Cation Vacancies Enable Anion Redox in Li Cathodes.

Conventional Li-ion battery intercalation cathodes leverage charge compensation that is formally associated with redox on the transition metal. Employing the anions in the charge compensation mechanism, so-called anion redox, can yield higher capacities beyond the traditional limitations of intercalation chemistry. Here, we aim to understand the structural considerations that enable anion oxidation and focus on processes that result in structural changes, such as the formation of persulfide bonds. Using a Li-rich metal sulfide as a model system, we present both first-principles simulations and experimental data that show that cation vacancies are required for anion oxidation. First-principles simulations show that the oxidation of sulfide to persulfide only occurs when a neighboring vacancy is present. To experimentally probe the role of vacancies in anion redox processes, we introduce vacancies into the Li2TiS3 phase while maintaining a high valency of Ti. When the cation sublattice is fully occupied and no vacancies can be formed through transition metal oxidation, the material is electrochemically inert. Upon introduction of vacancies, the material can support high degrees of anion redox, even in the absence of transition metal oxidation. The model system offers fundamental insights to deepen our understanding of structure-property relationships that govern reversible anion redox in sulfides and demonstrates that cation vacancies are required for anion oxidation, in which persulfides are formed.

Structural Diversity in 2-(2-Aminoethyl)pyridine-Based Lead Iodide Hybrids.

While 2D metal-organic hybrids have emerged as promising solar absorbers due to their improved moisture stability, their inferior transport properties limit their potential translation into devices. We report a new hybrid containing 2-(2-ammonioethyl)pyridine [(2-AEP)+], forming a 2D hybrid with the composition (2-AEP)2PbI4. The organic bilayer comprises of (2-AEP)+, which is arranged in a face-to-face stacking that promotes π-π interactions between neighboring pyridyl rings. We also demonstrate the structural diversity of 2-(2-aminoethyl)pyridine-based lead iodide hybrids in solution-processed films. This report highlights the importance of solution-processing conditions in trying to obtain single-phase films of hybrids containing dibasic organic species.

Temperature dependence of radiative and non-radiative decay in the luminescence of one-dimensional pyridinium lead halide hybrids.

The photoluminescence properties of organic-inorganic pyridinium lead bromide [(pyH)PbBr3] and iodide [(pyH)PbI3] compounds were investigated as a function of temperature. The inorganic substructure consists of face-sharing chains of PbX6 octahedra. Diffuse reflectance spectra of the compounds show low energy absorption features consistent with charge transfer transitions from the PbX3 chains to the pyridinium cations. Both compounds display extremely weak luminescence at room temperature that becomes strongly enhanced upon cooling to 77 K. Broad, featureless low energy emission (λem > 600 nm) in both compounds have large Stokes shifts [1.1 eV for (pyH)PbBr3 and 0.46 eV for (pyH)PbI3] and are assigned to transitions from self-trapped excitons on the inorganic chains whereas emission at higher energy in (pyH)PbBr3 (λem = 450 nm) is assigned to luminescence from a free exciton state. Analysis of data from temperature-dependent luminescence intensity measurements gives activation energies (Ea) for non-radiative decay of the self-trapped excitons in (pyH)PbBr3 and (pyH)PbI3, (Ea = 0.077 eV and 0.103 eV, respectively) and for the free exciton in (pyH)PbBr3 (Ea = 0.010 eV). Analysis of temperature dependent luminescence lifetime data indicates another non-radiative decay process in (pyH)PbI3 at higher temperatures (Ea = 0.17 eV). A large increase in the luminescence lifetime of (pyH)PbI3 below 80 K is consistent with thermalization between triplet sublevels. Analysis of the luminescence power dependence for (pyH)PbI3 shows superlinear response suggestive of quenching by static traps.

Reversible Intercalation of Li Ions in an Earth-Abundant Phyllosilicate Clay.

The phyllosilicate family of clays is an intriguing collection of materials that make ideal models for studying the intercalation of alkali ions due to their layered topology and broadly tunable composition space. In this spirit, we present a hydrothermal method to prepare a layered iron phyllosilicate clay, Fe2Si4O10(OH)2, and an evaluation of its electrochemical performance for the (de)insertion of Li ions. Through careful structural refinement, we determined that this iron clay contains a 2:1 stacking sequence, which is directly analogous to the widely studied mineral montmorillonite, with the crystallites adopting a platelike morphology. Cyclic voltammetry and galvanostatic cycling reveal reversible insertion of lithium into the interstitial layers via a solid solution mechanism. Comparison of ion (de)intercalation with reports on other clay systems like muscovite, KFe2.75Si3.25O10(OH)2, which features a rigidly bound interlayer cation, demonstrates that controlling the net charge on the layers with phyllosilicate minerals is a route to enabling reversible cationic intercalation within the structure.

