Materials’ Methods: NMR in Battery Research

Improving electrochemical energy storage is one of the major issues of our time. The search for new battery materials together with the drive to improve performance and lower cost of existing and new batteries is not without its challenges. Success in these matters is undoubtedly based on first understanding the underlying chemistries of the materials and the relations between the components involved. A combined application of experimental and theoretical techniques has proven to be a powerful strategy to gain insights into many of the questions that arise from the “how do batteries work and why do they fail” challenge. In this Review, we highlight the application of solid-state nuclear magnetic resonance (NMR) spectroscopy in battery research: a technique that can be extremely powerful in characterizing local structures in battery materials, even in highly disordered systems. An introduction on electrochemical energy storage illustrates the research aims and prospective approaches to reach these. We particularly address “NMR in battery research” by giving a brief introduction to electrochemical techniques and applications as well as background information on both <i>in</i> and <i>ex situ</i> solid-state NMR spectroscopy. We will try to answer the question “Is NMR suitable and how can it help me to solve my problem?” by shortly reviewing some of our recent research on electrodes, microstructure formation, electrolytes and interfaces, in which the application of NMR was helpful. Finally, we share hands-on experience directly from the lab bench to answer the fundamental question “Where and how should I start?” to help guide a researcher’s way through the manifold possible approaches

High-Rate Intercalation without Nanostructuring in Metastable Nb₂O₅ Bronze Phases

Nanostructuring and nanosizing have been widely employed to increase the rate capability in a variety of energy storage materials. While nanoprocessing is required for many materials, we show here that both the capacity and rate performance of low-temperature bronze-phase TT- and T-polymorphs of Nb₂O₅ are inherent properties of the bulk crystal structure. Their unique “room-and-pillar” NbO₆/NbO₇ framework structure provides a stable host for lithium intercalation; bond valence sum mapping exposes the degenerate diffusion pathways in the sites (rooms) surrounding the oxygen pillars of this complex structure. Electrochemical analysis of thick films of micrometer-sized, insulating niobia particles indicates that the capacity of the T-phase, measured over a fixed potential window, is limited only by the Ohmic drop up to at least 60C (12.1 A·g^–1), while the higher temperature (Wadsley–Roth, crystallographic shear structure) H-phase shows high intercalation capacity (>200 mA·h·g^–1) but only at moderate rates. High-resolution ^6/7Li solid-state nuclear magnetic resonance (NMR) spectroscopy of T-Nb₂O₅ revealed two distinct spin reservoirs, a small initial rigid population and a majority-component mobile distribution of lithium. Variable-temperature NMR showed lithium dynamics for the majority lithium characterized by very low activation energies of 58(2)–98(1) meV. The fast rate, high density, good gravimetric capacity, excellent capacity retention, and safety features of bulk, insulating Nb₂O₅ synthesized in a single step at relatively low temperatures suggest that this material not only is structurally and electronically exceptional but merits consideration for a range of further applications. In addition, the realization of high rate performance without nanostructuring in a complex insulating oxide expands the field for battery material exploration beyond conventional strategies and structural motifs

Selected Overtone Mobility Spectrometry

A new means of acquiring overtone mobility spectrometry (OMS) data sets that allows distributions of ions for a prescribed overtone number is described. In this approach, the drift fields applied to specific OMS drift regions are varied to make it possible to select different ions from a specific overtone that is resonant over a range of applied frequencies. This is accomplished by applying different fields for fixed ratios of time while scanning the applied frequency. The ability to eliminate peaks from all but a single overtone region overcomes a significant limitation associated with OMS analysis of unknowns, especially in mixtures. Specifically, <i>a priori</i> knowledge via selection of the overtone used to separate ions makes it possible to directly determine ion mobilities for unknown species and collision cross sections (assuming that the ion charge state is known). We refer to this selection method of operation as selected overtone mobility spectrometry (SOMS). A simple theoretical description of the SOMS approach is provided. Simulations are carried out and discussed in order to illustrate the advantages and disadvantages of SOMS compared with traditional OMS. Finally, the SOMS method (and its distinction from OMS) is demonstrated experimentally by examining a mixture of peptides generated by enzymatic digestion of the equine cytochrome <i>c</i> with trypsin

