Mixed oxides of titanium and niobium: the crystal structure of TiNb24O62(TiO2.12Nb2O5)

Chemical and Structural Factors Affecting the Stability of Wadsley-Roth Block Phases

Wadsley-Roth phases have emerged as highly promising anode materials for Li-ion batteries and are an important class of phases that can form as part of the oxide scales of refractory multiprinciple element alloys. An algorithmic approach is described to systematically enumerate two classes of Wadsley-Roth crystallographic shear structures. An analysis of algorithmically generated Wadsley-Roth phases reveals that a diverse set of oxide crystal structures belongs to the Wadsley-Roth family of phases. First-principles calculations enable the identification of crystallographic and chemical factors that affect Wadsley-Roth phase stability, pointing in particular to the importance of the number and nature of the edges shared by neighboring metal-oxygen octahedra. A systematic study of Wadsley-Roth phases in the Ti-Nb-O ternary system shows that the cations with the highest oxidation states segregate to octahedral sites that minimize the number of shared edges, while cations with the lowest oxidation state accumulate to edge-sharing octahedra at shear boundaries.

Rapid and Reversible Lithium Insertion in the Wadsley-Roth-Derived Phase NaNb13O33

The development of new high-performing battery materials is critical for meeting the energy storage requirements of portable electronics and electrified transportation applications. Owing to their exceptionally high rate capabilities, high volumetric capacities, and long cycle lives, Wadsley-Roth compounds are promising anode materials for fast-charging and high-power lithium-ion batteries. Here, we present a study of the Wadsley-Roth-derived NaNb13O33 phase and examine its structure and lithium insertion behavior. Structural insights from combined neutron and synchrotron diffraction as well as solid-state nuclear magnetic resonance (NMR) are presented. Solid-state NMR, in conjunction with neutron diffraction, reveals the presence of sodium ions in perovskite A-site-like block interior sites as well as square-planar block corner sites. Through combined experimental and computational studies, the high rate performance of this anode material is demonstrated and rationalized. A gravimetric capacity of 225 mA h g-1, indicating multielectron redox of Nb, is accessible at slow cycling rates. At a high rate, 100 mA h g-1 of capacity is accessible in 3 min for micrometer-scale particles. Bond-valence mapping suggests that this high-rate performance stems from fast multichannel lithium diffusion involving octahedral block interior sites. Differential capacity analysis is used to identify optimal cycling rates for long-term performance, and an 80% capacity retention is achieved over 600 cycles with 30 min charging and discharging intervals. These initial results place NaNb13O33 within the ranks of promising new high-rate lithium-ion battery anode materials that warrant further research.

Stable Multicomponent Multiphase All Active Material Lithium-Ion Battery Anodes

Due to their high energy density, lithium-ion batteries have been the state-of-the-art energy storage technology for many applications. Energy density can be further improved by engineering of the electrode architecture and microstructure, in addition to more common improvements via materials chemistry. All active material (AAM) electrodes consist of only the electroactive material that stores energy, and such electrodes have advantages to conventional composite processing with regards to improved mechanical stability at increased thicknesses and ion transport properties. However, the absence of binders and composite processing makes the electrode more vulnerable to electroactive materials with volume change upon cycling. Also, the electroactive material must have sufficient electronic conductivity to avoid large matrix electronic overpotentials during electrochemical cycling. TiNb₂O₇ (TNO) and MoO₂ (MO) are electroactive materials with potential advantages as AAM electrodes due to relatively high volumetric energy density. TNO has higher energy density, and MO has much higher electronic conductivity, and thus a multicomponent blend of these materials was evaluated as an AAM anode. Herein, blends of TNO and MO as AAM anodes were investigated, where this is the first use of a multicomponent AAM anode. Electrodes that had both TNO and MO had the highest volumetric energy density, rate capability, and cycle life relative to single component TNO and MO anodes. Thus, using multicomponent materials provides a route to improve AAM electrochemical systems.

Niobium pentoxide based materials for high rate rechargeable electrochemical energy storage

The demand for high rate energy storage systems is continuously increasing driven by portable electronics, hybrid/electric vehicles and the need for balancing the smart grid. Accordingly, Nb₂O₅ based materials have gained great attention because of their fast cation intercalation faradaic charge storage that endows them with high rate energy storage performance. In this review, we describe the crystalline features of the five main phases of Nb₂O₅ and analyze their specific electrochemical characteristics with an emphasis on the intrinsic ion intercalation pseudocapacitive behavior of T-Nb₂O₅. The charge storage mechanisms, electrochemical performance and state-of-the-art characterization techniques for Nb₂O₅ anodes are summarized. Next, we review recent progress in developing various types of Nb₂O₅ based fast charging electrode materials, including Nb₂O₅ based mixed metal oxides and composites. Finally, we highlight the major challenges for Nb₂O₅ based materials in the realm of high rate rechargeable energy storage and provide perspectives for future research.