Magnetic-Field-Induced Dielectric Anomalies in Cobalt-Containing Garnets.

Here we present a comparative study of the magnetic and crystal chemical properties of two Co2+ containing garnets. CaY2Co2Ge3O12 (which has been reported previously) and NaCa2Co2V3O12 both exhibit the onset of antiferromagnetic order around 6 K as well as field-induced transitions around 7 and 10 T, respectively, that manifest as anomalies in the dielectric properties of the material. We perform detailed crystal-chemistry analyses and complementary density functional theory calculations to show that very minor changes in the local environment of the Co ions explain the differences in the two magnetic structures and their respective properties.

Canting of the Magnetic Moments on the Octahedral Site of an Iron Oxide Garnet in Response to Diamagnetic Cation Substitution.

Using neutron powder diffraction and magnetic susceptibility measurements, we report on the preparation and characterization of the temperature- and field-dependent properties of CaY2ZrFe4O12, a composition closely related to the high-temperature ferrimagnet Y3Fe5O12. By diluting the concentration of paramagnetic ions on the octahedral sublattice of the garnet structure, we find temperature-dependent canting of the magnetic moments. This reflects the importance of the octahedral sublattice in mediating the magnetic interactions between the tetrahedral sites and offers insight into a large number of competing magnetic interactions in the garnet structure.

Influence of the Cubic Sublattice on Magnetic Coupling between the Tetrahedral Sites of Garnet.

We present a study on the nuclear and magnetic structures of two iron-based garnets with magnetic cations isolated on tetrahedral sites. Ca2YZr2Fe3O12 and Ca2LaZr2Fe3O12 offer an interesting comparison for examining the effect of increasing cation size within the diamagnetic backbone of the garnet crystal structure, and how such changes affect the magnetic order. Despite both systems exhibiting well-pronounced magnetic transitions at low temperatures, we also find evidence for diffuse magnetic scattering due to a competition between the nearest-neighbor, next nearest-neighbor, and so on, within the tetrahedral sites. This competition results in a complex noncollinear magnetic structure on the tetrahedral sublattice creating a mixture of ferro- and antiferromagnetic interactions above the long-range ordering temperature near 20 K and suggests that the cubic site of the garnet plays a significant role in mediating the superexchange interactions between tetrahedral cations.

Solution Deposition of a Bournonite CuPbSbS3 Semiconductor Thin Film from the Dissolution of Bulk Materials with a Thiol-Amine Solvent Mixture.

There is considerable interest in the exploration of new solar absorbers that are environmentally stable, absorb through the visible, and possess a polar crystal structure. Bournonite CuPbSbS3 is a naturally occurring sulfosalt mineral that crystallizes in the noncentrosymmetric Pmn21 space group and possesses an optimal band gap for single junction solar cells; however, the synthetic literature on this quaternary semiconductor is sparse and it has yet to be deposited and studied as a thin film. Here we describe the ability of a binary thiol-amine solvent mixture to dissolve the bulk bournonite mineral as well as inexpensive bulk CuO, PbO, and Sb2S3 precursors at room temperature and ambient pressure to generate an ink. The synthetic compound ink derived from the dissolution of the bulk binary precursors in the right stoichiometric ratios yields phase-pure thin films of CuPbSbS3 upon solution deposition and annealing. The resulting semiconductor thin films possess a direct optical band gap of 1.24 eV, an absorption coefficient ∼105 cm-1 through the visible, mobilities of 0.01-2.4 cm2 (V·s)-1, and carrier concentrations of 1018 - 1020 cm-3. These favorable optoelectronic properties suggest CuPbSbS3 thin films are excellent candidates for solar absorbers.

Multielectron, Cation and Anion Redox in Lithium-Rich Iron Sulfide Cathodes.

