity is retained after 100 cycles. We carry out a combined solid-state ²³Na NMR and first-principles DFT study of the evolution of the structure of β-NaMnO₂ upon electrochemical cycling. The as-synthesized structure contains planar defects identified as twin planes between nanodomains of the α and β forms of NaMnO₂. GGA+U calculations reveal that the formation energies of the two polymorphs are within 5 meV per formula unit, and a phase mixture is likely in any NaMnO₂ sample at room temperature. ²³Na NMR indicates that 65.5% of Na is in β-NaMnO₂ domains, 2.5% is in α-NaMnO₂ domains, and 32% is close to a twin boundary in the as-synthesized material. A two-phase reaction at the beginning of charge and at the end of discharge is observed by NMR, consistent with the constant voltage plateau at 2.6–2.7 V in the electrochemical profile. GGA+U computations of Na deintercalation potentials reveal that Na extraction occurs first in α-like domains, then in β-like domains, and finally close to twin boundaries. ²³Na NMR indicates that the proportion of Na in α-NaMnO₂-type sites increases to 11% after five cycles, suggesting that structural rearrangements occur, leading to twin boundaries separating larger α-NaMnO₂ domains from the major β-NaMnO₂ phase

Characterizing Oxygen Local Environments in Paramagnetic Battery Materials via ¹⁷O NMR and DFT Calculations

Experimental techniques that probe the local environment around O in paramagnetic Li-ion cathode materials are essential in order to understand the complex phase transformations and O redox processes that can occur during electrochemical delithiation. While Li NMR is a well-established technique for studying the local environment of Li ions in paramagnetic battery materials, the use of ¹⁷O NMR in the same materials has not yet been reported. In this work, we present a combined ¹⁷O NMR and hybrid density functional theory study of the local O environments in Li₂MnO₃, a model compound for layered Li-ion batteries. After a simple ¹⁷O enrichment procedure, we observed five resonances with large ¹⁷O shifts ascribed to the Fermi contact interaction with directly bonded Mn⁴⁺ ions. The five peaks were separated into two groups with shifts at 1600 to 1950 ppm and 2100 to 2450 ppm, which, with the aid of first-principles calculations, were assigned to the ¹⁷O shifts of environments similar to the 4i and 8j sites in pristine Li₂MnO₃, respectively. The multiple O environments in each region were ascribed to the presence of stacking faults within the Li₂MnO₃ structure. From the ratio of the intensities of the different ¹⁷O environments, the percentage of stacking faults was found to be ca. 10%. The methodology for studying ¹⁷O shifts in paramagnetic solids described in this work will be useful for studying the local environments of O in a range of technologically interesting transition metal oxides

New Insights into the Crystal and Electronic Structures of Li_1+<i>x</i>V_1–<i>x</i>O₂ from Solid State NMR, Pair Distribution Function Analyses, and First Principles Calculations

Pair distribution function (PDF) analyses of synchrotron data obtained for the anode materials Li_1+<i>x</i>V_1–<i>x</i>O₂ (0 ≤ <i>x</i> ≤ 0.1) have been performed to characterize the short to medium range structural ordering. The data show clear evidence for the magnetically-induced distortion of the V sublattice to form trimers, the distortion persisting at even the highest excess Li content considered of <i>x</i> = 0.1. At least three distinct local environments were observed for the stoichiometric material LiVO₂ in ⁶Li nuclear magnetic resonance (NMR) spectroscopy, the environments becoming progressively more disordered as the Li content increases. A two-dimensional Li–Li correlation NMR experiment (POST-C7) was used to identify the resonances corresponding to Li within the same layers. NMR spectra were acquired as a function of the state of charge, a distinct environment for Li in Li₂VO₂ being observed. The results suggest that disorder within the Li layers (in addition to the presence of Li within the V layers as proposed by Armstrong et al. <i>Nat. Mater.</i> <b>2011</b>, <i>10</i>, 223–229) may aid the insertion of Li into the Li_1+<i>x</i>V_1–<i>x</i>O₂ phase. The previously little-studied Li₂VO₂ phase was also investigated by hybrid density functional theory (DFT) calculations, providing insights into magnetic interactions, spin–lattice coupling, and Li hyperfine parameters

Probing Oxide-Ion Mobility in the Mixed Ionic–Electronic Conductor La₂NiO_4+δ by Solid-State ¹⁷O MAS NMR Spectroscopy

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>I</i> = 5/2) solid-state NMR spectra of the mixed-conducting solid oxide fuel cell (SOFC) cathode material La₂NiO_4+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 La₂NiO_4+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 La₂NiO_4+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

²H and ²⁷Al Solid-State NMR Study of the Local Environments in Al-Doped 2‑Line Ferrihydrite, Goethite, and Lepidocrocite

Although substitution of aluminum into iron oxides and oxyhydroxides has been extensively studied, it is difficult to obtain accurate incorporation levels. Assessing the distribution of dopants within these materials has proven especially challenging because bulk analytical techniques cannot typically determine whether dopants are substituted directly into the bulk iron oxide or oxyhydroxide phase or if they form separate, minor phase impurities. These differences have important implications for the chemistry of these iron-containing materials, which are ubiquitous in the environment. In this work, ²⁷Al and ²H NMR experiments are performed on series of Al-substituted goethite, lepidocrocite, and 2-line ferrihydrite in order to develop an NMR method to track Al substitution. The extent of Al substitution into the structural frameworks of each compound is quantified by comparing quantitative ²⁷Al MAS NMR results with those from elemental analysis. Magnetic measurements are performed for the goethite series to compare with NMR measurements. Static ²⁷Al spin–echo mapping experiments are used to probe the local environments around the Al substituents, providing clear evidence that they are incorporated into the bulk iron phases. Predictions of the ²H and ²⁷Al NMR hyperfine contact shifts in Al-doped goethite and lepidocrocite, obtained from a combined first-principles and empirical magnetic scaling approach, give further insight into the distribution of the dopants within these phases

Identifying the Structure of the Intermediate, Li_2/3CoPO₄, Formed during Electrochemical Cycling of LiCoPO₄

In situ synchrotron diffraction measurements and subsequent Rietveld refinements are used to show that the high energy density cathode material LiCoPO₄ (space group <i>Pnma</i>) undergoes two distinct two-phase reactions upon charge and discharge, both occurring via an intermediate Li_2/3(Co²⁺)_2/3(Co³⁺)_1/3PO₄ phase. Two resonances are observed for Li_2/3CoPO₄ with intensity ratios of 2:1 and 1:1 in the ³¹P and ⁷Li NMR spectra, respectively. An ordering of Co²⁺/Co³⁺ oxidation states is proposed within a (<i>a</i> × 3<i>b</i> × <i>c</i>) supercell, and Li⁺/vacancy ordering is investigated using experimental NMR data in combination with first-principles solid-state DFT calculations. In the lowest energy configuration, both the Co³⁺ ions and Li vacancies are found to order along the <i>b</i>-axis. Two other low energy Li⁺/vacancy ordering schemes are found only 5 meV per formula unit higher in energy. All three configurations lie below the LiCoPO₄–CoPO₄ convex hull and they may be readily interconverted by Li⁺ hops along the <i>b</i>-direction