Synthesis and electrochemical properties of pure LiFeSO4F in the triplite structure

Exploring new belousovite-related zinc and cadmium alkali sulfate halides: synthesis and structural variability

Fourteen new belousovite-related compounds, AZn(TO₄)X (A = K, Rb, Cs, Tl, NH₄; T = S, Se; X = Cl, Br, I) have been prepared via melt and evaporation techniques by reacting AX and ZnTO₄ either at high temperatures or in hot aqueous solutions. They adopt the layered structure of the belousovite archetype, and constitute a morphotropic series. The apophyllite-type layers in these structures undergo different corrugations, most pronounced in the case of CsZn(SO₄)I. In addition, during the study two species unrelated to belousovite, namely Na₄Zn(SO₄)₂Cl₂ and Cs₂Cd₃(SO₄)₄, were found with framework crystal structures having different topology and belonging to new structure types.

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.

Challenges in computational evaluation of redox and magnetic properties of Fe-based sulfate cathode materials of Li- and Na-ion batteries

Several Fe-based sulfates have been proposed recently as cathode materials characterized by a high average operating voltage (i.e. Li2Fe(SO4)2 and Na2Fe2(SO4)3) or low fabrication temperature (e.g. Na2Fe(SO4)2·2H2O)). In this work, we apply three methods to evaluate the redox potentials and magnetic properties of these materials: (1) local density functional theory (DFT) in Perdew-Burke-Ernzerhof parametrization; (2) rotationally invariant DFT  +  U; and (3) DFT  +  U with magnetic exchange, suggested herein. The U parameters used for DFT  +  U calculations have been evaluated by using a linear response method (this applies to DFT  +  U as well as DFT  +  U calculations with a magnetic exchange term). Moreover, we have performed adjustments of U and, for the case of magnetic exchange, J parameters, to find better agreement with experimental measurements of redox and magnetic properties. We find that a self-consistent DFT  +  U/linear response approach yields quite overestimated redox potentials as compared to experiment. On the other hand, we also show that DFT  +  U calculations are not capable of providing a reasonably accurate description of both redox and magnetic properties for the case of Li2Fe(SO4)2, even when adjusted U parameters are employed. As a solution, we demonstrate that a DFT  +  U methodology augmented by a magnetic exchange term potentially provides more precise values for both the redox potentials and the magnetic moments of the Fe ions in the studied materials. Thus our work shows that for a more accurate description of redox and magnetic properties, further extensions of the DFT  +  U method, such as inclusion of the contribution of magnetic exchange, should be considered.

Thermodynamic Properties of Polymorphs of Fluorosulfate Based Cathode Materials with Exchangeable Potassium Ions

FeSO₄ F-based frameworks have recently emerged as attractive candidates for alkali insertion electrodes. Mainly owing to their rich crystal chemistry, they offer a variety of new host structures with different electrochemical performances and physical properties. In this paper we report the thermodynamic stability of two such K-based "FeSO₄ F" host structures based on direct solution calorimetric measurements. KFeSO₄ F has been reported to crystallize in two different polymorphic modifications-monoclinic and orthorhombic. The obtained enthalpies of formation from binary components (KF plus FeSO₄ ) are negative for both polymorphs, indicating that they are thermodynamically stable at room temperature, which is very promising for the future exploration of sulfate based cathode materials. Our measurements show that the low-temperature monoclinic polymorph is enthalpically more stable than the orthorhombic phase by ≈10 kJ mol^-1 , which is consistent with the preferential formation of monoclinic KFeSO₄ F at low temperature. Furthermore, observed phase transformations and difficulties in the synthesis process can be explained based on the obtained calorimetric results. The KMnSO₄ F orthorhombic phase is more stable than both polymorphs of KFeSO₄ F.

An Exhaustive Symmetry Approach to Structure Determination: Phase Transitions in Bi2Sn2O7

The exploitable properties of many materials are intimately linked to symmetry-lowering structural phase transitions. We present an automated and exhaustive symmetry-mode method for systematically exploring and solving such structures which will be widely applicable to a range of functional materials. We exemplify the method with an investigation of the Bi2Sn2O7 pyrochlore, which has been shown to undergo transitions from a parent γ cubic phase to β and α structures on cooling. The results include the first reliable structural model for β-Bi2Sn2O7 (orthorhombic Aba2, a = 7.571833(8), b = 21.41262(2), and c = 15.132459(14) Å) and a much simpler description of α-Bi2Sn2O7 (monoclinic Cc, a = 13.15493(6), b = 7.54118(4), and c = 15.07672(7) Å, β = 125.0120(3)°) than has been presented previously. We use the symmetry-mode basis to describe the phase transition in terms of coupled rotations of the Bi2O' anti-cristobalite framework, which allow Bi atoms to adopt low-symmetry coordination environments favored by lone-pair cations.

Improved electrochemical properties of tavorite LiFeSO4F by surface coating with hydrophilic poly-dopamine via a self-polymerization process

PDA coated Li_1−xFeSO₄F shows improved electrochemical properties due to the highly hydrophilic and elastic properties of poly-dopamine.

Unveiling the electrochemical mechanisms of Li2Fe(SO4)2 polymorphs by neutron diffraction and density functional theory calculations

The structure of electrochemically active Li_1.5Fe(SO₄)₂ with difference Fourier maps highlighting the two lithium positions, which are also represented in the unit cell.