Conventional Li-ion cathodes store charge by reversible intercalation of Li coupled to metal cation redox. There has been increasing interest in new materials capable of accommodating more than one Li per transition-metal center, thereby yielding higher charge storage capacities. We demonstrate here that the lithium-rich layered iron sulfide Li2FeS2 as well as a new structural analogue, LiNaFeS2, reversibly store ≥1.5 electrons per formula unit and support extended cycling. Ex situ and operando structural and spectroscopic data indicate that delithiation results in reversible oxidation of Fe2+ concurrent with an increase in the covalency of the Fe-S interactions, followed by reversible anion redox: 2 S2-/(S2)2-. S K-edge spectroscopy unequivocally proves the contribution of the anions to the redox processes. The structural response to the oxidation processes is found to be different in Li2FeS2 in contrast to that in LiNaFeS2, which we suggest is the cause for capacity fade in the early cycles of LiNaFeS2. The materials presented here have the added benefit of avoiding resource-sensitive transition metals such as Co and Ni. In contrast to Li-rich oxide materials that have been the subject of so much recent study and that suffer capacity fade and electrolyte degradation issues, the materials presented here operate within the stable potential window of the electrolyte, permitting a clearer understanding of the underlying processes.

Influence of Dimethyl Sulfoxide on the Structural Topology during Crystallization of PbI2.

Hybrid metal-organic halides are an exciting class of materials that offer the opportunity to examine how fundamental aspects of chemical bonding can influence the structural topology. In this work, we describe how solvent adducts of lead halides can influence the crystallization and subsequent annealing of these hybrid phases. While the size and shape of organic molecules are known to govern the final topology of the hybrid, we show that the affinity of solvent molecules for Pb ions may also play a previously underappreciated role.

Synthesis, Crystal Structure, and Cooperative 3d-5d Magnetism in Rock Salt Type Li4NiOsO6 and Li3Ni2OsO6.

Two new transition metal oxides with the nominal chemical compositions of Li4NiOsO6 and Li3Ni2OsO6 were successfully synthesized. Both compounds crystallize in an ordered rock salt structure type in the monoclinic C2/m space group. The crystal structures were determined using both synchrotron X-ray and time-of-flight neutron, powder diffraction data. In both phases, Ni2+ ions are present while oxidation states of osmium are +6 and +5 in Li4NiOsO6 and Li3Ni2OsO6, respectively. Ni2+ ions in the hypothetical fully ordered phase form a honeycomb arrangement in the ab crystallographic plane and these hexagons are centered by osmium ions. The magnetic layers are separated along the c axis by the octahedra, which are centered by Li+ (or Li+/Ni2+, depending on the chemical compositions). Crystal structure refinements reveal that there is some degree of mixed occupancy in cationic positions. Temperature dependent magnetic susceptibility data for both phases show ferrimagnetic transitions with predominant antiferromagnetic (AFM) interactions among 3d electrons of nickel and 5d electrons of osmium. Iso-thermal magnetization loops as a function of the applied magnetic field below the transition temperatures confirm the ferrimagnetic nature in magnetic transitions. Temperature dependent heat capacity data, however, did not exhibit any anomaly in either phase, indicating the absence of long-range magnetic ordering. The lack of long-range order for both Os5+ and Os6+-based compounds was also confirmed by low temperature neutron diffraction data down to 10 K. Temperature dependent AC magnetic susceptibility data in various frequencies for both samples indicate that Li4NiOsO6 exhibits spin-glass-like behavior, while the transition temperature for Li3Ni2OsO6 is nearly frequency independent.

Room Temperature Metallic Conductivity in a Metal-Organic Framework Induced by Oxidation.

Metal-organic frameworks (MOFs) containing redox active linkers have led to hybrid compounds exhibiting high electrical conductivity, which enables their use in applications in electronics and electrocatalysis. While many computational studies predict two-dimensional (2D) MOFs to be metallic, the majority of experiments show decreasing conductivity on cooling, indicative of a gap in the electronic band structure. To date, only a handful of MOFs have been reported that exhibit increased electrical conductivity upon cooling indicative of a metallic character, which highlights the need for a better understanding of the origin of the conductivity. A 2D MOF containing iron bis(dithiolene) motifs was recently reported to exhibit semiconducting behavior with record carrier mobility. Herein, we report that high crystallinity and the elimination of guest species results in an iron 2,3,6,7,10,11-tripheylenehexathiolate (THT) MOF, FeTHT, exhibiting a complex transition from semiconducting to metallic upon cooling, similar to what was shown for the analogous CoTHT. Remarkably, exposing the FeTHT to air significantly influences the semiconducting-to-metallic transition temperature (100 to 300 K) and ultimately results in a material showing metallic-like character at, and above, room temperature. This study indicates these materials can tolerate a substantial degree of doping that ultimately results in charge delocalization and metallic-like conductivity, an important step toward enabling their use in chemiresistive sensing and optoelectronics.