Structural Evolution and Atom Clustering in β‑SiAlON: β‑Si_6–<i>z</i>Al_<i>z</i>O_<i>z</i>N_8–<i>z</i>

SiAlON ceramics, solid solutions based on the Si₃N₄ structure, are important, lightweight structural materials with intrinsically high strength, high hardness, and high thermal and chemical stability. Described by the chemical formula β-Si_6–<i>z</i>Al_<i>z</i>O_<i>z</i>N_8–<i>z</i>, from a compositional viewpoint, these materials can be regarded as solid solutions between Si₃N₄ and Al₃O₃N. A key aspect of the structural evolution with increasing Al and O (<i>z</i> in the formula) is to understand how these elements are distributed on the β-Si₃N₄ framework. The average and local structural evolution of highly phase-pure samples of β-Si_6–<i>z</i>Al_<i>z</i>O_<i>z</i>N_8–<i>z</i> with <i>z</i> = 0.050, 0.075, and 0.125 are studied here, using a combination of X-ray diffraction, NMR studies, and density functional theory calculations. Synchrotron X-ray diffraction establishes sample purity and indicates subtle changes in the average structure with increasing Al content in these compounds. Solid-state magic-angle-spinning ²⁷Al NMR experiments, coupled with detailed ab initio calculations of NMR spectra of Al in different AlO_<i>q</i>N_4–<i>q</i> tetrahedra (0 ≤ <i>q</i> ≤ 4), reveal a tendency of Al and O to cluster in these materials. Independently, the calculations suggest an energetic preference for Al–O bond formation, instead of a random distribution, in the β-SiAlON system

Electrochemistry in the Large Tunnels of Lithium Postspinel Compounds

Lithium spinels (LiMM’O4) are an important class of mixed-cation materials that have found uses in batteries, catalysis, and optics. Postspinels are a series of related framework structures with the AMM’O4 host composition that are formed with larger A-site cations, typically under high pressure. Postspinels have one-dimensional tunnel structures with pores that are larger than those in spinel and triangular in cross-section, but they are relatively unexplored as intercalation electrodes. While lithium postspinels have been previously found to be thermodynamically stable only at high pressures, we have identified a synthetic pathway that produces the lithium-containing materials at ambient pressure using an ion-exchange process from the corresponding sodium postspinels. Here, we report the synthesis and a survey of the electrochemical properties of 10 new lithium CaFe2O4-type postspinel compounds where M = Mn3+, V3+, Cr3+, Rh3+, Fe2+, Mg2+, Co2+ and M’ = Ti4+ and/or Sn4+. Although complete delithiation is not achieved during electrochemical cycling, many of the lithium postspinels have substantial charge storage capacity in Li battery cells owing to the ability of the large framework tunnels to accommodate more than one lithium ion per formula unit. Multiple redox couples are accessed for LiMnSnO4, Li0.96Mn0.96Sn1.04–xTixO4, Li0.96V0.96Ti1.04O4, Li0.96Cr0.96Ti1.04O4, and LiFe0.5Ti1.5O4. Compositions with moderate or poor lithium cyclability are also discussed for comparison. Redox mechanisms and trends are identified by comparing this new redox-active framework to related spinels, ramsdellites, and ‘Na0.44MnO2’ structures, and from density functional theory (DFT) electronic structures. Operando diffraction shows complex structural responses to lithium insertion and extraction in this postspinel framework. A DFT framework was proposed to identify promising lithium postspinel phases that could be accessed metastably under ambient pressure conditions and to assess their stability to lithium insertion and extraction. This work suggests that CaFe2O4-type hosts are a promising new class of lithium-ion energy storage materials