Structural Stability from Crystallographic Shear in TiO2-Nb2O5 Phases: Cation Ordering and Lithiation Behavior of TiNb24O62

The host structure and reversible lithium insertion and extraction of an intercalation compound, TiNb₂₄O₆₂, are described. Neutron diffraction, applied for the first time to TiNb₂₄O₆₂, allowed an accurate refinement of the complex block superstructure, particularly with respect to the oxygen sublattice. Analysis of the transition-metal sites revealed significant cation ordering in the mixed-metal oxide. Electrochemical analysis demonstrated highly reversible lithium intercalation with ca. 190 mA·h·g^-1 after 100 cycles (C/10 rate, 3 months). The effect of the potential window on the capacity, polarization, and reversibility was carefully examined; a minimum voltage limit of 1.1-1.2 V is critical for efficient and reversible cycling. The galvanostatic intermittent titration technique revealed three solid-solution regions, with different lithium diffusivities, in addition to the two-phase plateau that was clearly observed in the V versus Q discharge/charge profile. Lithium-ion diffusion decreases by over 3 orders of magnitude from the dilute lithium limit early in the discharge to the lithium-stuffed phase Li_37.5(1.0)TiNb₂₄O₆₂. Nevertheless, prior to lithium stuffing, TiNb₂₄O₆₂ possesses intrinsically rapid lithium-ion kinetics, as demonstrated by the high-rate performance in thick films of ca. 10 μm particles when interfaced with a carbon-coated aluminum foil substrate. The TiO₂·Nb₂O₅ phase diagram is examined and electrochemical results are compared to related superstructures of crystallographically sheared blocks of octahedra in the TiO₂·Nb₂O₅ homologous series including the H-Nb₂O₅ end member.

Porous TiNb24O62 microspheres as high-performance anode materials for lithium-ion batteries of electric vehicles

TiNb₂₄O₆₂ is explored as a new anode material for lithium-ion batteries. Microsized TiNb₂₄O₆₂ particles (M-TiNb₂₄O₆₂) are fabricated through a simple solid-state reaction method and porous TiNb₂₄O₆₂ microspheres (P-TiNb₂₄O₆₂) are synthesized through a facile solvothermal method for the first time. TiNb₂₄O₆₂ exhibits a Wadsley-Roth shear structure with a structural unit composed of a 3 × 4 octahedron-block and a 0.5 tetrahedron at the block-corner. P-TiNb₂₄O₆₂ with an average sphere size of ∼2 μm is constructed by nanoparticles with an average size of ∼100 nm, forming inter-particle pores with a size of ∼8 nm and inter-sphere pores with a size of ∼55 nm. Such desirable porous microspheres are an ideal architecture for enhancing the electrochemical performances by shortening the transport distance of electrons/Li⁺-ions and increasing the reaction area. Consequently, P-TiNb₂₄O₆₂ presents outstanding electrochemical performances in terms of specific capacity, rate capability and cyclic stability. The reversible capacities of P-TiNb₂₄O₆₂ are, respectively, as large as 296, 277, 261, 245, 222, 202 and 181 mA h g^-1 at 0.1, 0.5, 1, 2, 5, 10 and 20 C, which are obviously larger than those of M-TiNb₂₄O₆₂ (258, 226, 210, 191, 166, 147 and 121 mA h g^-1). At 10 C, the capacity of P-TiNb₂₄O₆₂ still remains at 183 mA h g^-1 over 500 cycles with a decay of only 0.02% per cycle, whereas the corresponding values of M-TiNb₂₄O₆₂ are 119 mA h g^-1 and 0.04%. These impressive results indicate that P-TiNb₂₄O₆₂ can be a promising anode material for lithium-ion batteries of electric vehicles.

Self-Assembled Hemicapsules with Inherent Functionalities: Modeling of a Supramolecular Electrostatic Self-Assembly

An approach to functional self-assembled hemicapsules is described consisting of the use of multivalent (valency > or = 4) and divalent components, the functional groups of which have a relatively weak binding affinity. Electrostatic self-assembly of tetrakis(pyridiniummethyl)cavitand hemispheres (H) and doubly charged anions (A) in polar media gives rise to an equilibrium mixture that consists, as detected with ESI-MS, of hemicapsule H2A3, capsule H2A4, and other ion-pair associates. Fitting 1H NMR data with a model that includes (hemi)capsules and ion-pair associates gave an effective molarity (EM) for the intramolecular assembly of the host (H) with sulfate (A) of 0.19 +/- 0.02 M and binding constants of the functional [2 + 3] hemicapsules H2A3 and the [2 + 4] capsules H(2)A(4) in methanol of 3.25 x 10(12) M(-4) and 3.45 x 10(15) M(-5), respectively. A substantial amount of the functional [2 + 3] hemicapsules H2A3 with respect to [2 + 4] capsules H2A4 is present in solution, with ratios of H2A3 to H2A4 of 5.67-0.43 in the studied concentration range (0.1-25 mM of [H]tot). The [2 + 3] hemicapsules H2A3 built with sulfate linkers incorporate guests between the closely positioned pyridinium planes.