Improved Cycle Stability and Rate Capability of Graphene Oxide Wrapped Tavorite LiFeSO₄F as Cathode Material for Lithium-Ion Batteries

Tavorite LiFeSO4F has been regarded as a promising alternative to LiFePO4 due to its high Li ionic conductivity. To overcome the low electronic conductivity of LiFeSO4F, we prepared a graphene oxide (GO)/LiFeSO4F composite material by the solvothermal method. The GO wraps on the surface of LiFeSO4F and links the adjacent particles, thus providing an effective network for electrons transport. As a result, the electronic conductivity of the material is improved from 8.16 × 10(-11) S cm(-1) to 1.65 × 10(-4) S cm(-1). In addition, the GO depresses the side reactions of the electrode and electrolyte, promotes the charge transfer reactions at the electrode/electrolyte interface, and facilitates the lithium diffusion in the electrode. The GO-wrapped LiFeSO4F exhibits much better electrochemical performance than the pristine material. It showed a discharge capacity of 113.2 mAh g(-1) at the 0.1 C rate with 99% capacity retention after 100 cycles. In addition, the material is able to deliver 85.1, 73.4, and 30.3 mAh g(-1) at high current rates of 1 C, 2 C, and 10 C, respectively.

Chemical and structural indicators for large redox potentials in Fe-based positive electrode materials

Li-ion batteries have enabled a revolution in the way portable consumer-electronics are powered and will play an important role as large-scale electrochemical storage applications like electric vehicles and grid-storage are developed. The ability to identify and design promising new positive insertion electrodes will be vital in continuing to push Li-ion technology to its fullest potential. Utilizing a combination of computational tools and structural analysis, we report new indicators which will facilitate the recognition of phases with the desired redox potential. Most importantly of these, we find there is a strong correlation between the presence of Li ions sitting in close-proximity to the redox center of polyanionic phases and the open circuit voltage in Fe-based cathodes. This common structural feature suggests that the bonding associated with Li may have a secondary inductive effect which increases the ionic character of Fe bonds beyond what is typically expected based purely on arguments of electronegativity associated with the polyanionic group. This correlation is supported by ab initio calculations which show the Bader charge increases (reflecting an increased ionicity) in a nearly linear fashion with the experimental cell potentials. These features are demonstrated to be consistent across a wide variety of compositions and structures and should help to facilitate the design of new, high-potential, and environmentally sustainable insertion electrodes.

A symmetry-mode description of rigid-body rotations in crystalline solids: a case study of Mg(H2O)6RbBr3

The application of rotational symmetry modes to quantitative rigid-body analysis is demonstrated for octahedral rotations in Mg(H2O)6RbBr3. Rigid-body rotations are treated as axial-vector order parameters and projected using group-theoretical methods. The high-temperature crystal structure of the Mg(H2O)6RbBr3double salt consists of a cubic perovskite-like corner-sharing network of RbBr6octahedra with isolated MgO6octahedra at the perovskiteAsites. A phase transition occurs at 411 K upon cooling, whereupon the MgO6octahedra experience a substantial rigid-body rotation, the RbBr6octahedra are translated but not rotated, and both types of octahedra become slightly distorted. The MgO6rotation has three orthogonal components associated with theX5−, Γ4+andX1−irreducible representations of the parent Pm{\overline 3}m space-group symmetry which, given the weakly first-order character of the transition, appear to be strongly coupled. Parametric and sequential refinements of the temperature-dependent structure were conducted using four model types: (1) traditional atomicxyzcoordinates for each atom, (2) traditional rigid-body parameters, (3) purely displacive symmetry modes and (4) rigid-body rotational symmetry modes. We demonstrate that rigid-body rotational symmetry modes are an especially effective parameter set for the Rietveld characterization of phase transitions involving polyhedral rotations.

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.

Understanding and promoting the rapid preparation of the triplite-phase of LiFeSO4F for use as a large-potential Fe cathode

The development of new electrode materials, which are composed of Earth-abundant elements and that can be made via eco-efficient processes, is becoming absolutely necessary for reasons of sustainable production. The 3.9 V triplite-phase of LiFeSO(4)F, compared to the 3.6 V tavorite-phase, could satisfy this requirement provided the currently complex synthetic pathway can be simplified. Here, we present our work aiming at better understanding the reaction mechanism that govern its formation as a way to optimize its preparation. We first demonstrate, using complementary X-ray diffraction and transmission electron microscopy studies, that triplite-LiFeSO(4)F can nucleate from tavorite-LiFeSO(4)F via a reconstructive process whose kinetics are significantly influenced by moisture and particle morphology. Perhaps the most spectacular finding is that it is possible to prepare electrochemically active triplite-LiFeSO(4)F from anhydrous precursors using either reactive spark plasma sintering (SPS) synthesis in a mere 20 min at 320 °C or room-temperature ball milling for 3 h. These new pathways appear to be strongly driven by the easy formation of a disordered phase with higher entropy, as both techniques trigger disorder via rapid annealing steps or defect creation. Although a huge number of phases adopts the tavorite structure-type, this new finding offers both a potential way to prepare new compositions in the triplite structure and a wealth of opportunities for the synthesis of new materials which could benefit many domains beyond energy storage.