Metallic Conductivity in a Two-Dimensional Cobalt Dithiolene Metal-Organic Framework.

Two-dimensional (2D) metal-organic frameworks (MOFs) have received a great deal of attention due to their relatively high charge carrier mobility and low resistivity. Here we report on the temperature-dependent charge transport properties of a 2D cobalt 2,3,6,7,10,11-triphenylenehexathiolate framework. Variable temperature resistivity studies reveal a transition from a semiconducting to a metallic phase with decreasing temperature, which is unprecedented in MOFs. We find this transition to be highly dependent on the film thickness and the amount of solvent trapped in the pores, with density functional theory calculations of the electronic-structure supporting the complex metallic conductivity of the material. These results identify the first experimentally observed MOF that exhibits band-like metallic conductivity.

Reversible Capture and Release of Cl2 and Br2 with a Redox-Active Metal-Organic Framework.

Extreme toxicity, corrosiveness, and volatility pose serious challenges for the safe storage and transportation of elemental chlorine and bromine, which play critical roles in the chemical industry. Solid materials capable of forming stable nonvolatile compounds upon reaction with elemental halogens may partially mitigate these challenges by allowing safe halogen release on demand. Here we demonstrate that elemental halogens quantitatively oxidize coordinatively unsaturated Co(II) ions in a robust azolate metal-organic framework (MOF) to produce stable and safe-to-handle Co(III) materials featuring terminal Co(III)-halogen bonds. Thermal treatment of the oxidized MOF causes homolytic cleavage of the Co(III)-halogen bonds, reduction to Co(II), and concomitant release of elemental halogens. The reversible chemical storage and thermal release of elemental halogens occur with no significant losses of structural integrity, as the parent cobaltous MOF retains its crystallinity and porosity even after three oxidation/reduction cycles. These results highlight a material operating via redox mechanism that may find utility in the storage and capture of other noxious and corrosive gases.

Vibronic Structure in Room Temperature Photoluminescence of the Halide Perovskite Cs3Bi2Br9.

We report a study on the optical properties of the layered polymorph of vacancy-ordered triple perovskite Cs3Bi2Br9. The electronic structure, determined from density functional theory calculations, shows the top of the valence band and bottom of the conduction band minima are, unusually, dominated by Bi s and p states, respectively. This produces a sharp exciton peak in the absorption spectra with a binding energy that was approximated to be 940 meV, which is substantially stronger than values found in other halide perovskites and, instead, more closely reflects values seen in alkali halide crystals. This large binding energy is indicative of a strongly localized character and results in a highly structured emission at room temperature as the exciton couples to vibrations in the lattice.

Synthesis, Crystal Structure, and Magnetic Properties of the Highly Frustrated Orthorhombic Li4MgReO6.

In an effort to understand the structure-property relationship in magnetically frustrated systems, an orthorhombic analog of the S = 1/2 Re-based oxide Li4MgReO6 has been successfully synthesized and its physical properties were investigated. Li4MgReO6 had been previously synthesized in a monoclinic system in an ordered NaCl structure type. That system was shown to exhibit spin glass behavior below ∼12 K. The crystal structure of the latter phase was determined using powder X-ray diffraction data. A structural model was refined in the orthorhombic Fddd space group that resulted in cell dimensions of a = 5.84337 (7) Å, b = 8.33995 (9) Å, and c = 17.6237 (2) Å. The magnetic ions, Re6+ (S = 1/2), consist of various arrangements of interconnected triangles and trigonal prisms that offer potential for geometric magnetic frustration. Temperature dependent magnetic susceptibility reveals an AFM transition below ∼2 K along with a ZFC/FC divergence suggestive of spin freezing. The Curie-Weiss fitting parameters to the paramagnetic regime result in θ = -124 (1) K, which is indicative of predominant AFM interactions. A frustration index of ∼62 is in accordance with a highly frustrated magnetic ground state. Zero field (ZF) µSR data provides evidence for the onset of magnetic order below 4 K, along with the evidence for dynamical fluctuations up to 5 K. Moreover, longitudinal field (LF) µSR data reveals a complete decoupling in applied field at 2 K, which is indicative of static order in most or all of the volume fraction at ∼2 K, with partial ordered volumes coexisting with dynamical fluctuations up to 5 K. Estimates of the relative strengths of various magnetic exchange pathways at the level of spin-dimer analysis for this novel system are calculated and are compared to those of the previously reported values for the monoclinic analog.

Long-Range Antiferromagnetic Ordering in B-Site Ordered Double Perovskite Ca2ScOsO6.

A new Os-based B-site ordered double perovskite with the chemical composition of Ca2ScOsO6 was successfully synthesized. The crystal structure of the title compound was determined by employing the powder X-ray diffraction method and was found to crystallize in the monoclinic P21/n space group with the cell constants of a = 5.4716(1) Å, b = 5.6165(1) Å, c = 7.8168 (1) Å, and ß = 89.889 (2)°. The temperature-dependent magnetic susceptibility data suggest that this novel S = (3)/2 compound undergoes an antiferromagnetic transition at ∼ 69 K. Fitting the high-temperature susceptibility data (100-300 K) to Currie-Weisse behavior showed C = 1.734 emu·K/mol (µeff = 3.72 bohr magnetons) and θ = -341 K, which is indicative of dominant antiferromagnetic interactions. Temperature-dependent specific heat measurements exhibit a λ shape anomaly at 69 K, which is consistent with a long-range ordering of the spins. Because of a triangular arrangement of antiferromagnetically ordered magnetic ions, the system exhibits some degree of geometric magnetic frustration (GMF), but not strongly. Spin-dimer analysis, employing extended Hückel theory, reveals that a dominant exchange interaction exists (along the a crystallographic axis in perovskite layer), which violates the perfect condition for GMF.

Paracrystalline Disorder from Phosphate Ion Orientation and Substitution in Synthetic Bone Mineral.

Hydroxyapatite is an inorganic mineral closely resembling the mineral phase in bone. However, as a biological mineral, it is highly disordered, and its composition and atomistic structure remain poorly understood. Here, synchrotron X-ray total scattering and pair distribution function analysis methods provide insight into the nature of atomistic disorder in a synthetic bone mineral analogue, chemically substituted hydroxyapatite. By varying the effective hydrolysis rate and/or carbonate concentration during growth of the mineral, compounds with varied degrees of paracrystallinity are prepared. From advanced simulations constrained by the experimental pair distribution function and density functional theory, the paracrystalline disorder prevalent in these materials appears to result from accommodation of carbonate in the lattice through random displacement of the phosphate groups. Though many substitution modalities are likely to occur in concert, the most predominant substitution places carbonate into the mirror plane of an ideal phosphate site. Understanding the mineralogical imperfections of a biologically analogous hydroxyapatite is important not only to potential bone grafting applications but also to biological mineralization processes themselves.

Magnetic coupling in a hybrid Mn(ii) acetylene dicarboxylate.

The design of ligands that mediate through-bond long range super-exchange in metal-organic hybrid materials would expand chemical space beyond the commonly observed short range, low temperature magnetic ordering. Here we examine acetylene dicarboxylate as a potential ligand that could install long range magnetic ordering due to its spatially continuous frontier orbitals. Using a known Mn(ii)-containing coordination polymer we compute and measure the electronic structure and magnetic ordering. In this case, the latter is weak owing to the sub-optimal ligand coordination geometry, with a critical temperature of 2.5 K.

Design and preparation of materials for advanced electrochemical storage.

To meet the growing global demand for energy while preserving the environment, it is necessary to drastically reduce the world's dependence on non-renewable energy sources. At the core of this effort will be the ability to efficiently convert, store, transport and access energy in a variety of ways. Batteries for use in small consumer devices have saturated society; however, if they are ever to be useful in large-scale applications such as automotive transportation or grid-storage, they will require new materials with dramatically improved performance. Efforts must also focus on using Earth-abundant and nontoxic compounds so that whatever developments are made will not create new environmental problems. In this Account, we describe a general strategy for the design and development of new insertion electrode materials for Li(Na)-ion batteries that meet these requirements. We begin by reviewing the current state of the art of insertion electrodes and highlighting the intrinsic material properties of electrodes that must be re-engineered for extension to larger-scale applications. We then present a detailed discussion of the relevant criteria for the conceptual design and appropriate selection of new electrode chemical compositions. We describe how the open-circuit voltage of Li-ion batteries can be manipulated and optimized through structural and compositional tuning by exploiting differences in the electronegativity among possible electrode materials. We then discuss which modern synthetic techniques are most sustainable, allowing the creation of new materials via environmentally responsible reactions that minimize the use of energy and toxic solvents. Finally, we present a case study showing how we successfully employed these approaches to develop a large number of new, useful electrode materials within the recently discovered family of transition metal fluorosulfates. This family has attracted interest as a possible source of improved Li-ion batteries in larger scale applications and benefits from a relatively "green" synthesis.